How to Jet Your Carbs - Article

[climber11]

Found this on the net recently: See any errors?

HIGH SPEED JETTING

High speed performance (50 MPH and up) is controlled by the main jets. To make sure you have the right main jets, take your bike out to an open highway where YOU can safely (and legally) open the throttle ALL the way in fourth or top gear. Roll that baby full open and let the bike get to top speed. Again, do this only if you are experienced and comfortable running at top speed, and you are not in danger of injuring yourself or someone else.

A. If at top speed or before, the bike runs rough or begins to hesitant and buck, your main jet is TOO BIG. The mix is too rich (more fuel than the optimum 1 part fuel to 14 parts air) and you are getting an uneven burn and poor performance. Install smaller main jets and take the bike for another test run.

B. If before top speed the engine is running smooth, but has no power, seems to be running hot, and you can't push the bike to top speed, then your main jets are too SMALL. You are running too LEAN a mix (less fuel than the optimum 1 part fuel to 14 parts air) for maximum engine performance. Put in LARGER main jets and take her out for another test run.

MID-RANGE OR MID-SPEED TUNING
Your carburetor needle controls mid-range performance. The needle tapers down to a point that fits into the main jet. The needle is lifted upward (along with the slide) by the carb diaphragm as the air flow increases through the carb. As the needle moves upward it is withdrawn from the main jet, allowing more fuel to mix with the increased air volume.
All that is needed to modify mid-range performance is to place (or remove) one or more small, thin washers between the diaphragm and the head of the carb needle where it's held in the diaphragm. Adding washers effectively RAISES the tapered needle further out of the main jet at all engine rpms and allows more fuel to mix with air to create a richer mix. Removing a washer effectively leans out the air-fuel mix.
There's usually no need to replace the stock needles since they have proven over time to provide good performance and good gas mileage. If you do use a third party needle for increased performance, then anticipate lower gas mileage (in the 28-32 MPG range, versus the 36-42 mpg you can get with stock needles).

Mid-range tuning is accomplished mainly by how the bike performance feels to you. If there's a smooth increase in power as you roll the throttle, then you're there. If there's a slow response or there's no power, then the mix may be too LEAN and you may need to add one or more washers to richen the mix. If the bike accelerate with power, but feels rough, you may be too RICH and need to remove one or more washers to lean out the mix. Now take the bike out for a test spin.

[tkent02]

Found this on the net recently: See any errors?

HIGH SPEED JETTING
High speed performance (50 MPH and up) is controlled by the main jets. To make sure you have the right main jets, take your bike out to an open highway where YOU can safely (and legally) open the throttle ALL the way in fourth or top gear. Roll that baby full open and let the bike get to top speed. Again, do this only if you are experienced and comfortable running at top speed, and you are not in danger of injuring yourself or someone else.

A. If at top speed or before, the bike runs rough or begins to hesitant and buck, your main jet is TOO BIG. The mix is too rich (more fuel than the optimum 1 part fuel to 14 parts air) and you are getting an uneven burn and poor performance. Install smaller main jets and take the bike for another test run.
B. If before top speed the engine is running smooth, but has no power, seems to be running hot, and you can't push the bike to top speed, then your main jets are too SMALL. You are running too LEAN a mix (less fuel than the optimum 1 part fuel to 14 parts air) for maximum engine performance. Put in LARGER main jets and take her out for another test run.

MID-RANGE OR MID-SPEED TUNING
Your carburetor needle controls mid-range performance. The needle tapers down to a point that fits into the main jet. The needle is lifted upward (along with the slide) by the carb diaphragm as the air flow increases through the carb. As the needle moves upward it is withdrawn from the main jet, allowing more fuel to mix with the increased air volume.
All that is needed to modify mid-range performance is to place (or remove) one or more small, thin washers between the diaphragm and the head of the carb needle where it's held in the diaphragm. Adding washers effectively RAISES the tapered needle further out of the main jet at all engine rpms and allows more fuel to mix with air to create a richer mix. Removing a washer effectively leans out the air-fuel mix.
There's usually no need to replace the stock needles since they have proven over time to provide good performance and good gas mileage. If you do use a third party needle for increased performance, then anticipate lower gas mileage (in the 28-32 MPG range, versus the 36-42 mpg you can get with stock needles).

Mid-range tuning is accomplished mainly by how the bike performance feels to you. If there's a smooth increase in power as you roll the throttle, then you're there. If there's a slow response or there's no power, then the mix may be too LEAN and you may need to add one or more washers to richen the mix. If the bike accelerate with power, but feels rough, you may be too RICH and need to remove one or more washers to lean out the mix. Now take the bike out for a test spin.

LOW OR SLOW SPEED JETTING
Now you can tune for low end performance. If you are getting a poor jump or hesitation off the line, or a lot of backfiring on deceleration, then you need more fuel through the pilot jet. You increase fuel flow through the pilot jet by turning the pilot jet screw OUT a half turn on each carburetor. It's best to start at 2 1/2 turns OUT on the pilot screws and then increase the turns OUT 1/2 turn at a time, and go for a test run. Do this until you get the performance you want and/or there are no back fires on deceleration. If you go more than 5 or 6 FULL turns out, you will probably have to install a LARGER pilot jets.

You DON'T need to go top speed to test the mains...
No mention that you can damage your engine running it hard while it is too lean...
There is no mention of plug chops...
Specific recommendations about how many turns of some screw without even saying what bike it is for.
Nothing there really of any use.

[chef1366]

Check out the factory pro website.

throttle / 7.5k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!

• • Select Best Main Jet
• To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the hardest pull at high rpm.
  • If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.

• • • If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
      • In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
        • Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
• 2. Midrange (full throttle /5k-7k)
  Step 1 (Best Main Jet) must be selected before starting step 2!
  • • Select best needle clip position
  • To get the best power at full throttle / 5k-7k rpm, adjust the needle height, after you have already selected the best main jet.
    • If the engine pulls better or is smoother at full throttle/5k-7k in a full throttle roll-on starting at <3k when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
    • If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
    • If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
      • Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.

• 3. Low end (full throttle / 2k-3k)
  Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!
  • • Float height (AKA fuel level & how to..)
  • To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
    • Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
    • If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
    • If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
    • Example: change float height from 15mm to 14mm to richen up that area.

• • • REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
      • Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
        • Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
      • Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
    • If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here
• 4. Idle and low rpm cruise
  • • Fuel Screw setting (AKA mixture screws)
      • There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit. We do have them available separately, too. 800 869-0497 to order -
    • Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation. (pj tuning information)
    • Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.
      • If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.
      • Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
        • NOTE: A rich problem gets worse as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
        • NOTE: A lean problem gets better as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!

[tkent02]

Check out the factory pro website.

throttle / 7.5k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!

• • Select Best Main Jet
• To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the hardest pull at high rpm.
  • If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.

• • • If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
      • In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
        • Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
• 2. Midrange (full throttle /5k-7k)
  Step 1 (Best Main Jet) must be selected before starting step 2!
  • • Select best needle clip position
  • To get the best power at full throttle / 5k-7k rpm, adjust the needle height, after you have already selected the best main jet.
    • If the engine pulls better or is smoother at full throttle/5k-7k in a full throttle roll-on starting at <3k when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
    • If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
    • If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
      • Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.

• 3. Low end (full throttle / 2k-3k)
  Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!
  • • Float height (AKA fuel level & how to..)
  • To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
    • Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
    • If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
    • If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
    • Example: change float height from 15mm to 14mm to richen up that area.

• • • REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
      • Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
        • Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
      • Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
    • If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here
• 4. Idle and low rpm cruise
  • • Fuel Screw setting (AKA mixture screws)
      • There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit. We do have them available separately, too. 800 869-0497 to order -
    • Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation. (pj tuning information)
    • Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.
      • If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.
      • Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
        • NOTE: A rich problem gets worse as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
        • NOTE: A lean problem gets better as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!

Again, no mention that this is only for CV carbs, no mention about plug chops, no mention that you can fry your engine running it hard when it is too lean.

[chef1366]

Again, no mention that this is only for CV carbs, no mention about plug chops, no mention that you can fry your engine running it hard when it is too lean.

I used this to learn how to jet with Keith teaching me about plug chops. I agree that a holed piston is doing no one any good.

[KEITH KRAUSE]

Found this on the net recently: See any errors?

HIGH SPEED JETTING


High speed performance (50 MPH and up) is controlled by the main jets. To make sure you have the right main jets, take your bike out to an open highway where YOU can safely (and legally) open the throttle ALL the way in fourth or top gear. Roll that baby full open and let the bike get to top speed. Again, do this only if you are experienced and comfortable running at top speed, and you are not in danger of injuring yourself or someone else.

A. If at top speed or before, the bike runs rough or begins to hesitant and buck, your main jet is TOO BIG. The mix is too rich (more fuel than the optimum 1 part fuel to 14 parts air) and you are getting an uneven burn and poor performance. Install smaller main jets and take the bike for another test run.

B. If before top speed the engine is running smooth, but has no power, seems to be running hot, and you can't push the bike to top speed, then your main jets are too SMALL. You are running too LEAN a mix (less fuel than the optimum 1 part fuel to 14 parts air) for maximum engine performance. Put in LARGER main jets and take her out for another test run.

MID-RANGE OR MID-SPEED TUNING
Your carburetor needle controls mid-range performance. The needle tapers down to a point that fits into the main jet. The needle is lifted upward (along with the slide) by the carb diaphragm as the air flow increases through the carb. As the needle moves upward it is withdrawn from the main jet, allowing more fuel to mix with the increased air volume.
All that is needed to modify mid-range performance is to place (or remove) one or more small, thin washers between the diaphragm and the head of the carb needle where it's held in the diaphragm. Adding washers effectively RAISES the tapered needle further out of the main jet at all engine rpms and allows more fuel to mix with air to create a richer mix. Removing a washer effectively leans out the air-fuel mix.
There's usually no need to replace the stock needles since they have proven over time to provide good performance and good gas mileage. If you do use a third party needle for increased performance, then anticipate lower gas mileage (in the 28-32 MPG range, versus the 36-42 mpg you can get with stock needles).

Mid-range tuning is accomplished mainly by how the bike performance feels to you. If there's a smooth increase in power as you roll the throttle, then you're there. If there's a slow response or there's no power, then the mix may be too LEAN and you may need to add one or more washers to richen the mix. If the bike accelerate with power, but feels rough, you may be too RICH and need to remove one or more washers to lean out the mix. Now take the bike out for a test spin.

Anything wrong with this article? Yes, the very first sentence about the main controlling things starting at 50 MPH.
I didn't bother to read another word after seeing that first sentence.

[greg78gs750]

this raises a question for me. i have read a MILLION threads about jetting, and have absorbed the wisdom of mr keith krause (who, in my opinion, comes somewhere just below frank in my hierarchy of gs deities..along with nessism, bwringer, cliff, and maybe a few others...) , well, if not like a sponge, then at least like a paper towel. Is there a thread somewhere that pulls together all the general jetting/tuning info together, like the carb rebuild series or the stator papers, that we could make a sticky or put in the garage section?

maybe included could be a diagram of the pathways in the carbs, so semi-retarded people like me can get our heads around exactly what is happening where and when? i am talking about a comprehensive, thorough document distilling the wisdom of the gods into a holy (or would it be unholy???) guide to gs carburetion paradise!!

maybe this already exists in some form, i am sure it does, but lets get it front and center!!

just an idea,

greg

oh, and everytime i see "keith krause" by a post, i read it first.....thanks keith!!

[srsupertrap]

Check out the factory pro website.

throttle / 7.5k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!

If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.

This statement from the Factory Pro is somewhat eyeopening in my opinion not from a jetting standpoint but rather a mechanical aspect: Aluminum's coefficient of expansion is roughly 2X that of the steel and the Bearing are steel, cams are heat treated and jumping on you GS and performing a "cold" wide open throttle pass from 6500 to 8500 RPM with conventional dino oil would take its toll on some mechanical components resulting in glittery oil.

[chef1366]

This statement from the Factory Pro is somewhat eyeopening in my opinion not from a jetting standpoint but rather a mechanical aspect: Aluminum's coefficient of expansion is roughly 2X that of the steel and the Bearing are steel, cams are heat treated and jumping on you GS and performing a "cold" wide open throttle pass from 6500 to 8500 RPM with conventional dino oil would take its toll on some mechanical components resulting in glittery oil.

I hope you don't mean in your garage. I do this on a great staight away. Uphill is even better. Full throttle as long as possible and hit the kill switch without closing the throttle. Coast to the side and pull a plug.

[climber11]

hmmm

seems like I started something. I too would be up for a collaborative sticky on carb tuning. The reason I posted this article, was that it was one of the first ones I read that actually made "some" sense outside the more obvious flaws. Getting something GS specific would really help me wrap my head around things. We could even leave the PODS discussion out if you want!!

Scott

[Suzuki_Don]

seems like I started something. I too would be up for a collaborative sticky on carb tuning. The reason I posted this article, was that it was one of the first ones I read that actually made "some" sense outside the more obvious flaws. Getting something GS specific would really help me wrap my head around things. We could even leave the PODS discussion out if you want!!

Scott

We would need to divide it into VM and CV though. I'd estimate there are a lot more bikes out there with CV carbs and sometimes I feel the VM people are a little forgotten. Though Keith usually comes to the rescue and helps out with his excess of knowledge. But yes it's a great idea.

[srsupertrap]

seems like I started something. I too would be up for a collaborative sticky on carb tuning. The reason I posted this article, was that it was one of the first ones I read that actually made "some" sense outside the more obvious flaws. Getting something GS specific would really help me wrap my head around things. We could even leave the PODS discussion out if you want!!

Scott

The information is here its just not centralized. If you perform a Carb jetting search on the GS Forum you will come back with 20 Pages of previous discussions on the subject. Most of the jetting symptoms & topics have been discussed previously you just have to spend considerable time reading up & digesting all the information.

Before turning screws, swapping jets and moving clips I would suggest starting @ square one: Understand the engineering theory which governs all carb operation, Bernoulli's Principle. Then move to posted articles which cover all the Carb components & circuits from Pilot (Slow) Jets, Fuel Screws (if applicable), Air Mixture Screws, Needle Jets, Jet Needles, Needle Clip position, Throtle Valve cut away, main jet and how they cut in, overlap and work together. For instance I have seen the topic of which way to turn Air Screws: well on the VM Carbs turning the screw in richens the mixture and turning the screw out, opens the passage and leans the mixture. The opposite is true for CV Carbs but don't memorize which is which just find were the air screw is actually located on the carb body and coupled with the appreciation of the theory you cannot be stumped.

So the next step is the application of dialing in a specific set of carbs on a specific motorcycle (with 4 into 1 exhaust & Pods) and this it were it becomes extremely complex. It takes a substantial amount of reiterative testing & retesting because no two motorcycles are exactly the same or operate at the same altiude & humidity levels all of which affect jetting. A 4 into 1's also throw a curve ball into the equation because the exhaust wavefront propogates differently than the OEM exhaust which is often described by owners as a "flat spot". Read chef1366 recent threads on the subject of jetting to gain an appreciation.

IMO, that's why many owners shy away from carb "performance" modifications.

[Flying Fish]

I came across this website recently http://www.motorcyclecruiser.com/tec...ion/index.html
Seems to know what he's talking about but I'm definitely no carb maestro.
Can anyone comment on the usefulness of this article?

[greg78gs750]

The information is here its just not centralized. If you perform a Carb jetting search on the GS Forum you will come back with 20 Pages of previous discussions on the subject. Most of the jetting symptoms & topics have been discussed previously you just have to spend considerable time reading up & digesting all the information.

Before turning screws, swapping jets and moving clips I would suggest starting @ square one: Understand the engineering theory which governs all carb operation, Bernoulli's Principle. Then move to posted articles which cover all the Carb components & circuits from Pilot (Slow) Jets, Fuel Screws (if applicable), Air Mixture Screws, Needle Jets, Jet Needles, Needle Clip position, Throtle Valve cut away, main jet and how they cut in, overlap and work together. For instance I have seen the topic of which way to turn Air Screws: well on the VM Carbs turning the screw in richens the mixture and turning the screw out, opens the passage and leans the mixture. The opposite is true for CV Carbs but don't memorize which is which just find were the air screw is actually located on the carb body and coupled with the appreciation of the theory you cannot be stumped.

So the next step is the application of dialing in a specific set of carbs on a specific motorcycle (with 4 into 1 exhaust & Pods) and this it were it becomes extremely complex. It takes a substantial amount of reiterative testing & retesting because no two motorcycles are exactly the same or operate at the same altiude & humidity levels all of which affect jetting. A 4 into 1's also throw a curve ball into the equation because the exhaust wavefront propogates differently than the OEM exhaust which is often described by owners as a "flat spot". Read chef1366 recent threads on the subject of jetting to gain an appreciation.

IMO, that's why many owners shy away from carb "performance" modifications.

yes, well, this is EXACTLY what i am talking about.....it is here, but not centralized. like doing a google search.....lot of good 2,148,584,234 hits does me... i know there are infinite (well, ok, maybe not infinite....but several? a lot? ) of combinations of pipes, filters, pods, jets, etc. etc. etc. but even to get the basic, stock, setup, or even just "carb theory" (which circuits do what, how the air and fuel flow through the carbs, heck, i would LOVE to see a diagram of where all the tiny orifices and passageways go and what they do....) would be helpful to us newbies, and maybe save you guys (keith, seriously, you are a patient dude....) some head banging answering stupid questions.....](*,)](*,)](*,)......

btw, my 78 750 RUNS!!!!!!!!!!!!!! just need to tune those carbs.......

greg

[KEITH KRAUSE]

This statement from the Factory Pro is somewhat eyeopening in my opinion not from a jetting standpoint but rather a mechanical aspect: Aluminum's coefficient of expansion is roughly 2X that of the steel and the Bearing are steel, cams are heat treated and jumping on you GS and performing a "cold" wide open throttle pass from 6500 to 8500 RPM with conventional dino oil would take its toll on some mechanical components resulting in glittery oil.

Yes. Not smart to run any motor harder if the oil's not hot enough. No point in evaluating performance either or checking the plugs until the bike is fully warmed up.
But I think what he's trying to get across is that generally, if the performance (while on the main jet) improves somewhat or improves a lot as the bike warms up, then that could suggest you need a richer main since conditions do richen as a motor heats up.
His choice of words could be better.