Jetting advice needed

[JimmyD]

I've got a '78 GS 750. It has a 4 to 1 Yosh exhaust and 4 k&n filters running stock jets. Any ideas to get me started towards the correct jetting? I'm at sea level. Thanks

[Mike_H]

Go buy a dynojet kit...

[chef1366]

Go buy a dynojet kit...

Ditto
Buy that and then repost and we'll help you get it dialed in.

http://www.dynojet.com/jetkits/motorcycle/suzuki.aspx

[andrewclaycomb]

His bike doesn't have CV carbs. I have almost the same setup and would also be interested in some tuning tips. Also will some GS750 carbs off of an 80s CV model match right up to the older 70s bikes?

[JimmyD]

I checked the Dynojet web site but only CV carbs listed. Any other ideas? Thanks

[chef1366]

Follow steps in order....First, dial in:

• 1. Top end (full 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!

[ta473]

jet question

i went to this link http://www.wgcarbs.com/ under tech forum, ask the same question there youll get a very fast and good reply

[KEITH KRAUSE]

i went to this link http://www.wgcarbs.com/ under tech forum, ask the same question there youll get a very fast and good reply

With all respect to George, he seems to have a great carb cleaning/rebuilding business going on, but much of his jetting advice, especially on VM carbs, is too lean in my opinion. Perhaps he has made changes though?
JimmyD, I've always found the Dynojet jet needles to be the way to go, but, they're not available. So give the stock needles a try. My best guess at your correct jetting...
127.5 or 130 Mikuni mains, I'd probably go with the 130's,
Place the jet needle e-clip in the bottom position (#5 furthest from the top if you have stock needles), which raises the needle the highest it can go. Be sure the thicker plastic jet needle "ring" goes directly above the e-clip and the thinner plastic spacer goes under the clip as this is the factory order,
Bench synch the carbs. If needed, do a search here for some detailed VM synch advice I've made at this site.
Try adjusting the pilot fuel screws underneath to 1 1/2 turns out from LIGHTLY seated. Be sure the sharp tips are not damaged. If no joy at as much as 2 1/2 turns out, then you need to go up to 17.5 pilot jet and return the screws to about 1 turn out and test,
The side air screws are always adjusted using the highest rpm method. Initially set them to 1 3/4 turns out and then fine tune with the method,
Be SURE to remove the two floatbowl vent lines and leave the ports open. This is necessary to avoid fuel starvation, especially at high speeds/windy conditions, when running K&N's. This is also recommended in the Dynojet jet kit info.
A vacuum synch is then necessary to get accurate plug/performance reads.
NGK B8ES plugs, gapped at .027",
Be sure the valves clearances are between .03mm and .08mm and the ignition timing is correct before the vacuum synch. Don't over-oil the K&N's. If you have the K&N ovals, I like to mount them so the middle two are "up" and the outer two are normal or "down". Better breathing, just my opinion.
Test at full throttle for the mains, 1/3 throttle for the jet needles, minimal throttle position for the pilot circuit. Chop test for best reads. Do what the plugs/performance say.
Don't expect the jetting to go well unless the bike is otherwise tuned well as I've said. Clean carbs, good inner o-rings from Robert Barr and correct float levels are mandatory too. I always suggest new o-rings for the manifolds too. Of course, the manifolds must be in good condition too. Apply a coat of hi-temp bearing grease to the o-rings. If you replace the useless Phillips screws that hold the manifolds with Allens, them you can correctly torque the manifolds to 6 ft/lb.
Please get back to us if we've helped you with the jetting.

[JimmyD]

Great! I'll make all the checks to the bike and order the jets. I'll let you know. Thanks for the help