GS550 VM carb pointers

[byrdman76]

I have a 79 GS550 with pods, a V & H meg, 102.5 mains, and a complete Dyna ignition with coils.

Valves are in spec, carbs were synced.

I had my needles in the lowest position and had my fuel pilot screws 1.5 turns out. Mixture screws were around 2 out.

Idle and 6000 rpm and above were great. 3500-4500 rpm the bike would eventually begin to load up on a long ride about 50 miles and sometimes loose a cylinder. Get it out on the HWY and it would clean its self up.

I figured i would try to raise my needle one position and re-tune. I fired the bike up and it started revving pretty high. I could bring it down but i would have to adjust my mixture screws 1/2 - 3/4 out which would load really quick. Any more out on my mixture screws and I couldn't keep the idle down below 2-3000rpm (throttle adjustment not making contact)

I'm going to adjust my fuel pilots to 2 full turns out and see how it does tonight but I was looking for a little input from you guys.

Thanks,

Byrd.

[hampshirehog]

You need to ignore what the bike is doing at different revs, it's how much twist on the throttle that tells you how your carbs are behaving. If you had a decent slow run before you should leave that area (fuel screws and air screws) alone, and if you had good WOT then your mains are probably fine (though 102.5 does seem a bit big).

Sounds as if your needle or cutaway is the problem but you need to tell us what is happening at throttle opening positions (1/8, 1/4, 1/2).

[byrdman76]

The rev's occured on their own after I lowered the needles down a notch. No throttle twist, and I adjusted the idle adjustment to be completely off. Still the bike would rev once I opened the air screws more than 3/4 open.

The bike revs like it has an intake leak between the carbs and head (running lean I suspect). The o-rings on the intake boots have been replaced recently and the boots are in very good shape. So I'm pretty sure there are no intake leaks.

I would say 1/4 throttle is where the bike begins to load up - but like I said it takes about a 30 - 50 mile ride to begin to notice performance issues. I lowered the needle to try to correct this issue.

I had hoped to close the fuel screws by 1/2 turn in (1.5 out total) and be able to bring the bike back in range on my idle circuit. This would hopefully allow me to be able to use the new needle adjustment to correct my midrange issues.

Thanks,

Byrd

[byrdman76]

I meant to say take the fuel pilot screws out to two full turns from 1.5 in my last post. Regardless I did and it didn't correct the issue. It helped a little I could get it to settle down and idle but if i touched the throttle it would rev to 5 grand unless i blipped the throttle and i could coax it to settle back out.

My question is am I better to return the needle to to the lowest setting and try to reduce the pilot to 1 full turn out, or should I leave the needle where it is and take the fuel pilot out more and see if I can tune it in that way.

Any thoughts?

Thanks,

Byrd

[KEITH KRAUSE]

Sounds like you're rich on the jet needles.
I'm not sure if your factory set jet needle e-clip position was # 3 groove from the top, or #2 from the top. In either case, I believe 2 positions richer than factory will work or get you closer. I'm also not sure what exact parts your jet needle assembly has but if you have the design that uses 2 plastic spacers (1 thicker and 1 thinner), then be sure the thicker spacer is placed directly on top the e-clip and the thinner is under the clip.
If any doubt as to what parts you should have, look up Bikebandit's diagrams or ask someone here with more info.
Test the jet needles mixture at 1/3 to 1/2 throttle position range only. Check the plugs and do what the plugs/performance say to. Keep in mind that accurate plug/performance reads also depend on a good vacuum synch. The VM carbs slides must be synced again every time you disturb the jet needles.
Also, be sure the side air screws are carefully adjusted using the highest rpm method.
Your stock #15 pilot jets may be too lean but always try to make them work with richer pilot fuel screw (underneath) adjustments first. Example, if your stock factory pilot fuel screw position was 3/4 turn out, try approx' 3/4 more, to 1 1/2 out. If you continue to need more richening after minimal throttle position testing but you've reached apoprox' 3 to 3 1/2 turns out, then you need to go to a 17.5 pilot jet.

[byrdman76]

Sounds like you're rich on the jet needles.
I'm not sure if your factory set jet needle e-clip position was # 3 groove from the top, or #2 from the top. In either case, I believe 2 positions richer than factory will work or get you closer. I'm also not sure what exact parts your jet needle assembly has but if you have the design that uses 2 plastic spacers (1 thicker and 1 thinner), then be sure the thicker spacer is placed directly on top the e-clip and the thinner is under the clip.
If any doubt as to what parts you should have, look up Bikebandit's diagrams or ask someone here with more info.
Test the jet needles mixture at 1/3 to 1/2 throttle position range only. Check the plugs and do what the plugs/performance say to. Keep in mind that accurate plug/performance reads also depend on a good vacuum synch. The VM carbs slides must be synced again every time you disturb the jet needles.
Also, be sure the side air screws are carefully adjusted using the highest rpm method.
Your stock #15 pilot jets may be too lean but always try to make them work with richer pilot fuel screw (underneath) adjustments first. Example, if your stock factory pilot fuel screw position was 3/4 turn out, try approx' 3/4 more, to 1 1/2 out. If you continue to need more richening after minimal throttle position testing but you've reached apoprox' 3 to 3 1/2 turns out, then you need to go to a 17.5 pilot jet.

Thanks Keith thats exactly what I was looking for (how far can i take the fuel pilot screw out before I need to go up the pilot jet)

The needles originally were set up with the e-clip in the in the 3rd notch. I dropped the e-clip down to the bottom to the richest position when i set the carbs up initially. Small washer on bottom, large washer on top.

Like I said the bike runs great at WOT and idle, but my mid range (1/4-1/2 throttle) will load up after time. Therefore I raised the e-clip one notch to try and remedy this small problem.

I will keep the needle position where its at and try to open the fuel pilot screw 1/2 turn at a time until I reach 3.5

If this doesn't work would it make sense to try and raise the needles back up to the highest position and close the fuel pilot screw? How low can you go on the fuel pilot screw 1/2 turn out?

Or should I keep the needles where they are if my midrange plug chops look good and increase the pilot jet to try and correct my lean condition on the pilot circuit?

One more question - What is making the bike high rev (5000rpm) when when I apply any throttle? I can blip it and bring it back down to idle - am I missing something or is this just a lean condition on the pilot circuit. I am positive that the intake area is sealed.

Thanks for your help.

Byrd

[byrdman76]

I ran the fuel pilot screws 3.5 turns out with pretty much the same results - too lean.

I raised the needles back up and adjusted the fuel pilot screws to 1 full turn out. The bike seems to run better - only time and plug chops will tell.

I did also realize that my issues were more around the 1/8 throttle or less leading me to believe that maybe I just had my pilot circuit a little too rich having the fuel pilot screws 1.5 turns out.

I'll keep you posted.

[KEITH KRAUSE]

If the bike is in otherwise good tune, you know a pipe (if freer flowing) and the pods will require a minimum of 1 position richer on the jet needles and usually more than that (1 1/2 to 2 positions). I can't say I'm surprised that 2 positions may be resulting in a too rich mixture. If so, then you know position 4 1/2 (by using an approx' .022" spacer) or position 4 will work. I'm thinking you aren't testing correctly though. Seems to me you're combining the pilot and jet needle circuits and not properly separating the two circuits for a positive test.
Since it appears the main jet is operating well, test at 1/3 to 1/2 throttle and you can determine what to do with the jet needle position. Then worry about the pilot circuit, though I do suggest some typical ballpark adjustments (pilot fuel screws approx' 3/4 additional turn out from "stock" and the air screws set using the highest rpm method).
I'm not sure what you mean about your bike "revving high" at 5K when you apply any throttle. You're saying if you open the throttle at all, the rpm's will jump to 5K and stay there?? But a quick blip will force the bike to idle normally again (which should be approx' 1,050 rpm's)? That would sound like a throttle cable or mechanical issue making something stick under certain conditions. A faulty advance timing mechanism can cause similar symptoms too. Hit the ignition with a timing light to see if the ignition timing is operating smoothly, which is part of basic maintanance checks before attempting to re-jet. If good, check cables and carb throttle pulley/slides for correct operation.

[byrdman76]

If the bike is in otherwise good tune, you know a pipe (if freer flowing) and the pods will require a minimum of 1 position richer on the jet needles and usually more than that (1 1/2 to 2 positions). I can't say I'm surprised that 2 positions may be resulting in a too rich mixture. If so, then you know position 4 1/2 (by using an approx' .022" spacer) or position 4 will work. I'm thinking you aren't testing correctly though. Seems to me you're combining the pilot and jet needle circuits and not properly separating the two circuits for a positive test.
Since it appears the main jet is operating well, test at 1/3 to 1/2 throttle and you can determine what to do with the jet needle position. Then worry about the pilot circuit, though I do suggest some typical ballpark adjustments (pilot fuel screws approx' 3/4 additional turn out from "stock" and the air screws set using the highest rpm method).
I'm not sure what you mean about your bike "revving high" at 5K when you apply any throttle. You're saying if you open the throttle at all, the rpm's will jump to 5K and stay there?? But a quick blip will force the bike to idle normally again (which should be approx' 1,050 rpm's)? That would sound like a throttle cable or mechanical issue making something stick under certain conditions. A faulty advance timing mechanism can cause similar symptoms too. Hit the ignition with a timing light to see if the ignition timing is operating smoothly, which is part of basic maintanance checks before attempting to re-jet. If good, check cables and carb throttle pulley/slides for correct operation.

I marked the throttle range in 1/8th's from 0 to WOT. This helped me realize that my issue was more in the pilot circuit range. The pilot screws were adjusted to 1.5 turns out - this seems to have been too much even with the pipe and pods. I reduced it back to 1 turn out and it seems much happier since i did this in conjunction with raising the needles back up.

The lower needle position was causing the idle to race - I made sure my slides, cables and throttle were all clean and correct since i first thought the same thing "somethings hanging". The needle position was causing the issue with high revs. Once I raised the needle back up all the problems with revving up went away. Its crazy how a slight adjustment like lowering the needle can have such a big effect on how the bike runs.

I'll let you know what results I get from my upcoming plug chops.

Thanks,

Byrd

[KEITH KRAUSE]

The way the VM carbs are designed, the jet needle doesn't regulate or have any effect on anything close to true factory recommended idle.
Adjusting the jet needle will not cause the idle to race. Normal idle is completely controlled by the pilot circuit. Looking at different jetting charts available, you'll see that the jet needle controls jetting at approx' 1/4 to 3/4 throttle positions. There may be a small amount of "overlap" effect when transitioning from the cut-away to the jet needle, but it's small.
The order of the VM type jetting circuits/regulation is starter circuit, pilot circuit, cut-away, jet needle and main jet.
So I know your efforts may suggest that your jet needle position changed your idle, but it's not possible. Only way it could happen is if something else happened while you made changes. You may have inadvertently done something that resulted in a change or possibly, compensation.
This isn't meant to say you don't know what you're talking about, it's only meant to help you understand how your carbs are designed to operate.

[byrdman76]

The way the VM carbs are designed, the jet needle doesn't regulate or have any effect on anything close to true factory recommended idle.
Adjusting the jet needle will not cause the idle to race. Normal idle is completely controlled by the pilot circuit. Looking at different jetting charts available, you'll see that the jet needle controls jetting at approx' 1/4 to 3/4 throttle positions. There may be a small amount of "overlap" effect when transitioning from the cut-away to the jet needle, but it's small.
The order of the VM type jetting circuits/regulation is starter circuit, pilot circuit, cut-away, jet needle and main jet.
So I know your efforts may suggest that your jet needle position changed your idle, but it's not possible. Only way it could happen is if something else happened while you made changes. You may have inadvertently done something that resulted in a change or possibly, compensation.
This isn't meant to say you don't know what you're talking about, it's only meant to help you understand how your carbs are designed to operate.

Not at all Keith - I learn more everyday, and hope to continue with helpful resources like your self. I have allot to learn man :-D

Question for you - there are three holes in the center of the slide. there is also a washer with these same three holes and a tit on it. It mounts between the slide and the "connecting rod"

Which way does the tit face (towards the needle or away from the needle) and what is the purpose of the tit?

Also I assume that the three holes should align with each other so that the one hole that dosent have a screw is wide open to eliminate any vacuum or suctions issues.

Thanks again for all of your help,

Byrd

[KEITH KRAUSE]

Not at all Keith - I learn more everyday, and hope to continue with helpful resources like your self. I have allot to learn man :-D

Question for you - there are three holes in the center of the slide. there is also a washer with these same three holes and a tit on it. It mounts between the slide and the "connecting rod"

Which way does the tit face (towards the needle or away from the needle) and what is the purpose of the tit?

Also I assume that the three holes should align with each other so that the one hole that dosent have a screw is wide open to eliminate any vacuum or suctions issues.

Thanks again for all of your help,

Byrd

The dimple on the thin brass plate goes down. There are some earlier ('76/'77) plates where the dimple rests in a depression (small hole) on the plastic spacer that sits directly on top the jet needle e-clip. I'm pretty sure your '79's plastic spacer (actually called a ring) doesn't have the little hole.
The dimple creates an uneven surface for the jet needle assembly to press up against and the needle cocks to one side under the spring pressure. This is important because if not under this spring pressure, the needle would shake/rotate itself and soon become damaged.

[KEITH KRAUSE]

Oh, and yes. The 3 holes will align when the plate is assembled correctly.