GS500 Exhaust/Air Filter/Rejet Help

[JonnyFoxtrot]

Lo all, as some of you may have read in my other threads I'm a new owner of a 1992 GS500E and so far really enjoying the bike (minus the paint scheme which I'm not mega keen on, Purple with Pink decals). It's only got 12k miles and its a relatively unabused example owned from new by a family friend and barely used at all in the last 2 years. (Pics to come sometime soon)

So far I've done all the general maintainance I'm confident doing i.e oil and filter change, clean and oil chain, new iridium spark plugs and a good clean!

I'd been lurking on various GS500 sites for a while before I got it, trying to get as much info as I could from posts and the GSTwins Wiki and already had a new exhaust (a UK performance full exhaust from a company called Viper), K&N lunchbox air filter and new jets for rejetting ready to go before I'd even collected the bike.

All the mods went on in one go, full exhaust, lunchbox filter and I rejetted to 40Pilot/150Mains/Needle set 1 setting higher/3Turns out on Air screw (stock is 140/122.5/NeedleStandard/2-2.5Turns), whilst also giving the carb a good clean. This is pretty much the standard setting for the carb as many GS owners report with a lunchbox and exhaust and I know loads of people run it fine. The GS500's are known to run lean from standard and obviously with the extra air in/out from the mods the above setup is quite a bit richer.

BUT

The bike now runs like absolute pap. When its still cold and warming up and I come to a stop the bike will nearly or completely stall out so I have to hold the revs open slightly at a stop to stop it dying. Its also plagued with rough spots through the range, sometimes bucking, struggling or overcoming a flatspot, and power seems to be topping out at about 9k revs (11k limit). Once the bike is warmed up it will sometimes have an even stranger problem of when I come to a stop (or start the bike up again if its been off but still warm) the revs will shoot up on their own to about 4k and then over 6-7 secs slowly return back to the 1.2k idle. I also changed the spark plugs after the new setup and the old plugs were black with soot so I'm guessing its running rich, though to be fair I dont know how they looked before I installed the new setup. I will check how the new plugs are looking and update this tonight.

I'm loathed to put it back to stock but if I cant sort out the problems then I'm going to bite the bullet and just put it back to normal to save the headache with stock filter and stock exhaust with a slightly more ideal jet setup (40/125/1Washer/3Turns). I can try to have a fiddle with the air mixture screws though I'm not sure how to really diagnose optimal settings for the screw. Am I right in thinking that with the air mixture screw all the way closed before your about to turn it out however many turns it is at its leanest and as you turn the screw to your desired setting 1-2-2.5-3 turns that your making the mixture richer as you go? Or have I got it the wrong way round?

I've also been told to check for air/exhaust leaks so will aslo check this tonight and update.

Any advice on the problems I've got with the current setup would be greatly appreciated and I appreciate they are pretty bike specific problems but I'm hoping the cold idle dying and the warm random rev shootin up problems may be quite generic and something i've screwed up and can fix.

Also yes I have posted the same on the GS forums

TIA!
Jon
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'89 Honda NS125F (SOLD) -> '92 Suzuki GS500E (SOLD) -> '03 Kawasaki ZX6R B1H (SOLD) -> '96 VFR 750 RC36 (SOLD) -> '02 Honda CBR1100XX Blackbird

[Odie]

sounds like a leak between the carbs and the inlet manifolds, tighten the clips up unless the inlet rubbers have got to hard, if they have, take them off, put them in boiling water to soften them up and then refit
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You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[JonnyFoxtrot]

[D O G]

[P.addy]

Just to help...the GS500 I had...and the 2nd one i had... both were absolute pigs to warm up... always took a while! Also...they must all be very prone to air leaks as mine had the same

[JonnyFoxtrot]

Thanks for the input guys, appreciated. At least I know its most likely going to be the air leak then.

What's the best way to test for it? Spray WD40 round suspect areas and look for bubbling?
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'89 Honda NS125F (SOLD) -> '92 Suzuki GS500E (SOLD) -> '03 Kawasaki ZX6R B1H (SOLD) -> '96 VFR 750 RC36 (SOLD) -> '02 Honda CBR1100XX Blackbird

[Odie]

[0l0dom0l0]

I would pull out the plugs again. If they are black, you have over compensated for the mods you have done. if this is the case Personally i would put the mixture screw to 2.5 turns out and drop it down a notch on the needle to lean it out abit. if the plug is still coming up black, then drop it down to a 145 main jet.

This should cure the problem, but thats obviously only if this is the problem in the first place. your symptoms do sound like that of leaking inlet rubbers, so thats the place to start as advised.

[D O G]

[JonnyFoxtrot]

Hi all, thanks for all the tips so far, sorry its been a while since my last post but I know have an update.

So I checked for any air leaks and found none. The exhaust still leaked from the manifold from time to time so I put that back to standard and figured I might as well put the stock airbox back on as well. Because of this I rejetted the carbs back to stock (or the ideal stock setting as given in the wiki) to 40/125/1Needle/3Turns. BUT the thing is still running like crap and really rich. I put brand new plugs in again and they fouled almost straight away with a black sooty deposit which was sometimes wet with petrol/gas. The exhaust was coughing and spluttering even at a decent idle (around 1200) and the revs would come down quite slowly if you rev it.

So I'm at a loss again. The bike is basically all back to stock, still running like crap and now I've no idea where to start. Two questions though:

When I rejetted for the second time I tried to check the float settings but found this quite a difficult task to get exact. I tried the usual method of carb upside down, lean it over until the metal clip of the float needle started to touch the metal tab of the floats. Then I tried checking the height using adjustable calipers measuring from the rim of the carb at gasket level to the top of the float plastic. They seemed to be only 1-2mm out but when i tried to adjust them by bending the metal tab it seemed to make hardly any different to the height even with the tab bent harshly up or down. Is there something I'm doing wrong?

Second question is only the same one again about the idle air mixture screw. If its screwed all the way in to then make adjustments out to 3 turns am I making it 3 turns richer or 3 turns leaner? As in could i just screw it to say 4 turns and this would be leaning the mixture to compensate for the richness or would I just be making it richer and therefore I should be screwing it in to only 2 turns.

As always any help/advice appreciated
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'89 Honda NS125F (SOLD) -> '92 Suzuki GS500E (SOLD) -> '03 Kawasaki ZX6R B1H (SOLD) -> '96 VFR 750 RC36 (SOLD) -> '02 Honda CBR1100XX Blackbird

[Bezzer]

The idle screw is just that- affects the idle circuit and has no real affect on mid and upper ranges of the throttle so it will be having no bearing on it's general running or making it as rich as you say it is.
Idle screw - low revs
Needle - middle throttle range
Main jet - upper throttle range
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I used to be indecisive but now I'm not quite so sure.

[JonnyFoxtrot]

I get what your saying and to be fair the running is not too bad up the top, its actually the idle and low revs that concerns me the most. The bike should be lean as standard (apparently the GS500's all are) so the jets should be spot on. The only things I can think of are the air mixture idle screw and the float heights?
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'89 Honda NS125F (SOLD) -> '92 Suzuki GS500E (SOLD) -> '03 Kawasaki ZX6R B1H (SOLD) -> '96 VFR 750 RC36 (SOLD) -> '02 Honda CBR1100XX Blackbird

[Odie]

EASY TEST to set up your mixture screws.

If at 4,000rpm; (steady state riding)
lean surging happens you need to richen the mixture (going outward 1/4 turn at a time.

A rich problem gets worse as the bike heats, lean gets better as the bike heats up.

Now to do the test: Here’s how it works (and it will get you dead spot on!)

Set your idle below 1,000 (6-700rpm works good)
1) Blip the throttle quick, lightly. If the idles rises up then drops below the set point you selected (6-700), you are too RICH, you need to go IN with your mixture screws 1/2 turn at a time.

2) If you blip the throttle and the needle rises, but then hangs for a second before dropping to the perfect set point of 6-700rpm you are too LEAN, you need more fuel, go OUT 1/2 turn at at a time with your mixture screws.

Now use your head. If you turn the mixture screws 1/2 turn one-way or the other and the problem goes back and forth from lean to rich, you need to turn less.......try 1/4 turn.

Once done, you should be able to set your idle back to normal, give a blip and have it rise fast, and drop fast right to the perfect spot. Now your mixture screws are set

If you run into problems and it just won't work. Make sure you have not intake leaks, bad exhaust leaks. Intake leaks (carbs to motor) cause the most frustration.
____________________
You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[Odie]

Mixture screw settings:

Uk models: K, L M, N and P = 2 1/4 turns out
UK models: R and S = 1 7/8 turn out
UK models: T and V = 2 turns out

Main jet sizes:

UK models: K, L, M, N and P = 120
UK models: R and S models: left = 125, right = 122.5
UK T and V models: 115
____________________
You are so ugly when you were born the doctor said "I?m gonna drop it, if it falls is a rat, if it flies is a bat."

[0l0dom0l0]

If everything is back to stock and its running very rich, then just try putting the after market parts straight on. Change the plug before you start it up for the first time with the mods, take it out after a ride and see what it looks like?

[JonnyFoxtrot]

Thanks guys, solid advice as usual.

Will try your guidance on the carbs odie and report back. Ta!
____________________
'89 Honda NS125F (SOLD) -> '92 Suzuki GS500E (SOLD) -> '03 Kawasaki ZX6R B1H (SOLD) -> '96 VFR 750 RC36 (SOLD) -> '02 Honda CBR1100XX Blackbird