1/2 -wot stumble lag

[Greg manton]

i have a 1994 gs500e with 4400 kms on it. out for my first ride since i got it and it just stumbles with wot, very peppy up to half throttle then falls on its face. would this not be the main jet and needle circuit. and would the carbs being out of sink cause this. im a newby to dual carbs. almost feels like misfire but i really think its fuel.

[invictus43]

Well, a bike that old with low mileage, I think you have to consider other things than just main jets. Main jets don't dirty up THAT much with age..the idle goes more than the main jets. You're missing fuel, air or spark. I'd start with fuel..check the float levels, make sure the floats aren't leaking. Check to make sure the filter isn't clogged. Make sure mice haven't nested in your air box (seriously). Make sure the diaphragms in the carbs haven't cracked or torn. The previous owner may have modded it so you may want to pull the carbs apart just to check on the jets sizes and needles.

Could also be spark..weak battery maybe..bad plugs.loose battery connection. I don't think really the spark is the problem though.

Good luck!

[boyoboy]

hi. i think you're on the right track - half throttle to wide open would have you on the main jets. on the needles would be at a steady cruise.

does the bike have an aftermarket exhaust? is the airbox stock? carb boots well sealed?

you could open the carbs, check that they're clean, check the main jet sizes.

i don't think you would get the bike falling flat at WOT just from out of sync carbs. rough and not smooth running would be an indicator for out of sync.

4400kms? 1994? hmmmmm......

[Greg manton]

i would love to dig into taking this bike apart but i vcant find my starting point. i know the seat must come off first but where the hell are the bolts. i know im missing something. rear farings pop off at the front but you cant access the bolts. on street bikes woulld it be normal to pull the inner rear fender to get to the air box.? sorry i feel foolish ive had dirt bikes apart my whole life and cant beleve how well hidden the access is

[Greg manton]

and it really has 4400 kms old guy lost interest and kept it in his garage what a shame

[invictus43]

i would love to dig into taking this bike apart but i vcant find my starting point. i know the seat must come off first but where the hell are the bolts. i know im missing something. rear farings pop off at the front but you cant access the bolts. on street bikes woulld it be normal to pull the inner rear fender to get to the air box.? sorry i feel foolish ive had dirt bikes apart my whole life and cant beleve how well hidden the access is

Generally the airbox stays in..you can remove the filter but the box itself is usually tough to get out. I don't know that bike specifically, sorry. Usually you can pull back the airbox rubber boots on the carb inlets and that gives you enough space to get the carbs out.

boyoboy is right about they carb synchronization. That will usually show up as a throttle response and smoothness issue. It might stumble slightly as you go off and on the throttle, but not a BIG stumble..more of a slight hesitation. And you'll get a vibration at steady throttle.

[Greg manton]

ok this bike is nasty to disasemble i got it apart air filter like new. the carb slides must be vacuume actuated, are they? they do not move when throttle opened. and this is a vibralion its like im bogging the heck out of it, and only one cyliner fires. but its not under load more with throttle position. how do you sinc the carbs?

[Greg manton]

spark nice and blue. no carbon tracking or cracks on wires or coils.plusgs look ok right side plug has some powdery carbon but left plug nice white/tan

[Greg manton]

diaphram in carbs looks good cant see ant air leaks in intake boots main jets clear so im back to how do you sink carb's

[kneedragger88]

Not carb sync. Thats a problem that presents as a rough idle it wont make any difference. Was it always like this or did it just happen?
Check the float level. Seeing as spark is good and intake track for air is good the problem is either mecahical or fuel related. Check the petcock for flow.
The fact that it runs well until the engine asks for max fuel flow it looks like that may be your problem. An easy way to confirm this is while riding open throttle to the stumble, add choke. Does it improve? If yes then you just confirmed a lean condition.
If you need it looked at give me a shout I work cheap.

[Greg manton]

i just got the bike so i'm not sure how long it has been happening i took it for a short test drive and chalked it up to bad fuel because the bike has been sitting for years. i just tried wot with choke and it bogged down and would have stalled out. i tried a plug chop like i would on a 2 stroke at 8 grand thats as high as it would pull and right plug covered in powdery carbon. im thinking mabey electrical only under load left plug carbon but still tan it is only runing on left cylinder i would guess. do cdi boxes act this way? and hey kneedragger where are you located and when are you free. this sucks bikes down and almost the end of the season arrr!!

[invictus43]

i just got the bike so i'm not sure how long it has been happening i took it for a short test drive and chalked it up to bad fuel because the bike has been sitting for years. i just tried wot with choke and it bogged down and would have stalled out. i tried a plug chop like i would on a 2 stroke at 8 grand thats as high as it would pull and right plug covered in powdery carbon. im thinking mabey electrical only under load left plug carbon but still tan it is only runing on left cylinder i would guess. do cdi boxes act this way? and hey kneedragger where are you located and when are you free. this sucks bikes down and almost the end of the season arrr!!

The fact only one cylinder is fouled is a good clue. It's obviously over-rich or lacks spark. You can try swapping the plugs between clinders to see if the plug is bad. You can also swap the coils to see if that's the problem. My GSXR back in the day had wonky coils that would cause breakdown at higher rpm. Or that carb may have a problem..float level or needle and seat issue..although you would expect to see that at idle as well. Try the coils.

[bubbaloo]

If carbs are clean, fuel filter clean? I would change the plugs , one plug could be breaking down. If plugs are new, your getting lots of fuel and it's still breaking down then it could be the CDI box. The slides are vacume operated so fuel metering is increased as required. It sounds like an ignition problem to me. You are going to have to work your way through and eliminate each possible cause till you find the problem. If you need a hand PM me I'm not too far from you.

[boyoboy]

kneedragger has a very good rep on gta ...give him a try -

[boyoboy]

ok this bike is nasty to disasemble i got it apart air filter like new. the carb slides must be vacuume actuated, are they? they do not move when throttle opened. and this is a vibralion its like im bogging the heck out of it, and only one cyliner fires. but its not under load more with throttle position. how do you sinc the carbs?

oh man...if you think the gs500 bike is nasty to work on.......actually they don't getmuch easier to work on than a gs500. its one very simple bike to work on. do a quick compression test on the bike if you can borrow a gauge. also swapping the coils is an easy check - as suggested by another poster.

[Greg manton]

well im use to motocross bikes few bolts and the whole bike is apart. i am going to switch the coils just avent had time yet. I will do a compression test but cross my fingers its ok. no valve noise and at 4400 kms it should be mecanically sound. also low comp should show up at all rpms i would think and the bike runs like a dream up to 3/4- wot

[boyoboy]

well im use to motocross bikes few bolts and the whole bike is apart. i am going to switch the coils just avent had time yet. I will do a compression test but cross my fingers its ok. no valve noise and at 4400 kms it should be mecanically sound. also low comp should show up at all rpms i would think and the bike runs like a dream up to 3/4- wot

i hear ya....myself - i try to do the fast, easy and inexpensive checks first.

keep us posted on this - as a gearhead i'm really curious to the cause of ...

best of luck to you.

[Greg manton]

ok found something by accident!!. switched coils and of course when doing that i had to remove fuel and the tank. reassembled to go for a ride, pull out on to the street and same thing. i went to the end of my street turned around and let her have it(so i could do a plug chop) and the bike does a wheelie man was i suprizeed. i realized i had not put fuel in the tank so now i know i'm overfueling or will these bikes brighten up without fuel(i guess not without spark). now the carb slides are moving with throttle what do you guys think i should be looking for obstructions. anyone know the correct jets for this bike?

[Jayv]

Silly question how old is the gas in the fuel tank seeing the bike has low kms on it.

[invictus43]

Silly question how old is the gas in the fuel tank seeing the bike has low kms on it.

1994..why? Is that a problem?? That's an interesting development. Could be bad gas..but why don't both cylinders crap out? Strange bike..