'95 GS500 Everything Stock - Bike idles fine, good plugs, mix screw 3 turns out, when I rev above 4500 rpm (approx) the bike dies, if i release the throttle the bike will sometime huff and puff and return to idle, sometime it just dies completely.

Things I've checked:
I checked the air filter, removed it, no difference (checking for blocked filter)
Floats are spot on using the drain hose check. Carbs are clean, jets are clear.
Valves are adjusted within tolerance.
I think it has to be a fuel issue, I tried it with the choke on as well and still the same issue.
Compression is around 135 on both with a cold engine.
Intakes seem to be fine. The petcock is good, On/Res/Pri, same result.
Carbs are balanced.
Strong spark from the plugs.
Looking through the Airbox I can see the piston on both carbs lifting up when I rev it, as well as the butterflies opening.

I could ride the bike around my yard all day if I stay below 4500rpm, but as soon as I rev it past that it dies on me.

-Could this be a diaphragm issue? (if the piston is moving up from the vacuum of reving it could this still be an issue)
Any help would be appreciated.

Have you thought about the mixture screw? Maybe its too rich or lean i forget what symptoms are associated with each. I know this isn't much information but something to think about. Is everything else stock on the bike?

could be sticking slides or incorrect idle rpm setting. what do you have the hot idle set to? should be 1200-1300

QuoteHave you thought about the mixture screw? Maybe its too rich or lean

Mixture screw seems to be right, according the plugs which I think is the most accurate way to check..? the plugs are close to perfect. I have fiddled between 2-4 turns and 3 out seems to be right.

Quote from: black and silver twin on April 26, 2010, 11:51:45 AM
could be sticking slides or incorrect idle rpm setting. what do you have the hot idle set to? should be 1200-1300

The idle is set at around 1300. Like I said the bike runs fine below 4500rpm. throttle is responsive and no coughs or sputtering. Slides seem good.

Hi,sounds like carbs to me,poss`slide needles not in correctly,wrong height.O rings under the top caps in? diaphram?blocked pilot jets.

Quote from: JAY W on April 26, 2010, 02:01:58 PM
Hi,sounds like carbs to me,poss`slide needles not in correctly,wrong height.O rings under the top caps in? diaphram?blocked pilot jets.

Well I havent taken the diaphragm side of the carbs apart yet, haven't had to, so the o rings are still in, jets are okay. I guessed the diaphragms are torn or pin holed. Was just curious if there may be another reason for this. Before I start chasing more gremlins in the carbs.

on my bike with this same symptom it was the needles and tubes clogged up with old gas crud.   The needles were actually stuck in place in the tubes, and the tube holes were completely clogged.

I would pull the carbs, and check for blockage  in all the passageways with compressed air

Hey guys thanks for the suggestions.
-Blew out the carbs with compressed air, checked the diaphragms(held up to the sunlight to check for pin holes..no tears or holes), needles seem to seat fine, checked by blowing air through them, slides seem fine, the vac o-rings are in and good.  jets are clear, (older bike so only two jets, pilot and main) blew all the passages out, cleaned the crud. so now i'm at a loss so to speak.

could this be a petcock issue? I have a clear inline filter and can see there is constant gas getting to the carbs, but perhaps when the vacuum is increased is when the issue occurs. i might try bypassing the petcock all together and run the fuel straight from the tank to the carbs for a quick test. any thoughts on this?

I've looked through the Haynes manual and it just talks about the obvious cylinder issues and air filter, plugs, valves etc. all help is very appreciated, this is my second GS so i'd like to think i know my way around the bike to some intermediate degree.  

Banana in the tailpipe?

Is someting being sucked up over the airbox inlet?  Over one of the carb mouths?

Tank venting properly?  Which would bring us to the petcock issue.

California models sometimes need the rollover valve cleaned (by the battery box), or the canister disconnected (when it gets full of gas the charcoal inside becomes mud and clogs, not allowing any venting).

I have the tank off and to the side, nothing blocking the carbs, I guess i can leave the gas cap open and see how that affects it with the venting.

Last valve adjustment?

Tried the cap open, by passed the filter and the petcock, running straight from the tank. No difference.
It's definitely starving for fuel, it revs to around 4500 -5500 rpm, any higher and it will start to die from starvation. With the choke on and a very slow increase in throttle I could get it up to around 8500rpm before it would starve of fuel and conk out.

Quote from: scratch on April 27, 2010, 03:00:27 PM
Last valve adjustment?

I checked the valves last week, all were within tolerances. I'll check again now before I run out of daylight and report back. Maybe I made a mistake

Checked the valves, all is within spec. It must be the carbs. Guess i will try and soak them. Like I said before I blew out all the passages. Cant think of anything else to do.

it could possibly be a clog in the in-tank petcock.  pull it off and give it a good shot of air through both passages

I actually tried that yesterday, took it off, washed the tank out and cleaned the tank petcock, no issue with that, comes out nice and strong.

what size jets are you using? maybe youre running too lean?

whatever the stock is, the bike is all original. I think they are 37.5 and 122.5, something like that.

do the electricals cut out too? or just the motor but lights and all are still on?

just another random question to find more symptoms.. xD

for sure bypass your petcock....I never got my right cylinder to fire reliably until I replaced the old 93 factory original with a Honda dirtbike tap.    Instantly had full performance afterwards.