Motor Vehicle Maintenance & Repair Asked by Iqbal Khan on March 24, 2021
I recently did a head gasket repair on my 1986 Kawasaki GPZ1000RX and now that everything is back together it starts up fine, idles fine, and revs high without a problem while in neutral, but when I put it into gear and started driving it it’ll drive fine when giving it slight throttle, but when I get to the higher revs, or when I open the throttle too quickly it bogs down badly, and starts stuttering, but eventually starts revving higher and leveling out. It sounds as if my carbs were dirty or clogged but they were literally cleaned a day prior to me test driving the bike.
I previously cleaned the carbs thoroughly before the second time cleaning it and it had sat for a while after that so I cleaned them again and found a little bit of dirt and stuff, and after trying it out after I cleaned it the bike bogs down a little less, but it’s still noticeable, especially around the 6k range. The bike also dies whenever I press the choke in while it’s warmed up, even slightly. I tried starting the bike with the choke and it’ll work until it warms up a little bit, then anything after that kills it.
This never happened before, only just now. Bike used to have perfect throttle response before taking everything apart, and now it just seems to be bogging down more. I’m sure I got the timing right, and everything is in order, just not sure on what could be causing the bogging down, unless a carb passage is blocked by a foreign particle or something.
Side note: the air box fittings are also a little warped and they don’t fit completely over the carbs, this didn’t affect the carbs much before but I’m.not sure if it could be affecting it now along with other things maybe.
Any help would be great, thanks!
You are describing a lean condition
The more you open your throttle the more vacuum you are creating in your carburetor venturi and your intake manifold. When you are operating at higher RPM any unmetered air leaks into your system can become more obvious.
Unmetered air is air that is getting into your system after the fuel has been delivered. If you have unmetered air getting into your system between the butterfly of the carburetor and the cylinder head this will create a lean condition.
All of the rubber components of the fuel system like vacuum hoses and intake manifolds that you mount the carburetors to are made of rubber. Since your MC is 30 years old now if none of those components have been changed out they are more than likely highly degraded and possibly cracked in places. When you disassembled the engine and removed the head you must have removed the rubber intake manifolds. After 30 years of being in the same position it's very possible that they cracked a bit while pulling them off. As well, you could have missed some vacuum lines when you reassembled the fuel delivery system.
Here is a QA regarding lean and rich setting of MC carburetors. About halfway into the response there is relevent information regarding how to test for air leaks.
Action Items
Check all of your vacuum lines and vacuum plugs for carburetor synchronization. the vacuum plugs are on the head just after the rubber intake manifolds. The petcock has a vacuum line as well as part of the emission system.
Check the manifolds for cracks
Ensure the bands used to tighten the manifolds down on the carburetor are secure and have not bound up the manifolds.
Fix your airbox mounting to the carburetors
You said
the air box fittings are also a little warped and they don't fit completely over the carbs
Your airbox is metering air and is the first step in a process of consuming air and fuel. The system requires the resistance of the air filter in order to get the proper vacuum to 'suck' the fuel out of the float bowl and create the proper venturi effect.
The mounting of the airbox issue you have will create a small lean effect. This might be a game of inches for you so just get that one out of the way. It's not correct, you are getting dust and debris in your engine that is doing slow damage by not having proper fitment. Fix it so you know it's not contributing to your issue. Pick the low hanging fruit first.
One last thing
Do not go and start adjusting anything at this point. It ran fine before. There is something wrong with the assembly or a component. Do not adjust your floats. Get it back to where it was. The moment you start tweaking everything is the moment you lose the state of the vehicle. Move onto that activity as a last resort. Troubleshoot, gather data, draw a conclusion, remediate.
Good luck
Correct answer by DucatiKiller on March 24, 2021
It sounds a lot like a fuelling issue to me, I'd suggest a systematic approach to the carbs. Get the workshop manual for the bike and try the following:
Answered by KnowledgeQuest on March 24, 2021
Go a size smaller with main jet. You may be getting too much fuel at mid rpm. My XL350 had a flat spot at 3500rpm. Going from a 130 to 120 main jet smoothed things out.
Answered by Normie on March 24, 2021
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