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!

  • have you had any luck with resolving your issue? May 26, 2016 at 9:51
  • Haven't had a chance to work on the bike yet, hopefully tomorrow or this Weekend I should have some time, I'll update you on what I find!
    – Iqbal Khan
    May 26, 2016 at 14:17
  • Any updates with your bike? Jun 6, 2016 at 16:45
  • 1
    My bad, forgot to update in these comments lol But I got everything sorted out fine, followed some of the trouble shooting tips from your answer, and I cleaned the carbs one last time to be sure, also cut some mounts up on the air box so I could push it more flush up against the carbs without them stopping it, and made sure to screw it down real tight, Bike works amazingly now, there's still a slight vacuum leak from 1-2 fittings but at least now I know exactly what it is, just gotta decide if im gonna fix it or buy a new airbox, thanks for your help man!
    – Iqbal Khan
    Jun 6, 2016 at 17:16
  • I'm glad it's fixed! Have a great day! Jun 6, 2016 at 17:35

3 Answers 3


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

  • eegads this is a good answer! :-)
    – Moab
    May 25, 2016 at 22:33
  • Makes a lot of sense, I heard a lean condition can cause it to heat up more too, and the bike was running hotter than usual at idle so this would make sense, gonna go ahead and try to fix the airbox fittings or buy a new one, and start going through the trouble shooting to see if I can find any vacuum leaks, ill keep you updated on my findings! Thanks for all the help
    – Iqbal Khan
    May 26, 2016 at 14:20
  • Great. Feel free to ask new questions as you go down your path. I worked on a lot of these in the late 80's and early 90's. Best of luck to you! May 26, 2016 at 17:30
  • I had a chance to check the bike out and since I had time I pulled the carbs, cleaned the jets and made sure the floats were all set right, I also checked the vacuum tube leading from the top of the valve covers to the carbs and one connection was loose so I tightened it, made sure everything else as in order and put my air box back on and fixed some of he connection issues so now it fits lot snugger. The bike idles way better, doesn't die with he choke too, but under load it still bogs down around 5-7k from, bogs down alot on full or half throttle too
    – Iqbal Khan
    May 29, 2016 at 21:47
  • From what I've read people at its a jet issue or something being dirty or clogged it seeing that I've cleaned it so much I doubt this. I tried spraying some areas down with carb cleaner to ee if it idled weird while running, and nothing, I know it is running s little lean cause it's a little hotter and backfires slightly but I can't find what could be causing it
    – Iqbal Khan
    May 29, 2016 at 21:49

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:

  • Check fuel supply
  • Check float level
  • Check idle mixture
  • Check that jets are clear
  • Check that all vacuum hoses are routed and connected correctly
  • Check that carb diaphragms are intact and needles slide easily
  • Check that air filter is clear
  • Fuel supply is working how it should, jets should be clear, I cleaned them very recently, all the hoses were routed correctly, and the diaphrams are good and slide well, I haven't looked at the actual air filter much, but would it make much of a difference seeing that the air box boots are only about halfway connected? The idle was also set to about 500-600 Rpm, I still have to check the book on the correct idle speed, but the bike seemed comfortable idling at this speed.
    – Iqbal Khan
    May 25, 2016 at 14:05
  • As for the float level, if it was letting too much gas in, could a sign of that be a lot of gas being dumped out of the carb overflow tubes? If so how does one set the float level? Pretty sure there's a bunch of guides on it, just wondering if you had any tips
    – Iqbal Khan
    May 25, 2016 at 14:06

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.

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