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Discussion Starter · #1 ·
This problem has me stumped. Been trying various fixes for months now. Any advice is appreciated

Kawasaki GT550 G4, Keihin CVK32.

Problem
It idles like crap. It hunts a little. Won't keep stable, and revs hang. There is no sign of an air leak. Here's some more detail...

Here's what i've found
• Mixture screws have an effect, but do not kill the engine when all the way in.
• Revs hang at a quarter throttle or more, even with mixture screws all the way in.
• Starts from cold only with choke - as expected (so not an issue).
• Spark plugs - straw colour.
• All 4 exhaust headers getting hot.

What i've checked/tried
• Throttle cables - Removed, so not hanging.
• Exhaust leaks - Checked, none found.
• Exhaust - stock.
• Coils, leads and spark plug caps and spark plugs - new and tested as good.
• Points - not applicable (bike does not have them).
• Timing - correct.
• Valve clearance - Within spec.
• Valve spring length - Fine.
• Sticking valves - Checked, possible but unlikely.
• Valve seats - possible. Checked visually, no corrosion.
• Piston rings - Replaced.
• Compression test - unequal between pistons, but just within spec.
• Air leaks between carb and engine - None found.
• Carbs disassembled and cleaned - Yes, with carb clearer spray (no ultrasonic however). Passages checked with compressed air.
• Carb kit used - Yes, new stock size jets 92 main, 35 pilot. New float needles etc.
• Carb diaphragms - checked and ok.
• Float height - 17mm as recommended (tried lower and higher).
• Floats tested in water - they do float.
• Throttle shaft air leaks - Unlikely (replaced felt seals with rubber o-rings).
• Choke shafts worn - unknown (disassembled, cleaned and rebuilt).
• Carbs synced - Yes.
• Carb vacum nipples - capped.
• Airbox - on bike and new air filter.

Possible causes
• Blocked pilot circuits? - unlikely. Turning mixture screws out increases revs and hanging. Also compressed air makes its way through.
• Carbs not synced properly? - i'm using a cheap set of carb sync gauges. I'm pretty sure it's not this, but may be wrong
• Sticking valves messing with the timing? - Checked when cold and move freely, but might stick when hot? Not sure how to check
• Wrong needles in carb, so pulling fuel through main jet at idle making it rich? - currently had N76Q

Please let me know if this info narrows down my issue to a specific area. I've obviously missed something here that I think is fine that is not!

Cheers all.
 

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"• Mixture screws have an effect, but do not kill the engine when all the way in"

indicates uncontrolled/unmetered fuel flow..............I'll suggest float/seats not fully sealing. Sorry, a carb "do-over" is in your future.
 
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Discussion Starter · #3 ·
"• Mixture screws have an effect, but do not kill the engine when all the way in"

indicates uncontrolled/unmetered fuel flow..............I'll suggest float/seats not fully sealing. Sorry, a carb "do-over" is in your future.
Thanks for the tip. I'll replace the float needles, check the seats and see if that helps resolve it.
 

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Discussion Starter · #4 ·
Update: I've replaced the float needles. I've also inspected the float seats and polished them with compound on a Q tip in a drill. No change in outcome.

It's worth noting that there is no overflow of fuel from the bowl/carb into the air box or anywhere else, so I think the fuel is successfully getting to the bowls and stopping when the needles seat.

If this is in fact a rich condition (which the seated mixture screws point to), I can only think of a few ways too much fuel could be getting to the engine (given the other findings):
1. Leaking enricher circuit
2. Needle collar is too big for needle
3. Float height is wrong (Having trouble finding info on the Keihin CVK32 for the GT550, but 17mm seems to be the online opinion). I've tried setting the fuel height by the clear tube method, and this works out closer to 19.5mm, which is a huge difference, so this has me a little confused also).

Am I missing anything obvious?

Thanks in advance
 

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Discussion Starter · #5 ·
Update: Strong diagnosis theory.

I believe the butterfly plates are worn/incorrect for the carbs. Even in the fully closed position they seem to show a gap too large around the edges for my liking. When I put a torch behind them way too much light creeps around the edges than I would expect.

This would mean there is an internal air leak which is consistent with the symptoms and makes sense why no matter how many times I clean and retune the carbs, or check the valves, timing etc the revs would hunt. And why the mixture screws don't kill the bike at idle.

I can't be 100% certain this is the issue but I'm 90% certain.

I have replaced the carbs with another set now, and the only difference between them I can find is the butterfly gap when fully closed. Other than that, all circuits are equally free-flowing, both overhauled and cleaned, both have the same new carb kits transferred from one to the other.

Hopefully this will help anyone else who comes across the same issue to consider it as one of the possible causes of the symptoms and save them some some time to rule it out/confirm the problem.

Cheers all.
 
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