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Discussion Starter · #1 ·
So I've got a 77 kz650 I've been working on. I have an issue where off idle it's running really rich. Fouls plugs and stalls often enough that I don't want to go for a ride.
It'll start up fine on choke, then after a minute or 2 I can shut the choke off and it'll idle nice and rev up nice. But after a couple of minutes the idle will still be ok but the second I touch the throttle it sputters, some black smoke out the exhaust and sometimes it stalls. If it doesn't stall it'll then rev up and rev nicely with no black smoke or anything. I can then let off and it'll return to idle. But next time I go touch the throttle same thing. It seems that the 0-1/4 throttle area is wicked rich. I actually bought a colortune and verified it's doing exactly this and all 4 cylinders are doing the same thing. Idle mixture is properly set and at 3-4k rpm fuel mixture is also correct.

Bike has original airbox but 4-1 exhaust with a single baffle.

Throughout the last year I've replaced coils wires caps and spark plugs, relay installed to give full voltage to coils, points condenser and verified advance working right and ignition timing correct with a timing light, battery and regulator, rebuilt the carbs properly with new jets and even went back to old now cleaned OEM jets just in case the aftermarket ones weren't made quite right, new floats and set float height the proper wet way(2 of the old floats didn't float anymore...), new carb to head boots and vacuum plugs, dynamic carb sync, head gasket and cylinder base gasket(was leaking compression now good with 5psi between cylinders), set reset and rechecked valve clearances and valve timing, new fuel in tank.

I'm sure I've missed stuff too but basically I've gone over everything 3 times if not more and the engine I can guarantee is healthy, the bike had tons of issues when I got it. It was cheap, I'm a young mechanic with the desire to learn and I've almost never touched carbs or points before this bike but I'm sure everything carb and point related I've done right. Spark is really strong.
Main jet is a 102.5 and pilot is a 15 with the secondary pilot jet being a 50. The manual I have for this bike actually states 100 for the main jet and 16 for the pilot but everything I've read online stated what was in there is correct and I doubt going richer on the pilot would make it better... And at this low throttle I don't see the main jet doing much
My carbs have a pilot fuel screw and I've messed with it plenty, if I close it almost all the way (lean) the idle sucks but the issue is slightly less worse.
I've lowered the needle one notch, it was at 4. I think it helped a little but barely noticeable and being that midrange seems to have the proper Afr I don't really want to lower it more.

I've recently learned about the pilot bypass outlet that seems to feed fuel into the carb when you start to open the throttle. It's fed from the pilot jet and not affected by the pilot fuel screw so I've ordered a couple size down pilot jets to see if that does anything. Just seems strange that I'd have to go leaner than stock... I've also read about changing to slides with different cutouts but they seem to be expensive and near impossible to find....

So assuming this isn't too much to read, any experts out there have an opinion? Ami right in thinking a smaller pilot will help because it's sucking too much through the pilot bypass hole?

Thanks for your time!
-Alex
 

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Didi you soak your carb bodies, in a "Berryman's" type solution. You could have a clogged passage way. I would leave everything stock until I got the bike running before I started changing thigs. That could open up a can of worms. Those idle jets and tubing have to be very clean, and 90% of the time they are frozen, and non adjustable anymore. :)
 

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Discussion Starter · #3 ·
I didn't soak them but they have been 100% apart with every removable part removed multiple times. No frozen jets, no corrosion and I have blown and sprayed through every passageway multiple times. The only passageways that should make it run rich would be if the air jet passageways were clogged so I've played extra close attention to these and they are immaculate. Also made sure the choke is closing fully though I'd expect if it wasn't it would affect idle.

Been scratching my head pretty good, and everyone I talk to has great ideas but nothing solves it....

I closed the pilot fuel screw completely yesterday and was able to go for a ride, idle was a bit lean but still running rich off idle just not stalling as often. I don't think it's supposed to be running this well with the pilot fuel screw completely closed so gas is coming in from somewhere it isn't supposed to.... Float levels still good.
 

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Discussion Starter · #7 ·
Thanks for that chart, I've found a couple but that one seems more complete!
My bike is a 77 kz650b1 and as far as I can tell engine and carbs are original and stock other than a 4-1 exhaust.
I did run it with no airbox and carbs completely open and it didn't make any difference, still way rich.
I'm now thinking worn needle Jet... There's is some play between needle and jet with throttle closed, how snug is the needle supposed to be in there? I'm going to have another look and try to get the Jets out, though they seem to be pressed in.
 

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Kawasaki and Haynes service manuals refer to your bike having a pilot air screw. Are you certain yours is a fuel screw?
Is the screw accessed from underneath the carb or from above? Picture if you have them would help.

Did you replace all O-rings in the carb. I believe there are 3 or 4 different O-rings in each carb, not counting the ones on the main fuel supply line linking the carbs.
 

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Discussion Starter · #9 ·
It is a fuel screw, the early carbs had fuel screws. The later better ones had air screws, I've read about ones with both but never seen.
It's underneath on the engine side.

All o rings are new
 

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Discussion Starter · #11 ·
I did actually. Bike is running great right now.
I changed the carbs for the slightly newer version with the air screws(off a 1978 I believe). A hundred times easier to tune and an immediate difference so I'm sure my carbs had a passage that was maybe worn too big or something allowing too much fuel in at idle. That fixed 80% of my issues.
The other 20% was fixed by re gapping my points on the larger side of spec. This caused a slightly advanced timing, maybe a couple degrees more advanced than spec. Made idle and low rpm much nicer, high rpm feels about the same.
I have since upgraded to a 1980something kz750 electronic ignition because the points kept wearing where they run on the crank and every 500km or so I'd have to re gap them or it would start stuttering a bit.

Hope this helps. This bike is proof that sometimes one issue is caused by 15 different things. Make sure your ignition system is perfect and then try to very slightly advanced it. Also make sure your carbs are immaculate inside and if you still have the fuel screw style, junk em.
 

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I did actually. Bike is running great right now.
I changed the carbs for the slightly newer version with the air screws(off a 1978 I believe). A hundred times easier to tune and an immediate difference so I'm sure my carbs had a passage that was maybe worn too big or something allowing too much fuel in at idle. That fixed 80% of my issues.
The other 20% was fixed by re gapping my points on the larger side of spec. This caused a slightly advanced timing, maybe a couple degrees more advanced than spec. Made idle and low rpm much nicer, high rpm feels about the same.
I have since upgraded to a 1980something kz750 electronic ignition because the points kept wearing where they run on the crank and every 500km or so I'd have to re gap them or it would start stuttering a bit.

Hope this helps. This bike is proof that sometimes one issue is caused by 15 different things. Make sure your ignition system is perfect and then try to very slightly advanced it. Also make sure your carbs are immaculate inside and if you still have the fuel screw style, junk em.
Thanks for replying!
We've been running it lately and it seems to smoke a bit on closed throttle deceleration. Fouling plugs over 600kms or so. Valve seals this winter for sure. Goimg to keep an eye out for some air screw carbs. I think these ones were short lived before a change was made. I think that any fuel screw carbs exibit this problem. Thanks again for replying!
 

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Discussion Starter · #13 ·

Multiple pages if you want to read through most of my diagnosing.

If it's fuel fouling plugs the valve seals won't help, but if they're oil fouling then obviously yes.
 

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Thanks again. I'll register on that forum and check out what you've done. The issue with this bike was fuel fouling. But recently we've been seeing some smoke during fairly aggressive deceleration. We've been through everything- new coils, points, spark plugs, carb cleaning numerous times, did the WG relay mod, and thoroughly tuned the timing and carbs. We have the air box, new air box boots, air filter and tested the intake runners for leaks a few times. We are getting a fluffy black soot build up and have a flat spot just above idle. So much so if you pull away without being a bit aggressive on the throttle you could stall the bike. If you hold the rpms at about 1700 for a few seconds then increase the rpms briskly you will get black smoke from the exhaust. There is no blue smoke on a cold start only during deceleration. These carbs seem to be the issue. They have a secondary jet (50) below the #15 idle jet, which to me anyway proves that they had issues with this carb set and were trying to lean the pilot circuit. Which just happens to be where the flat spot and black smoke come from. Everywhere else in the rpms the bike works well. Looking forward to checking out what you've been through and hope to glean some good Intel. Thanks so much.
 

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Discussion Starter · #15 ·
That sounds very similar to the original issue I had which went away after changing the carbs. I'd look for a used set with air screws. I managed to find a rough set for 100$ that just needed cleaning and some o rings.
 

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I've read your diagnosis and that is pretty much the same journey we have been through. I found a set of '77 air screw carbs that are in unmolested condition. We are going through them this weekend with a ultrasonic clean and replace any questionable parts then see how they work. We'll also recheck the points to see if they have changed any in the last 600kms. If we get an 80% improvement that would be substantial and probably good for the rest of the riding season. If it doesn't foul the plugs we'll be super happy. Electronic ignition will happen this winter.
 
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