TOPIC:

Hi, I'm looking for advice on odd problems I'm having with my 1980 (more or less) KZ440A. Factory Keihin CV carbs, factory jetting (per the service manual) and factory airbox/filter. Electronic ignition from a later KZ440.

When I got the bike, it didn't run great, but well enough to get me home. So, I decided to rebuild the carbs as a first step. They were actually pretty clean inside, but the o rings were all past their prime, and one of the idle screws had a bent tip. I ultrasonically cleaned the bodies, blew air and carb cleaner through all the passages, reassembled with new K&L carb kits, bench synced the throttle shafts, set the idle screws to 2 1/2 turns out, and set the float levels to just below the float bowl seam after installing them on the bike. I have rebuilt a lot of carburetors, including several other sets of CV carbs, so I'm pretty sure I didn't screw anything up in this process.

Since then, the bike will barely run, mostly on the right cylinder only, and will die if any throttle is applied. The only way it will rev above idle is if the choke/fast idle is engaged. I've done the following:

New OEM rubber carb-to-head manifolds
New NGK B7ES plugs gapped to spec
Overhauled ignition advance unit
Checked for vacuum leaks with propane and carb cleaner and haven't located any
Confirmed that valve timing is correct relative to crankshaft
Adjusted valves
Tested compression on both cylinders (within spec)
Tested for spark at both plugs
Swapped diaphragms and pistons between the two carbs (no change... left cylinder still won't fire reliably)
Disassembled carbs again and confirmed that no passages are blocked
Re-checked float bowl levels (no change)

Any ideas? I'm running out of them...

Thanks,
Andy

Sounds like carb problems. Did you remove the bleed pipes/main nozzles and pilot screws and make sure all the internal passages are clear?

Yes, the carbs were completely disassembled (except for the throttle and choke shafts) and everything was ultrasonically cleaned. All passages have been blown through with compressed air and Brakleen (twice now) to confirm they were clear.

Try closing the gap on the plugs.
There is a possibility of a weak ignition system having trouble under cylinder pressure.
It's harder to fire a plug under pressure with a lean mixture as opposed to a rich mixture.
And it's easier to fire the plug outside of the cylinder.
Your better running with the choke on made me think of that.

Have you checked the coil and what color is the spark.


.

Good ideas, thanks. The spark seemed like a good strong blue, but I will test the coil anyway. I guess I'll also tear the carbs down again, just to make sure that there isn't some sort of recurring contamination plugging the pilot circuit over and over.

Because the problem started right after the carb rebuild, I've been assuming it was fuel related... but I guess that could be a red herring.

Any other ideas are welcome...

Thanks,
Andy

Is there ever much chance of leakage around the throttle and choke shafts with these carbs? I didn't remove them as it appeared there were no seals or o-rings to replace, unlike the Bing CV carbs I am usually working on.

You might check each and every carb jet (main, pilot, idle) with a small piece of wire, maybe something around 28 gauge, to make sure that wire passes all the way through. Or, pull each one and hold it up to a bright sky and make sure it is clear.

On my KZ200, at one point, the idle jet plugged. If I revved it high it would run ok above 4000 rpm, but, let it get below 4000 and it would die. The idle jet was plugged solidly (after 8 years of storage). I replaced it and it went back to its nice smooth idle and throttle response.

Check each and every jet hole to make sure they are completely clear. The wire is a good way.

Note: Running air through it is not as good as using a wire that just fits through the hole. Air can get through a constriction, but, the wire will stop. If it does, replace the jet. Some folks drill the plugged jet out but then you have to find an exact match diameter drill bit and have a very good drill press. I just replaced mine for $3 bucks.

Yeah, I've done that a couple times now. Blown through with compressed air, and held up to the light to verify. They're always clear, but it's possible that there's some debris in a carb passage that is slowly breaking loose and re-plugging things. Going to take them apart again and see if I can find any new blockage.

I'm wondering if maybe the transfer ports in the carb body are plugging over and over.

Understood.

Question, in your original post you write: "and set the float levels to just below the float bowl seam".

But, there should be an exact specification for the gasoline level relative to the bottom of the gasket at the top of the carb. On my old KZ200 it is 4-6mm. I set mine to 5.0 mm.

If yours is just below the lip of the carb, it is probably running too rich. Check your manual or ask someone to check the manual for your bike to get the range, then, get a thin piece of paper and slide it up against the gasket, and, mark the distance to the middle of the spec and set the gasoline float height with a small, clear tube. This is a pain. Yes.

I've not rebuilt any 440 carbs but I've done a lot of Keihin 750 carbs and those don't use throttle shaft seals.

My strong advice for anyone rebuilding carbs is to always break the down to carb body level and soak for a few hours in carb dip. Getting all the gunk out deep in the passages can't be over stressed.

Sorry, I should have been clearer about the float level. The spec for the KZ440 is 3mm-5mm from the bottom edge of the carb. I set them at 4mm... the edge of the float bowl, where it bolts on, is right at 4mm thick, so it makes a handy built-in gauge.