Revs won't come down - KZ750 E2 1982 + Keihins

Hey guys!

I've been battling with this 1982 Kawasaki KZ750 E2 all summer and now that the problems are all almost tackled down, I'm still having a bit of a trouble with my carbs. At least I suspect the carbs.

The thing is, when bike is warmed up and idles nice, I give it a nice twist of a throttle and the revs wont come down. I can get them down sometimes applying some choke, sometimes that doesnt help but screwing idle screw out helps.
If I screw idle screw out enough, the bike doesnt leave revs high and comes down normally but then I don't have idle and the bike dies. Then if I screw idle screw in just a bit by bit, first revs stay high but come down after a while without doing anything and after screwing idle screw in a little more (about to point where the bike idles nice again) the revs stay up and wont come down.

Carbs are standard Keihins which came with this model, as far as I know.
Jetted to 140/70 pri+secondary main, 35 pilot (which i made larger, I'll explain below), mixture screw 1.5 turns out.
K&N Invidual pods. Can't adjust the needle, seems like it's non-adjustable.
Vacuum ports are capped off.

So thought it must be really lean to do that and I used small drill to enlarge the 35 pilot jet to 40. Couldn't find pilot jets for this carburetor models from any shop for the life of it so had to do it like that. That gave me a little smoother idle and easier start up but didn't affect on rev hanging up.

The throttle valves seem to work normally, the cable moves nicely and closes the valves too so that can't be the reason.

Any ideas? The next size of my drill is 0.5mm and i'm hesitating to drill pilot jet out any more. Symptom is definitely for lean running but can it be the pilot jet?

All this testing has been done the bike parked with gastank removed and with external gas tank. Carbs are also freshly synced.

-Sami

A hanging idle suggests lean. Most common reason is vacuum leaks from the carb boots, but since you modified the intake and exhaust, that's an unknown as well.

Dynojet makes a jet kit for your bike. That will save some trial and error.

The factory Kawasaki manual for your bike calls for setting the pilot screws at 2.0 screws out, but that's a little lean so I always go 2.5 turns out and then tune for the highest idle. Be sure to vacuum sync the carbs too.

When the bike was idling normally, I sprayed some brake cleaner on boots to see if they were faulty but idle stayed normal so the boots seem to be intact.

Forgot to mention that carb vacuum ports are capped off (i'll edit this to the post above) 'cos not running a vacuum tap anymore, but those seem to be solid as well.

Backing the pilot screw might smooth out the idle even more but it didn't seem to affect on the rev hanging issue, tried to go as far as 4 turns out half a turn at a time and no effect. Also the sync was made with vacuum gauges and it turned out ok.

You should check the carb holders for leaks. They will eventually develop tiny cracks in the mating surfaces between the holders and the cylinder head that can cause them to leak at idle. These cracks cannot be seen with the carb holders in place. I suggest you try the following. Get the engine warmed up and temporarily set the idle as low as possible without killing the engine (lower than 900 rpm if possible). The lower the better because with the idle very low the slides are nearly shut and the vacuum inside the carb holders is at its highest. Then spray carb cleaner around the carb holders, especially where they mate with the cylinder head and where the carbs attach to them. Carb cleaner works best for this test as it doesn't leave an oily mess with WD40 or evaporate so quickly that it is not drawn into the leaks as can happen with propane or starting fluid (ether). If the engine dies or reacts to the carb cleaner, you need to replace the carb holders. Carb holder leak related problems are noticeable at idle but generally don't have much impact when riding because the throttle is open. If you find you need to replace the carb holders soak the screws with Kroil for a couple days before attempting to remove them, and use JIS standard, NOT Phillips, screwdriver bits, and this will greatly improve the odds that you will not break or strip a carb holder fastener as so many have done. Let me know if you need info on JIS bits, I have a good link for making your own at no cost. Ed

I've tried to check those boots with brake cleaner repeatedly 'cos they seem to be usually the first place to look for super lean symptoms but no matter how much i spray them, it doesn't affect the idle. It's really weird. Is brake cleaner bad stuff for that, should i go for carb cleaner instead?

Spraying junk on the boots is an unreliable method to try to find vacuum leaks. I'd replace the boots preemptively since they are not overly expensive yet so important.

140/70/35 with stock N01A needle is what WiredGeorge recommends for Keihins CV34 + Pods + 4into 1 on the kz750.
Bigger than #35 slow pilot jets are now available at JetRUs (#38 and #40).
A bigger slow idle jet won´t do much for idle... as the misxture is controlled by the mixture screw and you can go up to 3.5 turns out. But it will help with idle to 1/8 throtle, specially with a cold engine.
I suggest (as Nessim already said), change the carb holders.

Okay I'll take another look on those carb boots, it really feels like something like that is wrong now instead of the jetting. But why the revs come down normally if I back out the idle screw then, if the boots were leaking shouldn't it do the rev hang all the time no matter where the idle screw is?

Oh and JetsRUs doesn't ship to Finland anymore for some reason It's a real bummer, it was such a good place to get stuff. I found some replacements from a place called PJ Motorsports though, but not pilots. Got real good service from there too anyhow!

Are you sure the 4 choke butterflys valves are closing and opening all together? Whe you apply choke, one axle is moved by the the lever and the other should be moved by a very weak spring.

Okay had a nice chance to test this out immediately 'cos I'm at the garage.

The choke butterflies close on both sides, so that should be ok.

I warmed up the bike and let it run on low idle and sprayed some more brake cleaner on those areas you designated. I noticed the bike was starting to stall. Then it picked up when brake cleaner dried and ran normally. I sprayed some more and the bike started to stall again, and picked up after a while again.

Shouldn't the revs go up if those boots leak, seems like something is happening 'cos it bogs down?

Nessism wrote: Spraying junk on the boots is an unreliable method to try to find vacuum leaks. I'd replace the boots preemptively since they are not overly expensive yet so important.

That may be true for minor leaks. In my case I set the idle speed very low and sprayed carb cleaner around the carb holders where they seal against the cylinder head. When I did that the engine immediately reacted. In fact, the engine momentarily stopped running. My carb holders actually looked fine when mounted on the engine, but when I removed them I found many hairline cracks in the sealing surface.

If the carb holders are original I agree that it is a sure bet they will need to be replaced even if that doesn't solve the immediate problem. Ed