It got worse! but better too?

I was checking out some of the advice in the "high revs" thread to bring my own down a bit. Replaced the boots, revs got higher. Then I sealed with permatex, smoothed out the rough spots when in open choke, but didn't drop the rpm. Thought maybe I should put the airbox on. Revs went HIGHER! Now it idles at 7000+. Spark plugs show a light rusty color. One mix screw is at full rich (with it full lean she rev'd at closer to 8000, and the other is un-adjustable.) Timing is currently full advance (rev'd higher when retarded) and the idle screw is at slowest speed. I've done spray cleaning but am too chicken to tear down the CVs. Cleaner sprayed on boots and airbox tubes doesn't change rpm anymore.

It's a '79 KZ400H with the stock Keihin CV carbs and straight pipe on the exhaust end.

I hate to sound like a smart ass, but 7,000+ rpm cannot be considered idling. It seems obvious to me that the slides in your carbs are not operating properly or your throttle cable or idle set screw is holding them open. Your engine will simply not run at 7,000 rpm with the slides closed. It's not possible for enough air to squeeze under them to support that high an rpm. I don't know what carbs your bike has, but if your carbs have butterflies instead of slides, then the butterflies are being held open. The main point is, your carbs must not be closing all the way to idle position. If they are closing, you have a tremendous air leak between them and the head. Ed

I checked for a leak in/around the boots/holders of the carb and haven't found anything there. The butterfly of the throttle does appear to be closing all the way, and the cables have a little (6mm or so) bit of play in them. Choking it makes it rev higher, and it seems that the more I went after leaks or too much air, the more it wants to race. Maybe the diaphragms have finally gone kaput?

Maybe. If your carbs are the butterfly type, and those are closed, there's a lot of air coming from someplace to feed 7,000 rpm. Maybe someone with similar carbs can chime in as I'm only familiar with slide / non-CV type. Ed

I've been looking at going with a slide since a new carb may be in order, any idea on what size I should step down to? This project is my daily so WOT power is not what I'm looking for.

Ok its been a long time; this is a 400 twin right? I used to have one. If memory serves the tappets are adjustable? lIke I said in my thread and high rev, if the engine is gettin hot, and the carb bowls are fairly cold at that rpm, I would check your valve clearances. I would think if the diaphrams had failed the needles wouldn't even raise. Again its been a long time but I seem to remember my 76 not having diaprams but a metal piston that raised and lowered, similar in function to a cv diaphram.

www.kz400.com/Workshop%20manual%20bilder...nual%20p.150-155.pdf


There is the part of the manual for your carburetors

Ed ( Godfrey)

Yeah the 400 is a twin. I didn't check to see if the bowls were getting cold, I'll check that when I wont wake the neighbors, and I should probably check the clearances anyways. Before I replaced and sealed the boots it idled at a good speed (though it took awhile to warm up), but only ran smoothly if I choked it, and showed evidence of being lean on the spark plugs. Now it runs smooth with choke off but suffers this major RPM problem. From frying pan to fire?

Just a thought.... Do your carb holders have vacuum nipples for synching carbs, and if so, are the rubber caps that cover them in place? Ed

I just got done finding out I had 2 tight clearanced exhaust valves on my 750, and that was enough to keep the valves barely open when it warmed up and then it would race to 5 grand. I had the air cleaners off so could see the slides were down, but with the exhaust flow it sucked the air right past the slides anyway, and with that kind of vacumn the carbs got plenty cold. and Yeah it would be way lean. I had my 400 for a lot of yrs, put 40k miles on it, with minimum problems.

I only am suggesting this cause its something besides an air leak that could cause the issue, I certainly am not suggesting that an air leak would not be the first and most obvious culprit. It seems you have explored that though.

Godfrey

No nipples. Just a pair of screw holes. There isn't much in the way of vacuum on my bike. I could try sealing the vacuum ports that aren't in use. I think that was mentioned and I haven't looked at that or tracked them all down yet.

Oh, something else! I noticed a bit of a pop, very muffled, possibly coming from the carb. Wasn't synced up with any backfires, and only started after I changed the boots/holders. With the airbox on now its hard to tell if it is still doing it though.

Hurr durr, I feel like an ass. Today I tore down the bike to access everything around the carb/intakes, and decide to check on that butterfly. Yeah it was snapping back "all the way," into the stupid idle screw, leaving a small little crack. The idle screw wouldn't back out any further by hand but did with a screwdriver, now it works. I gave it a quick run in my parking lot and it didn't start racing to 5k right off the start and settled around 1000-1500 when I pulled off the choke. Thanks guys! When I wake up today I'm going for a spin.