Vacuum problem still there! Hopeless!

Many of you have followed this problem with my 1982 KZ750H3. Recently rebuild. When I rev it past about 2500-3000rpm, it's stays there, even though the throttle cable and Keihin CV34 butterflys return to closed smoothly and completely.

At this point I've checked and fixed everything. EVERYTHING! Cleaned the carbs (and they are very clean now), checked the slides for damage or broken diaphrams, checked and sealed the carb holders, checked the advancer, checked the vacuum caps and the vacuum hose to the fuel tank, checked...checked...well, I can't think of anything else to check!

Problem still there. It runs very smoothly till I reach the rpm's mentioned above. Won't come down unless I kill the engine for a few seconds, or put it in gear and let the clutch out with the brakes on (and sometimes this doesn't work).

[And I'm not really sure how this can happen when the butterfly's are completely closing?-Er, maybe-I can see that the linkage to the butterfly shafts are closing anyway, and the butterfly's close all the way on the bench. And with them closed, where is the air AND fuel coming from to run at 3000rpm or often higher?]

Totally totally stumped. Can anyone think of anything I haven't tried yet? Thanks!

You say "recent rebuild..."

What did you rebuild?

polkat wrote:

Many of you have followed this problem with my 1982 KZ750H3. Recently rebuild. When I rev it past about 2500-3000rpm, it's stays there, even though the throttle cable and Keihin CV34 butterflys return to closed smoothly and completely.

At this point I've checked and fixed everything. EVERYTHING! Cleaned the carbs (and they are very clean now), checked the slides for damage or broken diaphrams, checked and sealed the carb holders, checked the advancer, checked the vacuum caps and the vacuum hose to the fuel tank, checked...checked...well, I can't think of anything else to check!

Problem still there. It runs very smoothly till I reach the rpm's mentioned above. Won't come down unless I kill the engine for a few seconds, or put it in gear and let the clutch out with the brakes on (and sometimes this doesn't work).

[And I'm not really sure how this can happen when the butterfly's are completely closing?-Er, maybe-I can see that the linkage to the butterfly shafts are closing anyway, and the butterfly's close all the way on the bench. And with them closed, where is the air AND fuel coming from to run at 3000rpm or often higher?]

Totally totally stumped. Can anyone think of anything I haven't tried yet? Thanks!

Could try enrichening carb pilot circuits.

Vac from carb provides vac to petcock, not tank -- probably what was meant, but just being sure.

Has WD40 been copiously sprayed onto all likely air suspect areas while engine idles to see if any smoke comes from the exhaust.

Still sounds like an air leak -- classic symptoms.

If pilot adjust screws are supposed to have 0-rings on this model carb, would assure the 0-rings are in place and in good condition. Carbs without 0-rings that are supposed to have such 0-rings may leak air.

Is ignition timing spot on, with F mark aligned with case mark at idle rpm? When throttle is blipped, does advancer immediately return to F mark alignment at idle rpm?

Good Fortune!

Did you check the seals at the ends of the throttle shafts? Spray some brake clean, or carb cleaner right on the end of each throttle shaft on all of the carbs. Look at the parts break down and you will see the small throttle shaft seals I'm talking about, they get cracked and split, the only other vacuum leak that I can think of would be what you have been checking over and over.

I'm having the same issue. I'll let you know if I have any luck figuring it out.

Yea, let me know if you find anything Thanks for the responses guys. I'll try the WD40 test again around those bushings, and if I don't find anything, I plan to pull the carbs off and try again.

Out of Cycle World Mag.....

Old Man Rock, I'll be trying the WD40 test again this afternoon. Tried it once before without success (no smoke). I don't think it's plugged idle jets as it idles very smoothly at good rpm when it's not stuck revved up. I did replace the little O rings when I cleaned the carbs and the whole pilot circuit was quite clean.

When I had the carbs on the bench, I checked each slider for smooth operation. They were fine, showing little wear with no hanging areas or roughness, and slipped back to closed position nicely.

Maybe I'll find something this time with the WD40.

Has the question been asked/answered???

Have you properly tuned the mixture screws, and vacuum balanced the carbs correctly?

Not just turned them out to some pre-determined/random number of turns, but actually properly tuned!

And use a manometer to set vacuum @ idle.

When you release the throttle and the slides close, the carbs return to the mixture screws/pilot circuit.
That circuit is too lean for one or more of many reasons, plain and simple.

TUNE the screws.

GL!

Yep, did that with my homemade manometer. Wasan't hard, and as I said, it idles beautifully and runs good till around 2000-2500rpm's, then sticks there. Doesn't seem to be a pilot problem. When it does this, I have physically grabbed the throttle shafts to see if they are down. They are.

Tried the WD40 idea again today. Couldn't find anything. Lots of thanks to those hewre who have tried to help. I realize that is hard to diagnose a problem from afar, particularly when it doesn't sound like anything's wrong, so thanks again.

I'm pulling the carbs off again tomorrow. What should I be looking for that I haven't already? If I had a hidden tear in a slider diaphram, what kind of symptoms would that cause? And how does one check out the 'hidden' pilot air jet (page 153 factory manual-top left drawing-item #10)?

polkat wrote:

...idles beautifully and runs good till around 2000-2500rpm's, then sticks there. Doesn't seem to be a pilot problem. When it does this, I have physically grabbed the throttle shafts to see if they are down. They are.

Tried the WD40 idea again today. Couldn't find anything...pulling the carbs off again tomorrow. What should I be looking for that I haven't already? If I had a hidden tear in a slider diaphram, what kind of symptoms would that cause? And how does one check out the 'hidden' pilot air jet (page 153 factory manual-top left drawing-item #10)?

Likely already done, but would assure FSM default settings on pilot mixture screws.

While stuck at the too high idle, what is effect of adding some choke?

If carbs are like the image shown below, which is the "hidden" pilot air jet referred to?

Good Fortune!

[Click on image to enlarge view.]





[Add checked items to Pick-List] REF NUMBER PART NUMBER DESCRIPTION QUANTITY US CA CN REMARKS
[View Supersession]
[Add to Picklist: 220B0408]
220 220B0408 SCREW PAN HEAD 4X8 16
[View Supersession]
[Add to Picklist: 220B0412]
220A 220B0412 SCREW-PAN-CRSOSS,4X12 16
[View Supersession]
[Add to Picklist: 461F0400]
461 461F0400 WASHER SPRING 4MM 16
[Add to Picklist: 16019-024]
11009 16019-024 "O" RING,FLOAT BALL 4
[Add to Picklist: 11012-1128]
11012 11012-1128 CAP,POLYESTER 4
[Add to Picklist: 13091-1050]
13091 13091-1050 HOLDER,NEEDLE JET 4
[Add to Picklist: 16005-1012]
16005 16005-1012 TOP,MIXING CHAMBER 4
[Add to Picklist: 16007-1022]
16007 16007-1022 SCREW,NEEDLE SET 4
[Add to Picklist: 16009-1062]
16009 16009-1062 JET NEEDLE,#N01A 4
[View Supersession]
[Add to Picklist: 16014-1011]
16014 16014-1011 SCREW,PILOT ADJUST 4
[Add to Picklist: 16017-1059]
16017 16017-1059 NEEDLE JET 4
[View Supersession]
[Add to Picklist: 16030-1007]
16030 16030-1007 FLOAT VALVE 4
[Add to Picklist: 16031-1001]
16031 16031-1001 FLOAT 4
[Add to Picklist: 16041-1022]
16041 16041-1022 SHAFT,CHOKE,L.H 1
[Add to Picklist: 16041-1023]
16041A 16041-1023 SHAFT,CHOKE 1
[View Supersession]
[Add to Picklist: 16126-1039]
16126 16126-1039 VALVE,VACUUM 4
[View Supersession]
[Add to Picklist: 92009-1095]
92009 92009-1095 SCREW,DRAIN 4
[Add to Picklist: 92022-1003]
92022 92022-1003 WASHER,PLAIN 4
[View Supersession]
[Add to Picklist: 16044-007]
92037 16044-007 CLIP,FLOAT VALVE 4
[Add to Picklist: 92043-1002]
92043 92043-1002 FLOAT PIN 4
[Add to Picklist: 92055-1002]
92055 92055-1002 O RING,PILOT ADJ 4
[Add to Picklist: 92055-1053]
92055A 92055-1053 "O" RING,DRAIN SCREW 4
[Add to Picklist: 92055B-1088]
92055B 92055B-1088 UNAVAILABLE IN PRICE BOOK 4
[Add to Picklist: 92062-1011]
92062 92062-1011 NOZZLE,MAIN 4
[Add to Picklist: 92063-1069]
92063E 92063-1069 MAIN JET,2ND,#125 4
[Add to Picklist: 92063-1070]
92063F 92063-1070 MAIN JET,PRI,#62 4
[Add to Picklist: 92063-1072]
92063G 92063-1072 MAIN JET,2ND,#120 4 (RACING USE ONLY)
[Add to Picklist: 92063-1073]
92063H 92063-1073 MAIN JET,2ND,#122 4 (RACING USE ONLY)
[Add to Picklist: 92063-1074]
92063I 92063-1074 MAIN JET,2ND,#128 4 (RACING USE ONLY)
[Add to Picklist: 92063-1075]
92063J 92063-1075 MAIN JET,2ND,#130 4 (RACING USE ONLY)
[View Supersession]
[Add to Picklist: 92064-1001]
92064 92064-1001 PILOT JET,#35 4
[View Supersession]
[Add to Picklist: 92066-1051]
92066 92066-1051 PLUG,PILOT SCREW 4 X US
[Add to Picklist: 92081-1002]
92081 92081-1002 SPRING,PIROT ADJ SCRW 4
[View Supersession]
[Add to Picklist: 92081-1412]
92081A 92081-1412 SPRING,VACUUM VALVE 4

polkat wrote:

...1982 KZ750H3. Recently rebuild. When I rev it past about 2500-3000rpm, it's stays there, even though the throttle cable and Keihin CV34 butterflys return to closed smoothly completely...checked the advancer....

checked the advancer --- If not already done, would remove, clean, inspect, and lube advancer, to assure it isn't sticking in the advance position.

This would include verifying that the springs seem tight and okay.

Good Fortune!