Carb Gurus, a little wisdom please...

Ok, service manual says a 1984 ZX750 should be idling between 1000RPM and 1100RPM. If I have the idle set to factory spec. the throttle constantly gets stuck at 3000RPM upon my deceleration if I was riding above that. I can get it unstuck by just revving high and then letting it drop than it will resume idle. The funny part is if I have the idle set below factory spec (900RPM to 1000RPM) this does not occur. Can somebody explain if this is a normal occurence or if I have yet another carb issue.

BTW: The throttle cable does not stick, that returns to idle position just fine.

Definitely not normal, and sounds like the classic symptom of some kind of vacuum leak.

Paging Dr. WiredGeorge.........

If I have to track down another vacuum leak I am going to dropkick this thing off an overpass.

The old standby test is with engine idling to spray WD40 or propane gas around carb holders and vacuum caps, while noticing whether the spray makes any difference in idle characteristics or smoking out the exhaust, as this would indicate that the spray is being sucked into the fuel mixture (from the outside) before the mixture gets into the combustion chamber.

Where spray is being sucked in, outside air is being sucked in, which is to say "air leak."

An air leak often results from old worn out cracked and deteriorated carb holders or vacuum caps. But can also result from a loose carb holder clamp. And there are of course other sources of air leaks, such as faulty o-ring on pilot screw, faulty coke plunger pad, cracked vacuum hose to petcock, and undoubtedly others.

Good Luck!

I have the same problem on my bike. except it does it at any and all rpms! Doesn't matter! Checked for vacuum leak yesterday, couldn't find any, will try again tonight when back from work. I am hoplessly hoping to find a vacuum leak somewhere... Just had the carbs done by WiredGeorge, so I am kind of ruling out trouble with the carbs... But needless to say, Check again for a vac leak, soak the carb holders quite well with WD40. Good luck!

This is my 3rd post today to same question:P :huh:

1) Synchronize carbs. Way out of sync can cause all kinds of funny things

2) Revs sticking a bit can also mean lean mixture. Try to open air/fuel mixture screw a bit like 1/4 turns.

3) Raise needle 1/2 notch to give it some more mix.

These helped on my bike. And no! I´m no carb/engine guru at all. Just learned this the hard way:lol:

I'm leary of actually taking apart the carbs. I know the bike is already running rich by reading the plugs (doesn't bug me, I will be putting pods on sooner or later), but i'm not sure what else has been done. I keep finding new things as I go along. I will do the WD40 test later this afternoon and report back.

Ok, found one source of the leak. One of the little vacuum caps on my carbs was melted. It turns out my ex-girlfriend thought it would be cute to spray them with the carb cleaner that eats rubber (thats what I get for trying to teach her what does what), but I must still have a leak somewhere I can't find cause the throttle is still sticking like before. I did the WD40 test and didn't find anything after I replaced all the vacuum caps (just in case). I inspected the carb boots and they are sealed and don't appear to be cracked, melted, torn, etc. Anything else I can check other than the carb innards?

Are carbs synchronized??? Please check this since one or more carbs can easily be so much off that revs stick and runs like crap.

I know this problem too well....

The throttle cable does not stick, that returns to idle position just fine -- both cables should have a little slack.

I know the bike is already running rich by reading the plugs -- if not already done, would check service floatbowl fuel levels using the clear plastic tube test. But thinking an excessively rich mixture isn't causing the eratic idle rpm.

will be putting pods on sooner or later -- foolish mortal :lol: , but then again, some people love to suffer.

Meanwhile, still suspicious of an air leak in carb holder area (not intake hose from airbox). Sometimes propane gas works better than WD40 for testing purposes.

Good Luck!

Patton wrote:

will be putting pods on sooner or later -- foolish mortal :lol: , but then again, some people love to suffer. :unsure:quote]

Yeap, definately.....Pods....... No compromise, no mercy:P

Pod people are cool......they make sure airboxs are available for the rest of us!