Mikunis won't idle down

I'm just about lost knowin what to do with this. Just put a KZ750 engine with an 810 kit in my bike,head is cut for 10:1 , its ported and has a set of web cams 118 grind; also a set of Mikuni 34 RS carbs.When I fire it up I can get it to idle around 1000-1200 rpm , but when I crack the throttle it revs to around 5 grand and stays there. I know the slides have come back down, as I have the air cleaners off and can observe it. I hate to think the idle circuit is pluged I've run carb cleaner thru the air jet on the front of the carb and it spuirts out the exit hole on the back side of the slide. Hard to believe that an idle curuit can provide enough fuel to make the bike run that fast. Any Help is appreciated TY.

Freeplay in the throttle cable? Dirty advancer?

Sounds like a vacuum leak or dirty carbs?

I would be checking for air leaks in addition to the stuff that Kidkawie recommends. Did you re-use your old intake manifolds or buy new ones? Does the idle ever return to normal or just stay at 5000 forever?

It doesn't take hardly any fuel at all to maintain 5000rpm when there is no load on the bike, as in sitting in neutral in the driveway. The pilots will totally provide enough fuel with no load.

Godfrey wrote: ...KZ750 engine with an 810 kit...Mikuni 34 RS carbs...idle around 1000-1200 rpm , but when I crack the throttle it revs to around 5 grand and stays there. I know the slides have come back down...

Hello, Godfrey, and WELCOME to KZr!

A "racing idle" may also be caused by an air leak, meaning intrusion of outside air into the fuel mixture provided by the carbs.

Often discovered sites for air leaks are old, hard, cracked, worn-out carb holders, and sometimes a loose vacuum plug, or a defective vacuum hose to petcocks.

May test for an air leak by spraying WD40 onto suspected areas while engine is idling, and watching for exhaust smoke or change in rpm.

Good Fortune!

Also check for loose carb holder clamp.

are you running two throttle cables?

assuming no air leaks then synching the carbs could sort it.

TY all for your suggestions , I have somewhere to look and when I have this solved I'll let you know what it was; I got the engine just as it is, so any of these are a potential.

Well the advance mechanism is free and working ,I sprayed gum out carb cleaner around the carb boots(manifolds)and there was no change in rpm or visible change in the exhaust. I removed the carbs and disassembled the pilot system, jets, adjuster screws, and cleaned them all though nothing appeared to need cleaning,blew about 100 lbs of air thru the system, reassembled it all and reinstalled them on the bike.I then fired it up got it idling around 1000 rpm and just let it run as the engine got heat in it the rpm slowly raised to 2k and then took off to 5 k and stayed there till I turned the idle speed thumbscrew , it slowed to 2 k and killed.I am running the petcock in prime position, and there is no vacumn hooked to the pump on the petcock. Back to square one, no vacumn leaks that I can find;

have you tried hooking up the vacuum to the petcock and taking it off of prime?

Did you start off with a "Bench Sync" to set the carbs all the same? Setting the height of the slides so all are equal at near closed throttle? Also the pilot adjustments the same turns out, and the float levels all the same using the clear tube fuel height method?
Then maybe disconnect the throttle cable and try to get the Hot idle sorted out, before going further.