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High Idle And Won’t Return

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I’ve got a 1974 Datsun 620 L18 with a recently rebuilt stock Hitachi DCH340 and haven’t had major problems before this. So this afternoon I pulled my carburetor and did I complete cleaning, I know it’s not the correct way but I reused all the old gaskets because the rebuild was lest than 3 months ago and I rarely drove it for the first two months. I pulled all the jets and such and cleaned the outside as best I could. When I put the carb back on and started it I didn’t notice any major problems besides mixture which didn’t surprise me. But after it got warm and I adjusted the mixture it was idling fine but when I gave it throttle it would jump to at least 1500 rpms from what the motor sounds like. But it doesn’t return to the normal idle unless I put it in gear and basically manually drop it to normal idle speed (sounds like 800ish). I’m gonna try and pull the carb again tomorrow and see if anything is clogging the jets or something like that but I have absolutely no idea what could cause this. Any help would be appreciated.

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OK, so here is what should happen when starting the engine cold. If you start with a cold engine, you step on the gas peddle in preparation to starting. This does two things. It squirts raw fuel into the intake from the accelerator pump, and it releases the choke which had cooled and reset itself. When the choke is released and closes by stepping on the peddle it also releases the fast idle cam which drops down between the throttle speed adjustment and it's stop. This holds the throttle open slightly and raises the engine speed during warm up, usually 5-10 min. depending how cold it is.  While warming, the choke relaxes and slowly opens fully and at this point if you again step lightly on the gas the fast idle cam pulls up out of the way and the throttle can now close on it's warm idle speed stop and the engine idles normally.


Did you remove or adjust the BCDD? This thing...


Or that white set screw the the nut on it?



BTW there's nothing wrong with reusing old gaskets if not torn. None of them seal in gas just vapors.

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Take the little rubber cover plug off the middle and turn the screw anti clockwise to reduce the sensitivity of the BCDD.


The BCDD is turned on by high intake vacuum when decelerating. Normally the mixture goes so lean that is doesn't burn. The BCDD is a little mini carburetor within a carburetor that opens to supply gas and air during this time. When vacuum returns to normal is it supposed to shut off but sometimes it doesn't and you have a slight runaway idle. If this fixes the problem then this was the problem.    

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