Swapped Carbs Truck idles high

[Bugeye]

ok with the carb that was on my truck i couldnt get the damn thing to idle at all, swapped carbs with a known good carb, replaced all the gaskets, made sure it was tight, looking through the sight glass the fuel is at the line where its supposed to be, did like the book said and turned the mixture screw lean, then started to turn the idle speed screw, truck will idle at 1500-2000 rpms no problem, when I try to bring it down to 850 like the book says it stumbles and dies. What the hell am a I doing wrong? there is nothing wrong with the engine internals, compression and cylinder leak down are damn near perfect, I also replaced the worn out throttle return spring. My timing is 10 degrees btdc, Im at loss right now, what do you guys think? I don't really like asking for help, but I need it Please.

[datsunaholic]

Most every time I've had an engine that wouldn't idle below 1200 RPM is was a big vacuum leak. One was a rusted out, disintegrated PCV. One was a EGR that someone had thoughtfully "Removed" and plugged with bondo (which fell out). One was a cracked intake manifold. And I've had lots due to leaky carb base gaskets (or someone putting RTV on the gaskets).

 

I've also had carbs that the idle circuit was plugged so that the only way it would run was with the throttle cracked open. Heck, one was so simple as the idle cut solenoid wire had fallen off.

[Bugeye]

thanks for the info, now here is another symptom, I drove the truck around for about a half hour (to go get gas) and when I came home, the truck was idling real low, I throw the tach and dwell on it and it is idling at 450-500 RPM's, I give it a little bit of gas because its stumbling and getting ready to die and the RPMS jump back up to 1500-2000 and hold? Im going to have to wait until the first of the month to buy some new intake/exhaust manifold gaskets, I have 2 spare intakes here, hopefully the mani isn't cracked

[datsun dreamer]

My 620 does the same thing:(! I just got used to it:D! Try the Choke:)!

[Bugeye]

choke is wide open, I dont mind it idling that high, just that it needs to idle at 600+/- 100 rpms for the damn smog tests

[datsun dreamer]

Does your truck have a throttle position Sensor? If so, Check:)

[Bugeye]

trucks a 79, no computers, no tps,

[moparvwfreak]

my 73 seems to be doing this. ahvent had a tach on it yet but i do know it idles WAY ot high. high enough i can start out in 1st gear WITHOUT touching the gas. i still have alot of work to do on mine. need to find my carb cleaner and hit the mating surfaces to see if i have a vac leak. that'll be tomorrows job if the weather gets better. i wish i had a covered spot that wasn't taken up by a motor swap project...

[racerx]

maybe I missed it but what kind of carb? stock or 32/36?

[datzenmike]

Re read more carefully.... Sounds like the idle cut solenoid isn't working or hooked up. It's the hex shaped thing with the Red wire in the picture below.

 

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Have someone turn the key to ON/OFF/ON/OFF (not start). You should hear a click sound from the solenoid if it is hooked up.

[moparvwfreak]

i will check that as soon as it stops raining. ok after i finish my seat install.

[dat521gatherer]

make sure your throttle cable has some slack. also make sure your linkage isnt binding. loosen the 4 nuts lift the carb up 1/4 inch or so and watch to see if the throttle linkage retracts any. i've had this happen before. also if the carb has been rebuilt it could have been done wrong. this also happened to me, damn carb was racing like no tomorrow.

[moparvwfreak]

well i got my little L idleing right. it was a problem with the BCDD not adjsuted right. i connected my tach and vac guage to the truck as recommended but put the vac guage inline with the switch for the BCDD. yes at teh manifold side ont he switched side. took the truck for a drive, adjusted and now it drops back down to idle like it should. my truck is now ildeing at 800 RPM, at 19.x invac and at 10* timing. now hopefully it will work good now.

[Bugeye]

ok found out what my problem is, i have a vacuum leak at the base gasket, thank you to colman propane....as for the bccd, its non adjustable for cali vehicles (go figure right) so tomorrow Im off to get some more new gaskets and for the hell of it a carb rebuild kit