Idle mixture adjustment screws barely turned open

[Gene Letke]

Recently put a rebuilt carburetor from NCI (National Carburetor Inc) on my 70 C10 350 and it was running very rich (had black smoke). The Idle mixture needle screws were found at 5-1/2 turns open. I also recently had the timing set. I also check every vacuum line, After numerous Idle mixture adjustments, I ended up with both screws at 1/2-3/4 open to get a smooth idle. I read numerous threads, articles, and watched numerous videos on adjustments and no one ended up with such a low number of turns open. My real question is this, seeing it now idles really smooth with no smoke, starts and runs just fine, should I quit worrying that something is still not right.?

[Steeveedee]

Probably, but you can look into the bores to see if any fuel is dripping from the venturis. One thing about some rebuilders is that they may use any throttle plate with any float bowl that fits, and the calibration may not be correct for a particular engine.

[Ironangel]

Whats the idle rpm and what is the timing set at? (Both with the vacuum canister disconnected and the vacuum barb plugged...?) In answer to your final question, "your probably close..."

[Minkota]

Verify that there are no vacuum leaks (carb gasket, intake gasket, misc vacuum lines, trans modulator, vacuum line plugs, ect...). make sure to check the brake booster for vacuum leak as well.

[Gene Letke]

i did check all the vacuum lines and the brake booster unit. I had the timing set by a license mechanic. I have played with the idle speed several times during adjustments, sure seems about right, idles fine in gear.
This checking the venturies, I assume this is with it running. So when idling no gas should be leaking from the venturies only when accelerating.

[garyd1961]

I have a brand new Holley that is the same way, runs good at 3/4 turns. I found that the shaft for the secondary butterflies is sucking air. I had the carb for over a year before I found it. Thought of putting a bushing kit in it but since it's running good I just left it alone.

[Gene Letke]

i just checked the primary venturies (with the engine running) and did not see any gas leaking from the venturies. All I can say is, this truck the way I have the idle mixture screws set sure idles real smooth.

[AussieinNC]

Originally Posted by Gene Letke

Recently put a rebuilt carburetor from NCI (National Carburetor Inc) on my 70 C10 350 and it was running very rich (had black smoke). The Idle mixture needle screws were found at 5-1/2 turns open. I also recently had the timing set. I also check every vacuum line, After numerous Idle mixture adjustments, I ended up with both screws at 1/2-3/4 open to get a smooth idle. I read numerous threads, articles, and watched numerous videos on adjustments and no one ended up with such a low number of turns open. My real question is this, seeing it now idles really smooth with no smoke, starts and runs just fine, should I quit worrying that something is still not right.?

I have seen rebuilt carbs with idle screws with different tapers on them...usually due to a new " seat" surface being machined into the housing....5 1/2 turns out is way too much....


Personally, if it isnt broken, it doesnt need fixing...

Better than having the screw backed right out just to get it to idle...

[geezer#99]

Originally Posted by Gene Letke

I had the timing set by a license mechanic.

That’s the root of your condition. Likely set to factory spec which results in the need to increase the idle speed which uncovers the transition slot. The mix screws get close to being useless like that as it’s starting to idle on the power circuit.
A small (likely 8 degrees) increase in your initial timing will allow the idle screw on the carb to be backed off enough to close the throttle, square up the transition slot and make your mix screws work normally at close to 1 1/2 turns out.
Yours is close. Might be a tad rich at idle. But runs all right like it is. You could get a timing light and fine tune it or just drive it and enjoy it. Sometimes best to leave it like it is. I always fine tune to the nth degree.
Info in this link about transition slots. On a Holley but most carbs are the same.


http://www.crankshaftcoalition.com/w...EI_distributor

[Steeveedee]

See post #2 here, it talks about the transition slot-

https://cliffshighperformance.com/si...p?topic=1845.0

[dmjlambert]

Did the carburetor come directly from National Carburetor or did you buy it at some other store? They set up the carburetors on an engine and they should require no adjustment except to perhaps idle speed screws and choke. If it came some someplace else I suppose somebody at the store could have screwed with it or it could have been a customer return. You can ask National Carburetor questions, I have found they have good tech support and customer service. I've returned my lifetime warranty carburetor to them for fixing once.

[Gene Letke]

I bought the carburetor directly from NCI. I originally had an issue with it hesitating and had to send it back. They completely went through it again and sent it back. Other than it running rich out of the box, it seems fine.

[RichardJ]

>>They set up the carburetors on an engine and they should require no adjustment except to perhaps idle speed screws and choke.<<

That statement is far beyond incorrect.

CARBURETORS ARE NOT PLUG AND PLAY.

Idle speed screws, Idle mixture screws and choke are always adjusted to the engine the carb is installed on.

Speed and mixture adjustments you see in a manual or carb rebuild sheet are bench adjustments that are made for the purpose of easily starting the engine the first time.

You start the engine, set the timing, then tailer the speed and mixture adjustment to that engine. If the timing needs to be readjusted a second time, you will almost certainly have to readjust the speed screw and possibly the mixture screws as well.

Some choke and high-speed idle adjustments are made on the bench, others are done on the engine, some running, some not, some engine cold, some engine hot. Different carbs and different choke systems require different adjustments and if done properly will almost always improve drivability, if done properly. Particularly when the engine is cold and while warming up.

If you think carbs are PLUG and PLAY, your results are a crap shoot.