Carb issue...
 

Finally getting a chance to drive my 68 firebird 400 HO . Engine runs great, I've only romped on it twice so far, the issue is in the upper RPM's it will not settle down to idle, engine races. I turn off engine, re-start, idles fine. No binding of linkage any ideas?

Carb is a late model Q-Jet...

Secondaries hanging open? Also look at the mechanical advance in the distributor hanging up. Seems that it would be running rough as well though…

It’s likely that the mechanical advantage section of your distributor be it HEI or points type is hanging up.

This inner shaft is what gets pulled around by the advance weights and the springs are what rotates this shaft back to idle timing position- that is unless the grease / lube around that inner shaft is dried up and stiff .

Also if the distributor is been fitted with aftermarket light advance springs they tend to fatigue rather fast and then can not pull this shaft back .

If it’s a original points type distributor then while you have it apart to relube that inner shaft then be sure to take a good look at the under plate wire the comes in a goes to the points.

It’s very common for the insulation to dry rot off of this wire.

When that happens the wire can move while the motor is running and then short out which then kills the spark.

First thing i would check in this instance Is make sure the fast idle cam on the choke side is not engaged, Air cleaner off so you can see familiarize yourself with the choke fast idle linkage then look for the fast idle screw being engaged on stepped cam ramp when this happens then go from there

choke stove may be shot maladjusted MIA., fast idle screw could be turned in to far, stuck corroded operations

I had this exact same thing happen to me with my '57 Chevy with a Rochester 4GC. Okay - different engine, different carb, but hear me out.

I did a fresh rebuild of a 4GC I got off eBay (replacing the Edelbrock that was on the car when I bought it). Set everything exactly to specs, including the clearly misadjusted secondary lockout (keep that in mind). Took it out and it ran great until I stomped on it and let off the accelerator - it kept going.

Fundamental to what happened is understanding the nuances of carburetors.

The choke is held open by a combination of relaxed thermostatic spring and vacuum break. When you stomp on it, vacuum drops to near zero, so the vacuum break can allow the choke to close a bit.

In my case, that's exactly what happened. The choke partially closed at full throttle, and the now-correctly adjusted secondary lockout tab jammed the secondary throttle linkage, allowing the primary to close but holding the secondary partially open.

If you road test it, a few things to try.

First, if it happens, shut off the ignition and safely coast to the side of the road without touching the accelerator. Pop off the air cleaner and see if either the secondary or primary throttle plates are stuck partially open. Then, see what's hanging them up.

Second, if you're in a situation where you can't do that, kick the accelerator pedal a few times and see if it comes down.

You can also try wiring the choke fully open and see if the problem disappears.

Based on my experience described above, I would check for a leaking vacuum break diaphragm and misadjusted secondary lockout. Some vacuum diaphragms have a controlled vacuum bleed, but if you can't pump it up with a handheld vacuum pump (Mityvac or equivalent) try a new diaprhragm.

In my case, the vacuum break operated with a brass piston instead of a diaphragm. I just re-disabled the secondary lockout like the carb's previous owner and avoided stomping on it with a cold engine.

X2! Put a timing light on it. Check at idle, record the number . Slowly rev the engine to 3000 then close throttle. The timing should quickly return to the exact number. Every time.

If it doesn't the do the same test with the vacuum advance disconnected and plugged. Recheck the idle, record the number and retest. It should quickly return to the same number.

If it doesn't return to the same number or it lags and returns slowly its time to pull and inspect the distributor.

The distributor is an old one that I had available, thinking the issue is the dizzy. I'll pick up another to replace it. I'll let you know what the result is. Carb is a professionally built & tested unit.

Is there an aftermarket distributor worth buying for a street engine?
I ask because I have same issue and after swapping to a known good carb same issue. I followed vcho455’s advice and can see after revving slowly to 3k rpm and returning to idle I pick up a few degrees (6 ish) advance and it’s different amount after each rev. It’s an original dizzy (‘68) with a Pertronix unit.

Did you read post number 4?

For that is the why your factory Dizzy is hanging up on you.

Yes Steve I did. My question was is there an aftermarket distributor worthy as a stock replacement that is not big $. The distributor I have likely has 200k miles on it and I would rather replace it than throw $ at it if it’s too worn. I’m thinking an HEI but there are many and all seem China made. A good OEM would be fine too but I’m not seeing any for sale.
Looking for suggestions from those who have replaced theirs.

I have had good luck with DUI distributors if running an HEI and casting numbers aren't important.

https://www.summitracing.com/parts/d...l/make/pontiac

The way it acts when it gets to a certain RPM range, it just won't return to idle to shift. New Distributor on its way, will have some time next week to swap it in. Engine pulls hard with a Muncie & 3.55 posi!

I like my www.progressionigniton.com distributor.

The SUMMIT brand distributors are fine