Erratic idle & stumbling acceleration
 

New to us '29 Tudor with '30 engine. My husband is sorting out our engine problems. Here's what we've got so far: Warmed up the engine for 3 minutes with a vacuum gauge connected to the wiper port on the intake manifold. Vacuum fluctuates 17-20 at idle. Did a compression test; results were 85, 86, 86, 89 (1-4). Set the used points to 0.020" and adjusted spark according to Les Andrews. Two notches down from full retard the light came on.
Auto-lite Plugs are gapped to 0.032. All are firing. Plugs are dry sooty black when we took them out.
Ran unlit propane torch around the intake manifold and carb mounting gasket without any improvement to the erratic idle.
He's thinking that cleaning/rebuilding the carb is our next step.
We welcome any suggestions.

Re: Erratic idle & stumbling acceleration
 

Try pulling the choke slowly while the engine idles, to see if that makes it run smoother. Or, you can do the same with your hand by slowly blocking off the carb air intake. If it smooths out then the carb is too lean at idle.

If it doesn't smooth out then turn the gas valve off while the engine is running and see if the engine smooths out as the carb runs low on gas. If it does, then the carb is running too rich.

Re: Erratic idle & stumbling acceleration
 

You may check your timing. I am no expert on this type of thing but I had similar problem with idle and set my timing back a bit and it now seems to have corrected problem. I also went through most of the things you mentioned you did and retarding the timing seem to help in my situation.

Re: Erratic idle & stumbling acceleration
 

http://www.autospeed.com/cms/article.html?&A=2393

great article on interpreting your vacuum gauge. it should hold steady at idle - when you say idle you mean ~600 rpm right?

according to that you may have a sticky halve or a burnt/non sealing valve or a misfire. - your leakdown test should be able to show you whats going on.

we hooked a vacuum gauge to a known good running model a engine and its idle vacuum was steady at 20in/mg, while cranking we were seeing 7-8 inches.

the idle number isnt too important as it varys depending on altitude. BUT none the less it should be rock solid steady.

if you dont know the history on the carb it cannot hurt to PROPERLY rebuild it. the throttle shaft is most likely worn and sloppy causing a vacuum leak.

Ive tried the unlit propane trick with mixed results, sometimes it works other times it doesnt appear to do anything....

Re: Erratic idle & stumbling acceleration
 

I would use Marco's timing instruction. http://www.abarnyard.com/workshop/timing.htm

Bob

Re: Erratic idle & stumbling acceleration
 

If the plugs are sooty, then you're to rich, or not getting good spark. What's the number of the plugs?

Re: Erratic idle & stumbling acceleration
 

if your plugs are black your running to rich.
i would look at the carb
rebuild it properly and use a set of flow tested jets from renners

Re: Erratic idle & stumbling acceleration
 

Thanks for all the suggestions.
We suspected a valve stuck open from the vacuum gauge results.
The compression test results didn't confirm this in our opinion.
The plugs are Auto-lite TL8 (not new).
We will look more closely at the timing and put in new points and rotor.
The distributor is the original. (Side question: there's no condenser. Is it some place else?).
A carb cleaning/rebuild with new jets is next.
Also draining all the gas and re-filling with fresh and MMO added. We have no history on the car, other than it sat for 3 years prior to our ownership.

Re: Erratic idle & stumbling acceleration
 

Put new plugs in. Gap them to 0.035"

You might not need new jets in the carb. The carb may need to be cleaned and install a new needle valve. Close the GAV and see if it is still rich (after cleaning).

You must have a condenser in the system or the spark will be poor. If you do not see one in the stock location, look on the "points plate" to see if they have modern points and condenser installed. If not, put on a condenser.

Fresh gas is always good. MMO never hurts (2 oz. per 5 gal.). Sitting 3 years can give you bad gas that runs poorly.

Re: Erratic idle & stumbling acceleration
 

Yes good advise.

Does your spark lever when you move it up or down make a difference in the way it idles? If not the above is very good advise.

Re: Erratic idle & stumbling acceleration
 

Condenser: I guess we don't know the stock location. There isn't one on the points plate.
Yes, moving the spark advance lever on the steering column makes a difference in how it idles. We have checked for full travel on the cutout on the side of the distributor.

Re: Erratic idle & stumbling acceleration
 

The stock condenser is the silver thing on the right side
of the distributor body. The round thing with the strap
with the three dimples in it.

Bob

Re: Erratic idle & stumbling acceleration
 

Black sooty plugs can miss-fire & cause most of your symptoms! ALSO, in the dark, check for spark jumping out of the top of the distributor cap. From the track going to #4 cylinder, it will jump up to #3 plug connector strap!!
Bill W.

Re: Erratic idle & stumbling acceleration
 

Bob C: thanks for the photo - found the condenser.

Re: Erratic idle & stumbling acceleration
 

Be careful to troubleshoot the issue rather than shotgunning it. You want to find the issue, verify it, and then correct that one thing before you start fixing other things. As you find things verified wrong, fix that one issue then move on to the next. Nothing worse than dealing with introduced problems.

Re: Erratic idle & stumbling acceleration
 

Bill W: just checked for spark jump in total garage backout. All good - no stray electrons. Appreciate the tip.

Re: Erratic idle & stumbling acceleration
 

with a spark tester in line of the coil wire while running does it get erratic when the car acts up. we recently had a run here on the dist plate pigtail wires either with broken strands or grounding. that would also make the plugs black from the erratic spark...

Re: Erratic idle & stumbling acceleration
 

I believe I have two verified problems at this point.
(1) Fluctuating vacuum at idle:
I'm suspecting worn throttle shaft, as was suggested.
(2)Too rich mixture.

I'm hoping a rebuilt/remanufactured carb, properly adjusted, will fix both of these.

Am I missing something?

Re: Erratic idle & stumbling acceleration
 

GOOD POINT TO REMEMBER! When plugs are sooty/black, we tend to blame an overly RICH fuel system, FIRST thing, instead of possibly ERRATIC spark.
Bill W.

Re: Erratic idle & stumbling acceleration
 

Could it be???
https://www.fordbarn.com/forum/showthread.php?t=168899