Idling a bit rough suddenly.

Pat Martone

Pat Martone

Senior Member
My Slant has begun to idle a bit rough in the last week or so. The car runs fine and does not stall out, but the idle is suddenly not as smooth as it used to be.
Is it an electrical or fuel problem; of course, that is the question!
I checked the points gap and it is fine.
I have an "A&L" supplied condenser, so I assume that is fine.
My timing is perfect, but I have noticed that at idle, the engine speed no longer changes dramatically when I move the spark rod from full-up to half-way down; it changes a little, but it used to be a more drammatic increase in idle speed when I moved the lever down after starting the car.
I noticed this change because sometimes while stopped at a red traffic light, I push the spark lever all the way up, and I recently noticed that the idle speed did not change as much as it used to when moving the spark lever up and down. (My spark lever linkage is working fine, and my carb was properly rebuilt 2 years ago by a local and reputable Model A enthusiast.) Perhaps I should go through the Les Andrews simple air mixture screw adjustment procedure?
I welcome your thoughts. Thanks!
 
Patrick L - Thanks for your thoughts; I did check those too; no vacuum leaks and my wiper is working fine, and the manifolds seem ok with no obvious leaks.
 
I was running my 20 year old lawnmower last weekend, it was going strong with some leftover chain saw 50:1 gas from Ace which is 92 octane gas and no ethanol. I had added some crap gas that had some MMO and stabilizer in it and when that gas began to feed into the carb. the engine began to run slower, it lacked power and started to miss!
 
I had the same situation yesterday. When I got to the guy's house, his A had no spark. I cleaned the points and got enough spark to start the car and back it out of the garage. I then stuck a nail in the distributor cap and held the coil wire 1/8" away as the engine idled. I could see the spark quit at times, so I knew he had points problems still. This was a modern top plate and points, and he had a rebuild original distributor in his trunk, so I installed that, and the problem was solved.
 
It sounds like ignition problems. I would look at the lever on the breaker plate. When the spark is retarded, the lever needs to move all the way to the right side of the window at the rear of the distributor cap and vice versa when the spark is advanced. If you aren't getting full movement in the distributor cap window, it will be necessary to loosen the clamps on the steering column and rotate the column tube untill you get full movement, both ways.
 
Ignition points tend to close up as the rubbing block on the arm of the points wear. If you installed a new set of points and set the gap at .020, after about a 100 miles you are likely to find the gap setting a .010. New points rubbing blocks wear significantly when first installed until a glaze forms on the block.

it is prudent to check the point gap each time you change oil as the block will still continue to wear. It is possible for the wear to be such that the arm will touch the cam and you will have to replace the points.

Set the point gap first, then the timing. As the points close up the timing will change. Restoring the point gap back to where you originally set it will restore the timing to where you set it.

Tom Endy
 
Thanks for all of the informative replies.

I wonder if a condenser can work intermittently, or it is usually a catastrophic fail?

If a condenser can fail intermittently, then I will try swapping the condenser again, if you recommend that?

(I think the other things are fine: points and gap, timing, and full-motion of the spark lever and distrutor plate rotation.)
 
Yes, sometimes condensers can fail intermittently, cool and work again for a short time. I don't think that the condenser would have anything to do with idle speed
 
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