Surging at idle with spark where it should be
 

I own three model A's. Do all of my own work, but this issue has me stuck. Distributor checks out with all the trouble shooting. The fuel system is good thanks to a brand new rebuilt Marvel. When I run the car it surges like its going to backfire through the carb unless I lay on the gas.

any thoughts?

Jon

Re: Surging at idle with spark where it should be
 

Look real hard for an intake air leak. Do the trick with starting fluid. Good luck.

Re: Surging at idle with spark where it should be
 

I agree with Jackson. Sounds like an intake leak. Another option is to use a LP torch, not lit, and direct that around manifold. Often times the carb intake will suck in some of the gas so you may want to connect a hose to the carb intake so it pulls its air from further away.

Re: Surging at idle with spark where it should be
 

How long has it been doing this? Could you swap the carb out just to eliminate that?

Re: Surging at idle with spark where it should be
 

Idle jet may be slightly plugged!

Re: Surging at idle with spark where it should be
 

A few years back our A always surged it idle when it got hot,I tried all the usual things over a couple years to sort it, finally got it sorted,
It was crook main thrust washers and cam thrust plunger spring allowing the crankshaft and camshaft to float back and forwards at idle,
fixed that and got the smooth idle back.
Lawrie

Re: Surging at idle with spark where it should be
 

Sticking value??? I had this problem when I pushed on the gas. It would backfire through the carb.

Re: Surging at idle with spark where it should be
 

Mine lopes at idle if the GAV is opened too much , causing a too rich condition . Try closing the GAV a bit . I run my Zenith or Tillotson carbs with the Gav opened one quarter turn . When I use model B Zenith carbs I run with the GAV completely closed after a brief warm up . Carburetor adjustment can vary with these old carburetors . I would experiment but wouldn't go over a quarter turn open with a warm engine .

Re: Surging at idle with spark where it should be
 

X2, usually on mine and the gav is set too rich I get some slight black exhaust.


Another possibility is the Carb Idle Mixture screw is set too rich. In extreme cases this can also cause a loud exhaust backfire (sound like a shotgun) when shutting off the motor, caused by unburned gas mixture in the exhaust manifold igniting.
If you pull a spark plug and the insulator is extremely black/sooty, you most likely are running rich.

Re: Surging at idle with spark where it should be
 

So where is spark where it should be for you?

Vince Falter indicates it should be fully retarded at start, then advanced slightly at idle. Then proportionately advanced to perhaps a little more than half way as the engine speed is increased.

http://www.fordgarage.com/pages/bdistributoradvance.htm

Early in my Model A experience I FINALLY got to where I felt the timing was correct - at least all the performance, lost motions, and "spark check" showed it was correct. However, on idle with the spark advanced to the top (my first mistake) the engine "rolled." I noticed when fully retarded, the engine ran tick-tick-tick like a fine watch.

For a long time I thought there must be something else here. There was - I was not running the engine properly! Now thanks to reading Vince, I know better.

Joe K

Re: Surging at idle with spark where it should be
 

Sorry for the delay on this. I did the torch trick, and sure enough the intake needed some love. Pulled here off and replained all decks. The issue is worse! I ran another full electrical check and all seems to be good. When I pulled the distributor, I did notice that there was quite a bit of water on the bottom of the housing. Could this be shorting out the distributor housing? Iv'e had this happen before on my other A. She ran like a top at full retard, but at operating level she'd run terrible; surging, missing, sometimes back firing. I changed the distributor housing to see if she shorted, and she ran great.

Re: Surging at idle with spark where it should be
 

The distributor housing is supposed to be grounded. However if you also have a lot of moisture inside the distributor that could cause a grounding or short condition. Try drying out the inside real good with a hair drier or take it all apart and wipe it out real good.

Re: Surging at idle with spark where it should be
 

A frustrating update!

I checked the intake manifold, and sure enough there was a leak. Problem solved? No! Next chased the distributor again. Cleaned it, new points, and ran all tests. Right where it should be. Fired the car up, and she ran great! drove it twenty miles, and when I went to stop at a stop sign she died, like someone turned off the key. Did some tinkering, and she started, got back on the road about 10 feet, same thing. After doing this three times in one mile to get it back home. I pulled the distributor off of one of the other cars running fine, same issue. I put a new cheapo repop switch on thinking it was an intermittent switch, now the car starts, runs and dies.

Help please!

Jon

Re: Surging at idle with spark where it should be
 

The good news: NO more backfiring thanks to the new intake!

Re: Surging at idle with spark where it should be
 

When it dies, is it due to loss of spark or is it fuel related? Determining that is the FIRST thing to do, its not always easy to tell. Pull a couple of the spark plug straps up so they have a small gap between them and the spark plug. Then let it idle and watch the spark. I would recommend you troubleshoot the problem and NOT start throwing parts at it. You will soon introduce new problems then you'll be worse off.

Re: Surging at idle with spark where it should be
 

Take a spark plug lead off the plug, place the lead about 1/4 inch from the spark plug post and start the car - Spark should be good blue flash, if it turns yellow or looks weak possible condenser or coil. Keeping checking it as you let it run and it starts having the issue. If weak yellow possible condenser or coil.

To make sure your coil polarity is wired properly.
http://1929modelaford.blogspot.com/2...-polarity.html

Disconnect the gas line from the A tank at the carb, hang a small gas tank with fresh gas from the hood/radiator rod and run a line to the gasoline carb Maybe possible you have rust or old tank sealer in your gas tank?

Then try a known good condenser.