engine misses

F

f balsiger

Member
engine misses at all speeds. good compression which I think eliminates valves , no vacuum leaks (intake) tends to blubber. Almost goes away with full retard on timing. New modern point and condenser. Blue spark. what could it be?
 
It is sometimes difficult to distinguish between ignition and carburetor problems. If you have a spare, it's quick to change out to see which has an effect on your problem. I had a similar issue recently and it was in the carb jets. Start with the simple things, especially if it had been previously running correctly.
 
Try running it in the dark, and watch for voltage leaking. Although, usually, if it’s intermittent spark, you’ll get backfiring. I assume your plugs are clean.
 
I replaced the coil, used modern points & condensor (Nu-rex plate and B style distributor cam), replaced the wires from the key to the points, and the coil HV wire to the distributor. The coil was last thing I dared to change and it made the most difference on mine. I think it was just weak. It was very old and slightly rusty. I also spent about two weeks off-and-on learning the timing inside and out. That edumacation alone got me out of a bind on one short trip to the post office. Wiring / Timing . . .

https://sacramentocapitolas.org/2017/10/18/model-a-wiring-diagram/

http://pvmafc.org/phpbb/viewtopic.php?f=8&t=4

I used a timing light to see where the timing was in relation to the crank shaft's position.
 
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What is the point gap?? Plug gap?? Do you get a nice spark from one of the plug wires and/or the coil wire??
WHY did you change to newer style points??
Did you have this problem before the change??
Paul in CT WELCOME ! ! !
 
At night, very dark, run the motor with the hood open, look for arcing. Also try a different Distributor body, once in a great while they develop sneak paths.
 
I remember someone posting a coil tester from the 30's era, Brent maybe. Is there a modern equivalent to test a coil? How do you know if your coil is weak? Surely there's a way to easily test it??
 
There are a lot of little things that can cause ignition problems. A sloppy distributor shaft, an uneven beaker cam, cap body electrode clearance with the rotor, a cracked rotor or body, a bad condenser, and there are others. I've even heard of pop out switches with broken wire strands in the armored cable causing problems. All the components have to be checked. Luckily, there are't too many things to check with a simple system.

An ignition coil has to be tested with a type of tester that will prepare it for test with a warm up and it tests it a lot like it works in the system. This is what was used back in the day. It's a Sun model AA3693.
https://www.worthpoint.com/worthopedia/sun-electric-auto-diagnostic-gas-station-coil

The only other way to test it is while it's running with an engine. The more modern units with the oscilloscope were also a good way to test them.
 
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katy said:
I bench test coils w/an old distributor in the vise, a 6 or 12 volt battery, and a spark voltage tester similar to:
https://www.amazon.com/Deal】OriGlam...iagnostic/dp/B06X9RC3PF/?tag=atomicindus08-20
Not perfect but gives a good idea of the coils output.
Click to expand...

On the farm we took a long green weed stem and while holding one end would lay it across the electric fence to make sure it was working. That should work here also, l'll bet.

I wouldn't recommend doing that if you have a weak ticker. Unless you don't mind checking out early. ;-)
 
Best to try to eliminate electrical before you start changing carb settings. To determine if the miss is only with one cylinder try disconnecting a plug and running the motor to see if you detect a difference. Do this for each cylinder and if you find one where the difference in minimal than you may have narrowed the problem to that cylinder.
 
I found a misfire that was driving me crazy. I thought it was the timing, points adjustment, coil, carb...nope. It was a little screw that came loose. In the distributor there is an upper plate that the contact points are attached to. One of the two screws came loose below the upper plate and it was shorting out a wire in the lower part of the distributor. Sometime it would run ok and sometimes it would misfire.
 
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