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Engine Misfiring & Starter Cranks Slowly When Hot

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Frameman 1949

Frameman 1949

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Hello all,
I’m have a problem with two issues. First is a misfire while driving that occurs at 4200 RPM. I have rebuilt the SU carbs and adjusted them to what I believe is correct, although I must admit to not being really sure of this. The engine has had the timing adjusted properly I hope.
The ignition points were replaced and the gap set to what my manual advised. The spark plugs are old but still working.
The engine was locked when I got the car and I got it freed up and running although the cylinder compression is a bit lotw as a result of the build up of carbon and powdery rust in the cylinders which is gradually improving.

My second problem is what I think is a heat soak issue with the starter motor. It works fine when first started but cranks very slowly when at running temps, 160 F. Allow to cool for 10 minutes and it will restart.
Any thoughts will be appreciated.
 
My first steps would be to replace condenser, points, and coil. In that order. Condensers do all kinds of inexplicable things for reasons you can't guess. The spring that closes the points can fatigue and not get the points closed at higher RPM. Coils, like condensers, do the inexplicable and tests do not necessarily duplicate operating conditions. Best test for a coil is replacement. They aren't that expensive. Carburetion is not likely to be your problem. There is a maxim sometimes repeated here that says 'When you are sure it is carburetion, rework the ignition again'.
Bob
 
bobhustead said:
My first steps would be to replace condenser, points, and coil. In that order. Condensers do all kinds of inexplicable things for reasons you can't guess. The spring that closes the points can fatigue and not get the points closed at higher RPM. Coils, like condensers, do the inexplicable and tests do not necessarily duplicate operating conditions. Best test for a coil is replacement. They aren't that expensive. Carburetion is not likely to be your problem. There is a maxim sometimes repeated here that says 'When you are sure it is carburetion, rework the ignition again'.
Bob
Click to expand...
bobhustead said:
My first steps would be to replace condenser, points, and coil. In that order. Condensers do all kinds of inexplicable things for reasons you can't guess. The spring that closes the points can fatigue and not get the points closed at higher RPM. Coils, like condensers, do the inexplicable and tests do not necessarily duplicate operating conditions. Best test for a coil is replacement. They aren't that expensive. Carburetion is not likely to be your problem. There is a maxim sometimes repeated here that says 'When you are sure it is carburetion, rework the ignition again'.
Bob
Click to expand...
Thanks Bob,
I had already changed the ignition points and condenser.
I just finished ordering the coil and plugs
 
On the starter when warm ... check the battery voltage Might be that the battery is losing charge from a faulty alternator/generator, so the starter gets lower voltage after a run, than when the car has been sitting. Just a thought anyway.
Tom M.
 
Breaking up at speed could also be impaired fuel flow, like a blocked line from the tank or clogged inline filter.
Bob
 
Good advice so far. My two ¢:

1) (misfire at high RPM) - Run the car in a dark--but well-ventilated--area and look for arcing around the distributor cap and, as suggested, the secondary wires. Aftermarket dist. caps are mostly crap; believe it or not OEM Lucas are best in my experience.

2) (sluggish starter) - This is a not uncommon problem. First, as suggested, make sure you have a robust battery. Unloaded voltage doesn't tell you much, test with a load tester or, better yet, a battery condition tester. Old-type starters--no fancy exotic metal permanent magnets for us!--get weak when the insulation on the field coils and armature breaks down with age and heat makes all things electric behave badly.

Battery tester (cheapest I found, may not be great quality): https://www.amazon.com/2Ah-220Ah-Re...ocphy=9032278&hvtargid=pla-1453557526312&th=1

If you decide a starter rebuild is necessary, most starter/generator shops can do it; nothing exotic about Lucas, or you can send to Star Electric: > Star Auto Electric - Home / British refurbished electrical components. <
 
My BT7 starter had the same issue. Like previously stated make sure your battery is good and you have good electrical connections.
I then replaced the Oilite bushings for the armature shaft and also replaced the small bendix spring.
That took care of it, spins the engine really good now.
 
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