persistent misfiring - help!

[Giles]

My A has suddenly developed severe misfiring as soon as it reaches operating temperature, with much backfiring of unburnt fuel through the exhaust and total loss of power. Condenser checks out ok. Points are clean and correctly gapped. Plugs ditto. I have tried another coil - a used one, but as far as I know good. No signs of tracking on the dizzy body or rotor arm.

Can anyone help me out with further tests I can run, theories, suggestions? I have had the car over 20 years but this is a new one on me ...

[pooch]

Check voltage at battery when at high idle.

[Patrick L.]

What do you mean by total loss of power ? The engine quit running and will not start ? Or, does it start but run poorly ? That'll make a difference as to where to start [ pun intended I guess] to figure this out.

[KGBnut]

I get the occasional backfire problem. When it happens, I pull the carburetor apart and check all the jets. I invariably find a piece of grit partially blocking one of them.

Good luck,

Ken

[Giles]

Patrick - good question - I meant that the engine continues to run, very irregularly, but generates insufficient power to move the car.

[wmws]

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Sounds like a condenser to me. How did you check it out to know it is good. Did you check it hot.

[Fullraceflathead]

Some Condensers can intermittently short when hot same as coils.
I'd change the condenser just to eliminate that too.

[Patrick L.]

OK. I'd recommend checking coil polarity [ if car is + grd then + coil primary lead goes to points]. I too would change condenser. If the coils are oil filled I would change with a tar or epoxy filled. Oil coils don not like being tipped upside down, neither do I.

I'd also check fuel supply for good flow. If the backfiring occurs at a higher road speed slow down and see if it goes away. If it does, thats a classic fuel deliver issue. loosen the gas cap to see if it helps. If yes, thats the problem. If not, fuel tank/ line/carburetor issue.

[Tom Endy]

Try a different top for the distributor. Sometimes they develop cracks and cause intermittent shorting.

Tom Endy

[Tom Wesenberg]

Yep, this is why some people carry a spare distributor, complete with cap and rotor, under the back seat.
They have it already timed also, so it's a simple slip in and go while on tour.

[Giles]

thanks folks - lots of helpful things to follow up. Since I last posted I have stripped the carb and checked all jets for obstructions. No change. Misfiring is equally bad at tickover, so I don't think it's fuel starvation. I have ordered a new coil, condenser and distributor cap ... we'll see if that does the trick.

[PC/SR]

Do a compression test to eliminate that as a problem. You seem to have done the fuel supply check. That leaves ignition.

[Giles]

Fitted new coil and condenser but problem is no better. Compression tests ok. Points are clean and correctly gapped - but could they nevertheless be acting up in some way? Worth replacing them perhaps? Plugs are also clean and correctly gapped, but quite old. I am starting to run out of options and getting increasingly baffled. I've looked after the car for many years and it's normally reliable...

[Big hammer]

Years ago two mechanic friends of mine did a tune up on one of their cars. When they finished they had a miss. All new parts points, plugs, condenser, cap, wires, rotor, etc.
It came down to the new plugs where bad causing the miss!

[steve s]

Some other straws to grasp at that you might not suspect:

I've had several ignition switches fail in an intermittent way. Try jumping around the ignition switch. Similarly, try jumping around the ammeter--this can be done without removing the dash by attaching clips from a jumper wire to the two wingnuts on the junction box.

Speaking of the junction box, if you're using a repro junction box, the screws that the wire lugs attach to can short out to the firewall if they are round-head screws and the box is not properly recessed for them.

Check that the pigtail wire in the distributor is not shorting to ground.

Speaking of shorting to ground, make sure that the engine block is properly grounded. Run an auxiliary grounding strap from a bolt on the top of the transmission to the ground post of the battery.

Try a different distributor body. This was the culprit for a friend's A.

Make sure that none of the spark plug straps are either touching or too close to the distributor body.

If you're using one of the little fuse blocks that mount on the starter, jump around it.

Steve

[john charlton]

I had the identical problem which baffled me . At last I changed the distributor cap and an instant cure . The car had been popping back through the carb as the wrong plug was being fired due to an internal short . The repro cap looked brand new . At some time I will do a post mortem on it to see what happened inside .

John in mild evening Suffolk County England .

[Tom Wesenberg]

Originally Posted by john charlton

I had the identical problem which baffled me . At last I changed the distributor cap and an instant cure . The car had been popping back through the carb as the wrong plug was being fired due to an internal short . The repro cap looked brand new . At some time I will do a post mortem on it to see what happened inside .

John in mild evening Suffolk County England .

Yep, I've seen it happen a couple times.

[100IH]

Then, if not ignition, check to see if the choke is staying completely open at higher RPM's. Just a thought if the ignition is not the issue.

[Giles]

Thanks guys - I'll check these options too.

[Patrick L.]

OK, I too would look at both ignition secondary and primary.

Secondary for bad rotor/cap

Primary for an intermittent open anywhere from terminal box thru ignition switch to distributor internal lead.