flathead possible miss only when hot?

[hombres ruin]

Hi guys ,I have a situation that I cant figure out and was wondering what your input would be.I have a 1950 8BA,offy heads and manifold,mallory electronic ignition and autolite spark plugs.When I start up the car it runs really nice,smooth, the idle is on point and its timed correctly,i will drive it maybe 30 mins back home and when I get there it sounds like its missing but not really bad,it doesn't stop,nor does it act like its going to stop.Its just not smooth as on start up and you can here a sort of interruption in the entire engine sound.When it cools down it starts up and the sound is gone.I re gapped the plugs and checked the wires to see if all was in tact and it is.The temp of the engine is 170 degrees.Any ideas as to whats up? thanks so much

[john mullen]

Possible a valve is sticking when its hot. try using some Marvel Mystery Oil in the gas. #2 check for vac leak I have had this happen before on a new rebuild with new guides and valves

[jkeesey]

What kind of coil are you running?

[GreenMonster48]

^^^ Was thinking the same thing. Coil getting hot or tired.

[BUBBAS IGNITION]

I think the guys that asked what kind of coil do you have is the correct answer.
Many times ( i get them every week) a engine builder/car owner will add a aftermarket distributor ( or stock for that matter) and then use the incorrect coil..

The one is see the most is the Mallory round can oil filled chrome coil sold by everyone.
they use two part numbers with identical looking coils.
The 29216 coil is a .700 ohm coil and is not considered a universal coil in their specs , meaning it wont work on everything..
The 29219 coil is a 1.4 ohm coil nad is considered universal in the detail specs meaning it would run ok in a lot of vehicles .

What happens with the low ressitance coil is it uses additional primary current based on the low ressitance. When cold the electronic module will withstand the extra current and run ok, after some time the excess current is controlled by a internal overload circuit shunting the excess current to ground preventing module early failure . This current limiting bypass usually makes the engine skip or have a slight missfire . The nature of the problem causes a skip on multiple cylinders and maybe not the same one each time....

So what coil do you have ???

[hombres ruin]

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bubba,here is the coil its a Mallory,also note that inside the coil there is a split in the metal,could this cause such a problem as described.Also here is the distributor.Thanks for the help guys.If this coil is wrong for my application,what would you suggest in its place.Thanks again for the help

[Joe Immler]

I had this problem and there was quite a bit on this site. Turned out to be valve adjustment. Check out the thread: Engine misses when hot.

[sortkaffekop]

An easy shot: Coil wired the wrong way.

[BUBBAS IGNITION]

Originally Posted by hombres ruin

bubba,here is the coil its a Mallory,also note that inside the coil there is a split in the metal,could this cause such a problem as described.Also here is the distributor.Thanks for the help guys.If this coil is wrong for my application,what would you suggest in its place.Thanks again for the help

Looks like thats the 219 coil which specs at 1.4 ohms. Does the coil get hot to the touch after running for a few minutes. If so add a 1.5 ohm resistor to the circuit.

The split in metal usually happens as the coil overheats and swell up, might be worth buying another coil...

The valves too tight when hot is another theory we have had over the years...

[jkeesey]

I would only suspect valves on a fairly fresh engine. If it has been in service for a long time you can pretty much rule that out as a problem.

[hombres ruin]

Thanks guys,the engine has 1800 miles on the rebuild,and has been in service for 8 yrs.Bubba I will get another coil,do you suggest the same one or is there another ,better coil I could get.I will replace the coil and use some MMO and then take it from there

[hombres ruin]

Well I picked up a new coil and will install it and see what happens. I think the coil was getting hot and tired. It developed a spilt internally. So I will keep you posted on what happens

[hombres ruin]

Well I installed the new coil which is a Mallory 219 made for my ingnition system.I took the shoebox out and ran ,took her to op temp and nothing,no miss,one block from my house nothing,one stop sign from my house and nothing.Get to my garage and put it in park and there is the miss.The temp was 175,idle is perfect,timed on point. It purrs along then drops out a bit,purrs then drops out.No miss when I am driving on the freeway or streets.Any ideas guys. I don't get ,why it likes to do this when I get home???

[George/Maine]

I think your car doesn't want to go home , If you only drive 225 miles a year that could be your problem. Try running it for longer runs like 75 miles non stop.
Put fresh gas in and idleing to temp doesn't count.

[hombres ruin]

George,i took the car out and ran her for a while,i didnt idle to temp in the garage.I hammered her.

[BUBBAS IGNITION]

Originally Posted by hombres ruin

Well I installed the new coil which is a Mallory 219 made for my ingnition system.I took the shoebox out and ran ,took her to op temp and nothing,no miss,one block from my house nothing,one stop sign from my house and nothing.Get to my garage and put it in park and there is the miss.The temp was 175,idle is perfect,timed on point. It purrs along then drops out a bit,purrs then drops out.No miss when I am driving on the freeway or streets.Any ideas guys. I don't get ,why it likes to do this when I get home???

Specs :

Looks like thats the 219 coil which specs at 1.4 ohms. Does the coil get hot to the touch after running for a few minutes. If so add a 1.5 ohm resistor to the circuit.

How hot is the coil ?? Try a resistor and then recheck. These coils are mostly off shore and can be a major problem...

Coil is cool when driving from extra air flow etc and time of build up with rpm....

[George/Maine]

Originally Posted by hombres ruin

George,i took the car out and ran her for a while,i didnt idle to temp in the garage.I hammered her.

If you were to get back on the street and drive another 5 miles would it run ok.
If you took one wire off spark plug it would be a solid miss.
I notice on my 8ba it misses at idle but runs ok when moving.
You could go back to points and try that.
The middle exhaust pipe could be causing a back fire, miss fire.
4 cylinder exit out 2 pipes.

[hombres ruin]

I have had a problem with the gas boiling after shut down and when the engine gets hot. Could this cause a slight stumble as the gas evaporates from the carb when hot,thus not delivering the right amout of fuel?

[hombres ruin]

I am going through the process of elimination so I will install the phenolic spacers,and use the correct ballast resistor for my Mallory ignition system,then see what happens.

[Ken Bull]

I went nuts with a similar problem ---Found bolts on intake manifold all turned approx. 1/8 " torquing to 25lbs. Found nut on one side of Resister under dash, a little loose - tightened & replaced Condenser. Has Skips rebuilt coil & all seems fine after 30+ mile trip. HOPE IT REMAINS RELIABLE.

[hombres ruin]

so to continue on..i have got a new coil and a new ballast resistor one that is recommended by Mallory for my magnetc breakerless dizzy.the carbs have been rebuilt and spacers placed to stop heat sink...The flathead runs really nice until it gets to its op temp and a small flat spot or stumble if you will starts again..I can maintain speed with no stumble and it occurs on idle ,it never wants to stall at all.could this be ignition such as a breaking down distributor,or spark plugs..some people have said vavles..I don't know what else to do