MTFCA Forum

I need help I'm in over my head I guess I've done everything you can do to make this car run right and it still has a miss rebuilt coils coil box two different sets of spark plugs two different timers two different sets of spark plug wires car totally rewired two different professionally rebuild carburetors this thing starts so easy it's unbelievable but I just can't find where that miss is coming from

Sounds like you've just about eliminated the ignition system as the problem. Does it miss all the time, or just at certain power settings? Have you tried richening the carb just a tad? A slightly lean mixture will cause an occasional "hiccup" that could be mistaken for an ignition miss.

I've tried two different carburetors from well-known rebuilder I've tried about everything I can the coils and coil box was built by a guy I'm not going to mention but they seem to be okay if you run the test on them but I'm still going to pull the coils out and have somebody else check them the only thing else I know to do after two or three timers three different sets of spark plugs two different sets of spark plug wires

Charles J wrote: ↑

Mon Aug 16, 2021 1:26 pm

I've tried two different carburetors from well-known rebuilder I've tried about everything I can the coils and coil box was built by a guy I'm not going to mention but they seem to be okay if you run the test on them but I'm still going to pull the coils out and have somebody else check them the only thing else I know to do after two or three timers three different sets of spark plugs two different sets of spark plug wires

You cant tune the miss @ idle? Does it miss @ hi speed? Have you tried tuning mix @ hi speed? Does the miss change @ these speed settings? Does the miss pattern change when hot?Do you have a heat pipe on? The float setting is a starting point & can require fine tuning based on idle & hi speed needle optimum settings. Few people know this. Researched from M. Fawnstock book.
What timer? Roller timers out of the box have issues. Timing cover is centered?

Just another thought, you might try running the engine through its paces in a darkened area, (dark garage, driveway at night etc). You might be surprised to see an occasional spark jumping across to ground. I chased an erratic misfire and it wasn't until I was driving/starting the car at night I noticed a spark jumping across the high voltage terminal to ground on the engine, (26-27). I won't go into all the gruesome details right now, but that convinced me that accessories on the engine are not necessary and can cause several problems. Just a thought, you might not find anything, but it could be worth a try. Kind Regards.

I had that issue and chased it around igintion. Mine was a leaking intake manifold. Just a small leak but it would miss irraticly.

Sometimes, the spark can jump through the wood of the coil box. I have replaced the wood with a plastic one which looks like wood. One of the problems with the original wood is that sometimes water can get to it if the car sets out in the rain or even when the car is being washed. The wet wood becomes a conductor and soon a carbon path is burnt in the wood so it will even conduct when the wood is dry. Sometimes the spark can be seen with the hood open in the dark with the engine running. It can cause a hit or miss depending on the compression of the engine and the spark gap in the plug. The current will take the easiest path depending on conditions at the time. So it might not even be a problem with the carburetor.
Norm

If you read my ad I've been through about three sets of spark plugs the coil box has been rebuilt all the coils have been refurbished I've had three different timers two different carburetors all new wiring I think I've been through about everything that I can figure that could even cause a problem oh yes and I run a compression check twice today they're all between 45 and 50 lb

We were having an occasional miss when driving that seemed to be at random, we changed our spark plugs. Cleaning the timer was next when we discovered an "occasional spark" was visible at the coil box. This is a "new and improved" car as well. The high voltage wires from the box to the plugs had a short. The metal at the end of the large wire at the box was sparking to the terminal directly below, low voltage wire.

Bending the high voltage wire away at the attachment gave greater separation and eliminated the sparking and miss.

Maybe this will help.
Nick

Were the coils adjusted for equal current draw or equal firing Time?

One other thing I can think of. Perhaps one of the contacts inside the coil box is not tight against the contact on the coil, or the coil box lid is not pushing the coils down tight. Does the engine continue to misfire even after it has warmed up?
Norm

A valve that is not seating well can cause a light miss. It might not show up on a compression check. A poorly-seating intake valve can mimic a manifold to block leak. Too much valve guide clearance can also cause a miss. A minor valve leak will usually only be evident at idle. A valve that lacks tappet clearance might not leak at all when cold, but could fail to seat fully with the engine warm, or vice-versa. A weak valve spring or sticky guide can also cause problems, as could a worn cam lobe.

Low compression in one cylinder?

Before doing any kind of tare down put your car in a garage at night and start the car up. As others mentioned check closely and see if you can see any sparking anywhere.
You didn’t mention the ignition switch or if it’s original or been rebuilt. If you have a slightly loose key in the switch that could be an intermittent issue.
When your car is running slightly move the key in both the mag and battery positions. If you can make the engine seem to miss or almost die that could be the issue with your car.
Good luck.

Pull your valve covers, check to see that all the pins are properly in their keepers, sounds silly but I had it once, it it puts a side load on the valve, it won't show up on a compression check but it will at speed, good luck,

Inductive crossfire at the plug wires?

Pull all plugs.
Do not clean them
number them 1 through 4, with "1" being the front of the engine
post a picture of the plug electrodes and indicate their number

Inductive crossfire is unlikely, but I suppose it could happen. I would arrange the plug wires so that they were at least an inch away from one another or anything else. If the car is a late type with throttle linkage above the cylinder head, be sure that the throttle linkage does not come very near or into contact with a plug wire at any point in the linkage travel. Even a slight cololant leak at the radiator or radiator cap could allow coolant droplets to affect the ignition system. Be sure the spark plug gap is not excessive.