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Old 05-31-2019, 09:34 AM   #1
Aviatur
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Default Stock 1950 8BA

Hey fellas, this one has me stumped.
Stock flathead with dual exhaust, holley 94, load-o-matic dist.
New fuel tank, flex line, rebuilt fuel pump and carb with 7.5pv and #51 jets, rebuilt dist with new vacuum diaphragm, points, condenser, cap, rotor.
Engine idles and starts perfect, great throttle response but with no load it has a miss. It comes in abouround 1500 and gets progressively worse and will backfire through exhaust. If you stomp on the gas it runs great.

Any ideas?? I am stumped.

Thanks.
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Old 05-31-2019, 10:14 AM   #2
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Default Re: Stock 1950 8BA

First thing to try is some Marvel Mystery oil, some in the gas ( 4 oz. or so, per 10 gal. ) some in the oil ( say 25% ). Assuming when you say stock, it has not had a rebuild recently, and perhaps has sat for a time. Good Luck. More suggestions will follow.
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Old 05-31-2019, 10:17 AM   #3
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Default Re: Stock 1950 8BA

When it starts to misfire, are you driving it or reving it up sitting still? I'm reading it as you are just reving it up. How does it run on the road?
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Old 05-31-2019, 10:21 AM   #4
51 MERC-CT
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Default Re: Stock 1950 8BA

What type plug wires are installed (solid wire, carbon core or other?)
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Old 05-31-2019, 11:29 AM   #5
Aviatur
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Default Re: Stock 1950 8BA

We started using MMO as soon as he got the car. Also ran one can of SeaFoam through it as well. The miss is when driving it of when just revving it up, and I am sure that is has solid wires. Ran it at night and checked for any sparking and could not find any.
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Old 05-31-2019, 11:30 AM   #6
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Default Re: Stock 1950 8BA

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When driving, as long as you are going uphill or against a wind it is fine. When you just drive on flat ground with no headwind and neither slow up or speed up is when the miss occurs.
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Old 05-31-2019, 12:01 PM   #7
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Default Re: Stock 1950 8BA

Had a similar problem with my 59AB. Replaced the carbon core with solid copper....problem went away. Every issue is different, but after trying everything else, this solved my problem. BTW, 6 volt ignition is stock with Bubba crab dist.
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Old 05-31-2019, 12:09 PM   #8
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Default Re: Stock 1950 8BA

We are going to try a new set of wires next. About the only thing we have not tried. Bad wires usually show up under a load. As limited as my flathead experience is, I do know that they defy common sense at times...LOL
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Old 05-31-2019, 12:12 PM   #9
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Default Re: Stock 1950 8BA

Just another thought here.... going to dual exhaust with glasspacks would the carb need to be jetted down a little bit or wouldn't it make much difference? Still have stock manifolds on it.
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Old 05-31-2019, 12:13 PM   #10
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Default Re: Stock 1950 8BA

When cruising the powervalve is closed giving you a leaner mixture...
So either you have a blocked nozzlebar or maybe timing a bit off or to weak spark to ignite the lean mix...lotīs of options to choose from...
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Old 05-31-2019, 12:26 PM   #11
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Default Re: Stock 1950 8BA

FLatheadmurre, good ideas, but I took the carb off and removed the nozzle clusters. Using a very small dental pick I was able to run through all of the ports and then follow up with spray carb cleaner. Checked to see that I had all 4 gaskets in place when putting it back together. Timing is spot on. The missing (if it gets bad enough) will pop through the exhaust which makes me think it was rich, but now I don't know.
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Old 05-31-2019, 12:31 PM   #12
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Default Re: Stock 1950 8BA

Stick a vacuum gauge on it while it's occurring. I'm leaning towards timing ( points set incorrectly would alter timing ) or a lean fuel problem .
At night I use a spray mist bottle and a few squirts high over the motor while running will really show the possible arc happening.
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Old 05-31-2019, 12:44 PM   #13
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Default Re: Stock 1950 8BA

Ggmac, the points were set to .015 but will make sure they didn't slip or change. I have an accurate McQuay Norris vacuum gauge about as old as the flathead. I will hook it up to the manifold vacuum and check after work. Also, I really like your idea of the water mist!!! I will be trying that tonight too. BTW, vacuum at idle is just over 19" but will check when it is missing and let everyone know.
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Old 05-31-2019, 12:50 PM   #14
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Default Re: Stock 1950 8BA

Like flatheadmurre, I suspect the misfire at lean mixture is the culprit. try adding a little choke to the carb when it occurs. Maybe the plug wires suppress the spark just enough to cause it.
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Old 05-31-2019, 12:56 PM   #15
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Default Re: Stock 1950 8BA

I had a miss at cruise in my stock ‘50 and I found a broken plug. I was checking them and when I pulled the wire, the electrode pulled out of the plug. I had no backfire though.... Mark
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Old 05-31-2019, 02:35 PM   #16
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Default Re: Stock 1950 8BA

I will doing more on it this weekend and will keep the groupl updated as to what I find.
Thanks.
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Old 05-31-2019, 03:15 PM   #17
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Default Re: Stock 1950 8BA

Lean conditions will pop back through the carb (back fire). If it does that too much, it will blow out the power valve which will then make a rich condition with possible after fire in the exhaust. Intermittent sticky valves will show up on a vacuum test.

While your at it, wiggle the distributor shaft and see if it has any play.This can cause an intermittent miss at about any rpm.
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Old 05-31-2019, 04:36 PM   #18
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Default Re: Stock 1950 8BA

Float level will have you chasing your tail.
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Old 05-31-2019, 05:09 PM   #19
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Default Re: Stock 1950 8BA

Sounds like a fuel problem. If it was ignition, stomping on the gas would put more demand on the ignition system and the misfire would still be there or it would be worse. No that I've said that it will end-up being the ignition system.

Hold the rpm at 1500 where it begins to misfire. Take a can of carb spray or a small propane bottle and spray a little into the carb and see if the misfire goes away. If does you have a fuel problem, if not its electrical or mechanical.

Next, run it again at 1500 where it is misfiring and pull one plug wire at a time with the best insulated pliers money can buy. Each wire pulled off should drop the rpm and make it misfire even more. If one wire makes little or no difference that is the cylinder with the problem.

Stick a vacuum gauge on it and see if it reads steady. That will tell you if the valves are opening and closing like they should be.

I borrowed this from another site about the causes of back firing in the exhuast. They make several good suggestions. There has to be fuel in the exhaust pipe for it to backfire. That can only happen if its too rich or a cylinder did not fire causing the gas flow on out into the exhaust.

Why does my engine spit or backfire?

First off, you need to determine whether it is backfiring out the exhaust or back up through the carb. We usually refer to an exhaust backfire as a "backfire", and backfiring through the carb as "spitting" or "coughing".
"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich and you have unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other and when this happens while the exhaust valve is open, it will ignite the rich / unburned mixture in the exhaust manifold and tail pipes and result in a big bang.
Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.
Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.
Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. It is also commonly caused by the accelerator pump inthe carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle it is probably the accelerator pump in the carb not working, plugged up or out of adjustment.
As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame.
When and how it backfires or spits will give you an indication for where to look.

Last edited by Flathead Fever; 05-31-2019 at 05:18 PM.
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Old 05-31-2019, 05:38 PM   #20
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Default Re: Stock 1950 8BA

Just a thought, there are two types of accelerator pumps, the blue ones and the leather. I had a blue one actually come apart before, caused similar symptoms. I prefer the original type that Daytona sells in their rebuild kits. Something to check if you have it apart.

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