1967 Backfiring Through Carb

[David H.]

Re: 1967 Backfiring Through Carb

They were black sooty carbon fouled. I am out of town now but will test them when I get back next week and post the results. I think they were intermittently firing. 5 of the plugs were light gray in appearance. The 3 were black sooty.

Lee

+1 on Mark E's "30 mile" comment.

Get someone with an oscilloscope to run a "relative compression test" on your engine. Fairly quick to do, with no disassembly required.

May point you to issue with those 3 spark plugs.

BTW: What cylinder # were carbon fouled?

Dave

https://www.youtube.com/watch?v=W_ns4BhaI1U

[Lee B.]

Re: 1967 Backfiring Through Carb

I was out of town, but I checked the spark plugs and all of them including the fouled plugs were at +/-5k ohms. When I changed the plugs, I also changed the cap and rotor. So not sure what the problem was. As far as compression goes, I do not know anyone who has a scope. The actual compression numbers for the cylinders are as follows:
1 170
2 153
3 180
4 160
5 155
6 163
7 160
8 175

[Mark E.]

Re: 1967 Backfiring Through Carb

I was out of town, but I checked the spark plugs and all of them including the fouled plugs were at +/-5k ohms. When I changed the plugs, I also changed the cap and rotor. So not sure what the problem was. As far as compression goes, I do not know anyone who has a scope. The actual compression numbers for the cylinders are as follows:
1 170
2 153
3 180
4 160
5 155
6 163
7 160
8 175

Not perfect but compression shouldn't be the cause. A couple of thoughts-

- It sounds like the car isn't driven much. Infrequent, short trips during which the choke is not fully open much of the time could cause carbon fouling.

- You checked plug resistance but spark plug cable resistance is a more common issue. Worth checking.

Since it now runs okay with new plugs, I would check the plug wires then make a point to run it for at least 20 minutes each time it's started. Then see if it continues to run okay... maybe pull one of the suspect plugs after a few hundred miles to see what's happening.

If plugs become oil fouled, start looking at valve stem seals. Those sometimes fail even with low mileage engines.

[Mark E.]

Re: 1967 Backfiring Through Carb

P.S. Make sure the heat riser is working properly. This is another possible cause for these symptoms.

[Ronald L.]

[Larry E.]

Re: 1967 Backfiring Through Carb

Ronald:Not sure why anybody would go this route. Don't need all that octane; 100LL Av Gas has all you need
with Low Lead which the engine was designed to use. Also it is cheaper and will last so much longer before
getting stale. Also safer by not having a big drum of gas around. Of Course JMHO>Larry

[Ronald L.]

[Larry E.]

Re: 1967 Backfiring Through Carb

Larry
Too many assumptions you make


I had a car sit 5 years stabilized
Started right up

Ronald: If you do a search on this board you will find that there are reports that a airplane
that was parked for 15++ years. Av Gas still tested good and plane started right up.
Of course all this is JMHO. Larry P.S. No additives are needed with 100LL Av Gas