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need help! 239 cui flathead inline 6


Guest Emil

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Guest Emil

I've been restoring my 1941 pontiac for the last couple of years into a running project.

got it up and running last year and had it out for a couple of test drives, but after converting to 12v I noticed the car was running real sloppy when idle, if I rev it up it balances out but since I've never had either flatheads or straight sixes before I can't tell if it's all good.

the engine is a 239 cui pontiac flathead from 1941 the original one for the car.

so here's the problem:

today I started going through the ignition and noticed that it's only running on 3(!) cylinders, I've got compression in two of the non-working ones and the third is all dead.

the real weird thing for me though is that the car runs but as soon as I take out any of the sparkplugs from the non functioning cylinders it dies, even if it's the one with no compression. I can see at least one of the valves moving in the non compressing cylinder and the other is at least moving behind the valve cover on the side.

all help is much apprechiated!

Emil

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By "taking out" a spark plug, do you mean you're disconnecting a plug wire from a spark plug? If so, try this instead; peel back the rubber boot from the end of the spark plug wire where it connects to the spark plug. With the engine idling, take a screwdriver and ground the connection to the engine block. Do you get the same result? Does the engine die?

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Guest Emil
By "taking out" a spark plug, do you mean you're disconnecting a plug wire from a spark plug? If so, try this instead; peel back the rubber boot from the end of the spark plug wire where it connects to the spark plug. With the engine idling, take a screwdriver and ground the connection to the engine block. Do you get the same result? Does the engine die?

if I take the cable off, nothing changes but if I remove the sparkplug from the engine it dies, it's like it need the compression in the non-working cylinders to run.

Emil

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Not related to your question Emil, but why have you changed to 12 volts? I would just leave it as 6 volt. Have you checked your valve adjustment? Flathead engines need less valve clearance than overhead valve engines and a camshaft which is not quite properly positioned can cause valves to be held open.

Terry

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Guest Emil
Not related to your question Emil, but why have you changed to 12 volts? I would just leave it as 6 volt. Have you checked your valve adjustment? Flathead engines need less valve clearance than overhead valve engines and a camshaft which is not quite properly positioned can cause valves to be held open.

Terry

it didn't have enough juice to turn the starter over at 6 volts, opted for conversion since I'm on a budget and had most of the stuff I needed laying around my garage.

I haven't checked it, but I have found the intake valve is not sealing on the third cylinder. I have an inspection booked for street use here in sweden on friday so will start tearing this down in a week to check all the valves and cam and all that.

after I open the cylinder it gets too much air through that hole and it kills the engine I belive.

Emil

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The spark plug is not stopping the car from running, it is something else. I can't tell what else you are doing from 5,000 miles away. It may have to with something you did wrong when you converted to 12V.

Can you do a compression test and get back to us? Test dry, then squirt a little oil down each spark plug hole and test again (1 or 2 squirts from an oil can).

What is your oil pressure?

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The spark plug is not stopping the car from running, it is something else. I can't tell what else you are doing from 5,000 miles away. It may have to with something you did wrong when you converted to 12V.

Can you do a compression test and get back to us? Test dry, then squirt a little oil down each spark plug hole and test again (1 or 2 squirts from an oil can).

What is your oil pressure?

the spark plug keeps it running, the intake valve is not sealing which makes the other cylinder suck all the air and fuel out of the third cylinder. if I take out the sparkplug it sucks way too much air in and it kills the engine. (this is atleast what I have concludes thus far)

did a compression test, the first two have compression and the third creates a vacuum. the last three are running.

oil pressure is good, at least judging from the original pressure gauge.

Emil

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Do you want the damn thing to run or don't you?

You have a problem and it's not the spark plugs. Test the compression wet and dry, and notice the oil pressure. You may just need a ring and valve job, you may need a complete engine rebuild. The compression test and oil pressure test will tell you which. Also how many miles on the car.

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Sorry if I was too blunt. The original post said need help! He got help. Then came a lot of palaver about whether a car will run with the spark plugs out (it won't).

I thought the idea was to make the car run right, evidently that wasn't it. I apologize for the confusion.

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Changing from 6 volts to 12 volts is not the best deal unless you are a little technically savvy. Most times the reason for doing so is hard starting. The battery cables should be 1 zero or better 2 zero in size with clean connections and grounds. I prefer to have the terminals soldered. Change the ignition coil too to 12 volts. The voltage to the gauges must be reduced to 6 . Do a compression test. There is an inherent problem with flat head motors .If the engine sat for too long the gasket tends to deteriorate between # 3 cylinder and #4 cylinder. Some time the cylinder head bolts passes through the water jacket which helps to speed up malfunction. So you get compression simultaneously in both cylinders. A compression test will determine that. Use a vacuum gauge while the engine is running. The first thing I would try is empty a quart of the engine oil and replace it with transmission oil. Start the engine and run, Transmission oil helps to loosen sticky valve and free passages. If successful drain oil and replace. The alternative is obvious. DO IT RIGHT THE FIRST TIME. Hope this help.

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In my book, fix the obvious first! I agree with the others above, why did you change to 12 volt. For hard starting with 6 volt, usually bad connections.

1. I would change back to the original as the cars worked fine when new.

2. Do a compression check.. dry then with a couple of squirts of oil in the cylinders. Compression checks are done with all the plugs out.

3. Determine which cylinder if there are any are more than 10% lower than the other/ highest one.

4. Remove the valve covers and inspect the rocker arms and see if they are moving and the valves are going up and down while cranking the engine.

5. If one of the cylinders are way out of spec or zero compression as noted above put a straw in the spark plug hole and see if the piston is going up and down.

If fortunate enough to have a boroscope put it in the sparkplug hole to see what is in the bad cylinder if there is a bad or low cylinder.

Please start here and give us a report and pictures if possible and there is enough knowledge with this group we can fix about anything.

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Hi Rusty,you are right on the ball. There are certain basic steps to be taken for poor running even in modern cars. Check gas in tank and getting to the carburetor, Check for spark at the ends of high tension wire and at plugs outside of engine. THE FIRST AND FOREMOST CHECK COMPRESSION. It is a must for every tune up. HOW ELSE CAN YOU TELL THE HEALTH OF THE ENGINE ? Poor compression ? Start tearing apart. Most electrical problems are because of bad grounds especially on 6 volts system GROUND, GROUND, GROUND. CLEAN, CLEAN. CLEAN. Solder all terminals if possible.

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