Technical need help with straight 6 flathead!!

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

 

take head off and make sure all the valves are free and happy

 

Did you take a compression test or just check for it by covering the plug hole?

 

It is surprising how those old flathead sixes continue to run smooth and quiet when practically worn out. Most complaints of low power and poor mileage are just because the motor is shot to blazes.

How is your oil pressure? How is your compression? If oil pressure is good you may get away with a ring and valve job. If the bottom end is worn you may need a complete rebuild.

Yours is a very simple type of engine and easy to rebuild. I don't know about parts prices for Pontiacs but rebuilding is usually cheaper than replacing the engine (unless the rod is sticking thru the block).

 

compression test, showed fairly good on first and second and none at all on third.

 

yeah, it's been sitting since I bought in 2008 and it fired right up last year, just put a 12v battery on the starter and it was easier to start than my daily driver.

oil pressure seems fine, dont know the actual pressure but it goes up if I'm revving.

any idea of what causes it to die when I remove the sparkplugs? I can remove the cables and nothing changes but if I loosen the plug it coughs and dies.

 

Huh. Now thats a question I hadn't thought about before. So I thought about it for a while until my brain started to hurt.

The only conclusion I could come to is that if an intake valve wasn't sealing, When you removed the spark plug in that cylinder you would create a massive vacuum leak. At least I think so. My brain still hurts a little bit.

I'd love to watch the manifold vacuum on this engine while it was running and see how it responds when a spark plug is removed,

 

dude! this might be it, if I plug the non-compressing cylinder with a rubber plug and then remove it wile idling I can hear a*****tion before it dies, as if there's vacuum in the cylinder.

I need to wait til next week to take it apart though, got a registration inspection to get my licensplates on friday.

thank you!

Emil


edit: I realized I was thinking backwards, again..

 

Sixes are smooth running.
Good luck

 

drain a qt. of engine oil and replace with ATF, remove side cover and make sure valves are operating properly. rings could be stuck on pistons.