Low Compression

Joe AZ

Joe AZ

Senior Member
I've got a new (to me) '40 Ford Coupe. I've had it about 6 months and have fought a number of issues regarding intermittent spark (new wiring harness to alleviate short between resistor and coil), and carburation problems (thanks Uncle Max). The Barn has helped me immensely in troubleshooting these issues.
I now have a dependable driver that I'm beginning to use more and more. It seems somewhat low on power, so I decided to check cylinder pressure. I was surprised to find a couple of readings as low as 50#, and none higher than 90#. This test was performed with engine cold, throttle blocked open.
I live at 5,500 feet, and I understand that altitude has an effect on compression. However, the 50# cylinders are pretty low, no matter how you look at it.
The car has a stock '40 engine that runs fine, idles fine, oil pressure is about 25# at idle and 45# at cruise. It doesn't visibly burn oil (sure leaks though), doesn't overheat, nor have I detected oil in coolant or vice-versa.
I'm planning on pulling the heads, but based on the facts above am I looking at valves, re-bore, or what?
I welcome your opinions.
Joe
 
Well Joe, I'd kind of put my money on leaking valves. What you should do is get or make an adapter to screw into the sparkplug hole that you can hook an air hose into. I just make them out of an old sparkplug. Get the piston somewhere close to top dead center comp stroke. Put air into the cyl, if you can hear it in the tail pipe the exh. valves is leaking, if it blows up the carb the int. valves is leaking, take the oil filler cap off, if there is a lot of blowby into the base the rings are bad. A stock little 221 engine in good shape should have 100# every cyl. Walt
 
Walt,
Sounds like a good test to determine low compression problems. How do you determine when a piston is on the top of the compression stroke? Thanks
John
 
oldford2 said:
Walt,
Sounds like a good test to determine low compression problems. How do you determine when a piston is on the top of the compression stroke? Thanks
John
Click to expand...
Hi John; Walt said close to TDC and the distributor will tell you that, Craig.
 
Better put it in gear or the air pressure will blow the piston down and the int. valve will open.
 
Walt Dupont--Me. said:
Well Joe, I'd kind of put my money on leaking valves. What you should do is get or make an adapter to screw into the sparkplug hole that you can hook an air hose into. I just make them out of an old sparkplug. Get the piston somewhere close to top dead center comp stroke. Put air into the cyl, if you can hear it in the tail pipe the exh. valves is leaking, if it blows up the carb the int. valves is leaking, take the oil filler cap off, if there is a lot of blowby into the base the rings are bad. A stock little 221 engine in good shape should have 100# every cyl. Walt
Click to expand...

Thanks Walt / Ron, sounds like introducing air pressure into the chambers could tell me quite a bit......Concerning Ron's comment about moving the piston down, how much air pressure are you talking about? I wouldn't think much would be needed?
Joe
 
You must log in or register to reply here.
Back
Top