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1940 Lasalle Compression check 1940 Lasalle

Started by jmscan06, September 13, 2015, 05:37:31 PM

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jmscan06

I checked the compression on my 1940 Lasalle while the engine was hot.   Readings are between 80 â€"110.  Per the manual the compression should be 155 lbs at 1000 rpms. But I don’t think that it was turning at 1000 rpms.  Battery  was fully charged.  Engine was turning good for a 6 volt battery.   Should I try jumping with a 12 volt battery?    If I do what precautions should I take?  I don’t want to damage  anything.

John Scanlan
CLC # 28099

James Landi

I would not recommend a 12 volt battery... you could destroy gauges...might consider taking all of the plugs out.  Make sure that your throttle is wide open! Your starter only provides you with 150 rpms... a more accurate measure of your engine's compression is a leak down test.   It involves some additional costs, but most professional mechanics have the tools to do it...click on this link:  https://mobiloil.com/en/article/car-maintenance/car-maintenance-archive/how-to-do-a-leakdown-test   Happy day, James

jmscan06

James

Yes I did have all of the plugs out.  I will take a look at the leak down test. 

Thanks

John Scanlan

camelot

I did the leak down test on my boat engines. I found it very difficult to get the engine to stay at tdc. When I applied the air pressure it rotated the engine and sent the piston down to the bottom. Be careful if you use a breaker bar on the crank bolt to get the engine to tdc. Be sure you remove it before applying the air! You have to be exactly at tdc, not piston at top of cylinder. There is a thing called dwell time where the crank moves and only the rod moves, not the piston. If the rod is at any point either side of true tdc the engine will rotate. Took me quite while but the results were amazing. The only thing I question about it is if you have score in the wall mid to low in the cylinder. Rings at the top of the piston won't leak by as if piston was low in cylinder. Just my $.02.
1939 Cadillac 7519
1940 Lasalle 5019
1962 Cadillac 6339 4w