Low compression on 365 GT 2+2

I had a PPI on a '70 365 GT 2+2 "Queen" on one of 12 cylinders. 11 cylinders were from 180-190 psi. One cylinder was 145 psi. Is the next step to get a leak down test to confirm if it is the cylinder rather than the valves? I have heard the comments that "it might just be some dirt in the cylinder that will work itself out", but that could be an expensive gamble...

 

How can you do a leak test on just the piston/liner if a valve could be leaking?

If you have a bad piston it will burn oil, blueish smoke will come out of the exhaust. When was the last time you had the valves adjusted?

 

The defect was found during a PPI. No blue smoke reported by mechanic.

 

I would drive around the block and then retest. If it is dirt, that should blow it out. If it is still low, the next step is a leak down test. That will tell you if it is the rings, the head gasket, or one of the valves.

Cheers Jim

 

if there is no blue smoke coming from the exhaust, most likely a valve adjustment will cure the problem. Reshimming the valves is simple and pretty easy fix. Use Volvo B230 8v shims (33mm diameter) , they are very cheap and come in more sizes than the Fiat/Ferrari ones.

 

Queen Mary's are single cam engines, they don't have buckets & shims, but rather rocker arms. Clearance is adjusted via a screw with locknut on each rocker.

 

So true, it is still early in the morning here Shims are for quadcam colombo units. That makes the adjustment even easier.

 

QM's have strong engines, I wouldn't fret over one cylinder being down on comp. My QM had comps all over the place & blew smoke, but it went hard enough. I rebuilt the engine, no more smoke but it didn't go any better!

If the seller is willing, get them to do the valve clearances, change oil and filters, go for a 20-30 mile drive, including a full load run up a long hill, then run a comp test again.

 

I have similar problems late at night, I blame the wine!

 

Ah right the leak down test should work fine because suppose a valve is sealing bad, it really shows up by air rushing out the cilinder. You should be able to hear this high rate of pressure loss, particularly at places close to the head and manifold. Please note that a test pressure fed of 10 bar is enough while say, 6 bar is too short. You'd probably want to use an adaptor with the sparkplug thread and a connector at the other end. I know there are compression testers out there that have couplers similar to the ones you get with the air compressor lines. Hope this helps.

 

Thanks guys. Any other comments or suggestions appreciated.

 

Did you drive it and retest? Did you do a leakdown test? I my opinion those are the key next steps. A less useful suggestion would be to boroscope it. A boroscope would tell you if the cylinder walls are scored or if there are other issues with the cylinder wall. Frankly though the leakdown test is a much better tool for figuring out why the compression is low.

Cheers Jim

 

Buy It, have fun. you wont even notice the low cylinder.