Low compression numbers?

[tinman760]

Hello all. The car is a '73 Spider 124. I did a compression test on the 1592, and came up with 80, 76, 76, 80. I am wondering if these are low numbers, or acceptable. Curious to me that they are all in a acceptable range. Plugs are kinda black, may be from excessive fuel, pouring it down the throat to get her to go. Replaced fp, and soon to replace the carb, as the acc. pump seems to not be working. Maybe rebuild the old Weber. Any and all thoughts welcome. Thanks ciao

[narfire]

seem low to me. my stock 2L was around 140-150 and now with a bit different cams I'm 120-130.
Do you have access to a leak down tester. See if what compression you have leaks down quickly or slowly.

[Kent124]

Mine are 152 across all cylinders for a 2L.

[tinman760]

Guess I'll do a leak down test. Seems kinda obvious, now. Might save a complete overhaul, tho. Thanks.

[kilrwail]

Those numbers are low, but make sure that you test compression with the engine fully warmed up. It makes a big difference.

[leftfield6]

My 2L was around 120 when tested cold. Bumped up to 150 +/- when warm.

I have a leak down tester, but haven't used it yet.

[engineerted]

A weak battery and/or not opening the carb fully will cause low readings also. Did you check the cam timing, this will affect the Comp numbers also.

Ted

[majicwrench]

Does the engine run?? If the compression is actually that low, it is gonna barely run, if at all. The good new is all the cylinder are about equal. Leak down test would not be the first thing I would do, check cam timing first.
Keith

[124JOE]

yes you mat have jumped a tooth on the timing belt

[So Cal Mark]

that compression is too low for the engine to run, I'd start with checking the cam timing. It's odd to have fairly even numbers that low unless the cam timing is off

[kilrwail]

You might want to calibrate your compression gauge with another (known good) engine. Did you drop it?

[tinman760]

The car runs when there is fuel present. Sounds pretty solid. The car has sat for years, tho. Gonna try the Marvel Mystery Oil trick, and check the cam timing also, before the leak down test. Thanks

[majicwrench]

If the engine runs strong you need a new compression tester.

[RRoller123]

I believe that engine has a 9.8/1 compression ratio, making nominal compression around 144 psi. This assumes constant temp, (PV=nRT) but is pretty close to what would be expected. Higher is often due to carbon deposits effectively raising the CR, lower can be (among many reasons) because the carb throttle plate is not fully open when the test is performed. I made that mistake on my Honda CB350, was getting about 75 psi across both cylinders, then realized I hadn't opened the throttle fully. Ran it again and the numbers were in the 150's.

[vandor]

I believe that engine has a 9.8/1 compression ratio, making nominal compression around 144 psi.

Why would it have 9.8? US Spiders were between 8 and 8.5:1.
A newly rebuilt stock 8:1 CR engine will make ~150 psi of compression, one with 9.8:1 CR will be around 180 psi.