What should 273 compression be

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bill paynter

67 Cuda
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I'm having problems with my car back firing so I finally decided to pull all the plugs and check cylinder presure all the plugs looked dead rich except for one
I kept thinking I had a distributer problem that's why it was back firing

my question is do any of you know what my stock cyliner presure should be for a 273 cid my reading ranged from 110-118 was wondering if it needed new rings, haven't checked leak down yet
 
that's sorta low, but it should still run fine. to check the rings, test a cylinder and get a number, then squirt a little motoroil in the cyl and try it again. If it goes up significantly, it needs rings.
 
hi, are the plugs oil or fuel fouled?? If fuel fouled, that will cause back firing and running rough. also, rich mixture will wash cylinder bores of oil. rings need oil to seal. maybe clean plugs or install a new set, just to see if cures problem. dry cylinder bores will show a low reading on comp gauge.
 
Did you check the compression with the throttle wide open??? Low compression will not cause a backfire. Carbon tracking in the cap?? Worn cam lobe?? Bad rocker arm?? These could be causes.
 
Spark plugs are deffenitly fuel fouled I went from a 495 CFM Holley Mech sec. to a 600cfm with vacum, thinking it would be better, I'm going to take the Jets down to 59's or 60's to see if that helps

I'm sure now the Back firing is from the bad plugs at first I didn't because I kept pulling the the No 1 plug out to look at but that was the only plug firng properly so I kept think it was somthing else

I'm still looking for that comprssion number I keep hearing 140-150 is where it should be but with this low compression motor and big cam I'm think it should be somewhere around 110-120
 
High compression is nice, but equal in all cylinders is what you are really looking for. Low compression engines will run good if properly tuned.
 
If that's a '67 engine in there, it has mechanical lifters. '68 and later had hydraulics. Check the valve lash, particularly on the exhaust valves. Backfiring occurs when the plug fires with fuel vapor in the cylinder and a valve is open. If the backfire is through the tail pipe, it's an exhaust valve. If it's through the carburetor, it's an intake valve. This assumes the distributor is set up properly.

On a warm engine, the valve lash should be .013 (intake) and .021 (exhaust). As things wear, it's not uncommon to have to re-set the valve lash. However, if the problem has been getting worse, it could mean that the adjustment screw on the rocker is stripped and not holding.
 
hi, in order to find the problem, you have to pull all the plugs out. same as comp testing just one cylinder. it doesn't give you a true picture of results or
where the problem is! you stated ,you changed the carb and plugs are fuel fouled. first place to look.
 
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