Ray Wheless

New build with multiple problems (low compression in one cylinder with rocker hitting valve cover on another)

5 posts in this topic

I am restoring a 67 mustang that I put a newly rebuilt 351c in.  It is bored .30 with an RV cam, Edlebrock Performer,, Holley 3310 four barrel.  Mallory Unilite.  The engine was not broken in properly because I didn't put in a new gas tank before firing up the engine and the cam went flat.  I have replaced the cam, lifters and push rods.   I have driven the car about 300 miles since I put the new stuff in.   I have a bad miss.  All cylinders are 180 PSI, except no. 6 is only 125.  I pulled the head off of the bank with the low compression and the valves checked out fine at a machine shop.  So, it is the rings on NO. 6 that are leaking even though No. 6 does fire.  However, No. 2 does not fire.  On No. 2, the exhaust rocker arm is hitting the inside of the valve cover. I assume that is related to that cylinder not firing.  I guess I better pull that head off and see if it is a valve on that side.  Any ideas about why that cylinder is not firing with 180 PSI, but the exhaust rocker hitting the valve cover.  All pushrods were new and all the same size.  And, considering the difference between 125 PSI and 180 PSI, should I tear the engine down or just drive it after I get NO. 2 firing by replacing what I am assuming is a bent valve on No. 2?

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Ray- thanks for the good puzzle. Your cylinder #2 does not have a bent valve if it has 180psi on compression. The valves on that cylinder are in the right place and sealing for compression. I'd leave the head on unless there's a better reason to pull it. To get it to fire go back to basics. It needs fuel spark and timing- I would start with verifying spark in that cylinder. The unilite uses a shutter wheel pickup- make sure it's not just poorly finished. Check the cap for internal issues. The rocker hitting the cover is strange; valve adjustment is first on my list of potential troublemakers. Did you only hear it, or did it give you a tangible sign of contact like a dent in the cover, or a shiny spot indicating contact? Do you have pedestal mount or did you convert to adjustable rockers? Candidates for low PSI are intake valve not opening, either intake or exhaust  valve too tight and causing a leak, bent rod or fried rings. On the reassuring side- If you had the head off - you would have probably seen damage to the cylinder surface from a broken ring, an upside down ring would use oil like crazy, and a bent rod would not allow the piston to rise to the top of the deck. Hopefully you didn't notice any of that. I lean toward a lazy intake - could be a flat lobe, could be too lose. It sounds like valve lash /preload but don't take it for granted. In answer to your other question- I would fix it right and not just drive it. It's too new with too much effort to not see complete and correct. 

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Exhaust valve on #2 jammed or way too loose: causes good intake stroke and compression but won't fire because exhaust never leaves the cylinder. 

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yeah a rocker loose or an exhaust lobe gone away makes more sense than rockers tight,

125 psi is still pretty decent and doesn't happen by accident

a popping sound out through the carb like reversion might occur if the exhaust side isn't letting anything out,

then it's just chewing it's coleslaw over & over again doing the best it can ... about 125 psi

any specifics on the cam grind? 

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