Need help with poor running engine
Thread Starter
Le Mans Master
Joined: Dec 2006
Posts: 5,154
Likes: 2
From: Ft Myers Florida
Need help with poor running engine
Ok engine is a chevy small block, but owner was told it was a 327 taken out of a 69 Corvette, we'll not get into it so just Chevy small block with HEI.
Car backfires through carb and surges while driving. It has new wires, plugs, looks like new cap. All 8 wires are getting spark all plugs are firing. Compression check shows about 150psi in all but no's 2 and 4 which are about 140. If I put my hand over the oil hole on the valve cover I get suction indicating a vaccuum leak. Car has only three vacuum lines on it, the pcv, brake booster and vacuum advance.
Good fuel pressure from pump.
I tightened the manifold and carb bolts, haw can I be 100% sure that what I'm dealing with is a manifold gasket leak? I tried spraying carb cleaner around the manifolds base put can't find any obvious sucking.
Any ides appreciated.
Car backfires through carb and surges while driving. It has new wires, plugs, looks like new cap. All 8 wires are getting spark all plugs are firing. Compression check shows about 150psi in all but no's 2 and 4 which are about 140. If I put my hand over the oil hole on the valve cover I get suction indicating a vaccuum leak. Car has only three vacuum lines on it, the pcv, brake booster and vacuum advance.
Good fuel pressure from pump.
I tightened the manifold and carb bolts, haw can I be 100% sure that what I'm dealing with is a manifold gasket leak? I tried spraying carb cleaner around the manifolds base put can't find any obvious sucking.
Any ides appreciated.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Backfiring through the carburetor and surging are lean conditions. Try to jet the carburetor a little richer. Vacuum in the crankcase would probably be the PCV system doing what it is supposed to do.
BigBlockk
Later.....
BigBlockk
Later.....
Thread Starter
Le Mans Master
Joined: Dec 2006
Posts: 5,154
Likes: 2
From: Ft Myers Florida
Someone put in new plugs and wires, but did not do the timing. I did do the timing it was way off advanced, so I set it to 6 and 8 btdc. It runs better but still does that stuff.
Thread Starter
Le Mans Master
Joined: Dec 2006
Posts: 5,154
Likes: 2
From: Ft Myers Florida
Really I was told if it sucks it has a vaccuum leak somewhere.
Instructor
Joined: May 2008
Posts: 154
Likes: 3
From: Indiana
Y-ME, I don't know what the book says for your base timing but if you have a cam in it yiou may want to advance some more. I have a 68 with 327 and Crane Z-cam. The book calls for 4 degrees but it would backfire through the carb till I went to 10 degrees. My base is now 14 degrees and it is happy. I did disconnect the vaccume advance because with the base timing at 14 the total timing was too high.
Racer
Joined: Dec 2007
Posts: 445
Likes: 2
From: Thomastown Mississippi
If you put the palm of your hand over the oil fill hole on the valve cover, you should feel a slight suction. That's normal. If it does the opposite and blows against your hand, it's an indication of worn rings. At least that's what I was told by an old Chevy mechanic. You can also lay a piece of paper over the hole in the valve cover and see if it sucks it down. Your compression looks OK though. I would be looking at the timing and carb. I would also triple check and make sure the spark plug wires are in the correct order.
Kenny C
Kenny C
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Check for vacuum leaks, first. Once you're sure there are none [or you've fixed them], check out the vacuum can on the distributor. If it is OK, reset the timing. If you still have the problem, either your carb has problems or you might have an intake manifold gasket leak.