Driving my self crazy. !970 LS5
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
Driving my self crazy. !970 LS5
I just finished a complete rebuild of a 1970 LS5 454/390HP. I have a miss. I have changed wires, plugs, checked compression, timing, vacuum leaks, coil, I even took the intake back off and heads to check valves etc. I am beginning to wonder if it is carburation. Can anyone tell me, should the base gasket under the carb. be wide open or should it be 4 holes for the butterflies. I ordered the gasket from Corvette Central and it is the open type. I also have a puffing sound from the right exhaust. I put in a high riser spacer instead of the control valve is this a problem. After 10 years of taking my time to do a frame off and everything new it is frustrating to not be able to solve this miss issue and the puffing sound in the exhaust. The distributor has an electronic conversion kit from Lectric Limited and I even took this off and tried the points and condenser. Help please.
Limping across the line
Joined: Oct 2002
Posts: 5,299
Likes: 109
From: Peoria Az
You running Headers or Manifolds? easy way to find the cylinder thats not firing is to spray (from a spray bottle) water on each tube by the exhaust port. Cold cylinder will be apparent by water staying on the tube. Another way to check is to pull each plug wire individually and RPM will not drop when you pull the wire to the dead cylinder
Racer
Joined: Feb 2007
Posts: 457
Likes: 2
From: Tucson Arizona
Have you run the car with the valve covers off and checked to see if the valves were actually opening and closing? I have seen calapsed lifters before, or a push rod out of place, or missing. If you are getting fuel and spark at the questionable cylinder, and the compression test is good, the intake valve might not be opening.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Only a couple of suggestions if you want one or two. First, since everything is new forget about more parts!. Consider you may have adjusted a valve too tight or a plug wire could be crossed.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Burnt exhaust valve?
Try this first. At night, start the engine and open the hood and see if you can see spark somewhere,..maybe a broken spark plug or bad plug wire or cracked distributor cap, etc.
Otherwise, I'd look at the valve train,..broken valve spring(s) or collapsed lifter (although this is usually noisy).
But this is sounding like a burnt exhaust valve to me,..hope not. Let us know what transpires.
Try this first. At night, start the engine and open the hood and see if you can see spark somewhere,..maybe a broken spark plug or bad plug wire or cracked distributor cap, etc.
Otherwise, I'd look at the valve train,..broken valve spring(s) or collapsed lifter (although this is usually noisy).
But this is sounding like a burnt exhaust valve to me,..hope not. Let us know what transpires.
Last edited by 73, Dark Blue 454; Jul 14, 2007 at 01:01 AM.
Navigator
Joined: May 2005
Posts: 9
Likes: 0
From: Everett Washington
I have experienced this same problem on a freshly installed engine in the past. The rough running engine (ignition miss?) and puffing through the exhaust was caused by the hydraulic lifters being adjusted too tight. I loosened up the rocker arm nuts 1/2 turn, and the engine ran perfect.
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Drifting
Joined: Jan 2002
Posts: 1,606
Likes: 1
From: Las Cruces NM
Have you run the car with the valve covers off and checked to see if the valves were actually opening and closing? I have seen calapsed lifters before, or a push rod out of place, or missing. If you are getting fuel and spark at the questionable cylinder, and the compression test is good, the intake valve might not be opening.
Pop the valve covers off and run the motor w/o them on. You can tell pretty quick if it's a lifter or, more likely, a cam lobe.
The best way is to disconnect the coil and have a friend crank on it while you watch the rockers.
On a fresh rebuild, if the cam wasn't broken in properly, there's a really, really good chance this can happen with today's zinc free oils.
A rocker nut too tight will do the same since that valve won't open right. If you did the static adjustment with the rockers not filled w/ oil, it's easy to overtighten.
C
Last edited by okinawa86vette; Jul 14, 2007 at 03:12 PM.
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
I haven't yet run with the covers off but I have gone back over the lifter adjustment procedure that calls for one turn after meeting pushrod resistance. I even backed them off a quarter turn.
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 43
Likes: 0
From: Elgin Ontario
Believe it or not I have now pulled the intake and right head of and had all the valves checked. Everything is good. Tried another distributor and wires. Only thing so far that has helped slightly was to disconnect the TCS system and run straight carb. vacuum to the distributor. Also took the carb completely apart and cleaned it and checked all settings.
Le Mans Master
Joined: Mar 2004
Posts: 7,502
Likes: 1,509
From: Little Rock AR
Pull the pasenger side plugs and see which one is not firing right.
What plugs do you have in there? Might be just too cold a heat range. If they are AC 43's I highly advise throwing in some 45's. The 43's are too cold. The 45's helped me get rid of a "puffing sound" I was getting at idle on one of my other cars.
Good luck!
-Mark.
What plugs do you have in there? Might be just too cold a heat range. If they are AC 43's I highly advise throwing in some 45's. The 43's are too cold. The 45's helped me get rid of a "puffing sound" I was getting at idle on one of my other cars.
Good luck!
-Mark.
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
Check timing ... I once had mine a couple of degrees (12) ATDC because i misinterpreted the direction the engine rotated in relation tot the timing on the damper, sounded like I had the engine knocking at idle..
I can imagine that if you have to little advance it could sound like you discribe.
I can imagine that if you have to little advance it could sound like you discribe.
Last edited by Belgian1979vette; Jul 24, 2007 at 12:48 PM.
