Engine Guru's - help needed
Thread Starter
Pro
Joined: Oct 2010
Posts: 606
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From: Mystic Ct.
Engine Guru's - help needed
Have a 71 350/270, absolutely stock!
Have a miss that I can't find.
Heres what I did so far: New plugs, new wires, new distributor cap & rotor, new points. Replaced carb.
Start engine and running rough, very rich, and you can hear the miss in the left exhaust pipe.
Did a compression test last night and here are the results:
#1 135 #3 140 #5 142 #7 140
#2 150 #4 150 #6 150 #8 145
Any thoughts?
Thanks
Russ
Have a miss that I can't find.
Heres what I did so far: New plugs, new wires, new distributor cap & rotor, new points. Replaced carb.
Start engine and running rough, very rich, and you can hear the miss in the left exhaust pipe.
Did a compression test last night and here are the results:
#1 135 #3 140 #5 142 #7 140
#2 150 #4 150 #6 150 #8 145
Any thoughts?
Thanks
Russ
Pro
Joined: Nov 2013
Posts: 730
Likes: 37
From: Catoosa Okla
Yes. The compression seems great on all cylinders. This rules out mechanical issues. Id pull a plug wire one at a time with the engine running and see if I had a cylinder not hitting. If so check spark plug and wires. If they are all hitting, about all that's left is the carb.
Safety Car
Joined: Jan 2009
Posts: 3,736
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From: anchorage ak
Yes. The compression seems great on all cylinders. This rules out mechanical issues. Id pull a plug wire one at a time with the engine running and see if I had a cylinder not hitting. If so check spark plug and wires. If they are all hitting, about all that's left is the carb.
Le Mans Master
Joined: May 2007
Posts: 5,007
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From: North tonawanda NY
About a week ago I noticed mine wasn't idling right either . I was constantly messing with the idle and I finally pulled the carb apart to find all kinds or crap floating around in the float bowls partially clogging jets on and off.... Something to look at. The ethonal gas turns to a clear jelly as it its up aluminum too...
Le Mans Master
Joined: Apr 2007
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From: Graceland in a Not Correctly Restored Stingray
Another method of finding a dead cylinder is to put some water in a spray bottle and squirt a small amount on each exhaust until you find one that doesn't steam. I like this a lot better than potentially getting bitten by live wires. Of course, it works better with headers, but you should still be able to tell with iron manifolds.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Another method of finding a dead cylinder is to put some water in a spray bottle and squirt a small amount on each exhaust until you find one that doesn't steam. I like this a lot better than potentially getting bitten by live wires. Of course, it works better with headers, but you should still be able to tell with iron manifolds.
Melting Slicks
Joined: May 2012
Posts: 2,192
Likes: 343
From: Allenton Wisconsin
My 6t8 had a misfire, did all of the above and found the problem to be excessive run out in the distributor shaft causing erratic dwell variations. Supposed I could of had the dizzy rebuilt but went with MSD ignition. Cleaned up immediately. Just something for you to check. Converting to the electronic pick up could also help. It doesn't care about shaft run out. T
Melting Slicks
Joined: Oct 2003
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From: Chapel Hill NC
2025 c3 ('68-'73) of the Year Finalist - Unmodified
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No guru here but a "miss" would lead me more to electric vs carb. Few times over the years I discovered I cracked a new plug during installation. Few times I found the new plug wires were not fully seated on plug or dizzy. Running in the dark helped me a few other times.
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Thread Starter
Pro
Joined: Oct 2010
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From: Mystic Ct.
Thanks for all the replies guys, really appreciate it.
My thinking is: if it were an electrical problem, wouldn't I hear the miss in both tailpipes?
Do you think, with the lower reading in #1, it could be a weak valve spring or sticking valve?
Thanks, will try all the above later today.
Russ
My thinking is: if it were an electrical problem, wouldn't I hear the miss in both tailpipes?
Do you think, with the lower reading in #1, it could be a weak valve spring or sticking valve?
Thanks, will try all the above later today.
Russ
Melting Slicks
Joined: Oct 2013
Posts: 2,569
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From: San Diego California
Another method of finding a dead cylinder is to put some water in a spray bottle and squirt a small amount on each exhaust until you find one that doesn't steam. I like this a lot better than potentially getting bitten by live wires. Of course, it works better with headers, but you should still be able to tell with iron manifolds.
From a cold engine, start it up & run it for 20 seconds, then off. Maybe the touch method can then point you to the missing cylinder. Otherwise, an infrared thermometer might tell you too.
Steve
Drifting
Joined: Mar 2011
Posts: 1,886
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Thanks for all the replies guys, really appreciate it.
My thinking is: if it were an electrical problem, wouldn't I hear the miss in both tailpipes?
Do you think, with the lower reading in #1, it could be a weak valve spring or sticking valve?
Thanks, will try all the above later today.
Russ
My thinking is: if it were an electrical problem, wouldn't I hear the miss in both tailpipes?
Do you think, with the lower reading in #1, it could be a weak valve spring or sticking valve?
Thanks, will try all the above later today.
Russ
Those cranking pressures are easily within tolerance. Chevrolet allows, I think, the lowest reading cylinder to be 75% of the highest reading. So, in your case, 135 divided by 150 is 90%. You're fine there.
Although, at 42 years old, I think that you really might have a cam lobe going away on you. That's a typical way for a SBC to need a rebuild.
Last edited by gcusmano74; Aug 18, 2014 at 09:24 PM.
Thread Starter
Pro
Joined: Oct 2010
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From: Mystic Ct.
Here is a video of the engine running:
https://www.youtube.com/watch?v=Mjz9...ature=youtu.be
As you can see, as soon as I let off the throttle, it quit running. The RPM's are fluctuating between 750 & drops down to 500 and it still smells terrible (lean/rich).
This thing is really starting to drive me crazy!
Thanks
Russ
https://www.youtube.com/watch?v=Mjz9...ature=youtu.be
As you can see, as soon as I let off the throttle, it quit running. The RPM's are fluctuating between 750 & drops down to 500 and it still smells terrible (lean/rich).
This thing is really starting to drive me crazy!
Thanks
Russ
Le Mans Master
Joined: Apr 2001
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
Have you pulled a valve cover yet? You could have a collapsed lifter, or wiped a lobe on the cam. There also could be a loose rocker, but you should hear that.
Le Mans Master
Joined: Oct 2006
Posts: 6,842
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From: Palmdale CA
True! Also, grab each rocker while it's hot and see if any are far looser than the others, that's how I found my loose rocker.
To see if you have a wiped cam or collapsed lifter, take off the valve covers and check each rocker. The one that's hardly moving, in comparison to the others, is the wiped lobe or collapsed lifter. Then, when it cools down, you pull the rocker arm, carefully pull out the push rod and then looking down into the lifter valley, go in with a pair of long needle noise pliers or a long pick in the lifter hole on top, pull the lifter and look at the lobe. If it's rounded, it's toast.
Le Mans Master
Joined: Oct 2006
Posts: 6,842
Likes: 285
From: Palmdale CA
Also, I highly recommend a spark plug tester, that works while the engine is running. You simply hold the end of it, against the plug wire and you will see a flashing light inside the glass tube. It will instantly tell you, whether a spark plug wire is dead or weak. It's shaped like a pen and very easy to use.
Racer
Joined: Jul 2007
Posts: 348
Likes: 12
From: Ocala, Florida
I had a weird miss on #2 cylinder, Checked plugs, plug wires, valves, changed distributer, used a heat gun on headers, removed carb and scoped the combustion chamber and found no problem still having a miss and a cold cylinder. last resort took carb 4150 holley apart, used air to blow out passages in carb and poof out came a piece of metal. Put carb back together and back on engine, and no miss or cold cylinder. Good luck JAY