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Help. Engine shakes badly at idle.

Huicho417

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12:12 PM
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Sep 15, 2020
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Location
San Antonio , Texas
On the way back from a car show yesterday I noticed the car had a slight hesitation and engine shake. I pulled off the highway and it felt like a bad misfire. This morning I replaced the distributor cap, ignition wires and checked the coil. I don’t think it is a misfire. The engine starts fine just feels like it is going to run out of gas but continuous to run. Could it be related to fuel or a vacuum leak? I did check compression on all cylinders and was happy to find it at 155 psi average. If it is related to fuel what can I check on the carburetor? It is a edelbrock 1402 and engine is 318 la.
 
Grab a can of carb cleaner, engine running mist a little over the throat of the carb... Does the engine pick up RPM's & smooth out temporarily? If so a vacuum leak is quite possible...

Try mist a little around the base of the carb, any effect? Check the intake gaskets, PVC system hose & valve... Power brake hose & check valve....
 
There are probably 50 different things that could cause an engine miss. You are going to have to do some diagnosis and repair. Good luck.
 
I assume since you checked compression you also checked the spark plugs.
 
Use a inferred gun and see if you have a "cold" cylinder, one that's not firing.
That's the high tech way... Water in a spray bottle works too... Squirt a little water at each port and if it doesn't instantly evaporate you have a dead/weak hole...
 
The most likely thing is a vacuum hose came loose and R/T’s test would help. Otherwise, it sounds like a plug or plug wire, but you appear to have inspected/replaced those. Could be a fuel filter as mentioned (just change it). If all that doesn’t do it, look at your fuel pump….but I’m guessing vacuum hose came loose
 
The most likely thing is a vacuum hose came loose and R/T’s test would help. Otherwise, it sounds like a plug or plug wire, but you appear to have inspected/replaced those. Could be a fuel filter as mentioned (just change it). If all that doesn’t do it, look at your fuel pump….but I’m guessing vacuum hose came loose
Or cracked... Had a cracked brake booster nipple a couple months ago... Not broken, cracked, couldn't see it but carb spray picked it up instantly...

Cylinder 5 was super weak, 8 was a little weak, 3&2 were barely effected...
 
Pull the fuel filter and cut it in half.
If there's junk in there then
I would suggest taking the carb off.
Set it on the bench, take a couple pictures of the linkage.
Remove the metering rod covers, linkage on the sides and take the top off.
Look inside for debris.

Has the fuel line ever been replaced?
Might have rust inside, mine did.
 
Thanks for the quick responses everyone. I will try to answer all questions. I did inspect the spark plugs when I did the compression check. A little dirty they cleaned up fine. I will inspect the fuel filter and take a look at all the hoses for a possible vacuum leak using carb cleaner. I did use the water spray method and there were at least three tubes from the headers that were not hot al all. What would cause a dead/cold cylinder?
 
Pull the fuel filter and cut it in half.
If there's junk in there then
I would suggest taking the carb off.
Set it on the bench, take a couple pictures of the linkage.
Remove the metering rod covers, linkage on the sides and take the top off.
Look inside for debris.

Has the fuel line ever been replaced?
Might have rust inside, mine did.
The fuel tank, steel line and rubber lines were replaced about three years ago when I installed the Edelbrock carburetor.
 
Thanks for the quick responses everyone. I will try to answer all questions. I did inspect the spark plugs when I did the compression check. A little dirty they cleaned up fine. I will inspect the fuel filter and take a look at all the hoses for a possible vacuum leak using carb cleaner. I did use the water spray method and there were at least three tubes from the headers that were not hot al all. What would cause a dead/cold cylinder?
Three cool/cold pipes with good compression points to one side of the carb being lean... That could be a vacuum leak or a plugged jet in the carb...

Look at how the intake manifold is designed, Assuming it's a dual plane, the two center cylinders on one bank and the two outboard cylinders on the other bank are fed by one side of the carb... If that matches up with your three cool/cold cylinders......
 
Three cool/cold pipes with good compression points to one side of the carb being lean... That could be a vacuum leak or a plugged jet in the carb...

Look at how the intake manifold is designed, Assuming it's a dual plane, the two center cylinders on one bank and the two outboard cylinders on the other bank are fed by one side of the carb... If that matches up with your three cool/cold cylinders......
Thank you.

In an attempt to eliminate all potential issues I would like to verify something. If I run the engine and remove the ignition wires one by one from the distributor cap while its on. Should there be a spark present on each one as separate it from the cap?
 
Well, went outside to take advantage of the dark setting to test it. When I pulled the first wires from the cap I could clearly see the blue spark, same with the following two. However the third and fourth I pulled had no spark at all. Guess I am back to an ignition issue. I did do the pertronix conversion to my distributor to switch from points to electronic. Not sure if this would add to the issues.
 
The ignition issues would likely be the plug wires or cap... The petronix can't identify cylinders... It could potentially cause a misfire but not on the same cylinder
 
The ignition issues would likely be the plug wires or cap... The petronix can't identify cylinders... It could potentially cause a misfire but not on the same cylinder
Also, a lean mixture in that cylinder could also cause a weak/no spark condition because high cylinder pressure requires higher firing voltage, but a proper A/F ratio lowers that required voltage..

Pull the distributor wire boots back but fully insert the plug wires in the cap.... Use a screwdriver with a wire clipped to it and ground... poke the screwdriver tip into each plugwire/cap/tower one at a time... Identify the weak cylinders, now one at a time watch each tower with the screwdriver tip held close to see if a spark jumps... I'm betting all eight have a spark...
 
Your suggestion in post #14 is a good one. To make the test more reliable, remove one plug lead at a time & stick a screwdriver shaft in the plug socket. Away from the carb, lay the shaft on the engine so that there is a 1/4" gap to the engine. Start engine & check for spark. Do all 8 cyls the same way.
 
I FOUND THE PROBLEM (i think)!!!

After still not getting spark from the cap to wires as I mentioned yesterday, I began to look at the Pertronix system. The black piece that sits under the rotor has a missing magnet that fell out, I'm guessing through centrifugal force. No sign of it rubbing against anything. If I can find it I will try to glue it back into place and hopefully that fixes the issue.

I spoke to Pertronix this morning and they said a magnet falling out would definitely have this effect.

distributor.jpg


pertronix.jpg
 
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