Rough Running Question

Mike in NRN IN

Mike in NRN IN

Senior Member
Here's the story:

I have been having trouble with my engine running rough while trying to maintain speeds 45 mph or higher.

Some specifics:


  • A stock head - rebuilt engine has about 700 miles on it.
  • Zenith Carb with flow tested jets
  • Modern Upper / Lower plate in the distributor
  • NAPA Points/Condensor
  • Champion W18 Plugs
  • Timing done according to the Les Andrews book and with my handy dandy rotor adjusting tool from Bratton's.
  • Dead Leg in the gas tank and have not had any rust come out during the last two clean-out drains.
  • Reproduction Gas Cap has been 'drilled' to allow open flow
  • Spark about 1/3 advanced
  • GAV open about 1/4 turn off the closed position
What happens. When I get to cruising speed, the engine begins to hesitate and the car feels like it's slowing down, then there is a bit of a surge, then a drop, then a surge, then a drop, then a backfire.

The whole feeling is like tapping the gas pedal and letting off about five or six times. I have tried opening the GAV and closing the GAV. I have tried advancing the spark and retarding the spark. Regardless of what I do, the result is still the same - surge/lull/surge/lull/surge/lull/surge/lull backfire.

No matter what I do, this happens until I drop the speed to under 45 mph.

Any advice on where to start looking for problems?
 
My quess would be the modern point plates,loose wire connection at the points or condensor or the wire touching ground. Others will chime in.
 
Check to be sure the fuel line doesn't stick too far out of the ferrule at the carburetor. Someone posted recently that you should be able to run a 1/2" fine thread bolt into the filter threads without it touching the end of the fuel line.

Joe
 
It sure kinda sounds like a fuel delivery issue[like Joe mentioned] or a float height problem. Remove the fuel line at the carburetor and check the flow for at least a gallon's worth of time just make sure its not a vent problem..Then check to make sure the fuel line isn't sticking too far into the carburetor, the fuel line should go just slightly beyond the compression/ferrule fitting.. If that doesn't do it, it looks like the carburetor will have to be looked at.. Generally, if the ignition condensor is bad or loose it'll just start backfiring rather quickly..
 
Inspect the carburetor float, I had this problem and it was a pin hole in the float.

mike
 
I agree sounds like a fuel problem. The gas cap maybe the culprit. I have seen it several times over the years. Try running he the car with the cap just set on the tank and see if the problem disappears.

Pete
 
check the gasket on the intake manifold to the block. I had a problem at high RPMs where enough air was getting in to cause the engine to run a little rough.
 
JoeWay said:
Check to be sure the fuel line doesn't stick too far out of the ferrule at the carburetor.
Click to expand...

I'm with Joe on this one as well as the other end of the line as also mentioned. The line only needs to extend passed the ferrule about 1/8". If you see 1/4" or more it isn't right.

  • Timing done according to the Les Andrews book and with my handy dandy rotor adjusting tool from Bratton's.
  • Spark about 1/3 advanced
Click to expand...

If that timing description is the same as I read several years ago then it's slightly retarded (about two clicks) but not enough to be a problem by itself. If 1/3 advanced is true then it is insufficient for that engine speed and should be no less than about 2/3 which would place the lever horizontal. With a stock head and ACCURATE timing you can have the lever fully advanced at 45 mph and above even under a load.

I'll be surprised if it isn't starving for gas.
 
I see you are using the modern upper / lower plate with condensor on upper plate?

Try this.

Remove upper plate and look at the lower plate where the condensor would normally be installed. I think you will find that the end that normally attaches to the condensor is rubbing against the side of the distributor housing. This shorts the ignition intermittently.

Just bend the plate back and reassemble the distributor.

I would post pictures but can not see anyway to do so on this reply page.


Drove me crazy until I found it,
 
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Maybe the manufacturer of this Repro part might want to fix this problem. I am sure that more than a few folks have had or will have this problem and will trash and "trash talk" the modern plates!
 
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BTW ... applogies to my IBM tech instructor. I meant to say "Form" the part NOT "BEND" it.

They told us to NEVER tell the customer that you bent a part to affect a repair... always say that you "formed it"!!

They had no sense of humor when it came to things that had to do with the company's image.
 
ex ibm typewriter/copier guy here, ever notice when they did a field fix the screw holes were already there? Yes, never bend, always reform
Paul in CT
 
Re: Rough Running -been there....

I had the exact same symptoms and found that he tank was filled with minute particles of rust. I drained and filtered the gas then repeated the sequence several times and it ran like normal again. Some time later I removed the tank and cleaned it, and sealed it with two coats of the alcohol/ethanol proof "aircraft" sealer available at parts houses, then let it cure for two months with the cap,gauge and fuel lines removed from the tank. If you install a new cork float with the gauge, be sure to dip it in sealer and allow time for it to dry thoroughly before assembly. I also used the type of filter which fits inside the tank opening at the fuel line connection and have not had a problem in 15 years of use. Of course I had to re-paint the outside of the tank after sealing the inside, and the "same color" did not match the rest of the car so I ended up re-painting the entire car ! :mad: Good luck !
 
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