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Rough Idle - What to check?

Chet Zerlin

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I am really trying to diagnose and fix this issue without having to bring the car back to my local (and excellent) mechanic.

Yesterday I drove my 1959 100-6 about 30 miles and everything was working beautiful. Engine pulled through 4,000 RPM's smoothly, gearbox and overdrive shifted cleanly.
When I got within about one mile from my shop I stopped at a red light and shifted into neutral and let the clutch out. Suddenly the idle RPM began to drop and the engine sounded like it was going to stall. I kept the revs up to prevent that and when the light changed I was able get moving again but now the engine didn't want to pull any RPMs and started "spitting" and backfiring. I was able to keep the car moving and drive to my shop but the engine was clearly running rough. Pulled it in and turned it off. Then waited about 5 minutes and cranked it up again. It started right up but would not hold an idle. A few minutes later I started it again and it maintained a 700 RPM idle but was running very rough.

So I'm understanding that the usual things to check would be fuel and ignition. My gas tank was almost full and considering how well it ran prior to this happening I'm thinking it's not a fuel or carburetor issue. So I'm thinking ignition?

What would cause an engine that was running so well to suddenly exhibit these problems? What would all of you suggest I check?

Thanks!
Chet
 

Healey Nut

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Re: what to check?

Fuel filter ?
Points issue ?
Cracked distributor cap .
bad rotor arm
loose HT lead
Dead spark plug .....the list goes on
 

John Turney

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Most likely electrical (they usually are).
A bad rotor usually means it won't run at all.
A loose HT lead or dead spark plug can be diagnosed by pulling the HT leads to the plugs one at a time (Make sure your hands are insulated or pull it with the engine off - it can be quite a bite otherwise). It there is no change when you do that, that is the cylinder that has the problem.
 
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Check your carb pistons: sometimes, a jet needle will come loose and drop into the jet and car will run on 3 cyls, then 'fix itself,' then break again.
 

Novamonte

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I experienced very similar symptoms recently. It turned out to be the cable from the dizzy to one of the plugs that had broken. I found out with the help of an ohmmeter.
 

mgtf328

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If the wiring is sound and the ground connections are good I would put in a new condenser and check out the coil.
AJ
 

Bob Hughes

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Chet

Open up the float chambers and look in, you might have - Low petrol levels - maybe a pump problem or dirt in the bottom of the chambers - blocking the entry to the jets.

If fuel level is low get a bottle of know quantity and disconnect the fuel feed to the carbs and shove it into the bottle, turn the ignition on and see how long it takes to fill up.

I experienced similar problems once, and after spending a small fortune with a specialist to almost no avail I found that I needed new needles in the carbs - I had run the car for nigh on 13 years with those needles but in the end I was over fuelling - new needles solved this problem. (the old needles were totally different to the new ones despite having the same markings) Just in case have a look at your spark plugs if they are sooty black you could be over fuelling.

:cheers:

Bob
 

trotti

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I had a very similar issue and the condenser was the culprit. After replacing, the car performed beautifully again.
 

Martinld123

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Intake manifold or exhaust manifold air leak will cause this. Mine went up and down 800 rpm until I tightened bolts. If you did any work in that area take a look and tighten bolts. Marty
 

roscoe

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One possibly telling thing you mentioned is that you preferred to not bring t he car BACK to your mechanic. While I empathize with the feeling, it begs the question of how recently your mechanic had the car and more importantly, exactly what was done to it. You might be able to focus in on what, if any, ignition or fuel system parts were removed/replaced/ disturbed. Not that it would necessarily be his/her fault if something they fiddled with failed but sometimes fiddling can accelerate the appearance of problems that were not yet appearing. The cars need fuel, air and spark delivered properly to run and there are probably more electrical possibilities than fuel potentialities but if the carb linkages were adjusted etc. something might have slipped. If the inside of the distributor was visited there are several things that could cause your issue. Please let us know what fixes it. I vote for a faulty condenser/rotor or an intermittant ground associated with the points or breaker plate in the distributor. Spitting and especially backfiring make me think ignition. An unburned fuel charge due to intermittant lack of spark can both sputter and cause backfire.
 
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