Sudden Rough Idle Syndrome.

MrMagoo · #1 (**permalink**) 05-13-2009, 01:08 PM

Here's the scenario: Cobra sits for about 3 months without being started. First startup: runs exactly the way it did last summer. Idles perfectly. After about 45 miles through second tank of gas (about 200 miles of driving this season), car goes in garage. Next morning, start it up, idles low and rough. I turn the idle speed screw just to keep it running, and drive it at highway speeds, where it seems OK, normal power etc.

After getting to my destination, I put the idle screw back where it was, and investigate. Still with the rough idle and low idle speed.

Fuel Pressure (post filter) is 6+. Good. Float Levels good. Choke open at temp. Good. Didn't check fuel filter, it has a few thousand miles on it and it;s clearly not blocked.

Carb is Quick Fuel 780 (Holley 4150 style). MSD ignition.

My first guess is that I got some bad fuel. Or perhaps some debris got into the system. I'm not flooding, so I am guessing the needle valves are not the problem. Maybe somewhere in the metering blocks?

I think the ignition is OK, cause it starts right up, cap looks normal, plugs normal.

Anything seem obvious to anyone here? I might try some fuel additive before pulling the carb and cleaning it out.

Thanks

ZOERA-SC7XX · #2 (**permalink**) 05-13-2009, 02:29 PM

Try two cans of dry gas. It's a start and a cheap possible remedy. You may have moisture in the tank that just got pulled up.
Also change the fuel filter.

Rick Parker · #3 (**permalink**) 05-13-2009, 03:08 PM

You may want to spray some carb cleaner in the brass air bleeds at the top of the venturis.
Secondly be sure the mechanical advance is not hanging up, may need a little grease on the flyweights.

Dangerous Doug · #4 (**permalink**) 05-13-2009, 03:58 PM

Check for a vacuum leak or a blocked PCV?

DD

MrMagoo · #5 (**permalink**) 05-13-2009, 04:41 PM

First thing I checked was the advance. It was all good in there. I have a rule: all problems are caused by some failure in the MSD distributor until proven otherwise

I will try the dry gas and the carb cleaner in the air vents, and check for vacuum leaks and report back.

Thanks as always!

mln385 · #6 (**permalink**) 05-14-2009, 06:08 AM

Before storage did you use a gas stabilizer? If not it is possible you have some shellac type substance clogging the carb. from the de stabilization of old gas.

Carb. may need rebuild...try some carb cleaner like seafoam in the fuel and run a few new tanks thru it and see if it cures the issue.

Marc

jhv48 · #7 (**permalink**) 05-14-2009, 07:14 AM

Possible piece of gunk in one of the idle mixture screws. Pull and blow passage out with carb cleaner.

CBattaglia · #8 (**permalink**) 05-14-2009, 07:25 AM

Last weekend I had a sudden lack of engine performance too. When I would blip the gas on down shifts the engine would bog. It happened right after a fresh tank of Sunoco 93. I ran through that tank (one day of spirited driving)and got an new tank of gas from a different Sunoco and she purrs like a kitten again. The towel heads that took over the gas station (coincidence

I doubt it) where I got the bad gas may have swapped out for the 89 garbage or had water in the batch.

Do the cheap stuff first, its usually something stupid and simple.

MrMagoo · #9 (**permalink**) 05-14-2009, 08:16 AM

OK, I now have sudden idle improvement syndrome...

Heres what happened this time: Car sits in parking lot all day. I go out, start the car up, experiencing about the same problems. As I am warming the motor up preparing to GTFO the revs fly up to the rev-limit... the accelerator got tangled up in the floor mat (bye bye floor mat). I kill the motor, no harm done. Start up again, problem gone.

I am thinking that Jim may have hit the right answer, a piece of crud (maybe broken off varnish) in the idle mixture circuitry. If it was a bad load of fuel, I doubt gunning the motor would have made a difference.

I am going to run a batch of SeaFoam through the fuel system to take care of any residual crud. At the end of the season, I'll do the Crankcase and the Intake manifold as well, change the oil, and put some Lucas stabilizer in the fuel tank.

Thanks again everybody, informative as always.

GlynMeek · #10 (**permalink**) 05-14-2009, 10:21 PM

This happened to me two weeks ago and I diddled with the carb for a while...Tried everything, richer, leaner, reset the choke but nothing made a difference...it idled badly and didn't 'sound' right. It ran dreadfully at low revs, but OK at above 2500 other than the odd 'miss' when I stabbed the gas hard.

Turned out for me that I had a broken push rod and NO compression in Cylinder 2...

Hope you don't have the same!

Glyn