dead cylinder at idle (but not under load)

si2t3m · 09-22-2003, 11:54 AM

Hi,

#3 cylinder is acting up. The cylinder is dead at idle but wakes up on acceleration. At constant rpm say at highway speed, i can feel the car jerking like #3 coming on and off. At contant rpm (lower rpm) cylinder is dead. Don't worry i don't drive the car around like this. I'm only troubleshooting!

Engine is a 3.0L with Webers. All this started when i lauched pretty hard (might overreved the engine as i have no rev limiter). Car was low on gas also.

I checked the cap and distributor and they are ok.
I have spark. I changed the plugs and it's still the same.

Fuel pressure gauge located just before the carbs shows 3psi.
Had crud in one of the fuel filters located before the fuel pump. I replaced it. Still the same.

I checked the compression and it was even on all cylinders. I'm I ok to say that I can rule out a bent valve (from a possible overrev) if compression is good and syncrometer hows that it's drawing the same amout of air as the others???

I checked the idle jet and main jets. Took the carb apart and checked for blockage in the idle circuit. none.

Checked with a propane around the base of the carbs. No implovement.

Inspected the manifolds for cracks. Nothing.

It didn't suck a gasket either.

I gotta confess that i'm clueless on what is going on.

I'm seeking for ideas!

What could cause #3 to be dead at idle, and pick up on acceleration and be intermitant at constant rpm (in the 3000-4000 rang)???

Marc-André

TIA

Marc-André

Rot 911 · 09-22-2003, 11:58 AM

I have had a bad plug wire lead to the kind of problem you describe. Try switching out the #3 wire and connectors. For that matter change out the #3 plug with a new one while you are at it.

jtf914 · jtf914's Garage

I know you went through the webers, but everything you have listed says fuel problem to me.

Idle jets provide fuel up to 2 or 3000 RPM, main jets take over from there, sounds like you are only getting consistent fuel from the accelerator pump......

-Justin

Formerly Steve Wilkinson · 09-22-2003, 01:19 PM

Agree with jtf914. Sounds like you've got some schmutz in the #3 idle circuit. Happened to me recently (also #3) and the symptoms were identical. And it was interesting that it happened suddenly, not a gradual on/off/getting-worse thing but one minute the car was running perfectly and the next it was going baa-da-baa at partial load (PMOs on an '83 SC). Having crud in the fuel filter is a bad sign. (I have three filters--one big diesel filter and two in-line filters.) I tiny bubble of water can do it too. I purged my #3 idle circuit with some goodly squirts of carb cleaner and the car ran perfectly.

Stephan

si2t3m · 09-23-2003, 03:43 AM

Thanks guys,

It's raining cats&dogs today but i took time to remove again the offending carb & manifold. I'll replace the insulator gaskets check to see if the manifold is straight and procede to cleaning the idle circuits again. I must of missed something the last time.

Marc-André

gregk1 · 09-23-2003, 06:29 AM

I had the same problem with my girlfreinds 70T, I took a compression test and it was real low, then I opened the throttle on the carb and It came up perfict, So I removed the carb and the throtle plates were off one was closed tighter than the other two, I made them all even with the allen screw collers between all the ports put the carb back on and it ran GREAT!!! and still does

si2t3m · 09-24-2003, 10:17 AM

DOH!!!

Was getting ready to tear the carb apart when i decided to take the idle jet out of it's carrier one more time.

First time i checked, i didn't take the idle jet out of the carrier but this time i did it and blew thru it the other way. I clogged the small orifice doing this. Cleaned it out and it's now running on all six.

Took the 914-6 to the office (using the long way to get there) to celebrate.

I'm one happy camper again!!!

Marc-André