Erratic Carb at idle

[Hack Richards]

350 cc engine HEI Edelbrock Carb electric choke TH350 Trans

Has a very rough idle when placed into gear. Idles better when in park or neutral but seems to surge just a little but whenever it is dropped into drive there is a very noticeable roughness.

Takes fuel great and screams on the highway....just the idle problem in drive and reverse.

Could this be a vacuum leak somewhere? or carb air fuel needs adjustment?mOr is it something in the ignition system?

Thank you in advance.

 

[1987 GMC Jimmy]

I’d check vacuum reading, curb idle speed/mixture, and base timing for starters. When’s the last time you did a tune up and replaced your air and fuel filter?

 

[Hack Richards]

new air filter now and fuel filter is only 1 year old. I cleaned the plugs last spring BUT I do suspect maybe a bad plug wire. May just go ahead with a tuneup and replacing the vacuum lines since they are about 40 years old to see what happens. Darn thing runs so well though except for at idle in gear.

 

[C10MixMaster]

new air filter now and fuel filter is only 1 year old. I cleaned the plugs last spring BUT I do suspect maybe a bad plug wire. May just go ahead with a tuneup and replacing the vacuum lines since they are about 40 years old to see what happens. Darn thing runs so well though except for at idle in gear.

you can take a squirt bottle with water to find vacuum leaks. spray down anything you suspect leaks hoses manifold carb base etc. then you hit a leak you can here it and the idle will change. that's also a good way to check plug wires. spray then with a mist if there bad they will spark n snap and drop a cylinder. Just be sure YOUR not grounded to the truck.

 

[1987 GMC Jimmy]

Are these platinum or iridium plugs you’re cleaning? I’d give the cuprous plugs a lifespan of around 30,000 miles. Maybe forty if you regap at 20K. Aforementioned test is a good way to check the wires for sure. If it’s really dark, you can see it if it’s consistently arcing. If all that stuff is cracked and disintegrating, I’d replace it. You can also make sure that there’s no corrosion or carbon tracking in the cap. I think that’s cover you for ignition, but the base timing should be checked, especially if issues started happening after winter ended. They can lose timing when it gets really cold out.

 

[Hack Richards]

Replaced all the vacuum lines. The last one I pulled off felt way to easy - - it was on the spark advance. I believe that was a leak. It started right up and while the choke was on I thought I had it perfect again. went for a ride and engine warmed up and it still has a rough idle - NOT as bad as before but rough whenever in drive or reverse. It runs smooth on the highway.

I'm thinking the timing may be off a little. It did backfire a little when I shut it off once.

The timing indicator is missing off the front of the engine. I hope I can find someone who is good enough to time by ear.

So to make a long story short. It is better but not as smooth as it once was. OH Oreillys did not have any plug wires. I may order some off the internet in a few minutes.

 

[1987 GMC Jimmy]

You remembered to check the one going to the trans modulator and make sure that it wasn’t sucking fluid? I’d go for the AC Delco wires. I’m not as much of a Delco fanboy as I used to be, but the ignition parts I would still file under ‘quality.’ I don’t know about timing it by ear, but it’d be easier than trying to install a new tab and set it up. Does it ping under heavy acceleration or run-on? It backfired through the carb? It could be too advanced. It might not hurt to look for a stuck valve or test for a burnt valve while I’m thinking about it. I think that can cause run-on, as can a worn timing set or heavy carbon deposits in the combustion chambers, valves, or EGR pintle.

 

[Hack Richards]

GUYS, I feel like slapping the STUPID right out of me. After pranking with this thing on & off for 2 weeks I finally discovered the Air cleaner gasket has a tear in it. I taped that gasket together and its runs/idles perfect.

All this after I replaced all vacuum lines new plugs & wiring.

Goes to show you that it all about the little things in life!