Thread: Rough Idle on my Chevy 350

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Hello again. Guy with the 48 buick here (Matt). I've noticed that the car idles very rough when it's cold. This morning it stalled twice before it caught it's breath. The guy said it could use a tune up. I've done a bit of googlight and i think it might be a little bit of a dirty carb? The car was only driven 400 miles in the last 5 years. I remember my grandfather said that when he drove his 66 chevelle that little the carb got gummed up and he needed to run a few gallons of racing fuel to clean things out and wake up the engine.

Any suggestions on where to start and steps to troubleshoot.

I saw a youtube video of a guy spraying carb cleaner into the float bowl then running the engine. Is that ok to do?

You can do that and it may help.. I think I'd just do a rebuild on the carb, you didn't mention what the carb is. 2 or 4? make?? Carb kits are usually under $50. and not hard to do if you follow the directions. It sounds like Accelerator valve or choke adjustment.

So there is another "minor" issue that i googled more out of curiosity than concern. Then i read a bit and now i'm concerned. The engine is Dieseling. There's a 2-3 second delay where the car keeps running. I didn't think anything of it and assumed it was some electrical lag. But now i'm seeing it can be a variety of things.

- Carbon: I but high test in it hoping that this will help. I haven't run it enough to get any result. I'll drive it for 10-20 mins today. I've read i can also buy an additive to help solve this problem.
- Spark plugs/ignition: One suggestion was to change out these and i'll be fine.
- Engine running too rich: Build up of fuel could be causing the problem. Adjust carb and it's fixed?

You said it's only been driven 400 miles in 5 years, right? How much have you driven it since you bought it? Before I did anything in the troubleshooting arena, and for sure before I started changing parts or buying things I'd change the oil, change the coolant unless you know it to be fresh, check the tranny fluid for color and sniff test and if OK (clean red, no burned smell) I'd take it out and drive it. Once it's warmed up to operating temperature run it up in rpm several times and see what you get. If you get any backtalk from it then head to the interstate (or similar open road) and run it above 3000 rpm for at least twenty minutes if you can do that without getting too far over the speed limit. That will clean off the plugs of any deposits, and might smooth things out for you. When you get back pull a few plugs and read the color - you can find a chart by Googling "spark plug color", and see if that tells you anything.

Not to worry you, but having the car sit that long likely means that it's had the same gasoline in it for a loooooong time, and that can be bad with todays ethanol blends. You might have eaten up anything rubber in the fuel system, but do the drive first, then worry about problems once you've got some real time info.

Good luck with the shakedown!! Oh, and just to clarify, 'high test' only burns slower than "regular" to eliminate detonation in higher compression engines. If your engine runs knock free on regular then you'll get less power and efficiency with higher octane fuels.

WOA! Thanks! I drove the car home (non highway trip for about 20 mins), then to the park and ride a few times (3 miles round trip). I'll take it out warm it up and take it on the highway.

I changed the oil last night with VR-1 racing oil 20W-50.

Thanks for the info. I really appreciate the help.

Visual inspection of plugs and rotor terminals might be in order.

Originally Posted by rspears

You said it's only been driven 400 miles in 5 years, right? How much have you driven it since you bought it? Before I did anything in the troubleshooting arena, and for sure before I started changing parts or buying things I'd change the oil, change the coolant unless you know it to be fresh, check the tranny fluid for color and sniff test and if OK (clean red, no burned smell) I'd take it out and drive it. Once it's warmed up to operating temperature run it up in rpm several times and see what you get. If you get any backtalk from it then head to the interstate (or similar open road) and run it above 3000 rpm for at least twenty minutes if you can do that without getting too far over the speed limit. That will clean off the plugs of any deposits, and might smooth things out for you. When you get back pull a few plugs and read the color - you can find a chart by Googling "spark plug color", and see if that tells you anything.

Not to worry you, but having the car sit that long likely means that it's had the same gasoline in it for a loooooong time, and that can be bad with todays ethanol blends. You might have eaten up anything rubber in the fuel system, but do the drive first, then worry about problems once you've got some real time info.

Good luck with the shakedown!! Oh, and just to clarify, 'high test' only burns slower than "regular" to eliminate detonation in higher compression engines. If your engine runs knock free on regular then you'll get less power and efficiency with higher octane fuels.

So I took her out and ran her pretty hard on the highway. When I got home and put her in the garage....no more dieseling!!!! Thanks everyone!

Originally Posted by Zombiebrew

WOA! Thanks! I drove the car home (non highway trip for about 20 mins), then to the park and ride a few times (3 miles round trip). I'll take it out warm it up and take it on the highway.

I changed the oil last night with VR-1 racing oil 20W-50.

Thanks for the info. I really appreciate the help.

FWIW 20-50 in not a good choice, stock 350 stick with the spec weight viscosity. If it has some whoop-ass and you lean on it often, Castrol 20w-40 will work great.

Might also add some Marvel Mystery oil the gas tank, good for valves and guides.

Originally Posted by pepi

FWIW 20-50 in not a good choice, stock 350 stick with the spec weight viscosity. If it has some whoop-ass and you lean on it often, Castrol 20w-40 will work great.

Might also add some Marvel Mystery oil the gas tank, good for valves and guides.

Another board suggests adding a quart of significantly thinner oil to balance out. Any suggestions?

Oh and how much mystery oil do I add per fill up?

Anybody in the seattle area that wants to walk met brought the tuning process for beer?

Sorry for all the posts but i have more updates! I ran her at idle yesterday, took off the air cleaner, and sprayed carb cleaner into the fuel bowl tube in short squirts. I would rev the engine as it started to sputter and then kept it running for 5-10 mins. HUGE difference!!! Much better start up, better idle, better acceleration. I think i still need to tune it but things are much better. Thanks for all the help as i continue to tinker!!!

Pepi mentioned Marvel's Mystery Oil in the gas tank, which I've used in the past, but another alternative to consider is a pint of SeaFoam. Follow the directions on the can for the amount of fuel to mix it with, and it's best to add it to the tank first, then add the fuel to mix things up. I've also used SeaFoam straight out of the can, pulling the large vacuum hose off of the power brake booster and letting the engine suck the SeaFoam out of the can opening. Don't submerge, just put the end of the hose over the can opening, and tilt the can enough for the vacuum to slurp it up. Note, do this OUTSIDE, NOT in the garage. It smokes to high heaven!!

Originally Posted by Zombiebrew

Oh and how much mystery oil do I add per fill up?

I do not have a bottle in front of me but it is roughly half the bottle pre 10 gals. Mixture rate is printed on same. It is easily found at Wally world. My fuel load is 15 gal, pour 1/2 of the bottle to that. Use it in the fuel systems of all my gas powered motors (4 cycle) .

some history:
Marvel mystery oil first came out in October 1923 to rid carburetors of deposits caused by unrefined gasoline. In World War II, Marvel mystery oil was actually used in ships, tanks, airplanes and other military vehicles. Marvel Mystery Oil has several uses in the automotive field. It can serve as an additive in the fuel system to help clean out any buildup from debris in the gasoline. You can also add it to engine oil at oil change intervals.


Amazon.com: Marvel MM13R Mystery Oil - 32 oz.: Automotive

32oz 8 bucks; in service as of 1923... good history there


Amazon.com: Sea Foam Motor Tune - Up: Sports & Outdoors


16oz 6 bucks;


coin toss ...

Originally Posted by rspears

Pepi mentioned Marvel's Mystery Oil in the gas tank, which I've used in the past, but another alternative to consider is a pint of SeaFoam. Follow the directions on the can for the amount of fuel to mix it with, and it's best to add it to the tank first, then add the fuel to mix things up. I've also used SeaFoam straight out of the can, pulling the large vacuum hose off of the power brake booster and letting the engine suck the SeaFoam out of the can opening. Don't submerge, just put the end of the hose over the can opening, and tilt the can enough for the vacuum to slurp it up. Note, do this OUTSIDE, NOT in the garage. It smokes to high heaven!!

To add to Roger's post - - - I had a 350 in my '55 and it got to dieseling so I used Seafoam according to instructions and "voila" problem solved.