I fired my freshly rebuilt 352 up again, and it has about 6 hours on it, I started it today, and got the timing in sink, it was finally running smooth for about twenty min. and all of the sudden, it started misfiring really bad, it sounds like it fires on about 2 of the 8 cylinders, it really scares me considering I am 16 and have all of my money tied up in the truck. When you give the truck some gas it smoothes out some, but at idle, it is really struggling, it is really bad, any ideas what is wrong? Please help? If you are familiar with the term dieseling( when you shut of the engine it sounds like it doesn't want to shut off, and runs real rough, that is what it sounds like)

 

if its dieseling it sounds like the timeing is off, the gas is still trying ot burn when its turned off, you might want to check the timeing again to make sure its right it could also be the carb sense it smooths out, you will just have to do some adjusting and get back to us.

 

from the sound the carb is dumping in way to much fuel check the carb to see if maybee your floats got stuck if you have a four barrel on it anything much over a 600 on top of a 352 unless it has quite a bit of cam will be too much if you are running an edelbrock id suggest gettin a calibration kit and adjusting down the jets, metering rods, and springs, also they make a fuel richness sensor it goes on after the headers or manifolds and works the same as an o2 sensor and tells you how rich your truck is running to help you adjust the mix

 

Well...He said it started off running smooth?? and its not now....and And it wants to keep running..sounds to me like the timing slipped some where....? Loose bolt on the dist. hold down? Jumped a tooth? Dist. cap cracked from not being seated right? and the list goes on... Good Luck..Russ

 

The motor is not dieseling, it just sounds like it is while running, to tell you the truth I put the air cleaner on top of the carb, and when I took it back off it started running rough

 

when my 390 was running terrible before i tore it down, it wouldnt run with the air cleaner on unless i kept giveing it gas, it must be something screwed up in your carb if its doing that.

 

Well it started running like that out of nowhere, the engine was running for 20 minutes, and while it was running, I placed the air cleaner on it, well after I took the air cleaner off after about a minute, that is when the engine started running really rough, I will tell you that, it is a new carb, new plugs, wires, cap, rotor, points, condensor, coil. The distributor is original though.

I finally had the timing right, and 20 min later it runs like it is about 180degrees off, it is bad,

After thinking maybe it is a clogged fuel filter???

 

What temperature is showing on the temp gauge? Better check for a vacuum leak because this can cause a lean running condition which raises combustion chamber temperatures. If the the combustion chambers get hot enough then just about any sharpe edge in the combustion chamber can be hot enough to maintain combustion for a bit even after turning the key off. A lean condition will cause the rough idle also. A vacuum leak usually has the largest effect on idle.

I have seen holes burned into pistons (don't ask me how I know) because of a lean running engine.

 

The temp is good and the oil pressure is good, I wouldn't think it is a vaccuum leak, because I have new vacuum lines, but anything is possible

 

Have you rechecked your timing? Easy stuff first. I agree that it sounds like your dizzy hold down was tight but not tight enough and your timing slipped. Check it and see what it is. Check all your vacuum lines and plugs to be sure none popped off or were knocked off. I'm sure you checked already but to be sure. I get so freaked out with a new motor like that it is easy to miss stuff. Then check your intake. and make sure it doesn't need to be retorqued.
Let us know what you find. Good luck

Ken

 

Checking everything that has an effect on engine performance should be done until you find the problem. Don't assum anything isn't the problem yet. Your original post makes it very clear that there is a rough idle and the engine continues to run after the ignition is turned off (dieseling). Very solid symptoms of a lean condition or timing being off.

You could find a vacuum leak anywhere in the induction system and from any vacuum line including the ones under the dash. The vacuum advance, PCV or even a power brake booster shouldn't be overlooked. One way to test for a vacuum leak is to just disconnect/plug off all vacuum operated items that aren't required to start and run the engine. Another is to spray brake cleaner, carb cleaner or WD-40 around some key areas where the carb, intake and heads seal together. A change in engine rpms will be evident if there is a vacuum leak in these areas. Dont' spray to heavy or the engine will get some vapors through the carburetor and it will change rpms.

Unless you built the carburetor then it is suspect as well. Not that you did anything wrong installing your new carburetor, but there could be some trash/debri in the carburetor blocking a fuel passage. It would be interesting if you had another carburetor that is known to work well and bolt it on just to see what the difference would be. I know too many times where that solved many wild goose chases looking for performance problems under many hoods.