dieseling

[ruberpoliceman]

My 86 305 5.0 thinks its a diesel . ive rebuilt the carb and set the choke and all that good stuff and it still wants to run after i cut the switch. Anybody got any Bright ideas? any suggestions are accepted. Thanks. Oh and im pretty sure its not the timing incase anybody was thinking that. Thanks again.

[silicon212]

Hmm... mayhaps perform an upper cylinder cleaning? You can get a can of stuff from a GM dealer for this purpose - basically you run the engine at a mild RPM while you slam this down the throat of the carb/TBI unit fast enough to cause the engine to stall. Then you wait a while and then restart the engine and see gobs of whitish smoke screen the neighborhood - at least that's how I remember it worked - it's been 17 years.

You can also look for vacuum leaks.

[kahjdh]

Yeah, i would clean up the cyclinders and the valve train. It can also be running too hot so maybe a colder thermostat?

[ruberpoliceman]

ive slamed it with 2 cans of seafoam, no 3(in carb and crankcase), and havent seen a diffrence as far as dieseling goes but it wouldnt hurt to do it again i suppose. And as far as it running hot, thats a good posibilitie, i do need another radiator cap and some antifreeze or water whatever, so that could be it even though its done this almost since ive had it wich has been about a year and some change.(ive never driven it as far as a few miles if anyone was wondering its been sittin in my yard).I think i will do another thermostat seeing how its not much of a problem on my time or my wallet and maybe thatll help something. You know what, i just thought about the vacuum thing and rememberd plugging a couple hoses, ill play around with those tommorow and hopefully see a diffrence. Thanks fellas. keep em coming

[randy78]

diesleing is caused by over advanced timing, hot spots in combustion chambers, and too high of idle speed
also too high of octane of gas can too

[silicon212]

Originally Posted by fastr68

also too high of octane of gas can too

That's backwards. Low octane causes dieseling, higher can prevent it. Octane is the rating of a particular grade of gasoline's volatility - the rate at which it evaporates. The higher the volatility, the higher the chance of explosion or flash fire from fumes. Octane rating is inverse to volatility - in other words, 92 octane gasoline does not evaporate as readily as, say, 87 octane. In the engine, higher octane gasoline takes more to ignite - which is why "knocking" or "pinging" tends to soften the higher you go (the knock or ping is power-robbing detonation, explosive preignition of the air/fuel in the cylinder before the spark).

Since higher octane is more resistant to detonation or preignition, it's equally more resistant to after-run (dieseling) - for the same reason as outlined above.

You pretty much hit on many of the causes of dieseling - there are many variables. Sharp angles in a combustion chamber can predispose an engine toward detonation and dieseling - case in point would be some parts I have in my own engine - 492 casting heads w/64cc closed chambers combined with the KB Hypereutectic reverse-dome pistons (a 'dish' in the shape of the combustion chamber as opposed to a full dish) which are flat in the area where the combustion chamber is flat - the "quench" area. These angles can have localized hot spots which can contribute to the issue (and in my case, do during the summertime when it's hot here).

Upper cylinder cleaners will eliminate most hot spots, except the ones that are caused by geometry as I stated above. You can use the GM stuff as I outlined, but there are other, not-so-safe ways of doing it that are just as effective and won't smokescreen a city block. One of these I had outlined in another post a while back, and it involves the use of about a cup (8 oz) or 1/4 liter if you're not in the USA, of water that's slowly fed into the carb/TBI throat while the engine is clocking in at about 2 grand. Unlike with the GM method, you do not want to stall the engine with the water method - if you pour the water into it too fast, or with the engine not cranking enough revs, there's a real chance that you could hydrolock the engine, resulting in bent and broken bottom-end parts as well as blown head gaskets - hence the "not-so-safe" statement above. If you know what you're doing with this, it works well. After this treatment, you get on a stretch of open road and WOT it to speed, then reduce to say 25mph, then WOT again, a few cycles as if you're breaking in a new engine. The water loosens all of the carbon deposits, and what doesn't come out at the time of the water treatment, will come out after doing this.

Vacuum leaks will also cause a dieseling-friendly scenario, as will high idle speed, a stuck throttle, a stuck choke on a carb that causes the curb idle speed setting to determine idle speed, timing as you stated, or even a plugged cat.