I finally finished building my first engine and got it in the car today. The engine started up immediately and sounded great. After the cam was broken in I shut the engine off and then a quick gust of gas vapors shot out of the carb. I took the car for a short drive through the suburbs(with open headers :D ) and when I got home and shut the engine off, the engine "desieled" again. My friend adjusted the mixture screws in the metering plate until the engine ran well at a lopey 750rpm idle. His explanation for the problem had something to do with the engine pulling more fuel than was in the idle circuit when the engine was shut off? I am carburetor illiterate so I can't give a more detailed explanation. My engine stats are as follows:
camshaft : 230intake duration 236exhaust at .48 and .49 lift. 110 lobe separation, 106 degree intake centerline.
carburetor: 3310 holley vac. sec. model 4160 with 72 primary jets, plate 134-9, power valve 6.5
Stock HEI distributor
static timing set at 10 degrees btdc
Rest of the stats are in my sig.
When I pulled the spark plugs after an hour of operation they were coated black with what looked like either burnt oil or fuel-soaked carbon. My friend( a retired Calfiornia vette mechanic of many years) suggested the use of an "anti-desieling solenoid" to hold the throttle shut when shutting off the engine. Anybody use this thing? Any help would be greatly appreciated!





[Modified by Imabadman, 5:18 AM 2/24/2002]

 

Did adjusting the idle screws help?

How much have you driven it since then?


[Modified by 45ACP, 6:26 AM 2/24/2002]

 

The Anti-diesling solenoid (AKA curb idle solenoid) actually sets the idle speed and when you shut the power off, it retracts allowing the carb to close fully. Factory set-ups used them in the late 60's-70's. because they had the same problem when the octane rating of fuel was dropping. With low vacuum settings on power valves used with performance set ups, when you shut the engine down, vacuum drops off and the power valve opens and delivers a very rich mixture (like is should) That is why the plugs are black. You could try another power valve just in case yours is blown from back firing (maybe that's what happened). Diesling can also be caused by timing too far advanced for the octane of fuel or by high cylinder temperatures, dut I assume you have been through that. :jester

 

The only time mine has ever dieseled is when I accidentally put 87 octane in it. I am running ~9.5:1 CR and the motor wants the 93 octane stuff. What kind of gas you using?

 

I always use the highest available octane - to keep the "nothing fancy"'77 from pinging more than anything - like mentioned prior completely closing the carb is the key.

With the Holley - if you've had to open the primaries a little much - (the slot should just barely show when viewed from the bottom -your carb literate friend will know what I mean)- to get the idle you want - you might need to crack open the secondaries to allow for a bit more air and this will also allow you to close the primaries some and hopefully give you the idle you need while closing enough to keep the motor from wanting to run on.

I've been running Holleys for 20+ years (am I getting old or what?) and will be glad to try and answer any specific questions - just email me.

good luck, congrat’s on getting the motor up and running - great feeling isn't it?

 

It won't cure the problem,but it will stop the effect.......turn the ignition off in gear BEFORE you put it into park or neutral....john :chevy

 

Thanks for the info, you guys are great! As for the octane rating, I had half a tank of 87 octane from before the new engine. I filled the tank the rest of the way with 92 octane and a bottle of octane booster. The booster was supposed to give me 7 more points of octane in a 15 gallon tank. The engine desiels only once upon shutdown, when it's running there isn't any problems. The engine only has about 10 miles on it max. Now that I think about it, the buildup on the plugs is probably from the well-oiled pistons and the fact that the rings haven't broken in yet (In addition to the power valve shot, thanks silervetteman). The engine responded well to idle mixture adjustment but I needed to set the idle high enough in the "park" position that the primaries are cracked a little as rrent observed. So what do you guys think is the best solution, crack the secondaries slightly or find a "curb-idle solenoid" ;) ? I'll check up to see how the 6.5 power valve is operating. Thanks again for the feedback everyone!

Oh, and yes rrent, the feeling is beautiful, taking into account the most powerful motor I have personally driven was the stock 180hp L48 engine! The sound and feeling of the open headers shaking the car was unforgetable. :D


[Modified by Imabadman, 5:31 AM 2/25/2002]

 

It's possible that you blew out the power valve in the carb. I had the same thing happen when I changed my cam. The valve went on the old carb after I had a small backfire. After that the plugs were coming out black becuase it was pulling fuel at idle. So maybe that's something you should check into. It only takes a few minutes to pull the front of the carb apart and check assuming that it's a holley or a demon. Try a 5.5 valve

 

Try retarding your timing a couple of degrees, and make sure your advance is hooked to a ported vacuum and not straight manifold vacuum. An adjustable timing light would really help you get your timing and advance curve setup right. A misadjusted carb and/or low fuel pressure can cause back-firing problems and poor running, but your description leads me to believe that you have a timing/advance problem with your distributor.