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Discussion Starter · #1 ·
Please help, I have a Chevy 350 motor with a Rochester Quadrajet. She has a run on issue. I have to stop her while in gear, even then, the carburetor will spray up and out a dangerous amount of gas with a hiss when she kills. I don't want a fire. How can I stop this? Thank you Tom
 

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is it dieseling or shooting gas out of the carb when you shut it off?
 

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It could be a bent valve, couple of back yard things you could try would be to pull each plug wire one at a time. You will feel the engine miss a little. If you pull one and the engine runs the same that could be the cylinder with the issue.
Next would be a compression test.
Then google how to do a vacuum test. A bent valve will cause the needle of the vacuum gauge to do some crazy stuff.
I drove around my old truck with a bent valve, It would run fine, but would run on sometimes when I got it hot and killed it.
I also figured out that there is a unique smell to the exhaust of an engine with valve issues. I recently told a friend that I thought he had valve issues on his motor when I walked up while he was checking it out. He said no way, he did a compression test and they were good. A few days later it started running worse and he pulled the heads. An exhaust seat came loose and and was flopping around between the valve and the head!
 

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Run on or dieseling is caused by a couple of things
Carbon buildup in the combustion chambers can cause it. They make stuff to pour down the carb that will clean that out.
Also. if your idle speed is too high, the engine will diesel.
 

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Where is the timing set? Tell us about your ignition system...
 

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This is what mike is talking about.
More Information for AIRTEX/WELLS 3H1025

The issue is you turn off the key which kills the spark so the spark is not the issue.
The issue is there is enough heat or compression in the chamber space to ignite the fuel air mix.
Now you can eliminate the heat or like stated could be carbon on pistons glowing causing the run on.

BUT a sure way to have the engine die right now is shot off all the air.

That rock auto part can be found on many vehicles..You apply voltage to it when the key is in the run position.
And attach it on the carb so that the plunger holds the throttle open.

When voltage is applied the plunger will come out .
Turn the key off and Zero voltage and the throttle slams shut (if you screwed the idle screw out) and the engine dies.

Basically too much throttle opening will cause it to run on.

You can increase timing for higher idle or adjust the idle mixture screws for a higher idle.
When you get it to idle higher then back off the throttle screw so the idle comes back down.. and she will usually die really quick when you shut off the key.

My 12:1 compression 350" with 245 psi cranking pressure and 24 initial timing would not idle at all when I turned the key off..Of course I had my idle set at 1100 rpm.
She just kept running .. no missing ,, it was just as if you did not turn the key off.

That Idle Stop Solenoid fixed it.
12:1 compression .028" piston to head clearance and the small 270H [email protected] .050 cam just did not want to die.
I feel that was the extreme limit on 91 pump gas and factory iron heads. I was pushing the envelope as much as I could.
 

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An MSD box without the diode installed on the alternator will make this happen also.
 

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What's your idle speed set at? If its 900-1000 rpm, try turning it down to about 650.
 

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Discussion Starter · #11 ·
Thanks for the info. on the idle stop solenoid, I will look into that to learn more. As far as the timing and idle speed numbers, I don't know I'm quite a novice at this.
 

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Tom,

If you fill out your info (location), maybe someone in your area would be able to check a couple of things out. i.e. if you were close to me I'd stop by with a timing light and tach and get your baseline settings for you. Much easier to trouble shoot if the baseline is known.
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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looks like he's in Wi, probably in a city that abbreviates as FTBG, fort something?
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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looks like Beaver Dam, Wi.
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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he was on here today:

Last Activity: Today 4:39 PM

but didn't update the thread.
 
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