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Discussion Starter #1
Can a high level of fuel in the carb contribute to pre-ignition (after turning off the key) on a very hot day?


I had problems with the needle seats not sealing (4V Holley) so I got new seats, floats (just to make sure they were not the problem) and filter. Those were the only changes. Not 100% I have the floats adjusted spot on...that's next. I've never had pre-ignition on either car...ever.
 

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Dieseling (run-on) is usually caused by hot spots in the cylinder. Glowing edges, valves, carbon buildup, whatever. Its most frequent cause is improper timing, causing things to get too hot in the combustion chamber while driving around.


Vacuum leaks can cause this too, because the mix is too lean as a result. Obviously improper carb tuning can also cause it. Are you using vacuum advance, and if so, are you on manifold vacuum or ported (timed) vacuum off the carb?
 

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Check Idle speed and timing. Make sure your engine is not overheating too. Dont just go by the gauge reading.
 

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Discussion Starter #5
Dieseling (run-on) is usually caused by hot spots in the cylinder. Glowing edges, valves, carbon buildup, whatever. Its most frequent cause is improper timing, causing things to get too hot in the combustion chamber while driving around.


Vacuum leaks can cause this too, because the mix is too lean as a result. Obviously improper carb tuning can also cause it. Are you using vacuum advance, and if so, are you on manifold vacuum or ported (timed) vacuum off the carb?
I have not changed the timing in a long time. Yes, using vacuum advance, believe it's ported. It is connected to the carb.


I have since backed off on the timing a bit.


We are moving this weekend and all the tools are packed up. - so can't check the timing.
 

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Discussion Starter #6
Check Idle speed and timing. Make sure your engine is not overheating too. Dont just go by the gauge reading.


I went thru the whole thermostat calibration routine with it in a pot of water and measured the resistance to get a curve. Then installed in the car and measured the resistance as the engine heated up and noted position on the temp gauge. Pretty confident it's not overheating. If it continues, I'll bring home an IR temp gauge from work and shoot the engine to confirm.


It was a very hot day and I suspect it may be too much timing.
 

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pre-ignition(knock or ping) is usually due to an excessive amount of timing...its unlikely too much fuel is the issue regardless of temp...too much fuel just means a rich mixture, which is going to be harder to ignite, not easier(assuming an AFR richer than say 12:1)
 

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Setting your base mechanical timing gives you the proper advance for wide-open-throttle, based on RPM. When your throttle plates are closed, very little air and fuel go to each cylinder, and even with compression, the mix will light and burn very slowly. This is why cars use vacuum advance. When there's high vacuum (very thin air/fuel mix), vac advance lights the mix sooner, allowing the combustion pressure to peak at the right time, letting the engine run smoothly.

Ported vacuum 'turns off' the vacuum advance while your car idles. It only allows advance when your throttle blades open a bit, like during cruise. As a result, if you set your timing to idle smoothly with ported vacuum, that means you've set your mechanical advance so high that it's idling well WITHOUT any vac advance. So, when you open the throttle blades (driving down the highway) and the ported vacuum adds in another 15 or so degrees of advance from your vacuum advance, suddenly your engine is very over-advanced. It will not necessarily be audible, but your engine is probably dealing with significant power loss, very high combustion pressure and temperatures, and detonation is likely.

Furthermore, at idle, if you go WOT, your mechanical advance is not properly set for wide-open throttle; it will be too far advanced. Accordingly, all the way through midrange, your timing will be too high - and that's assuming your final timing for mechanical is set where it should be.

These conditions frequently make a lot of hot spots in the combustion chamber that make engines diesel, even after the ignition's shut off.


I suspect your problem is either due to a vacuum leak, or it is a problem that has existed for some time due to ported vacuum, and perhaps is getting worse over time. Check your plugs, and see what they look like, to give yourself some more information.
 

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Discussion Starter #9
All I did was remove the carb. Pull both bowls off, replace the needle seats and floats and added a sintered bronze filter. Now all of a sudden I have pre-ignition. I'm still in the middle of moving and can't spend any time on the car.
 

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Then perhaps float levels are too low (making the carb run too lean) or the filter is restricting flow in some unforseen way. Or, you just have a vac leak under the carb.
 

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All I did was remove the carb. Pull both bowls off, replace the needle seats and floats and added a sintered bronze filter. Now all of a sudden I have pre-ignition. I'm still in the middle of moving and can't spend any time on the car.
You don't have pre-ignition. You have "dieseling" or the continued run-on of the engine without the benefit of spark. As stated before it is typically caused by high combustion chamber temperatures, hot spots due to carbon deposits and, most likely, throttle blades that aren't closed far enough at idle or a vacuum leak.

The knee-jerk reaction in such instances of "backing off the timing" only helps exacerbate the problem by raising combustion temperatures. If you didn't have to re-adjust your curb idle (increasing it) at the time you reinstalled your carb then I vote for vacuum leak or excess fuel dribbling into the throttle bores.
 
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