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Still backfiring...valves?

L

lochenjons

1/2 ton status
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Sep 13, 2006
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Location
Santa Cruz, CA
So my truck has been backfiring but only around 2000 rpm, less or more and it doesnt do it. I've adjusted air/fuel, checked timing, changed cap/rotor and its still there so I'm guessing valves. Anything else to try? And what is the exact process I'm supposed to do? I know I take the valve covers off and start the truck and tighten the bolt thats on the rockers I think untill its about to die or it sounds bad and then loosen about 3/4 of a turn or something? Something like that?
 
the way ive always adjusted mine was pop them off...start the truck LOOSEN them till they start clacking then tighten till it stops the 1/2 turn past that.

only around 2? hmm thats weird, it sounds more like a carb issue like past that the 2ndaries are opening and avoiding the problem...if the valves were out of adjustment it would get progressively worse at either lower or higher rpms...
 
broncoman6524 said:
the way ive always adjusted mine was pop them off...start the truck LOOSEN them till they start clacking then tighten till it stops the 1/2 turn past that.

only around 2? hmm thats weird, it sounds more like a carb issue like past that the 2ndaries are opening and avoiding the problem...if the valves were out of adjustment it would get progressively worse at either lower or higher rpms...
Click to expand...
The secondaries dont open at only 2000 rpm I didnt think besides if I give it more 2400 or so it wont backfire. It probably backfires at aroun 1800-2400. It doesnt mater if the truck is hot or cold.
 
Disconnect and plug your vacuum advance, make sure your using the correct firing order, check for vacuum leaks, reset your timing again (without the vacuum advance connected)... the usual stuff. Tune your carb while you're at it... set the floats, make sure your transition circuit is properly set, etc.
 
fried_guy said:
Disconnect and plug your vacuum advance, make sure your using the correct firing order, check for vacuum leaks, reset your timing again (without the vacuum advance connected)... the usual stuff. Tune your carb while you're at it... set the floats, make sure your transition circuit is properly set, etc.
Click to expand...
Firing order is correct, it wasnt but it is now:rolleyes:. I'm almost sure theres a vacuum leak (it surges at idle) but I cant find it. I've sprayed carb cleaner everywhere and over everything and tightened everything and sprayed more carb cleaner and the idle never changed. Carb, float, etc is tuned. Ill try and find the vacuum leak again I guess since I can;t check the timing again since I checked it at the shop at my school and school IS OVER!!! :D:D
 
When you set the timing did you disconnect and plug the vacuum advance? As far as the vacuum leak goes- bend your hoses all most in half. If they look cracked and dried up pretty good then replace them. vacuum line is cheap.

What is this "make sure your transition circuit is properly set"?
 
The transition circuit is a function of the carb that transistions from the idle circuit to the main circuit.

When you open the throttle plates more by pressing the go pedal 2 things happen. 1. velocity goes to crap and 2. vacuum drops. Because of those 2 things the gas that is normally atomized drops out of the a/f mixture and doesn't burn as effeciently (that's if it's not sticking to the manifold floor). The a/f is off for that fraction of a second and you need more gas or you will get a lean condition and possible a backfire.

That's what the squirter and cam are for. So as you press the skinny pedal and open the butterflies it'll squirt more gas into the carb. If you squirt too much gas you'll get black smoke out the exhaust and if you don't squirt enough... *pop* *stall* *surge*
 
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