Backfire

[DatDoug]

Problem: Truck backfires when letting off accelerator to shift. Runs and drives fine other wise. Its all stock 76 620 with L20b. Any thoughts comments.....??

[banzai510(hainz)]

those still still have points?

 

Condensor?????????

 

adjust the mixture and timming again.ck the vlave lash

 

water under dizzy cap?

[ariascarlos1990]

your anti-backfire valve might be out.

[]2eDeYe]

A small exhaust leak could cause that as well.

[DatDoug]

This is why i take off the smog crap and go with a weber. There are too many things that can go wrong on the stock systems! It does have points and the condensor is brand new. I just put on new muffler and tail pipe. It only does it when letting off the accelorator to shift. I will check timing next.

[datsun dreamer]

I had this same problem and It turned out to be my points. My truck backfires or I like to call it "pops" 'or "raps" cause I got a '18 Glasspack installed. I noticed more of a "hesitation" when accelarating than anything 'cause of pitted or burnt points.

[datzenmike]

your anti-backfire valve might be out.

 

Too right. Either missing, not working or the vacuum signal hose is off.

 

That 'smog crap' you removed probably included this important valve, which by the way has no effect on running performance, and only operates on deceleration. It corrects an over-rich condition that occurs on sudden closing of the throttle, like shifting. The fuel does not completely burn in the cylinder and will explode in the hot exhaust manifold, causing a backfire. The valve senses a high vacuum and draws filtered air from the air filter and directs it into the intake manifold to lean out the mixture.

 

The Smog Nazi

[DatDoug]

Too right. Either missing, not working or the vacuum signal hose is off.

 

That 'smog crap' you removed probably included this important valve, which by the way has no effect on running performance, and only operates on deceleration. It corrects an over-rich condition that occurs on sudden closing of the throttle, like shifting. The fuel does not completely burn in the cylinder and will explode in the hot exhaust manifold, causing a backfire. The valve senses a high vacuum and draws filtered air from the air filter and directs it into the intake manifold to lean out the mixture.

 

The Smog Nazi

 

Its all still there! I even took off the a.b.v. and put another one on from a previous tear-out. It still does the same thing. Like dreamer says it poops.

It sounds like it mite b the problem tho.

[datzenmike]

There should be a tiny hose from it to the intake to sense high vacuum. If it's off it won't work.

 

Maybe the Weber is over rich to begin with?

 

Adjust idle mixture a lean as poss and still idle good.

 

Turn the idle speed up 100 RPM.

[DatDoug]

I think you have been misinformed Mike. It is an all stock set-up including Hitachi carb.

[banzai510(hainz)]

adjust the pointS AGAIN.

 

good to know if this all of a sedden happened or was over time.

Here is why a eleltronic dizzy is the best.

Point dist can give alot of proplems over time, point gap dwell and bearing wear.

 

see if the shaft moves side to side. this might cause the points to open prematurely. still is a worn bushing inside. try differnt point gap settings

[DatDoug]

I fixed my backfiring problem. I cut the belt running the smog pump. No more

Backfire!:D