[moshbmxer2002]

I have a 1987 w150 with a 318 and i put a four bbl carb and intake on it and hooker headers and when i cruise down the road and the let off the throttle it back fires bad and sometimes stalls. And now it backfires all the time when just idleing and adjusting the carb now doesnt seem to do much at all. any help would be appreciated thanks.

 

[DiverDwnPowrRam]

I had a truck that would do the same, there are many causes for this but mine would backfire even at idle...come to find out one of the valves was burnt so bad it had a "V" cut into it.

 

[1972-D100]

It could be a burned valve, but it could also be a bad bushing in the distributor, a loose distributor, or a stretched timing chain. Most of the time a backfire is a timing issue, did you adjust your timing to match the new carb, intake and headers?

 

[B-300]

To lean of an idle mixture will cause backfire on deceleration. This is hard on the exhaust valves as a lean mixture burns slower and is still burning over the exhaust valve when it opens.
Also recheck the header bolts as the gaskets crush and will let air into the exhaust causing unnburned fuel to ignite just like the AIR pump system does.

 

[moshbmxer2002]

alright thanks. I think it might also be the fuel pump. so I'm gonna try that.

 

[DiverDwnPowrRam]

So you took the line apart and measured the flow?

 

[Speed Dragon]

Could be a worn out rocker arm also.

 

[3834B]

Are you still running a smog pump? Cross a plug wire? I assume it ran OK prior to disassembly?

 

[moshbmxer2002]

no i didnt check the flow but i figured since it had a two barrel the pump is smaller so i was going to get one for a 4 bbl. I dont have the smog pump anymore either and all the plug wires are good.

 

[3834B]

What size carb? Did it have issues prior?

 

[B-300]

Not likely a fuelpump or it would hesitate/ping when when you accelerate... The 2 and 4bbl pumps are the same.

 

[1972-D100]

The fuel pump for a stock 4bbl is the same one for the stock 2bbl... Unless you went huge with your carb, your fuel pump will most likely not be the culprit. Did the truck backfire before you did the swap? If not, then you probably have either a timing or vacuum leak issue. If it was doing it before the swap then Speed and others would be correct about the burned valves and or worn rocker arm....I'm betting it's your timing.

Anyways what Carb did you install?

 

[moshbmxer2002]

I put a carter thermoquad carb on. And it wasn't backfiring before i swapped it out and i adjusted the timing and i got it to the best way it would run and it still does it and i adjusted the carb to wear its all the way rich and it runs the best but it still acts like its not getting enough gas.

 

[1972-D100]

I put a carter thermoquad carb on. And it wasn't backfiring before i swapped it out and i adjusted the timing and i got it to the best way it would run and it still does it and i adjusted the carb to wear its all the way rich and it runs the best but it still acts like its not getting enough gas.

Was it a used or remanned Thermoquad? They have a bad way of having leaks on the plastic part of the carb, they get a little warped and will never seal again.

 

[Speed Dragon]

Yeah it's probably a carb issue.

Edit: Although, 4bbls take a good bit more timing than the 2bbls sometimes. Try setting the timing up to around 14 or 16 and see if it gets better (assuming you have it at the normal 10-12 degrees initial now).

 

[3834B]

Try setting the timing up to around 14 or 16 and see if it gets better

While base timing is a start, total timing needs to be checked.

He states it acts like it is running lean. This sounds like a vacuum leak.

 

[DiverDwnPowrRam]

try putting a vacuum guage on it and see what the needle is doing.

 

[moshbmxer2002]

alright ill try that also

 

[Speed Dragon]

While base timing is a start, total timing needs to be checked.

He states it acts like it is running lean. This sounds like a vacuum leak.

Improper timing can cause low vacuum also

 

[3834B]

adjusting the carb now doesnt seem to do much at all.

It's been my experience that when your carburetor adjustment nets zero gain it tends to be vacuum leak related.

Improper timing can cause low vacuum also

Vacuum leak versus "low vacuum" are two different entities.