LUVTruck.com

When I let off the gas I get a muffled backfire through the exhaust. It is starting to get more louder as time goes by. Is it caused by timing being off or to much gas.

could be burning rich or not enough back pressure in your exhaust.

_________________
1980 luv 1.8 finally running!!!...... and then like a moron i sold it
got another one!!! 1980 mikado. v8 swap here we come... nope divorce and cross country move happened.
got another one!!! 1980 4x4 mikado sport. rusty beater for now...

I would investigate your timing!!!!!!

check your EGR valve

Easiest would be to check the timing then adjust the carb. Also check out your spark plugs, if they are fouled they can cause a backfire. Then check for vacuume leaks as they can do some wierd stuff too.

_________________
1980 LUV 4x4, going under re-construction...

Easiest and cheapest thing to do before the other mentioned things would be to check the condition of your exhaust and the flange where it joins the manifold.

If the exhaust has a hole oxygen can get into the exhaust, oxygen allows unburnt fuel in the exhaust to ignite creating explosions in the exhaust and thats what a backfire is.

Get someone to stand at the back of the ute and cover the tailpipe over with a rag, then you climb under and check for leaks while they hold the rag over the exhaust. If you cover the tailpipe and the motor stalls youve got no leak, if there is a leak the gas will exit elsewhere and you should be able to find the leak.

My utes got a leak and it does it, nice rally car burble when you back off the gas, the more revs the more it burbles. If i hook up the air injection pump people hit the deck for fear of drive by.


Cheers, Bob.

_________________
Current project, 1979 KB Isuzu Ute.

Daily driver, '86 Rodeo Spacecab.
Chasing parts for Isuzu Impulse.

I had the same problem. All I did was tightened down the bolts going from the carb to the manifold and it stopped.

First and foremost, check for vacuum leaks. The problem has slowly gotten worse as time went on which sounds like an air leak that has gotten worse.
Any motor is suspect to vacuum leaks and all the tune ups in the world cant get rid of them.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

Seems like popping in the exhaust was a symptom every time I've had a problem with the air injection pump or its control valve. I noticed it especially between shifts when accelerating. It could be running rich, getting an exhaust leak, or just need a tune up too though. That is a tough symptom to pin down on any one thing for certain.

_________________
95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)

MEPR: Man, my 4x4 makes all other LUVs look good

Just wondering if I need to rebuilt my carb. It has been flooding out real bad and seem to be running rich which maybe causing the muffled backfire. I have idle screw set at 3 turnouts. If I put it at 2 turnouts it acts like it is starving for fuel.

Forgot to say that I have a weber carb with manual coke.

3 turns out is too much. The motor is drawing in extra air from somewhere. If you can, remove the mixture control valve and cap the big line going to the intake. If the motor smooths out, the problem is within the smog system. If it still runs funny, you most likely have an air leak somewhere at the intake manifold.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

Forgot to say that I have a weber carb with manual coke.

It runs rough when it first starts up when it is cold even if it is coked, but when it is warm it idles pretty smooth.

Small air leaks can idle smooth with the mixture screw out a little more than normal, but it can be even smoother.

Check for air leaks...

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

I would have to say the diverter valve, my 79 and 78 that I owned did that and the diverter valve was at fault.

_________________
Certified pilots, looking down on people since 1903.

Just wondering if the carb is getting to much fuel. When cold it would flood out real easy. Bad enough I would have to disconnect the fuel pump so it would not keep feeding fuel to the carb so I could get it started then when it was running I would have to plug back in the pump.before it would run out of gas. I have a napa fuel pump with a regulator set at 3 pounds. Not to sound dumb but what is a diverter valve.

One more note to add. The fuel pump is hooked up to the original wire that is connected to the fuel pump relay but it would still keep running with the engine not running that would cause the flooding so I had to put a oil pressure sending unit in that I ran the ground wire thru so the pump would only run with oil pressure present

If the carb was getting too much fuel, it wouldn't run at all once the motor warmed up. If the motor now runs ok once it's warmed up, the fuel pump is working just fine at 3 p.s.i.
It sounds like you have multiple problems with the motor.
A diverter valve (in this case it's called a mixture control valve) is a component that looks like a motorcycle carb located between the smog pump and the intake. These components are common to fail after time.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

Would you be talking about the air bypass valve. Mine doesn't have a mixture control valve . That was on the calif. model only.