Engine backfires at decel and stop, almost dies at stop... need help figuring out what this could be. (1 Viewer)

[cvan1971man]

Hello Mud’rs,

Got a 1973 FJ40 problem. My son came home for Easter and just headed back to college. When he pulled off to get gas at the college town, he said the engine started backfiring at deceleration and backfiring while stopped at a stoplight.

He said when at a stop, the engine would almost die like it was not getting gas, and when he would give it gas, it would respond as if he gave it a ton of gas.

Also while at a stop, there is a whine, almost like a vacuum whirr sound. He said he did not see any hoses off.

Unfortunately I am no where near where he is.

• 1978 2F Engine
• Pertronix Electronic Ignition
• Pertronix Flame Thrower Coil
• Holley 2BBL carb
• Desmogged

I cannot think of any other modifications that should be added to this list.

 

[Pighead]

Probably it's something simple and easy like a loose hose clamp on a big vacuum line or a loose dizzy hold down bolt. But somebody's gonna have to get their hands dirty to find out what.

 

[cvan1971man]

Probably it's something simple and easy like a loose hose clamp on a big vacuum line or a loose dizzy hold down bolt. But somebody's gonna have to get their hands dirty to find out what.

I didnt think about the distributor moving. Do you think it could be that?

 

[Pighead]

I didnt think about the distributor moving. Do you think it could be that?

It could be any one of a dozen things out of whack. Putting hands on things, like the dizzy, often helps me diagnose simple things.

 

[Engineer8000]

Brake booster diaphragm bad, can cause large vacuum leak when stepping on the brakes.

 

[Downey]

A blown power valve in the Holley could sound like that (loading up on fuel, never stopping the flow into the float bowl), but you’d have known about that way before now.

 

[cvan1971man]

any other ideas

 

[brooklyn]

Good advice so far — I was stumped for a while with a similar issue - checked for vacuum leaks and while leaning into engine bay feeling around I put some weight on the carb and the idle changed— carb mounting bolts were loose.

 

[MoCoNative]

I used to have issues like this when I ran a Holley. How is the brake booster line, and PCV line attached to the manifold? Usually the brake booster is attached in the rear runners, usually not an issue. As for the PCV, the Holley has its own nipple in the base for a PCV line. Toyota ran the PCV into the side of the manifold on the 2f manifolds or into a nipple on the phenolic carb spacer on the 1F engines, and in 1973. How is the carb mounted, with the 2F carb spacer and heat shield, or was the simpler 73 1F spacer used?

On mine, I had the nipple on the spacer capped off with a rubber cover and I used the Holley nipple. Every several months or so the craptastic rubber cover would rot out from the heat and gas fumes and develop a horrendous vaccum leak. A little backfire would cause it to fail faster. Only shows itself when you slow down at the stop sign or idling with poor idle or wanting to die, then you give it some gas and you will get a lean carb backfire. My final soloution before going back to Toyota carb system was a short piece of 3/8 hydraulic hose (never rots off like fuel hose) and bolt in the end with a hose clamp, no more surprise failures.

If you have this possible scenario, the cracked plug may not be evident at first glance.

 

[cvan1971man]

Turns out it was caused by a massive 4" long split in the hose that goes from the PCV valve to the back of the carburetor.

Thanks for all of your ideas and help