Backfiring through carb in F engine. (1 Viewer)

[Cleg]

I am experiencing backfiring through the carburetor on a recently re-assembled 1972 FJ40 with stock F engine. Any suggestions on what to try to eliminate the problem would be appreciated.
Pardon the long essay but I want to provide enough info for troubleshooting.

The timing has been set at 7 degrees BTDC at idle with the vac line to the advance diaphragm disconnected and plugged. The vac line runs to the vacuum port at the base of the carburetor just above the spacer plate.

The engine starts easily, with the choke when cold and by only turning the key when warm.
If I try to drive before it warms up it backfires badly & stalls.
With the choke left partway on it can be driven immediately.
Once warm it backfires about 1 or 2 times per mile. It most commonly occurs when running at almost no load at around 35 MPH (level ground) or when the engine starts to labor when climbing a hill.
Other than that the engine develops decent power & is responsive. It does not seem to backfire through the exhaust.

During the rebuild I decided not to mess with the block or head because it compression tested OK.
Everything that bolts to the long block was however rebuilt or replaced.

Factory rebuilt vacuum advance distributor assembly from drive gear up to cap, (without octane adjuster).
New ignition coil & ballast resistor, stock type but not OEM.
New plugs & wires. Old plugs looked healthy with light gray ash coating.
New OEM Aisin Fuel pump.
Stock 2 barrel carb rebuilt but not professionally.
New fuel filter & fuel lines from tank to carb.
Fuel tank was removed & flushed during rebuild.
Intake & exhaust manifolds, removed, blasted, machined flat while bolted together as a matched set & reinstalled with new gaskets. 99.9% sure there is no air leak at manifold gasket or carb base.
Professionally rebuilt brake booster.
Heat riser butterfly valve was replaced but without the bi-metal coil. For now it is fixed in the closed position, blocking heat to the intake base.
All new cooling system radiator & water pump.
Fuel is untreated currently mix of 87 & 93 octane, with ethanol.

The emission control system that was on it consisted of the evaporative loss tank behind the middle front seat, canister in engine compartment on the passenger side VSV solenoid box adjacent to carb on driver’s side, “computer” by steering column & associated piping for all. There was no air pump. I don’t know what belongs on it.

Too much air was getting into the intake through the VSV making it impossible to get a normal idle speed. Almost all is bypassed now with just 1 vacuum line to the VSV. The line has a restrictor plug in it with a .090” diameter hole drilled through it so it can pull some fumes out of the evap system.

Advancing the spark reduces the backfiring but makes cold starting difficult.
Setting it later than 7 degrees makes the problem much worse.

What do you think ???

Thanks.

 

[Waterboy]

Check your firing order. Sounds like plug wires are on in correctly.

Sent from the islands.

 

[1911]

If the timing is good and the plug wires are on correctly, then a backfire through the carb is almost always the result of too lean a mixture. Most likely an intake/vacuum leak, but possibly the carb jets in the wrong location or jets of an inappropriate diameter.

 

[Pighead]

What do you think ???

I think it sounds like you have been very thorough, and it's probably just a minor detail somewhere...
How many times have you adjusted your valves since reassembly? Do double-check your sparkplug wires firing order. Could you perhaps have mixed up the main jets in the carb? How about just running without the VSV?

 

[bgray]

Have you had this issue since rebuilding your carb or is it something new? If it ran fine after your carb rebuild and this issue is more recent, then I would suspect a vacuum leak.

I had an issue with my rig running lean at idle (didn't notice a problem it driving speeds) and it turned out to be a small leak in the PCV hose where it connects to the base of the carb.

Good luck.

Brian

 

[Pin_Head]

It sounds like it is lean on transition from partial throttle to accelerate.
Check:
Accelerator pump function.
Idle vacuum (could be a manifold vacuum leak and you are compensating for this by opening the throttle plate so it will pull fuel out of the main nozzle. )
Check for fuel flow from the main nozzle at idle. (If you see flow, it is not a good sign.)
Check the operation of the idle fuel cut off solenoid.

 

[Cleg]

Thank you all for the input.

I only drove the truck a few miles before taking it apart for the rebuild.
At that time air was leaking into the manifold from everywhere so I have no baseline of how the carb might have worked before I took it apart.
Mixing up the jets during the carb rebuild? Crazier things have happened. Thanks, I'll check.


The consensus seems to be that the mixture is probably too lean.

Is there an adjustment for the mixture? It is my understanding that the idle mixture adjustement has no effect on how it runs when the throttle is open.

I will also plug off the last of the VSV to see if that helps.


Question: Should there be enough vacuum at idle to advance the distributor when the vac line is reconnected after timing?

For the follow up questions above:
The idle fuel cutoff solenoid is working and moving freely.
I've never adjusted the valves on it.
The plug wires are connected correctly.
The accelerator pump w new piston was working properly at the time the carb was rebuilt but then it sat in a box for 2 years during the rebuild.

I'll recheck all of the above.

Thanks again

 

[Pin_Head]

Mixing up the jets during the carb rebuild? Crazier things have happened. Thanks, I'll check.


The consensus seems to be that the mixture is probably too lean.

Is there an adjustment for the mixture? It is my understanding that the idle mixture adjustement has no effect on how it runs when the throttle is open.

Only the idle mixture is readily adjustable and it has increasingly little effect as you open the throttle. You would need to change the slow jet or main jet to a bigger size to make it richer at transition. The secondary jets are usually larger, so mixing up the jets would make it rich and not lean.

Since this is all new, I would start with a complete tune up by the book, including valve adjustment, compression check etc.

 

[Cleg]

Elsewhere I read that running rich could also cause a backfire through the carb but I am having trouble understanding why. In an older post there was a suggestion of adjusting the float level to ensure that the fuel pump was not forcing fuel past the float valve. I've got to go back to the book to see what the jet sizes should be.
If I remember correctly there were spare jets stored inside the carburetor, I assume for swapping out for running at a higher altitude.

It has been 2 years since I worked on the carb so I do not remember the details but the venturi assemblies were separate small bridges. Did these also have different sized ports that I could have swapped?

 

[1911]

Elsewhere I read that running rich could also cause a backfire through the carb but I am having trouble understanding why.

Me too! If it backfires because it's rich, it will backfire in the exhaust because there is un-burned fuel there. But you have to be careful because backfire in the exhaust is more commonly caused by leaks in the exhaust manifold or header, or retarded ignition timing.

 

[MoCoNative]

Look in the carb and see if the acclerator pump is consistently squirting out a good stream.

If you rebuilt it and it sat, the leather cup on the end may have dried up. The fresh gas may get it swollen back out, but I have found that the rebuild kits have poor quality cups in them. I did a rebuild years ago, stuck it all back together. I had a vacum leak on the PCV line at the base of the carb and the accelerator pump was pretty weak. Both issues were causing bad backfiring through the carb and rough idle. Fixing the vacum leak cured the rough idle, but it would still pop off during transistion, would also kind of bog down and stall on a quick acceleration.

Long story short, pulled the cup out, flared it out more by hand, and soaked it in a little light oil. That seemed to create a better seal in the bore and a more reliable stream of fuel from the squirter. That stopped the lean backfire. Just being cold can cause the backfiring too until everything is totally warmed too. But I would check the accelerator pump out.

 

[Pin_Head]

Elsewhere I read that running rich could also cause a backfire through the carb ?

Not true. It might cause backfiring through the tail pipe.

BTW: Nice looking engine compartment. I'm sure the rest matches. Looks like it came right off the dealer's lot.

 

[67FJ40]

backfire through carb

I was having backfiring up through the carb really bad last year and it ended up being either a couple bad (new) spark plugs or i put too much anti-seize on them and they were not grounding to the block. Not sure which one it was, but after replacing the plugs it runs perfect. I ended up putting a timing light on every plug wire and watching it as i reved the motor. I could see the light miss when it backfired and knew what cylinder/cylinders it was doing it on. I have a thread on it if your bored and like to read. I know it sounds crazy but sure enough its fixed.

 

[Cleg]

I plugged the last remaining line supplying vacuum to the VSV,
Re-checked the distributor & wires to make sure all was tight and there was no sign of arcing inside the cap & no plug wires laying parralel against each other that might deliver a spark to the wrong cylinder.

I did not realize the venturi assemblies were specific to primary & secondary. The primary has a .350" ID and the secondary only .010" bigger at .360". Measuring both through the top of the carb I can confirm they are indeed in their correct bores. The main barrel has a coating of black soot on it from the backfiring.

The accelerator pump delivers a good squirt of fuel, maybe a little weak at top of the stroke but I think this is not the culprit. Took it for an 8 mile round trip to the dump and it backfired about 3 times, all while rolling along at about 35 - 40 MPH in 4th gear on level road. The accelerator at this point was being held in a steady position to maintain that speed... no accelerator pump involved.

I'll pull out the carbon-eater and partially disassemble to clean & measure the Jets.

Blasted a can of carb cleaner through the venturi with it running.
This new eco-friendly carb cleaner doesn't remove squat !!!!

I may swap on another coil & try that first.




Blasted some carb cleaner down

 

[Cleg]

Ran across my own old thread but now armed with the knowledge of the rest of the story.
Turns out is was in fact an air leak.
The gaskets between the carb base plate & manifold were not completely sealing. I had re-used the old gaskets figuring there was so much contact area there that it would not be a problem.
Cut 2 new gaskets at a cost of 10 minutes and $0.00 and the problem was gone.