Weber carb issue. Bogs down/backfires from carb on full pedal (1 Viewer)

[HawkGunner]

I set the timing after installing DUI HEI ignition, adjusted the valves, lean dropped the carb etc. As per instructions all over Mud.

Anyways, the 40 runs pretty well and idles nice and smooth. Doesn't overheat or anything. I can even get up and drive at highway speeds at what I consider a reasonably good pace in a 40 year old suped up tractor.

The issue is that if I give it any more than about half or 3/4 pedal, the motor stutters slightly and if I gas it hard and quick it will backfire out of the carb.

Backfiring from the carb is normally a sign of a lean mixture, but it doesn't do it unless I go hard on the pedal. I checked for vac leaks and found none, and I'm fairly confident I set the carb properly.

Could it be I need bigger jets? Is my secondary not opening properly? Are my jets crudded up?

EDIT: Additionally, the problem presents itself regardless of how far advanced or retarded the timing is (within reason/ from 7° to a bit more), and doesn't seem to get any better or worse.

I've also poured a bottle of seafoam in the tank last time I filled it up, hoping to clean out any small bits. I'll probably run a second bottle through after my next fill up.

 

[brian]

Any fuel pressure regulation?

 

[HawkGunner]

Any fuel pressure regulation?

I don't think so, but I havent changed the filter. Maybe that's it?

Now that you bring it up, it does seem a little like the begginings of a clogged fuel filter.

 

[Pighead]

I think what Brian is asking is, do you have a fuel pressure regulator between the fuel pump and the carb?

 

[HawkGunner]

I think what Brian is asking is, do you have a fuel pressure regulator between the fuel pump and the carb?

Ah, no. I don't. The line runs straight from the pump around the front of the head to the carb.

 

[Pighead]

I have heard that some of those weber carbs need fuel pressure regulated down to around 3 psi, and that a fuel pressure regulator will do just that. You might want to research that.

 

[HawkGunner]

I have heard that some of those weber carbs need fuel pressure regulated down to around 3 psi, and that a fuel pressure regulator will do just that. You might want to research that.

That's interesting, thanks! I'll research and try to get it nailed down.

If nothing else, I'm swapping for a Holley Sniper in a few months.

 

[David70FJ40]

My Weber did the same thing, stumbled then backfired through the carb when climbing a hill at highway speed. Removed the old gas lines and filters, three, removed the old fuel pump and added a new filter, electric pump and regulator. The gas tank is a new one I installed three years ago from BTB Products. The original tank was suspect at best so I removed it. (The instructions with the fuel pump stated Weber carbs need no more than 3 psi.) It ran pretty good for a little while then started to flood out at idle and was near impossible to restart. Found that one of the jets at the top of the fuel bowl had backed out and was laying in the bottom of the bowl, the other was loose and even one of the jets in the bottom of the bowl was about 3/4 turn loose. We have adjusted it and adjusted and adjusted, still idles rough and stumbles momentarily at highway speed when given additional pedal to climb a hill.

Timing is correct, vacuum lines are in place but the carb is still stumbling. The rebuild kit/tune-up kit I purchased was installed by the instructions with a new float. The inlet needle set was entirely different than what was in the carb originally and the new set would not allow the bowl to fill so the old one was reinstalled. I did call the Redline technical line who handles Weber and the call hasn't been returned yet.

Dumping the Weber for a Holley 350 cfm carb, an American carb someone can actually work on.

 

[Pin_Head]

Yes. Sounds like it is lean on acceleration.

It could be the wrong jets, but this is not likely if the carb was originally set up for your engine or it used to run fine.

It could be a plugged jet or jets. You would have to take the top off, remove the jets and check.

Depending on which Weber you have, it may be a synchronous two barrel, where both throttles open simultaneously, like on the 38 DGES.

 

[roadstr6]

My Weber did the same thing, stumbled then backfired through the carb when climbing a hill at highway speed. Removed the old gas lines and filters, three, removed the old fuel pump and added a new filter, electric pump and regulator. The gas tank is a new one I installed three years ago from BTB Products. The original tank was suspect at best so I removed it. (The instructions with the fuel pump stated Weber carbs need no more than 3 psi.) It ran pretty good for a little while then started to flood out at idle and was near impossible to restart. Found that one of the jets at the top of the fuel bowl had backed out and was laying in the bottom of the bowl, the other was loose and even one of the jets in the bottom of the bowl was about 3/4 turn loose. We have adjusted it and adjusted and adjusted, still idles rough and stumbles momentarily at highway speed when given additional pedal to climb a hill.

Timing is correct, vacuum lines are in place but the carb is still stumbling. The rebuild kit/tune-up kit I purchased was installed by the instructions with a new float. The inlet needle set was entirely different than what was in the carb originally and the new set would not allow the bowl to fill so the old one was reinstalled. I did call the Redline technical line who handles Weber and the call hasn't been returned yet.

Dumping the Weber for a Holley 350 cfm carb, an American carb someone can actually work on.
View attachment 1722216

Might I suggest a City Racer carb? I love mine! Bolt on. Adjust mixture screw and idle screw and you are done. No more fiddling. Rock solid carb.

 

[HawkGunner]

Yes. Sounds like it is lean on acceleration.

It could be the wrong jets, but this is not likely if the carb was originally set up for your engine or it used to run fine.

It could be a plugged jet or jets. You would have to take the top off, remove the jets and check.

Depending on which Weber you have, it may be a synchronous two barrel, where both throttles open simultaneously, like on the 38 DGES.

Its not a synchronious carb. The secondary doesn't actuate until the throttle linkage is rotated about half way or thereabouts.

I'll pull the top and clean out the jets. Any particular method you recommend? A sewing needle or drill bit, or something along those lines?

Might I suggest a City Racer carb? I love mine! Bolt on. Adjust mixture screw and idle screw and you are done. No more fiddling. Rock solid carb.

Unless "city racer carb" also means fuel injected throttle body.... im not terribly interested.

As much as I drive my 40, my next major move will be to swap the carb for a Holley Sniper. Provided that I can keep this Weber running until September or October.

I might get a city racer carb as a backup if the Holley takes a dump though. How much are they?

 

[roadstr6]

Its not a synchronious carb. The secondary doesn't actuate until the throttle linkage is rotated about half way or thereabouts.

I'll pull the top and clean out the jets. Any particular method you recommend? A sewing needle or drill bit, or something along those lines?



Unless "city racer carb" also means fuel injected throttle body.... im not terribly interested.

As much as I drive my 40, my next major move will be to swap the carb for a Holley Sniper. Provided that I can keep this Weber running until September or October.

I might get a city racer carb as a backup if the Holley takes a dump though. How much are they?

Hawk, I was actually suggesting the City Racer carb to @David1947 who said he was going to go with a good ol' American made Holley 350. Edit: I just remembered that he has an F engine. I guess he'd have to convert the linkage to the later style to make that work. FWIW the City Racer carb is $285.

Aftermarket 2F / 1.5F Carburetor for Land Cruiser FJ40 FJ60 - Made in Japan

 

[HawkGunner]

Hawk, I was actually suggesting the City Racer carb to @David1947 who said he was going to go with a good ol' American made Holley 350. FWIW the City Racer carb is $285.

Aftermarket 2F / 1.5F Carburetor for Land Cruiser FJ40 FJ60 - Made in Japan

Gotcha. Even so, $285 is a good price for a carb that works as advertised! I'll probably get one as a backup if it works as good as you say.

 

[Pin_Head]

I use a magnifying glass to check for obstruction and use nylon monofilament line to clean and carb spray cleaner to clean them out.

 

[HawkGunner]

I use a magnifying glass to check for obstruction and use nylon monofilament line to clean and carb spray cleaner to clean them out.

Thanks! I have both on hand. I'll take care of that this afternoon.

 

[Solace in Solitude]

As others have said, it sounds like a fuel delivery issue.

FWIW,

1. My DUI performs magnificently, with DUI LiveWires, timing at ~12*BTDC and Denso (3013) W14EXR-U Traditional Spark Plugs, gapped at .055”.

Installing my DUI also eliminated a vacuum leak (vacuum advance module), with the oem dissy, that caused significant power loss, as altitude increased.

2. I had the stumble-and-die issue early on, when under load (uphill with pedal buried)... it was presenting when the Aisan carb secondary kicked in, on my ‘78 2F.

Turned out to be a glass fuel filter (replaceable element) not sealing entirely... allowed mass quantities of air to be sucked in, when the secondary kicked in.

 

[HawkGunner]

As others have said, it sounds like a fuel delivery issue.

FWIW,

1. My DUI performs magnificently, with DUI LiveWires, timing at ~12*BTDC and Denso (3013) W14EXR-U Traditional Spark Plugs, gapped at .055”.

Installing my DUI also eliminated a vacuum leak (vacuum advance module), with the oem dissy, that caused significant power loss, as altitude increased.

2. I had the stumble-and-die issue early on, when under load (uphill with pedal buried)... it was presenting when the Aisan carb secondary kicked in, on my ‘78 2F.

Turned out to be a glass fuel filter (replaceable element) not sealing entirely... allowed mass quantities of air to be sucked in, when the secondary kicked in.

My DUI performs flawlessly, and is set up about like yours. My plugs are only gapped to .050 though. I figured I'd try that and see how it ran, then open it up another .005 if I felt it necesarry. In any case, the DUI HEI was a significant upgrade in terms of drivability. I even went from 10mpg to 13, which is probably more an effect of a working and efficient advance mechanism than anything else. Nonetheless, I love it so far.

I'm fairly certain this is a carburetor/fuel delivery issue though. Currently, it drives well enough for me to ignore it (especially since I'm fixin to do a TBI upgrade). So long as I remember not to hit the throttle too hard.

The honest truth is that I want to fix it because it's just not the way it should be, and I'm a stickler for those kinds of things hahaha

 

[David70FJ40]

Purchased a rebuilt Holley 2 bbl from Downey with his extras, bowl baffle, tall chimney, extra float bowl gaskets, throttle cable and mounting bracket and the all important carb adapter mounting plate. As much as I respect Roger and even though I have purchased a lot of stuff from him, the price for the Holley with the extras including shipping was enough less that I had to do it. Maybe next year I can manage the Sniper TBI.