Bucking from fuel delivery problem (1 Viewer)

[Driftingcloud]

All,

After having Jeeps my whole life, I finally got my first FJ40 (1978 NAS Mustard) that’s almost 100% stock. I absolutely love it, but have already encountered an issue.

The truck idles well and is fine under acceleration, but starting at about 40-45 mph when cruising steadily in third or fourth gear, it’ll begin bucking violently. It’s also especially noticeable pulling a hill at speed and requiring near full throttle to maintain my speed. Once it starts bucking, I have to lower my speed and gear on straightaways or finally get past the hill (usually in first or second gear and at a crawl).

It’s definitely related to fuel, like getting fuel starved. I’ve changed the fuel filter to no avail, and I’m contemplating the next steps. Does this sound like a vacuum advance issue? Any other thoughts or ideas?

Thanks for your time and experiential wisdom!

 

[rkymtnflyfisher]

Fuel level in the sight glass?


When it starts bucking, immediately push in the clutch and turn off the engine, once you're stopped check the fuel level in the sight glass on the carb.

Vacuum leaks?

Check for em.


Pull your oil dipstick and sniff it, make sure it doesn't smell like raw gas.

 

[Driftingcloud]

Fuel level in the sight glass?


When it starts bucking, immediately push in the clutch and turn off the engine, once you're stopped check the fuel level in the sight glass on the carb.

Vacuum leaks?

Check for em.


Pull your oil dipstick and sniff it, make sure it doesn't smell like raw gas.

I’ve got a good smoker for testing, just haven’t gotten there yet. I also haven’t tried checking the sight glass. Thanks for the ideas!

The truck was bare metal fully restored about 9 years ago, but has probably had less than 1,000 miles on it in that time. I’m guessing there are still some sorting issues.

 

[rkymtnflyfisher]

Start with the sight glass, it can tell you where to start looking.

 

[Driftingcloud]

Start with the sight glass, it can tell you where to start looking.

Thanks!

 

[777Gage]

@rkymtnflyfisher is correct with the diagnosis of your symptoms

I just had something very similar happen to me a few days ago. Ran fine on flat ground, but when I hit an incline or under more throttle, it would "stumble/buck or felt like it was choking".
Limped home and checked everything mentioned above but found something different. My fuel supply line had a small kink in it and under load or incline it was being further bent and fuel restricted.
So I ordered and properly mounted a new fuel filter and supply line - Fixed my problem
My issue pictured below:

 

[Driftingcloud]

@rkymtnflyfisher is correct with the diagnosis of your symptoms

I just had something very similar happen to me a few days ago. Ran fine on flat ground, but when I hit an incline or under more throttle, it would "stumble/buck or felt like it was choking".
Limped home and checked everything mentioned above but found something different. My fuel supply line had a small kink in it and under load or incline it was being further bent and fuel restricted.
So I ordered and properly mounted a new fuel filter and supply line - Fixed my problem
My issue pictured below:

View attachment 2319126

Thanks! I really appreciate all the insights.

 

[Advent]

What’s the inside of the gas tank and fuel filter look like? I ended up replacing my tank for the same problem - so much rust was plugging the filter that it would starve for fuel. It got REALLY bad before I replaced the tank; I was changing fuel filters at least once a week.

 

[Driftingcloud]

What’s the inside of the gas tank and fuel filter look like? I ended up replacing my tank for the same problem - so much rust was plugging the filter that it would starve for fuel. It got REALLY bad before I replaced the tank; I was changing fuel filters at least once a week.

That was my first thought. The fuel tank is very clean, though, and nothing looked out of normal on the fuel filter I replaced.

 

[samatulich]

Check and/or replace your fuel pump.