'77 FJ40 stumbling just off idle (1 Viewer)

[Fort Knox]

1977 FJ40. Replaced the stock carb with a Trollhole carb about 18 months ago. The problem actually started with the stock carb, but has continued after installing the THC carb.

Starts fine. Idles fine. For the most part, runs fine once it's moving. The problem is taking off from a stop when idling. When I hit the gas from a stop at idle, it tries to cut out or stumbles badly. If I pump it a little to get the rpm's up before letting out the clutch, it's good and then no problem at all going through the gears all the way up to "highway speed". It does this whether the engine is cold or hot. It does occasionally stumble a little if I slow down to take a turn and the rpm's get down low while in gear, then when I go to get back on the gas going through the turn it will buck a little.

I tried the carb replacement, which seemed at first to help, but quickly started exhibiting the same issues. I've replaced the fuel filter under the hood a couple of times, which seems to help a little at first, but not for long. My first thought is "crud in the gas tank" or fuel pump, but I would think that would affect it at other RPM's, not just right off idle. Site glass seems to show the fuel level right about in the middle most of the time.

Thoughts anyone?

 

[middlecalf]

Seems like a plugged or faulty accelerator pump, but you did swap out a new carb and still have issue. Do you see a squirt of gas in the Venturi when the pedal is depressed?

 

[Fort Knox]

Seems like a plugged or faulty accelerator pump, but you did swap out a new carb and still have issue. Do you see a squirt of gas in the Venturi when the pedal is depressed?

Yep, new THC carb, same issue. I'll have to check for the squirt.

 

[Fort Knox]

Bump.

 

[Travis 76]

Could be stuff in the gas tank. My truck had bad gas the fine particles went through the filter and clogged the carburetor. Drained the tank new filter and cleaned out the carburetor and it has been fine since. I thought it was a timing issue, chased all kinds of possibilities. Truck would run, just not well.

 

[Fort Knox]

Thanks Travis. It's strange. I would think that a clogged carb would cause some stumbling at other RPM's, but the issue seems to be just as I'm giving it a little gas off of idle. Can't really hurt to drain the tank and see if anything comes out, I suppose.

 

[brian]

what does the vac gauge say?

 

[Fort Knox]

what does the vac gauge say?

Um, it says "If you don't buy one of me and hook me up, I can't tell you anything."

 

[John Smith]

Hey Allen, All my carbs started doing this once the 10% ethanol gas was all there is, and I let my 40 sit for long periods. For me it was a sticky accelerator pump. I verified this by starting the truck and manually moving the throttle linkage. I could see the accelerator pump literally get hung up, then start to move slowly. I would then clean the deposits from the walls of the accelerator pump housing and it would work fine again. Also, your "transition slot" or whatever it is officially called could be clogged. @pinhead can explain what it does. But basically when you first get on the gas it acts as a "transition" for fuel flow is how I understand it. I've grown so tired of rebuilding my carbs I'm in the middle of doing a Holley Sniper install. Good Luck my friend!

 

[Fort Knox]

Hey Allen, All my carbs started doing this once the 10% ethanol gas was all there is, and I let my 40 sit for long periods. For me it was a sticky accelerator pump. I verified this by starting the truck and manually moving the throttle linkage. I could see the accelerator pump literally get hung up, then start to move slowly. I would then clean the deposits from the walls of the accelerator pump housing and it would work fine again. Also, your "transition slot" or whatever it is officially called could be clogged. @pinhead can explain what it does. But basically when you first get on the gas it acts as a "transition" for fuel flow is how I understand it. I've grown so tired of rebuilding my carbs I'm in the middle of doing a Holley Sniper install. Good Luck my friend!

Hey John! Been way too long buddy!

That sounds like some possibilities. The Sniper would be really cool, but probably not the time to drop that kinda coin at the moment.

 

[65swb45]

And the vacuum that’s says...

 

[Fort Knox]

And the vacuum that’s says...

I assume you’re asking the same thing Brian asked. I have ordered a vacuum gauge. It should arrive tomorrow.

 

[Solace in Solitude]

What @brian and @65swb45 said.

I had the same issue, when first I bought 44. Turned out the nuts on all four carb mounting studs were loose.

Brian and Mark could better explain... my presumption was that releasing the clutch caused enough of a lurch forward to cause the carb to lift slightly and break contact with the intake manifold... vacuum leak killed the engine.