1971 FJ40 stumbling when coming off idle (1 Viewer)

[JimBo71]

Hi Guys,

I've had a long time problem with my 40 (original F engine and carb) since I bought it, where it stumbles and will stall (or nearly stall) when the throttle is increased coming out of idle (the harder the throttle is pushed the more it stalls). Everything else seems to work well (idles well, never stumbles once out of idle). I rebuilt the carburetor a while ago, but when I got it all back together the problem still existed (maybe a little better but still there). The workaround has always been to lightly double tap the gas pedal before trying to get the revs up since that gets the engine rpms up without it completely stalling, but I would like to finally fix the problem.

Today I checked with a couple vacuum hoses disconnected and plugged (brake booster and transfer case), but the problem was still there. I also took some video of the carburetor operating during that phase where it stumbles. I have posted a link here (stumbles occur at 00:17 and 00:23). fj40 stumbling

There is definitely gas being squirted in from the accelerator pump, but after that the engine will bog down unless more gas is pumped in (by tapping the pedal multiple times to get more from the accelerator pump). Eventually if the throttle is held down long enough gas starts coming out of the main nozzle (video doesn't capture that) and that allows the rpms to start to increase. So it seems to me there is a point where the mixture is getting too lean and causing the stall (or near stall). My question is whether there should be more fuel coming out of the accelerator pump or the fuel should be coming out of the main nozzle sooner (or maybe something completely different)? Thanks for any advice!

 

[pithicus]

All I can think of is float level too low or a blockage somewhere. Assuming all the vac hoses are all good and in the right places.
Also check for wear in the butterfly bushing. Could be sucking air through there.

 

[DougAustinTx]

I would start by checking your vacuum reading at idle. Should be around 20 inches of mercury. If that is good, then verify your timing is set correctly.

 

[65swb45]

/this\

 

[JimBo71]

Cool, I've definitely set timing while checking vacuum in the past (probably most recently after the carb rebuild), but I'll double check those things!

 

[DesertFJ40]

I have a 71 that also had a similar problem. This was largely noticeable when I would be making a turn, had to slow down and suddenly accelerate when clearing the corner.

The the carb had been rebuilt, vaccum was good. Timing was set. Points and spark plug set. However, adjusting the valves would temporally fix the problem but they would re-tighten every 600 miles. I removed the intake/exhaust manifold and cleaned the exposed valves with diesel fuel. Left the diesel sitting overnight and used a toothbrush/gun cleaning rods to scrape off all the built up sludge.

Turns out the previous owner hardly drove it long distances and caused a lot of the oil to sludge over from minimal driving.

Now it's been 2 years and my Fj40 doesn't suffer from hesitation. I've checked the valve clearances 3 times in the past 1 1/2 years and they have not tightened up. Running the original 112 sized jet at elevation of 3000ft. Using non-ethenol gas.

Just another thing you might want to check

 

[JimBo71]

Looked at a few of the suggestions, the fuel in the sight glass is right around the half point so I think the float level is ok. I checked vacuum and timing and see around 20in steady idling right around 650rpm (pic attached). Timing is at about 5 degrees advance beyond the bb, and the vacuum retard on the distributor is capped off so as far as I can tell everything is tuned ok. I looked a little more at what's happening in the carburetor when it is faltering and captured that in this video.


The faltering occurs in the period between the accelerator pump squirting and the main circuit providing fuel, it's a little hard to tell from the video but that is a constant throttle application (it kind of sounds like the throttle has been let off but that is from the problem I am trying to track down). The thing I'm not sure of is whether this looks correct for this carb, video I've seen from other carbs show the accelerator pump and main circuit having some overlap in providing fuel (so there isn't a period where the engine is starved), but I'm not sure if my accelerator pump is not putting a strong or long enough stream, or the main circuit is taking too long to engage (maybe the power valve is not working properly but there is no problem with faltering at any other time, just coming off idle). Any more advice is appreciated and thanks in advance!

 

[katwalk]

JimBo - did you ever solve your problem. I am having a similar issue on mt '71.

 

[cstav86]

@JimBo71 @katwalk have yall figured this out? I have the same issue as well. I need to check timing. I rebuilt the carb, had the same problem, then replaced with a city racer carb and still have the issue, so I don't think its a fuel delivery issue. I am desmogged, and all vacuum lines are new and in good shape.

 

[JimBo71]

@cstav86 @katwalk yes, I finally got the off-idle stumbling problem fixed! I tried a lot of different things and I'm not sure if the solution ended up being one of them or multiple, but I have listed things below that I tried with more or less success

- removed carb and insulator block, smoothed out insulator block surface since there were some big gouges in it, sealed pcv inlet since it seemed like there were some cracks, put gaskets on insulator to carb/manifold interfaces (none there before) : no change observed

- increased and played around with accelerator pump movement (different springs and spacers) : some change observed, seemed to move the stumbling point around but stumbling still always there, ended up going back to original

- cleaned out all carb passageways using carb cleaner and air : no change observed

- swapped power valve from one that had come with keyster rebuild kit to the original since they seemed to behave a little different when activating them externally : no change observed

- increased size of primary slow idle jet from 0.5 to 0.8 : caused the stumbling to happen at higher rpms, which in some ways was worse since the stumble would happen during acceleration instead of just off-idle, ended up going back to original

- increased size of primary main jet from 1.12 to 1.18 (as specified in FSM for US models), increased secondary slow idle jet from 0.5 to 0.8 (as specified in FSM for US models), raised float level a couple of mm (fuel previously appeared to be in center of window or slightly below and now appears slightly above) : these changes, which were all done at once, are what fixed the problem but I'm not sure which was most important

in the end it appears the problem was leanness in the off-idle transition so enrichening the mixture was the key

also note, the keyster carb rebuild kit comes with a 1.12 primary jet and 0.5mm slow jet, but I was able to get those increased by carefully opening them up to the larger diameters manually with small drill bits and carburetor cleaning wires

 

[cstav86]

@cstav86 @katwalk yes, I finally got the off-idle stumbling problem fixed! I tried a lot of different things and I'm not sure if the solution ended up being one of them or multiple, but I have listed things below that I tried with more or less success

- removed carb and insulator block, smoothed out insulator block surface since there were some big gouges in it, sealed pcv inlet since it seemed like there were some cracks, put gaskets on insulator to carb/manifold interfaces (none there before) : no change observed

- increased and played around with accelerator pump movement (different springs and spacers) : some change observed, seemed to move the stumbling point around but stumbling still always there, ended up going back to original

- cleaned out all carb passageways using carb cleaner and air : no change observed

- swapped power valve from one that had come with keyster rebuild kit to the original since they seemed to behave a little different when activating them externally : no change observed

- increased size of primary slow idle jet from 0.5 to 0.8 : caused the stumbling to happen at higher rpms, which in some ways was worse since the stumble would happen during acceleration instead of just off-idle, ended up going back to original

- increased size of primary main jet from 1.12 to 1.18 (as specified in FSM for US models), increased secondary slow idle jet from 0.5 to 0.8 (as specified in FSM for US models), raised float level a couple of mm (fuel previously appeared to be in center of window or slightly below and now appears slightly above) : these changes, which were all done at once, are what fixed the problem but I'm not sure which was most important

in the end it appears the problem was leanness in the off-idle transition so enrichening the mixture was the key

also note, the keyster carb rebuild kit comes with a 1.12 primary jet and 0.5mm slow jet, but I was able to get those increased by carefully opening them up to the larger diameters manually with small drill bits and carburetor cleaning wires

Very helpful and that narrows my search significantly. I have another keyster kit laying around, so I'll start there. Thanks!

 

[largeone]

Good info....I think another thing to check would be making sure the emulsion tubes for the main jets are cleared. I did not do this when I rebuilt my carb, and had similar symptoms. I opened the carb back up and pulled the main jets to clean and the stumbling nearly went away. I used the 114 primary main jet at altitude and seems to be OK. I used the 60 power jet.