2F poor acceleration (1 Viewer)

[igotta40]

1974 FJ40 with 1975 2F, professionally rebuilt, 6700 miles on it. It idles fine, at speed it runs fine but when I step on the gas I get a miss, kind of a splashy weak acceleration. No backfiring, just kind of sputters it's way up to speed. Recently adjusted the valves, cleaned and gapped the plugs and set the points. I'm thinking maybe the carb has some junk in it? Fuel filter looks OK but I could change it. Aisin carb, also rebuilt when the motor was installed. Had been running great until the last 500 mles or so. Don't drive it much. 6700 miles on the motor since installed in December 2006. It started doing this at freeway speed, hard acceleration, high rpm's when I would open the secondary with my foot, but recently getting this weak acceleration and miss with only the primary throttle. Do I need to rebuild this carb again?
Sorry for the long post.

 

[kurtnkegger]

Vacuum advance on the dizzy? Look for leaks, or decent movement on the points mount plate (don't know what it's really called)

 

[KB45]

Interested to see what you find, I am having the same/similar issue with my FJ45. Maybe running rich/timing not quite right?

 

[Duke]

I had a similar issue with my Aisan carb when I rebuilt it. I tried two different carb kits from NAPA. Still no improvement. I bought a Keyster kit and replaced only the accelerator pump based on info I found on here and after a ton of reading. In my case it solved my problem completely. Runs smooth, accelerates fine, secondary opens when floored and takes off. I might also add that the float level be checked for accuracy. This can easily be screwed up when taking the carb top off repeatedly to replace the accelerator pump.

In the end the NAPA kits are poor quality in my opinion, I'm sure others will disagree. If you take a look at the acclerator pump in the NAPA kit and compare it to the Keyster you'll know what I'm talking about.

FYI, I'm no expert. This was only my experience based on the process of elimination after replacing jets, slow jets, idle solonoid, fuel pump, fuel filters and finally the accelerator pump. I might also add that a some who have done their own rebuild think they've done a proper rebuild by pulling the carb apart and cleaning it with carb cleaner spray. This is not sufficient. It needs to be dismantled and soaked overnight in carb cleaner to clean of years of built up varnish in some of the tinier places. Then soap and water wash and then carb cleaner spray in all the holes and then blow it out with air. I did this on mine several times due to several attempts to rebuild it. Jim C, Pinhead and several other gurus on here might have different advice. This is what I discovered for me.

Hope it helps and good luck!!

 

[Pighead]

It idles fine, at speed it runs fine but when I step on the gas I get a miss, kind of a splashy weak acceleration.

Gosh that could be a lot of things...you don't mention where it's timed, or even what distributor (vac retard?). I wouldn't tear the carb apart again until checking everything else. Remember, 1/2 of all carb problems are really electrical problems. We need pics too.

 

[Mach Schnell]

Poor acceleration out of a 2f...never. cough...v8....cough

No, Id really check the vacuum advance. I had a similar problem it sounds like.

 

[splitshot]

I agree with Pighead go there first, then see if you have a decent spray from the accelerator pump discharge.

 

[Splangy]

X2 on the vacuum advance and accel pump. Also check your secondary diapharm for leaks & proper operation.

 

[Pighead]

Poor acceleration out of a 2f...never. cough...v8....cough

Shut up. The problem here is worse-than-usual 2F acceleration (if that were possible). You just keep your V8 talk amongst your jet-set friends. Ludites are talking here...

 

[Mark W]

From the symptom described I would look at carb first. I would of course double check timing and points before tearing into the carb. But I expect it is the carb.

Initial problem sounds like restricted or blocked fuel flow through the secondary circuit. What you have now sounds like additional blockage of accel pump and/or primary circuit.

I recently had a aceel pump plunger come apart and some of the debris found it's way into the fuel bowl and from there into the primary and secondary circuits.


Also... the accel pump should not provide a "spray" of fuel. if it does, it is wrong. it should be a strong and solid stream like from a squirt gun.


Mark...

 

[igotta40]

Wow, thanks for all the replies! I'll double check the timing, etc. and have a look at the gas stream. I'll get back to you with my results and pictures.

 

[MtnClimber]

I had the same miss issue. My fix was properly adjusting the Mixture screws (Rochester). After that it ran awesome. And this was after I had checked everything else with no issues and nothing needed adjustment.

 

[Splangy]

I had the same miss issue. My fix was properly adjusting the Mixture screws (Rochester). After that it ran awesome. And this was after I had checked everything else with no issues and nothing needed adjustment.

Adjusting the idle mixture screw improved cruising acceleration?

 

[MtnClimber]

It completely got rid of the stumble.

The problem I had was any time the throttle was applied/increased, the engine would cut power immediately and then it would gain power again but was sluggish. If it sat at idle and I floored the pedal quickly, the engine would die. Once I adjusted the mixture screws making sure they were absolutely in sync with each other, the engine accelerates smoothly and runs great and at idle and very low RPMs I definitely have a lot more torque on the trail than before!

 

[subzali]

The idle mixture screw doesn't have a huge effect on cruising speed performance on an Aisan carb...

 

[cruiserdood]

any of you guys have experience with this with a Weber. i recently did an HEI dizzy, new plugs, set the valve clearences, which helped somewhat, have the right spark plug gaps, timing is advanced to 17 BTDC, as told by iron pig, played around for a couple of degree's in both directions, somewhat fixed problem...

yet, when going up hills, or basically starting out, anwhere, it back fires, the throttle feels spongy, the less gas, the better, and smoother the accel, runs fine at speeds

 

[DSRTRDR]

The idle mixture screw doesn't have a huge effect on cruising speed performance on an Aisan carb...

 

[FZJ80ismybaby]

no one on the weber carb. from cruiserdood?

 

[Hudso056]

i had the same issue with a weber 38 dgas, it was the power valve was gone. Rebuilt with new power valve and accelerator pump and it was back to new.

 

[Ry24000]

I gotta know. What was the problem?
I have the same issue and I started going through all the steps staring with checking for vacuum leak and then electrical. If that's not it then year down the carb again.