Gas starved 4th gear only?

[bucfan]

Hello,

My 75 40 has been running pretty good so been driving more than posting here-plus I didnt want to see something else I just couldnt live without. Anyway after a 30 mile drive and almost home, the FJ acted like it wanted to die. No backfiring or anything like that just a hesitation/ bucking type thing. Down shifted to 3rd and it went away. Back into 4th, hit the gas same thing. Pulled over, had a quick look, took off hard-no problems until 4th again. What I have done so far-cleaned carb with carb cleaner, checked timing (good). Still hestitates/stumbles in 4th only-and right after I shift to 4th and hit the gas. Back off into 3rd and you can run the rpms way up and no stumble.
Any ideas? specs: Non-usa dizzy and Weber 38 thats not very old. Thanks in advance.

Matt

 

[Pighead]

Fuel filter. (change it)

 

[fireflyr]

Are you running an electric fuel pump? Mine was running like that at one time. Started and ran fine, then would starve for fuel if you accelerated into 3rd gear (I only have 3 ). Fuel pump was hooked up to voltage regulator and only recieving 7 volts. Wish I could blame this one on the PO.

 

[bucfan]

will check/replace the filter. I am using the stock fuel pump with a fuel reg set to 2lbs for the weber carb. thx.

 

[fireflyr]

Here's a link for troubleshooting Webers:

http://www.webercarbsdirect.com/v/vspfiles/images/Weber Trouble Shooting Guide.pdf

It says not to exceed 3.5 psi on the fuel pressure regulator, I think I'm running 3 psi. Fuel starvation is never fun to diagnose. Good luck.

edit: just checked. i'm running 3.5 psi.

 

[3_puppies]

will check/replace the filter.

trace the fuel line and make sure some PO didn't "add" another filter somewhere.

have you tried running with the gas cap loose?

what's odd is the engine doesn't know how fast your driving, RPM's under load are the same 3rd to 4th gear? or am I not thinking that correctly?

 

[Pin_Head]

If there is no popping and spitting back through the carb, like you get with a lean condition, it could also be ignition related. The voltage required to make a spark increases with increasing engine load, so it could be a problem with the plug wires, cap, rotor and/or coil.

 

[Brian in Oregon]

Starving for fuel in 4th, which is cruise? Then it really ought to be starving when the rpm's are upped in third.

Since you are likely accelerating in the lower gears, perhaps the problem is not with fuel at all, but with distributer timing. It's advancing when accelerating, but not set right for sustained cruise. Check that the clamp is tight, and if you have one put a timing light on it.

 

[fireflyr]

When I had my 'problem' the bowl would fill and it would accelerate normally, then stumble when the bowl went dry, usually 3rd gear getting on the freeway. If I slowed, less demand would allow the bowl to fill back up. Try setting your fuel pressure regulator up to 3.5 psi. Maybe you aren't getting enough volume at 2 psi.

 

[Magnum44]

Well, you know it hesitates under load. It looks like all or most of the answers could be the problem or a combination of them. One thing for sure it's fuel or ignition which is why the older vehicles are much easier to diagnose. What have you recently changed or done to it. You said the Webber wasn't very old. Did this start right after the new carb was installed. Anything else that may have been changed? For example. I had an 80's model Chevy Blazer and decided to change out fluids and filters. The next day on the way home my fuel pump took a dive. I had to pull out the new filter (in the carb-Rochester) just to get me home. I changed the pump, reinstalled the filter and it ran like a top. For whatever reason the new filter must have been a little more restrictive (the old on disintegrated in my hand) and caused the pump to finally give out. Anyway, might be a starting point.

 

[bucfan]

Like you guys, I am perplexed on why only 4th. My thoughts were the same, the engine has no idea what gear it is in.
I checked out the idle timing and it was ok. The weber is only about a year old or so, installed new by me and was running fine until this. I will check out everyones suggestions and report back when I find the prob for future reference. thx

 

[tucker74]

Fuel filter. (change it)

X2 - when I first got mine going I had this same issue, only on the highway in 4th gear. It turned out the tank had rust scale and it would starve for fuel only when running at sustained high rpms (which are seen in 4th on the highway). I went through 3 filters before I just replaced the tank -

Good luck!
Tucker

 

[bucfan]

Just wanted to update this thread for future. Here is what I did.
Replaced fuel filter, followed the weber troubleshooting and removed the jets and idle mixture screws and blew air thru the holes in carb (thanks fireflyr). Put back together, let her warm up for 5 min or so and took for a 5 mile drive- zero hesitation. Hope its not a gremlin and I got it.
I should have just tried one thing and then drove but working 5-6 days a week, you gotta take advantage of what time ya have.
Thanks for the help.