Stumbling at high rpm uphill

Joined
Mar 27, 2003
Messages
6,369
Location
Georgia Tech
'74 FJ40 original 1.5F, with JimC rebuilt stock carb (rebuild was 15 years ago). FJ60 ignition system.
When driving up hills at sustained high RPM (might be first gear up a steep farm field, or 2nd gear up a steep paved mountain road, the engine starts to stumble. Since highway speeds in 3rd or 4th gear are not an issue I don't think it is solely RPM related. Otherwise no running issues, starts easily cold or hot, idles smoothly, runs 70 on the interstate (as long as it's flat).

I've wondered if it is a fuel issue- perhaps bowl height? Welcome any pointers as I begin to dive i
 

micruz60

Unleaded Regular
Joined
Nov 6, 2006
Messages
1,801
Location
East of the Crossroads
^^
and how old is the fuel pump?

*edit, is the fuel pump the rebuildable type?
 
Last edited:

Steamer

SILVER Star
Joined
Jul 20, 2009
Messages
1,834
Location
Miami, FL
Things I would check.
Fuel bowl level should be in the middle of the carb window.
Dirty fuel filter.
Timing.
Water in fuel tank.
Valve adjustment.
 
Joined
Mar 27, 2003
Messages
6,369
Location
Georgia Tech
Fuel level in the bowl seems "normal" at middle of window sitting still at idle on level ground.
I replaced the fuel pump with OEM sometime in the last 15 years ;)
Fuel tank is brand new (under one week).
Kinda wish i could put a gopro under the hood to see the carb window when it happens

I don't recall last time I changed the filter so I'll start there. This problem is new, and the carb hasn't been touched since JimC rebuilt it, how would bowl fuel level go down over time? I could see how it could go up if floats lost buoyancy.
 
Last edited:

flx

SILVER Star
Joined
Jan 4, 2019
Messages
710
Location
France
Fuel level in the bowl seems "normal" at middle of window sitting still at idle on level ground.
Fuel may be starving only when high demand.

If the stumbling happens in a situation where you can stop easily a test you can do is as soon as the stumbling start you shut off the engine (and clutch-in immediately after), then coast to park. Here you can check the fuel level in the bowl and make sure it's still in the middle.
 

micruz60

Unleaded Regular
Joined
Nov 6, 2006
Messages
1,801
Location
East of the Crossroads
Fuel level in the bowl seems "normal" at middle of window sitting still at idle on level ground.
I replaced the fuel pump with OEM sometime in the last 15 years ;)
Fuel tank is brand new (under one week).
Kinda wish i could put a gopro under the hood to see the carb window when it happens

I don't recall last time I changed the filter so I'll start there. This problem is new, and the carb hasn't been touched since JimC rebuilt it, how would bowl fuel level go down over time? I could see how it could go up if floats lost buoyancy.
If you have been running ethanol / RFG, it can degrade internals of the carburetor, along with evaporation the fuel bowl will lose contents.
Do you drive it often?
My experience many years ago, while running 60 mph over Bozeman pass, the engine would buck and surge, stalling out. The fix was to rebuild the OEM fuel pump with new diaphragms and gaskets.
HTH
 

Users who are viewing this thread

Top Bottom