Carburetor tuning for dummies

[Heirloom]

I need help with getting my 1966 F135 tuned for drive ability. Here's where I'm at:

Rebuilt engine. All rings and bearings are new, fresh hone, new valves and hardened seats.

I put a Keyser carb rebuild kit in the Aisin SD-40.

I replaced all ignition components (except the distributor itself, which I cleaned, lubed, and also replaced the centrifugal advance bushing).

I set the timing according to the 1966 FSM (thanks 'mud for the download).

Initially, I put the throttle butterfly in incorrectly, and vacuum was bypassing the idle circuit. That caused a very rich idle and other problems. After revisiting the instructions about throttle plate alignment in the FSM , the idle seems better. I also pulled the plugs and cleaned off the carbon from the previously rich idle.

My current issues are with stalling when I open the throttle. It will gain rpm if I ease into the throttle very gently, but it is still pretty low on power (bogs down in first gear) . This stalling occurs under load as well as free-revving the engine.

I just pulled the bowl cover off the carb and adjusted the step-up rod height to increase fuel delivery at lower throttle settings. This seemed to help a little, but not completely. Now, it will Rev without stalling to 30-40% throttle, but it backfires and stalls above 70% throttle, even if I slowly bring the engine up to speed. The FSM states that the step-up rod has a set-up protocol, but that procedure is not specified.

Does anyone have carb tuning expertise and can give me some advice? I appreciate it.

 

[Pin_Head]

I am not familiar with that particular carb, but check the fuel level in the bowl when it stalls. If it is empty there is a fuel deliver problem, like a clogged inlet, pump, fuel filter, tank. If the fuel level is normal, it sounds like you botched the rebuild and have the main jet, it's fuel passage or main nozzle clogged. There may also be a problem with the idle fuel circuit. Did you check all these tiny holes to make sure they were clear?

 

[Heirloom]

Bowl level stays good. The problem is instantaneous (not like it runs well until the bowl depletes). I blew through all passages with pressurized carb cleaner after I dipped the main body, and all seemed to be flowing as far as I could tell.

Do my symptoms seem like a lean mixture problem? I can get more fuel in with another step-up rod adjustment.

 

[Splangy]

Those carbs have an external ljnkage that operTes the metering rods. That linkage has a small plate with a hole for a small, curved linkage rod. That plate always gets buggered up where it mounts to the throttle shaft so the metering rods don't open correctly. Take a look for it in the bottom half of the carb.

 

[Old Red]

Have a look at these. Treeroot knows.

General info: OK you carb experts

Specific info: Aisan SD40 Big Window Carburetor

 

[Heirloom]

Those carbs have an external ljnkage that operTes the metering rods. That linkage has a small plate with a hole for a small, curved linkage rod. That plate always gets buggered up where it mounts to the throttle shaft so the metering rods don't open correctly. Take a look for it in the bottom half of the carb.

Yes, mine is set up as you describe. I made sure all the metering rod movements and linkages worked on reassembly. The top of the metering rod linkage (internal) has a bendable link to adjust where the metering rod is located, relative to the throttle input. One possible solution is to find out how to check this setting. The FSM images from Old Red's SD40 link show these parts and reference that a set-up procedure exists, but the procedure is not outlined in the manual.

 

[65swb45]

Rebuilt engine? Broken in? Compression? Vacuum?

As opposed to EFI [active fuel management] carburetion is passive fuel management, and very vacuum dependent.

 

[Heirloom]

Rebuilt engine? Broken in? Compression? Vacuum?

As opposed to EFI [active fuel management] carburetion is passive fuel management, and very vacuum dependent.

Vacuum is 18-20 in Hg, depending on idle mixture settings. Rebuilt, not running well enough to drive or break in.

 

[65swb45]

Trying to tune a carb on a motor that isn't broken in makes as much sense as squeezing jello in your hand.

And it is a recipe for junking your piston rings. I watched in horror (actually over the telephone) as downey ruined an awesome 2f I built for them that way.

Borrow someone elses carb that runs and break that motor in...while you still have a chance.

 

[Heirloom]

Trying to tune a carb on a motor that isn't broken in makes as much sense as squeezing jello in your hand.

And it is a recipe for junking your piston rings. I watched in horror (actually over the telephone) as downey ruined an awesome 2f I built for them that way.

Borrow someone elses carb that runs and break that motor in...while you still have a chance.

Point made. Thanks.