2F Loss of Power with speed/load - need advice (1 Viewer)

[Milkshake]

Good Morning,

I have a 1971 FJ40, and I am having some issues with the vehicle when I am trying to drive at higher speeds. The FJ40 starts and idles great, and drives very well in 1st and 2nd gear. When I am getting up to about 40mph and above in 3rd/4th gear, and I try to give more than about 20% throttle, I suddenly lose almost all power, and the vehicle starts bucking. If I lift off the throttle for a few seconds, the vehicle recovers, but will repeatedly do this if I try to accelerate.

Things to note:
The 2F is from a 1984 FJ60 - Desmogged
The carburetor is a Weber 38 DGAS (I know, but its the carb I have)
New plugs/wires during the engine swap (less than 400 miles ago)
Has the stock FJ60 Igniter/Coil/Distributor with dual diaphragm vacuum advance
Has a new tank, fuel lines, and mechanical pump

At first I thought it was a fueling issue due to the hard lines being constricted. This was correct, and I currently have the hard lines bypassed with rubber line for troubleshooting. This helped the problem a bit, gained maybe 5% more throttle at speed under load. My base timing is set to 12 degrees BTDC, and I have the HAC diaphragm blocked with the main advance hooked to manifold vacuum. The distributor is in fairly good condition, but the vacuum diaphragm does not hold vacuum.

I will pull the carb this afternoon and check for debris from the hard lines and double check the float level. Reading in the forums, I saw a few other cases with similarities, but nothing answering the following questions:

If my vacuum advance is inop, would it make enough of a difference above 2200 rpm or so to cause the power loss/bucking?
Or should I be digging into the carb deeper and possibly looking a re-jetting?

 

[charliemeyer007]

Sounds to me its running out of spark or fuel. I have a Rochester 2bbl and a straight mechanical advance Delco distributor.
Make sure your fuel pump is actually working and your fuel filter isn't plugged with water. How old is your gas?

 

[Pighead]

How old is your fuel filter?
Dizzy vacuum should come from ported vacuum not manifold vacuum. If the diaphragm doesn't hold vacuum it's just a vacuum leak to the manifold and should be fixed.

 

[pb4ugo]

If the advance vacuum diaphram wont hold vacuum, then you have a vacuum leak and the hose should plugged. Bucking, like surging? It sounds like your running out of gas. The float level could be low, clogged filter or the fuel pump is weak.

 

[Milkshake]

Sounds to me its running out of spark or fuel. I have a Rochester 2bbl and a straight mechanical advance Delco distributor.
Make sure your fuel pump is actually working and your fuel filter isn't plugged with water. How old is your gas?

If I pull the fuel feed line off the carb while running, I have good flow at about 6 psi on a gauge. Put in a new clear plastic filter, and verified that the pump isn't sucking air. The gas is about a month old.

 

[peesalot]

Main jet got junk in it.

 

[Milkshake]

How old is your fuel filter?
Dizzy vacuum should come from ported vacuum not manifold vacuum. If the diaphragm doesn't hold vacuum it's just a vacuum leak to the manifold and should be fixed.

Understood, looking at other diagrams for the Weber 38, looks like the port I have the line on is labeled "vacuum advance port". I'll check to see if the source is above the throttle plates or not. I've also just learned of the fuel screen on the feed of the carb itself, so I'll be checking that as well.

 

[Milkshake]

Main jet got junk in it.

Sounds likely. Seems like I've got a few carburetor items to run down, and I'll start looking at distributor replacement options based on @Pighead 's notes about the vacuum leak.

 

[Milkshake]

If the advance vacuum diaphram wont hold vacuum, then you have a vacuum leak and the hose should plugged. Bucking, like surging? It sounds like your running out of gas. The float level could be low, clogged filter or the fuel pump is weak.

Yes, like surging.
My plan after work today is:

1. Check inline filter for debris/clogs
2. Pull Weber feed filter and inspect for debris
3. check/set float level in the carb
4. Pull main jets and inspect/clear debris
5. Verify vacuum advance is hooked to ported source

 

[peesalot]

FWIW if you remove carb, don't just spray brake cleaner thru the main, clean it, I use syringe needles from the vet, they are the right size and screw onto the little plunger thing so you can get a blast of air or whatever you suck into it to blow debris out. I find the needle is just right to clear the bore. If you have the proper tip screwdriver to remove jets, I like to remove and clean as too many times taken carbs off 3 or more times cause we didnt remove, check, clean main jets. Don't use a tip that is marginal , it will fubar the jets.

 

[Pighead]

5. Verify vacuum advance is hooked to ported source

If the diaphragm won't hold a vacuum just block off it's vacuum source, run it on mechanical advance. Also, are you sure it's a vacuum advance dizzy? A '71 might be a vacuum retard. Here's two dizzys, one is vacuum advance one is retard, which one does yours look like?

 

[Milkshake]

If the diaphragm won't hold a vacuum just block off it's vacuum source, run it on mechanical advance. Also, are you sure it's a vacuum advance dizzy? A '71 might be a vacuum retard. Here's two dizzys, one is vacuum advance one is retard, which one does yours look like?

View attachment 3765410

The 2F was pulled from a 1984 Fj60 that had all the smog equipment short of the carb. It has the Toyota Igniter/Coil and Distributor with 2 diaphragm advance from the 60 series as well. Just like in this photo from the web.

 

[Pighead]

The 2F was pulled from a 1984 Fj60 that had all the smog equipment short of the carb. It has the Toyota Igniter/Coil and Distributor with 2 diaphragm advance from the 60 series as well. Just like in this photo from the web.View attachment 3765415

I see. Well, if one of those diaphragms will hold vacuum, use it. I was able to get a new single diaphragm advance unit for my Big Cap dizzy from @4Cruisers that was an easy install

 

[1911]

If my vacuum advance is inop, would it make enough of a difference above 2200 rpm or so to cause the power loss/bucking?

Lack of power for sure, and running warm at highway speed. "bucking" (backfiring?) also if the base ignition timing was too retarded.