Engine wizards I need your help, losing power at WOT, rebuilt engine. (1 Viewer)

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Just installed a rebuilt 2F into a desmogged 1985, the engine stumbles when you go WOT and loses power. It really has me scratching my head.
It has a new Aisin carb that was installed before I replaced the engine (by someone else) so I feel like it's highly unlikely there is an issue with it, I have verified the secondary is opening via the paper clip test.
It accelerates through the stumble but when it smooths out it doesn't have power to climb hills, at part throttle it cruises and accelerates perfectly. It idles perfect and revs up at idle completely normally, no stumble, only under heavy load.

Fuel filter is new (fuel looks clean)
Float bowl level is good
New DUI distributor (10 degrees, 7000' altitude.)
16" vacuum at idle, steady needle. Snap throttle test shows 2"-25"
Valves adjusted
Primary jet .155
Secondary jet .200

I am running it with the air cleaner off as well ruling out an air resctriction.

Hopefully a knowledgeable old timer can help me out here :) 🤞
 
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Seems like it’s starved for fuel at full throttle.
If the second butterfly valve opens at WOT but fuel doesn’t flow sufficiently in that barrel, the engine A/F mixture will instantly be way too lean and the engine will lose power.
While your secondary may be opening (verified with the paper clip test) fuel may not be flowing properly in that circuit.
 
How’s your fuel filter looking? Are you sure your fuel pump is operating correctly?
 
How’s your fuel filter looking? Are you sure your fuel pump is operating correctly?
It's brand new and clean. I assume so, the bowl is always full but I don't have a gauge connected.

I will probably swap the carb and see what happens.
 
It's brand new and clean. I assume so, the bowl is always full but I don't have a gauge connected.

I will probably swap the carb and see what happens.
That would be my next stop. Maybe some trash inside the carb preventing sufficient flow for WOT.
 
Recently, I pulled the top off my carb to replace a leaky plunger boot. I forget what they’re called but there are two little rubber seals about 1/8” tall that seal two ports from the lower and upper halves. They kept falling into the throttle body and I had to fish them out several times before I got them seated properly. Maybe you’ve got something like that going on?
 
distributor not advancing? Do you have vacuum advance? Is it hooked up correctly?

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Any vacuum leaks?
 
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Well I'm a dummy, After replacing the carb/fuel pump I left a vacuum cap off of the little tri port on the intake manifold which was causing it to run rough.

The good news is with a new fuel pump it's running great now.
 
OK. So you think it was the fuel pump then? The vacuum cap was taken off during the troubleshooting process?
 
OK. So you think it was the fuel pump then? The vacuum cap was taken off during the troubleshooting process?
Yes I took it off when I hooked the gauge up and didn't cap it after removing the gauge.

As you can see from my first post there were no vacuum leaks, except the one I created later 🤦‍♂️

The fuel pump was the culprit 👍
 
But earlier you said you already had a new fuel pump and problem remained. Was that one faulty?
New fuel filter I think...
 
But earlier you said you already had a new fuel pump and problem remained. Was that one faulty?
Yeah because I had accidentally created a vacuum leak AFTER replacing the pump, after I fixed the vac leak the truck runs as it should.

The pump that came off the truck didn't seem bad so I tried to reuse it, I swapped carbs, then I replaced the fuel pump with a new OE unit.
Interestingly enough my fuel pressure gauge only reads about 4psi, truck runs find though.
 
Well I'm a dummy, After replacing the carb/fuel pump I left a vacuum cap off of the little tri port on the intake manifold which was causing it to run rough.

The good news is with a new fuel pump it's running great now.
Welcome to the Boy Do I Feel Dumb Club. Where I'm not only an executive level member, I'm also a client.
 

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