Engine wizards I need your help, losing power at WOT, rebuilt engine.

  • Thread starter Travis Bickle
  • Start date
  • Watchers 2
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
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 :) 🤞
 
Last edited:
OSS

OSS

SILVER Star
Joined
Jun 30, 2017
Messages
4,045
Location
Oblivion
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.
 
cps432

cps432

Joined
Mar 4, 2016
Messages
2,212
Location
Boulder, Co
How’s your fuel filter looking? Are you sure your fuel pump is operating correctly?
 
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
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.
 
cps432

cps432

Joined
Mar 4, 2016
Messages
2,212
Location
Boulder, Co
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.
 
cps432

cps432

Joined
Mar 4, 2016
Messages
2,212
Location
Boulder, Co
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?
 
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
Swapped in a known good carb, same thing!
Going to try a fuel pump
 
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
New fuel pump, same thing.

😤
 
2mbb

2mbb

SILVER Star
Joined
Sep 1, 2003
Messages
5,311
Location
WC, CA
distributor not advancing? Do you have vacuum advance? Is it hooked up correctly?

1632248724914
 
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
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.
 
2mbb

2mbb

SILVER Star
Joined
Sep 1, 2003
Messages
5,311
Location
WC, CA
OK. So you think it was the fuel pump then? The vacuum cap was taken off during the troubleshooting process?
 
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
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 👍
 
Travis Bickle

Travis Bickle

Joined
Aug 29, 2018
Messages
377
Location
CO
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.
 
Spook50

Spook50

Skål
Supporting Vendor
Joined
Feb 16, 2005
Messages
6,678
Location
Spokane, WA
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.
 

Users who are viewing this thread

Top Bottom