Engine choking 351W

Hey guys long time supra owner here, new to the forum. I have 1991 Supra Sunsport with 351W(Pro-tec converted) It was freshly rebuilt(block replaced) about a year ago and has 24hrs on the rebuild. I have been having an issue this season and I'm trying to narrow it down. The symptoms are: runs fine at normal idle, won't accelerate(above 800rpms) until up to operating temperature, bogs/chokes when giving any throttle at any temperature, occasionally back fires. If you careful and slow you can accelerate.
The carb was replaced last year and properly stored at the end of the season
I have been through the gas(replaced filter, new gas, ethanol treatment) I have replaced a couple vacuum hoses. Im wondering if it could be due air starvation(choking). The flame arrestor is really dirty. In the next few days I'm going to pressure wash the flame arrester and see what happens. Please let me know if you guys have any other ideas.

I recently read on this forum re a 351 W with holley carb.
Writer said if you experience backfire even just once chances are you will need to replace the power valve.
You may wish to check this out further re same.
Good luck

Ok, so new symptoms. After cleaning the Flame arrestor the engine vapor locks upon every shutdown. That leads me to believe that it is running extremely rich? It accelerates smoothly to 2600RPM and won't go any further. It will smoothly ride at 2600rpm making about 14kts.

I know on the SEFI ones there are two switches one for oil pressure the other for coolant temperature that trigger a limp mode that limits RPM to ~2,500 if they are tripped. The one for the coolant caused me a bit of trouble in our '95 and is presently bypassed until I can source a replacement. Not sure if there's something similar on the 1991?

You say the engine is chocking but what I'm getting from the posts is that you have an enrichment issue. No enough fuel. Vapor lock is when engine is hot enough to boil the fuel in the lines and carb. That's a lean issue and boats don't normally get that hot. If an engine is chocking its getting too much fuel you will have lots of black smoke. ( A closed chock plate or high float setting )

Remove the flame arrester. With the engine off push on the throttle and look inside the carb, Choke plate open. You should see two streams of fuel through the complete travel of the throttle. If not you have a bad accelerator pump.

A few things to check that have already been mentioned.

1. The backfire is typically a lean issue and will wipe out the power valve in a Holley carb. Check this and the flow of fuel with the engine off as someone eases the throttle from idle to wot. As stated you should see a steady flow of fuel throughout the throttle range.

2. I would also check your timing as this can contribute to your issues.

3. Inspect your ignition system closely. Could, cap, rotor, plug wires, plugs can all be sufficient for idle but be crappy at anything above that. I’d also suggest putting a new set of plugs in it while you’re troubleshooting and then pulling them after each run to see the difference of each one.


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If you determine that it's vapor lock, I have a fix for you.

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