351w wants to die

[TheJ]

Some of you may know that I rebuilt a 351w for my brothers 84 F359 dump truck. It had been running fine when I decided to go the the transmission shop to make sure everything is working good. On the way it felt like it was going to die around 40 mph and I could go any faster regardless of ho much gas I gave it. I don't know if it is fuel or spark, maybe even heat or vacuum related. I can rev it higher at lower speeds and its fine but can't go above 40 mph. Any suggestions are appreciated.

Thanks for looking.

 

[Don C]

It sounds like fuel starvation to me. When it's not under load the fuel pump can supply enough fuel for it to be revved. I would start with the fuel filter and then check the output and pressure delivered by the fuel pump. If the truck sat for some time before the new engine was installed it's hard telling what may have been in the fuel tank. If the fuel filter is not clogged and the fuel pump still doesn't deliver the spec quantity it could be either a bad fuel pump or a clogged line or sock in the fuel tank.

Another possibility is ignition, if you have points, the rubbing block may be worn and they may not be opening enough.

Are the spark plugs new and correctly gapped?

Next I would check the coil for correct primary and secondary resistance, and then the coil output.

With something like this there are a variety of things that could be wrong, you just have to methodically check them all until you find it, starting with the most likely things.

 

[MotoArts]

With something like this there are a variety of things that could be wrong, you just have to methodically check them all until you find it, starting with the most likely things.

X2 ^^^

Also consider possible blocked catalytic converter(s) or muffler (it can happen).

 

[TheJ]

It sounds like fuel starvation to me. When it's not under load the fuel pump can supply enough fuel for it to be revved. I would start with the fuel filter and then check the output and pressure delivered by the fuel pump. If the truck sat for some time before the new engine was installed it's hard telling what may have been in the fuel tank. If the fuel filter is not clogged and the fuel pump still doesn't deliver the spec quantity it could be either a bad fuel pump or a clogged line or sock in the fuel tank.

Another possibility is ignition, if you have points, the rubbing block may be worn and they may not be opening enough.

Are the spark plugs new and correctly gapped?

Next I would check the coil for correct primary and secondary resistance, and then the coil output.

With something like this there are a variety of things that could be wrong, you just have to methodically check them all until you find it, starting with the most likely things.

I will add that I have been driving it and it just recently started doing this. The spark plugs are new and I gapped them to spec. It has a duraspark ignition system and I have a flamethrower ignition coil. As for the spark plug wires, when I was checking them one of the boots came off of the wire. Could that have been the problem? I ordered a new set of ford racing ones maybe that will fix it. I'll still go through and check everything else, along with pulling both tanks and taking a look at them.

Thanks for the help.

 

[VegasFastback]

Sounds like spark to me. I've had 2 vehicles where the wire has popped off the alternator and they exibited that exact problem. As the voltage drops on the dying battery the max MPH you can reach gets lower and lower.

 

[TheJ]

Well, I think I have figured out what is wrong. Both of the fuel filters are clogged so I'm going to drop the tank and try to clean it but we'll see how it looks.

Thanks for your help guys.

 

[TheJ]

Update, I just installed the new gas tank and it runs like a top now, I might need a new fuel pump though.

Here's the old tank

This is what was being pump out of the fuel pump.

New tank and filler neck.

Thanks for looking.

 

[c9zx]

Very glad you got it fixed and letting us know what the problem was. Chuck

 

[Don65Stang]

I'd like to see an engine pic J.

 

[TheJ]

I'd like to see an engine pic J.

Ask and you shall receive.

Quick video when I first got it running:



I posted the entire engine build here:

http://www.7173mustangs.com/thread-update-new-project?pid=90091#pid90091

 

[Don65Stang]

Cool! I'm glad my intake lives on.

 

[TheJ]

Cool! I'm glad my intake lives on.

That it does, the truck definitely has more torque because of it and the cam.