Ok fellas. I got a '90 F700 grain truck, truck has a 370 2v in it.

Here is the problem:

Filled the truck up completely full of gas. Drove 6 miles, it starts making a popping noise from carb, top speed of 65mph drops to 45mph-50mph. It then decides to backfire, run like absolute crap, if you hit the gas there is that "popping" sound from the carb.

Here is what I did to fix it yesterday. Had a few guys, 2 of which were mechanics, said to start with fuel filter. Has a spin-on filter on frame rail right under passenger door. Replaced that. Still ran like crap, so I then replaced the filter in the carb, still ran like crap. Then I put a new air filter on, still runs like crap.

Took it out for a nother test drive, all the sudden it started to run right. Right now its got just above a 1/2 tank of fuel left in it. Drove the truck back home at 65mph, best the truck has run in years.

This morning I fired up the truck, ran great. Decided to top off my gas tank and hit the road again. I got to the same exact spot I made it to yesterday when I filled up the tank full of gas, and it started running like **** again. I kept going today, I'm rather upset at this point. Drove 40 miles to my destination at a max speed of 45mph with lots of spitting an sputtering, poppin, etc. Once the fuel gauge got to 1/2 tank, truck runs just fine again........

What is going on with this thing???

I checked all the vaccum hoses and all that stuff, so I'm at a loss right now.....

 

the fuel pump may be causing an old rubber fuel line to collpase at high rpm. check for rotted hoses

 

Check the timing chain, distributor drive gear. Mosst likley the timing chain

 

pull the carb and check if the bowl has dirt or water in it,had this happen before

 

Ramboss does this truck have 2 fuel tanks. Just curious. I was thinking it was a timing issue for sure. Reminds me when I put the our on our old 534 irrigation engine ignition wires on wrong did the same thing. But I doubt if its a timing or a distributor issue at all. Since when it gets half empty it runs fine. A puzzle isnt it. Got to be a fuel issue for sure.. Either to much or not enough. Maybe forcing to much fuel in the carb when full but when half empty just the right amount. Maybe a defective fuel pump. When its running rough when the tank got half empty did it all off a sudden run fine? Im with gas hog check those hoses am maybe replace them. Then maybe the fuel pump. Just my 2 cents. Hope youget her running right. Do you use this truck every day or does it sit idle for long periods off time? As Fordegrt said check that carb an the fuel bowl.

 

Thanks everyone for the ideas and replies so far.

The truck has 1 tank that is right under the driverside door, and it has a built in step in it. I will have to check my fuel lines tomorrow and see what is up with them. I don't even know if it has a mechanical or electric fuel pump, haven't looked into it that far, anyone know?

I'm almost positive that when it is running rough it is gettin too much fuel, which might explain the backfiring/popping coming from the muffler.

The truck gets used a few times a week usually. Did sit for a month or so this winter with no use, but has been gettin used everyday almost since March with no issues.

 

Maybe a stupid question here but does it have a manual or automatic choke. Ok if you run the truck from a stop an gaining speed does it run rough then as well or is it when you get up to speed 45 plus MPH then it runs rough. At idle does it run rough. When you run the engine in neutral with no load how does it run. When you get the RPMs up does it run smooth, Sorry for all the questions. Just wanna narrow it down.

 

Getting water in with the gas at that pump?

 

is this truck fuel injected?

 

Truck has a 2barrel carb on it.

I don't think the gas is the problem, filling up at my tank in my yard. None of my other vehicles run like crap.

 

I'm not familiar with the 2V carb, but it could be a blown power valve in it (if it has one). You might find views and parts at this site: http://www.swapsupply.biz/Carburetor...ationguide.pdf

 

Ramboss did you figure out what was wrong with your truck. Just curious if anyone had the right answer on how to fix it. Sure hope it was something minor.

 

I actually haven't gotten to look at it yet. Been way busy planting soybeans and corn. It rained yesterday an today so will more than likely get it under the knife by Friday.

 

Keep us posted. Never turn down rain. When it stops it stops for a long time. Good Luck

 

Is it running really rich, i.e. black smoke out tailpipe? If so, possibly sticking carburetor float or float valve or power valve. Also check vacuum advance on distributor--remove and plug vacuum hose to distributor, put an external piece of vacuum hose on advance unit, start engine/let idle and apply oral suction to the external hose. You should be able to hold a vacuum on the line with the tip of your tongue and the engine speed should increase as suction is increased. A sticking open EGR valve might explain the popping/spitting if it's through the carburetor, due to causing too lean of a mixture. I don't think it's a cam timing problem because if it "jumped time" it wouldn't return to running ok at other times. Make sure carburetor is tightened securely to manifold.