In the past month or so, I've had a handful of times when my truck has lost almost all power. It'll still run and get me down the road, but I might not be able to get over 50 MPH on a straightaway with my foot on the floor. And, it misfires all over the place, and will backfire a little when I get off the gas pedal. I've been able to figure out that it's only happened when I'm running on the rear fuel tank and it's below about 1/4 full (about 4 gallons in the 16 gallon tank). It seems to start happening right after taking off from a stoplight, or going around a long curve, then it gets gradually worse. Hasn't happened when running off of the front tank.

With my engine swap, I run a carbureted system. So the only fuel pump in the system is the one driven by the engine.

It happened to me earlier today when I was almost home. Once I got home I looked under my hood at my fuel filter (I've got a clear one between the fuel pump and carb) and it had almost no fuel in it. Usually it stays almost all the way full. When I pulled the fuel line off of each side of it, there was a little bit of pressure that released. I guess that the fuel pump was just compressing air, and a little bit of fuel...?

Never had this issue before some time last month.

My diagnosis so far is that the piece on the end of the pickup in the tank is probably done for. It seems that it could keep the pickup from being exposed to much fuel when the tank got low. Does it sound like this may be my issue?

What else could it be? Could the fuel pump just be weak and not pulling well enough when the furthest tank is low? Other than the tank pickup, I don't know what it could be... Any ideas other than the tank pickup or is that probably it? Just want to figure out what it is before I spend some of the little free time that I have tearing into stuff...

 

I'm guessing you're going to need to remove the rear tank and check.
How did you extend the pickup down, could your clamp be leaking and its sucking air from the tank instead of fuel.
If it runs fine on front tank, its probably something to do with the rear lines or the modified pickup you have.
Let us know what you find out.

 

Yeah, I guess I'll end up dropping it. Not a big deal, and I don't mind doing it, BUT I hardly have the time right now, haha. Might find the time over the weekend or early next week. It's also the only truck I have right now, so I can't have it apart for too long...

A PO extended the pickup. I put in the front tank (about 5 months ago, not anywhere near the time of this issue), and I extended it with fuel hose with slits at the bottom and used 2 stainless clamps on the hose.

Could be the clamp leaking. It's been together like this for 15 years, so it's possible whatever was used has just failed with age.

Yep, fine on the front tank.

Sure thing. I'll post back. May post pics when I drop the tank.

 

Had the issue again today for the first time running on the front tank at about 3/4 full. Started starving after about 5 seconds of full throttle acceleration uphill. Drifting back down the other side of the hill it "caught up" with itself and was fine afterward until I did more heavy acceleration several minutes later then is was "surging" and starving on and off for about 30 seconds while just holding speed until I got all the way off the throttle and let it coast for a few seconds. Definitely a fuel restriction somewhere...

Took my carb off, took it apart, and cleaned it. There was plenty of junk in it even though it's only a year old. After seeing the junk in there, I'm thinking I might need to take off and clean the fuel pump. Maybe even replace it if it's bad. I really need a filter before the pump, and haven't had one there in the past.

 

I would have keep the two in-tank pumps to help the engine mounted pump. You can also add a frame pump with a filter. That is the way I had to run my 1982 460 in the mountains. Ran fine when I was not at higher altitude so I only turned the electric pumps when needed with a switch on the dash. The engine mounted pump will pull fuel OK through the electric pumps with them turned off.

 

Well, the engine mounted pump has been coin a fine job up until now. So I'd rather diagnose what happened to make it not perform well enough.

Yeah, I'd thought of doing this a while ago because it seemed like a lot for the engine pump to pull fuel from so far, but it always did fine. I'd like to get it working properly with the current setup, then I may add an electric pump right on the outlet side of the tank switching unit.

Thanks for the reply!

 

If you add an elec pump then you do not need the mechanical pump.
If your engine is built up any, I'd either suggest spending the money and upgrade your complete fuel system. The factory feed line is barely large enough to feed a decent engine.
Are you trying to suck fuel through the diverter valve, it may be sticking now and not completely switching correctly.
I'd still drop both tanks and check it out. If you put the intank pumps back in, just put a return style regulator and eliminate the mechanical pump.
My 86 1ton van has factory elec pumps in the dual tank, with a carb, and they work great.
My 89 mud truck I used the intank pumps and again, worked great.
I have 3/8 line on my 67 mustang, with a high output carter pump, not if enough of fuel line to suck gas through.
Just some examples.

 

I agree, but I think I'd keep both like Bill is saying. Should help eliminate possible vapor lock too.

It's all 3/8" rubber/aluminum through the system. I believe the original was 5/16 "plastic" like hose.

Yeah, it's an electric switching unit. Got it from NAPA, and wired it into the original dash switching toggle. Good for up to 10 PSI. Could be sticking, but wouldn't think it too much because it's only about 5 months old.

Thanks for the examples!

 

Got an electric fuel pump ordered. It may not end up being part of the setup, but it'll be a good testing device. Should have it in 3 days.

 

Well, it's been quite a while since I got everything fixed. I put the electric fuel pump in the system, and it didn't help anything. I took the carburetor off dug into it, and found that one of the stainless steel mesh filters where the fuel enters was clogged badly. Soaked the whole carburetor (minus any rubber or gaskets) in acetone for a while to get it all cleaned up inside smaller passages. Was all good after that. The engine pulled harder than I'd felt it in a while, so I guess it had been slowly clogging for several months.