Just when I thought it was running perfectly...
 

While I'm building my "dream" small block, I'm taking some pride in having the inline 6/250 in my truck running like a clock. Driving the 250 three-on-the-tree is fun and brings back a lot of memories learning to drive my dad's '68.

Anyway, my '69 was running perfectly until a couple of weeks ago. In Spring I installed a new coil and fuel pump. Rebuilt the carb last year. All ignition parts have been replaced in the last few years. I upgraded to a newer 1-wire alternator with a kit from LMC and it made a big difference. Like I said, up to a month ago the truck was running like brand-new - smooth acceleration right up to 80 MPH.

Lately I've been having a strange problem. The trough "coughs" a bit when I step on the gas, but not in a big way. The big problem happens when cruising at highway speeds. At first everything is fine and I can cruise at 65 MPH. Then without warning the truck starts acting like its running out of gas. I release the pedal, ease back into it and I can bring it back up to 60. Last night I went for a 30 minute drive and by the time I arrived I was down to 50 MPH.

The odd thing is that after the truck sits for a few minutes the problem goes away. But...I'll drive for 5-10 minutes and the 65, 60, 55, 50 problem starts all-over again. Even driving slowly around town seems to make the problem go away.

Any thoughts? I was thinking that maybe there is some crap in the fuel system that gets sucked-in at higher RPMS. I ran some Seafoam through the system but it didn't make a difference. The fuel is pretty fresh.

I was also thinking that maybe the problem is electrical but there is no warning light, lights aren't dimming, and the truck starts very well. Turning the lights on and off when driving didn't make any difference.

I'm going to blow out the fuel lines just to be sure. Any advice you migh have would be much appreciated - not much nice weather left here in Canada and I want to get in a few more cruises before the snow flies.

Re: Just when I thought it was running perfectly...
 

Fuel filter or the fuel sock in the gas tank would be a guess. Tank has accumulated some trash or sediment? Could even be something as simple as a rubber fuel line with old aged cracking/ kinked or sucking air when it is under a large demand. Just a couple of ideas for you.

Re: Just when I thought it was running perfectly...
 

Got a vented gas cap?

Re: Just when I thought it was running perfectly...
 

Sounds like it could be vapor lock, when the fuel starts vaporizing before it gets to the carb. Usually it can be fixed by moving the fuel line farther away from the motor.

The reason I think this is because you said it runs fine for a while after you shut it off, which would allow the motor to cool, thus the vapor lock not being a problem until the motor warms back up.

Re: Just when I thought it was running perfectly...
 

Make sure the screws that hold carb together haven't worked loose. Inline 6's are notorious for vibrating the carb apart since they have the small 2 bolt baseplate. I'd check the baseplate stud nuts as well, but usually they will sill be tight, but the body of the carb will be loose. The weight of the carb body and air filter cause them to work loose.