250 running badly.

[brian.kannady]

So I finally got my 67 with a 250 and 3 on the tree out today. I managed to keep things from breaking too catastrophically to keep running long enough to get out of the driveway. The engine runs fine in the yard (minor stumbling on initial acceleration) and revs just fine. On the road, however, it was almost undrivable. At WOT it stumbled really badly and got worse the longer it went, and by the time I got it turned around and headed back home it would only run with the choke fully out. I only managed to get about a mile on it. After I got it back in the yard, it was OK again. It is all stock except for the HEI conversion. It has a brand new fuel pump, new fuel filter, the HEI is brand new, the carb is a fresh reman. Has plenty of fuel in it. Timing is 4 degrees before just like the manual says, I have spark on all six cylinders. The idle speed screw is 1.5 out as the manual indicates for a starting point. I checked and there is plenty of fuel to the carb.

Any thoughts?

[70LonghornCST]

Brian, I will move this to the Engine and Drivetrain area for you. Can someone please help Brian?

[cableguy0]

How long did it sit for with the fuel that's in it? Modern ethanol blend fuel goes bad extremely fast. I have seen it gel up in less than a month without fuel treatment like star tron.

[brian.kannady]

the fuel in it has been there for probably 6 months. If that is the problem, would filling it up from 1/8 where it is now to full with fresh fuel take care of it, or do you guys think I need to flush the tank out and start completely fresh?

[Alex V.]

Even though you've gone through most of the fuel system, just from reading your description of the problem it sounds like there's a fuel flow problem somewhere. It runs alright at low RPM's on the reduced fuel flow, but when you get it throttled up and need more fuel, continuously, it slowly runs out of gas because supply isn't equaling demand. If it sat for 6 months with ethanol fuel in it, any part of the fuel system could be pugged up - if it was me, I'd probably dilute that 1/8 tank with a fill-up of at least 89 octane fuel, with a can of Seafoam, and start the troubleshooting by disassembling and cleaning the carb. out with carb. cleaner and replacing the fuel filter, then see where that gets you. It won't hurt to drain the tank, but I think another 15-18 gal. of fresh fuel will dilute it enough. Also, a clear fuel filter wouldn't be a bad idea, so you can see what, if anything, gets pulled out of tank for a better idea of what the old ethanol fuel did after sitting that long.

[cableguy0]

I would be worried about the fuel in the carb more than anything. That's where the fuel will tend to gel or turn bad the fastest. Your describing an extremely lean condition. Which would make me very suspect of the carb being gummed up or a fuel delivery problem. Any chance you can yank the carb apart and clean everything up? I would fill the tank with 93 octane as well as an additive like star tron to counteract the ethanol.

[68gmsee]

I agree it sounds like a fuel starvation problem. Or, it could be a really bad vacuum leak but I would take the carb off and take it back to whoever rebuilt it or to the parts store you got it from. Years ago I went through at least three before I got a halfways decent one. Now I buy cheap used ones and just rebuild them myself since they are easy to rebuild and the auto parts rebuilt ones are not worth the money they're asking for them.

[cableguy0]

Originally Posted by 68gmsee

I agree it sounds like a fuel starvation problem. Or, it could be a really bad vacuum leak but I would take the carb off and take it back to whoever rebuilt it or to the parts store you got it from. Years ago I went through at least three before I got a halfways decent one. Now I buy cheap used ones and just rebuild them myself since they are easy to rebuild and the auto parts rebuilt ones are not worth the money they're asking for them.

That's an excellent point that I didn't even think to post. Most of the parts house rebuilds are worse than the carb your replacing in the first place. They are being rebuilt by people who have no clue what a carburetor is or does. They are just told to put part A in Hole B and to repeat all day.

[geezer#99]

Bump your timing up first to 12 degrees btdc. Set it with the vac advance line plugged of course.

[Mike Wessinger]

Originally Posted by geezer#99

Bump your timing up first to 12 degrees btdc. Set it with the vac advance line plugged of course.

This is where I would start too. Just after I got my truck back on the road late last year, the shop that did my rebuild had the timing set at spec. It would run fine in the shop, and OK down the road, but after I got a few miles on the engine it started acting the same as the problem you have. Took it back in and we started timing adjustment by feel. Final turned out to be around 18 degrees, but she ran like a new truck.

[storm9c1]

I agree on the timing. 4 degrees in the book was for 1960s fuel and to hold back emissions. Nowadays, you can easily start at 8 degrees on that engine and work your way up from there. It may be OK to go to 12 degrees or so, just listen for pinging.

How high you can go will depend on your compression and load that the engine is under. For example, you may be able to go as high as 18 (as mentioned above) but then it might kick back when starting, or ping on steep hills, or ping with fuel that sat for a while and lost octane, etc. So keep that in mind.

I usually like to give myself a conservative ceiling with timing so that I don't damage the engine under an abnormal circumstance. That's why I'd only recommend about 12 degrees max. I've timed engines by ear before (on the side of the road too) and ended up at 18 degrees. LOL.

This is the one case where you have to disregard the specs in the FSM unfortunately.