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#1 |
Registered User
Join Date: Apr 2002
Location: Manitoba, Canada
Posts: 663
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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.
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Present: 2015 Tacoma. Yeah, not a GM, but I love it. 1969 GMC 32,000 - fix, drive, relax, fix... 2019 BMW R1250 GSA - Yahoooooooo 1979 Honda GL 1000 - retro touring at its best. Past: '05 Sierra 4x4 - Had 270,000 KM and running well when it was written-off by a stop sign runner. '94 F-150 from the "F word" company. I'll admit it...good truck. Sold what was left of it for $800 to a guy who came to pick it up at 11:00 PM with cash in hand. Hmm. '79 Sierra Grande (Black) organ donor - perfect rebuildable 4-bolt 350 and a good TH350. '76 Sierra Grande (Orange) - hate isn't too strong a word. Kid who bought it turned it into a hot rod. '68 C-10 R.I.P. - Dad's old truck...too far gone to resurrect. '59 C-50 - with hoist. Truck is gone, wife isn't. Nuff said. Last edited by bluec10; 09-22-2013 at 03:49 PM. Reason: grammar |
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#2 |
Registered User
Join Date: Apr 2000
Location: Bonne Terre, Mo
Posts: 3,005
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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.
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'69 Chevy Long Fleet & '71 GMC Short Step & Project "ODD BALL" "You can wash a pig, soak it in most expensive perfume. In the end it is still just a pig." ODD BALL build thread http://67-72chevytrucks.com/vboard/s...d.php?t=478629" |
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#3 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Just when I thought it was running perfectly...
Got a vented gas cap?
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#4 |
Registered User
Join Date: Jul 2012
Location: Ft Wayne Indiana
Posts: 1,204
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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.
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71 K20 64 GMC mud truck |
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#5 |
I have a radical idea!
Join Date: Jan 2005
Location: Sweet Home Alabama!
Posts: 6,513
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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.
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'67 C-30 Dually Pickup 6.2 Turbo Diesel, NP435 ‘72 C-10 SWB , 350 4bbl, TH350 '69 C-10 SWB , 250 L6, 3 OTT '69 GMC C3500, dump truck, 351 V6, NP435 '84 M1009 CUCV Military Blazer 67 C-30 Turbodiesel build thread http://67-72chevytrucks.com/vboard/s...d.php?t=254096 My trucks http://s226.photobucket.com/albums/d...ediafilter=all Member of the 1-Ton Club! |
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electrical issues, fuel, hesitation, starvation |
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