Truck Hesitates at 50mph - Coil? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bluec10 · 05-29-2009, 09:44 AM

I've got a '69 GMC with an inline 6. When I take the truck on the highway it works well, accelerating to speed. BUT when I get to 50 mph, that's it. It hesitates and will not accelerate further. It feels like an electrical problem because otherwise it drives well, but it could be fuel.

I've replaced the plug wires, distributor cap, rotor, points and condenser with no change. What next?

As a related issue, the coil on the truck seems to have an additional condenser attached. I've never seen this, so I was wondering if someone could explain.

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Name: coil.jpg
Views: 1608
Size: 57.7 KB

69GMCLonghorn · 05-29-2009, 10:24 AM

Buy a Pertronix Ignitor II coil and the part that replaces the points in the distributor. It will be the cheapest and best thing you could do to an old engine. If that doesn't solve it, replace the fuel pump and/or clean out the ol' carbatuter...

**WorkinLonghorn** · 05-29-2009, 10:26 AM

First thought is plogged fuel filter.Check that and if no improvement check that coil if you have a meter.They do go out. You could pull a plug wire off and hold it close to ground (or use a sparkplug held up against the intake mani,etc.) and see if the spark is real good. If a feeble spark,maybe replace the coil and the capacitor ( I think that's what that is,can't remember).

**leddzepp** · 05-29-2009, 11:57 AM

That additional part on your coil is to reduce noise heard through the radio. I have never seen it on the coil before...usually it is on the regulator.

ChevLoRay · 05-29-2009, 01:19 PM

I've seen 'em on the coils.

My experience with coils that are suspect is limited to having a bad coil wire, which breaks down just like turning off the key (under load).

If it isn't leaking or you can see it arcing (in the dark), I'd think it is okay if the engine will run.

You could try the Pertronix route or look for an HEI from an in-line 6, if you want to get away from what you have.

Stock72c10 · 05-29-2009, 01:18 PM

Is your vacuum advance working?

PanelDeland · 05-29-2009, 04:43 PM

I'll say HEI all the way.I couldn't believe the difference it made from my well tuned points dist to the HEI.It was a simple swap also.Here's everything you need to know to do it in an hour or so after you have the parts.Make sure you have a cold one handy since you'll have time for it,if you follow the directions exactly.
http://67-72chevytrucks.com/vboard/s...d.php?t=148346

Just so you know,I bought a coil in cap unit complete from the local "Pick and pull" for $40.00 a set of wires and new plugs.IIRC total for the swap was about $75.00, but I could have probably gotten by with the plugs I had.The coil,module and cap were in good shape so I got lucky.I would probably had a few more $$$'s in it if I had to replace any of them.

**WorkinLonghorn** · 05-29-2009, 04:53 PM

Really though,before throwing a bunch of parts at it,check the fuel filter,then TEST the coil.
A properly functioning and maintained points ignition will work just fine.

LONGHAIR · 05-29-2009, 05:22 PM

I am with the fuel filter guys on this one. What kind of RPM are we talking here?

The extra thing on your coil is the radio interference supressor. I have seen a lot of them mounted to the coil, but it's another thing that you can eliminate by swapping to HEI.
Like Workinlonghorn said, points will work fine, they just need more attention than HEI parts.

Longhorn Man · 05-29-2009, 05:35 PM

this sounds like a classic fuel issue, not electrical.
At 3000 RPMs in second gear, the fuel consuption is less, than 3000 RPMs in 3rd gear due to the engine working harder to overcome the wind resistance.
Since the complaint is described as "problems at 50 MPH" and not "problems at 2800 RPM", it points to a fuel related issue.

I would replace the fuel filter(s), and clean the carb with some gumout or berryman's B12 chemtool carb cleaner, and see what happens.

**WorkinLonghorn** · 05-29-2009, 06:22 PM

I give up.

**leddzepp** · 05-29-2009, 06:36 PM

Quote:

Originally Posted by WorkinLonghorn

I give up.

I know how ya feel dude!

factorystock · 05-31-2009, 01:04 PM

One more fuel filter to check.Some fuel tank sending units have a filter on the end of the pickup tube.

Shaky · 05-31-2009, 01:11 PM

Just something to check:

I had a '70 Bel Air with a 250 and points. The engine would die at high rpm. Long story short - the resistor wire was not working right.

A points coil requires 7 volts. If it gets too much voltage it doesn't work. Mine would see 7 volts at lower rpm, but when the engine spun, it would get 12 volts.

I solved it by slamming an HEI in there. Problem solved.

jim zag · 06-01-2009, 05:46 AM

How much pressure does the fuel pump make? Volume? It should pump a quart of fuel in maybe 30 seconds? FILTER? It sounds like the carb is running out of fuel.

rlm316 · 06-01-2009, 01:54 PM

Check the base gasket on the carb. Also check for any other vacuum leaks.

bluec10 · 06-03-2009, 04:01 PM

Thanks for your help - fuel filter it was! I ruled it out too early because I thought I'd put one in recently. If I did, there was sure a lot of crap in the gas tank because the inside of the old filter was coated. The truck runs like a champion- three-on-the-tree rules!

05-29-2009, 09:44 AM	#1
bluec10 Registered User Join Date: Apr 2002 Location: Manitoba, Canada Posts: 663	Truck Hesitates at 50mph - Coil? I've got a '69 GMC with an inline 6. When I take the truck on the highway it works well, accelerating to speed. BUT when I get to 50 mph, that's it. It hesitates and will not accelerate further. It feels like an electrical problem because otherwise it drives well, but it could be fuel. I've replaced the plug wires, distributor cap, rotor, points and condenser with no change. What next? As a related issue, the coil on the truck seems to have an additional condenser attached. I've never seen this, so I was wondering if someone could explain. __________________ 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.

05-29-2009, 10:24 AM	#2
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Truck Hesitates at 50mph - Coil? Buy a Pertronix Ignitor II coil and the part that replaces the points in the distributor. It will be the cheapest and best thing you could do to an old engine. If that doesn't solve it, replace the fuel pump and/or clean out the ol' carbatuter...

05-29-2009, 10:26 AM	#3
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: Truck Hesitates at 50mph - Coil? First thought is plogged fuel filter.Check that and if no improvement check that coil if you have a meter.They do go out. You could pull a plug wire off and hold it close to ground (or use a sparkplug held up against the intake mani,etc.) and see if the spark is real good. If a feeble spark,maybe replace the coil and the capacitor ( I think that's what that is,can't remember). __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

05-29-2009, 11:57 AM	#4
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 20,022	Re: Truck Hesitates at 50mph - Coil? That additional part on your coil is to reduce noise heard through the radio. I have never seen it on the coil before...usually it is on the regulator. __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man RIP FleetsidePaul

05-29-2009, 01:19 PM	#5
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Truck Hesitates at 50mph - Coil? I've seen 'em on the coils. My experience with coils that are suspect is limited to having a bad coil wire, which breaks down just like turning off the key (under load). If it isn't leaking or you can see it arcing (in the dark), I'd think it is okay if the engine will run. You could try the Pertronix route or look for an HEI from an in-line 6, if you want to get away from what you have. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

05-29-2009, 01:18 PM	#6
Stock72c10 classic=of recognized value Join Date: Sep 2008 Location: Wichita, KS Posts: 317	Re: Truck Hesitates at 50mph - Coil? Is your vacuum advance working?

05-29-2009, 04:43 PM	#7
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,993	Re: Truck Hesitates at 50mph - Coil? I'll say HEI all the way.I couldn't believe the difference it made from my well tuned points dist to the HEI.It was a simple swap also.Here's everything you need to know to do it in an hour or so after you have the parts.Make sure you have a cold one handy since you'll have time for it,if you follow the directions exactly. http://67-72chevytrucks.com/vboard/s...d.php?t=148346 Just so you know,I bought a coil in cap unit complete from the local "Pick and pull" for $40.00 a set of wires and new plugs.IIRC total for the swap was about $75.00, but I could have probably gotten by with the plugs I had.The coil,module and cap were in good shape so I got lucky.I would probably had a few more $$$'s in it if I had to replace any of them. __________________ The 47-present Chevrolet and GMC Truck Message Board Network,it's owners,moderators,members,and associates of any type should not be held responsible for my opinion. You can't fix stupid,not even with duct tape. "My appearance is due to the fact that "GOD" does punish you for having too much fun!" Barrett-Jackson has perfected alchemy,they make rust into gold! "You can lead a horse to water but you can't saddle a duck" "Cleverly disguised as a 'Responsible Adult' "Sometimes your Knight in shining armor is just a retard in tinfoil" Last edited by PanelDeland; 05-29-2009 at 04:46 PM.

05-29-2009, 04:53 PM	#8
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: Truck Hesitates at 50mph - Coil? Really though,before throwing a bunch of parts at it,check the fuel filter,then TEST the coil. A properly functioning and maintained points ignition will work just fine. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

05-29-2009, 05:35 PM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Truck Hesitates at 50mph - Coil? this sounds like a classic fuel issue, not electrical. At 3000 RPMs in second gear, the fuel consuption is less, than 3000 RPMs in 3rd gear due to the engine working harder to overcome the wind resistance. Since the complaint is described as "problems at 50 MPH" and not "problems at 2800 RPM", it points to a fuel related issue. I would replace the fuel filter(s), and clean the carb with some gumout or berryman's B12 chemtool carb cleaner, and see what happens.

05-29-2009, 06:22 PM	#11
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: Truck Hesitates at 50mph - Coil? I give up. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

05-31-2009, 01:04 PM	#13
factorystock Registered User Join Date: Feb 2007 Location: west coast Posts: 3,380	Re: Truck Hesitates at 50mph - Coil? One more fuel filter to check.Some fuel tank sending units have a filter on the end of the pickup tube.

05-31-2009, 01:11 PM	#14
Shaky Six in a row makes it go... Join Date: Mar 2009 Location: Vacationland: Maine Posts: 1,346	Re: Truck Hesitates at 50mph - Coil? Just something to check: I had a '70 Bel Air with a 250 and points. The engine would die at high rpm. Long story short - the resistor wire was not working right. A points coil requires 7 volts. If it gets too much voltage it doesn't work. Mine would see 7 volts at lower rpm, but when the engine spun, it would get 12 volts. I solved it by slamming an HEI in there. Problem solved. __________________ 67 C20 long step resto: http://67-72chevytrucks.com/vboard/s...d.php?t=342086 66 C10 long step build: http://67-72chevytrucks.com/vboard/s...=1#post3814790 CT to Alaska in a 67 C10: http://67-72chevytrucks.com/vboard/s...d.php?t=399224 “The height of sophistication is simplicity." - Clare Boothe Luce

06-01-2009, 05:46 AM	#15
jim zag ENGINE BUILDER Join Date: Apr 2006 Location: Shortsville n.y. Posts: 344	Re: Truck Hesitates at 50mph - Coil? How much pressure does the fuel pump make? Volume? It should pump a quart of fuel in maybe 30 seconds? FILTER? It sounds like the carb is running out of fuel. __________________ JIM ZAG

06-01-2009, 01:54 PM	#16
rlm316 Registered User Join Date: Aug 2006 Location: Woodstown, NJ Posts: 669	Re: Truck Hesitates at 50mph - Coil? Check the base gasket on the carb. Also check for any other vacuum leaks.

06-03-2009, 04:01 PM	#17
bluec10 Registered User Join Date: Apr 2002 Location: Manitoba, Canada Posts: 663	Re: Truck Hesitates at 50mph - Coil? Thanks for your help - fuel filter it was! I ruled it out too early because I thought I'd put one in recently. If I did, there was sure a lot of crap in the gas tank because the inside of the old filter was coated. The truck runs like a champion- three-on-the-tree rules! __________________ 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.



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