67 250 runs rough, backfires when shut down. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

67CHedges · 10-17-2003, 09:54 AM

My 67 C-10 250 three on the tree is not running as smooth as it used to.

The symptoms are:

1). It seems like it is running on 5.5 or five cylinders. You can really tell when you are trying to pull a hill.

2). When I shut it down after it operates under normal operating temperature, it has a loud backfire.

We have recently converted the distributor to a HEI pointless ignition and replaced the carb a few months before that. The carb has never worked as well as the old one.

I have tried to troubleshoot and have done the following.

1). reset the timing several times.
2). verified the wiring, distributor and coil.
3). I checked the compression to see if I had a valve problem. The book I have says the compression is 130 psi. From front to back, starting with the first cylinder, I range from 125-150. Cylinders 2-6 are within 5-10 pounds. The first cylinder is about 20 off from the others. Don't know if that is contributing to the problem or not, but wanted to check it anyway.

I have not changed the plug wires, they are new, but I know that doesn't necessarily mean they are good. I checked the gap on the plugs when I checked the compression, they were all close, if not right on.

I am frustrated and wondered if anyone has some tips before I hand it over to a garage. I checked some of the archieves and didn't see anything that addressed my issue.

I am wondering if we have the distributor off a tooth? Or if the new carb is the culprit.

Thanks in advance.

oldsub86 · 10-17-2003, 11:39 AM

Did the troubles exist before you converted to HEI?
Did the troubles exist before you changed carbs?

67CHedges · 10-17-2003, 12:12 PM

It never has run as good with the replacement carb.

I am a little leary of replacing it since it would run me around $100. That is a pretty expensive guess. There is supposed to be a good carb guy in town I was thinking taking it to. We have tweaked on it to our ability.

The old distributor was wore out. It would backfire some when going down a hill. The cam was wore almost in a perfect circle.

I was hoping it was something simple. This was my Grandpa's old truck. He bought it new, and I got it after he died. He would never give it to me while he was living. It only has 105K miles on it.

TP from Cntl PA · 10-17-2003, 04:07 PM

Maybe you got a bad plug? I have gotten Champions(Well, my buddy bought them.........I always use AC's) for our Farmalls that were bad, and they were brand spanken new!

My brother's 69 C-10 did the same exact thing.........But the shmuck who sold us the truck had put the plug wires on wrong...............It is a 250 and it likes to back fire, but I have the timing more advanced than it should be...............But man, pulling 300 bu gravity wagons up mountains with a 250 and 3spd, with 3.73 gears it runs like man out the straight exhaust stack we have on it!!!!!!!!!!!!11

Denny · 10-17-2003, 07:27 PM

It sounds like a timing problem to me. Might want to check if the distributor was put in at TDC for sure. Good luck.

kroodawg · 10-17-2003, 07:29 PM

sounds like a timing problem to me also

Blue Beard · 10-19-2003, 10:17 AM

Hope the plug wires are for the HEI.

And assuming that you eliminated the yellow wire from the old distributor to the starter when you converted.

67CHedges · 10-19-2003, 01:29 PM

I am pretty sure the wires are the right type.

As far as the yellow wire to the starter, it is still there.

What does that do?

Blue Beard · 10-19-2003, 01:33 PM

Disconnect the yellow wire from the starter...........its not needed.

67CHedges · 10-28-2003, 03:30 PM

Thanks Bluebeard. That helped it some. It is running better after I removed that wire than it has in a long time. I don't know why, but it is.

I may get her back to normal after some more timing and carb tweaks. She may be back to purring like a kitten soon

CPNEJR · 10-28-2003, 04:15 PM

I know my 67 backfired because the carb was junk. as for the rest I have no clue

67CHedges · 10-28-2003, 04:27 PM

It is a prime suspect.

The truck has only had 3 carbs in its lifetime. The original was not as good as the second. The second was the best carb I ever had. I had to replace it with the one I have on it now. It is a rebuilt model, like the second, but has never performed well, in my opinion. I hate to guess, pull it off, and replace it with another near $100 dollard rebuild. Unfortunately it is not like the old days when I had these things laying around everywhere and changed them out once a week (or weekend

) O 4 the good ol days

69C10Atl · 10-28-2003, 06:13 PM

How long ago did you change the HEI. If i'm not mistaken the position of #1 on a HEI is different than a point type distributor. At least it is on a V8.

67CHedges · 10-29-2003, 09:47 AM

2-3 months ago, maybe longer. I don't drive it too much, maybe once or twice a week, especially since it is not running as good as it has been, and drinking extra gas.

Where can I find the firing order, if it is different than the stock distributor?

Maybe I misidentified the distributor. The one I used was out of a 70 Nova 6 cylinder. It has an external coil, but no points. I was told that is what it was, but maybe that is not what it is.

69C10Atl · 10-29-2003, 02:05 PM

I'm not sure of the firing order for a 6cyl with an hei, but on a V8 the firing order is the same but the position for the #1 spark plug wire on the distributor moves over one plug clockwise. I found that out the hard way, just rebuilt Holley four barrel and rebuilt HEI, fired up truck it ran rough then backfired through carb blowing out my power valve. Once I found out the Hei is a little different I was so mad. Never did fix the Holley, just replaced it.

67CHedges · 10-29-2003, 02:37 PM

That would explain why it seems like it is running on 5 or 5.5 cylinders. I think we have it timed right. We used a light and used the settings indicated in my manual.

It does run better after I removed the yellow wire from the coil, but still is missing something.

I would figure it would run pretty nasty if the wires are one out or order.

Has anyone else had experience with this? Firing order that is.

10-17-2003, 11:39 AM	#2
oldsub86 Registered User Join Date: Dec 2002 Location: Canada Posts: 695	Did the troubles exist before you converted to HEI? Did the troubles exist before you changed carbs? __________________ 1968 Chevy - 292 with a powerglide

10-17-2003, 12:12 PM	#3
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	It never has run as good with the replacement carb. I am a little leary of replacing it since it would run me around $100. That is a pretty expensive guess. There is supposed to be a good carb guy in town I was thinking taking it to. We have tweaked on it to our ability. The old distributor was wore out. It would backfire some when going down a hill. The cam was wore almost in a perfect circle. I was hoping it was something simple. This was my Grandpa's old truck. He bought it new, and I got it after he died. He would never give it to me while he was living. It only has 105K miles on it. __________________ 1967 C-10 Custom, 250, 3 on the tree.

10-17-2003, 04:07 PM	#4
TP from Cntl PA Registered User Join Date: Apr 2003 Location: Central PA....In the Berg of Nisbet Posts: 790	Maybe you got a bad plug? I have gotten Champions(Well, my buddy bought them.........I always use AC's) for our Farmalls that were bad, and they were brand spanken new! My brother's 69 C-10 did the same exact thing.........But the shmuck who sold us the truck had put the plug wires on wrong...............It is a 250 and it likes to back fire, but I have the timing more advanced than it should be...............But man, pulling 300 bu gravity wagons up mountains with a 250 and 3spd, with 3.73 gears it runs like man out the straight exhaust stack we have on it!!!!!!!!!!!!11 __________________ '72 K-20 EFI'ed 250 Inline/4spd stake body, Farm Truck '71 C-20 Cummins Diesel Powered, In storage thanks to $5.00 diesel! '69 3500 GMC 305V-6/4spd, Still under reconstruction.......... Inlines Rule! 6 soldiers standing is better than 8 laying down!!!!!!!!

10-17-2003, 07:27 PM	#5
Denny Registered User Join Date: Jul 2002 Location: Cedar Hill, MO Posts: 426	It sounds like a timing problem to me. Might want to check if the distributor was put in at TDC for sure. Good luck. __________________ 1969 Chevy C10. Dakota Digital Gauges, 383 Votex Engine, MSD 8361 Distributor, MSD 6A Unit, Demon Carb, Phoenix Transmission 4L80E trans, 3:73 Posi Rear End Early Classics 6 Lug Disc Brakes and Spindles All Stainless Lines

10-17-2003, 07:29 PM	#6
kroodawg Registered User Join Date: Nov 2002 Location: Florida Posts: 42	sounds like a timing problem to me also __________________ 1968 SWB CST

10-17-2003, 09:54 AM	#1
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	67 250 runs rough, backfires when shut down. My 67 C-10 250 three on the tree is not running as smooth as it used to. The symptoms are: 1). It seems like it is running on 5.5 or five cylinders. You can really tell when you are trying to pull a hill. 2). When I shut it down after it operates under normal operating temperature, it has a loud backfire. We have recently converted the distributor to a HEI pointless ignition and replaced the carb a few months before that. The carb has never worked as well as the old one. I have tried to troubleshoot and have done the following. 1). reset the timing several times. 2). verified the wiring, distributor and coil. 3). I checked the compression to see if I had a valve problem. The book I have says the compression is 130 psi. From front to back, starting with the first cylinder, I range from 125-150. Cylinders 2-6 are within 5-10 pounds. The first cylinder is about 20 off from the others. Don't know if that is contributing to the problem or not, but wanted to check it anyway. I have not changed the plug wires, they are new, but I know that doesn't necessarily mean they are good. I checked the gap on the plugs when I checked the compression, they were all close, if not right on. I am frustrated and wondered if anyone has some tips before I hand it over to a garage. I checked some of the archieves and didn't see anything that addressed my issue. I am wondering if we have the distributor off a tooth? Or if the new carb is the culprit. Thanks in advance. __________________ 1967 C-10 Custom, 250, 3 on the tree.

10-19-2003, 10:17 AM	#7
Blue Beard Registered User Join Date: Jan 2003 Location: Washington State Posts: 8,834	Hope the plug wires are for the HEI. And assuming that you eliminated the yellow wire from the old distributor to the starter when you converted. __________________ 1970 Chev CST 2003 Harley Fatboy 1975 Chevrolet Step Van 1956 Chev Bel Air 1977 Blazer 2WD For Sale $3000.00 1978 Blazer 2WD For Sale $7000.00 1978 Silverado 2005 Monte Carlo

10-19-2003, 01:29 PM	#8
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	I am pretty sure the wires are the right type. As far as the yellow wire to the starter, it is still there. What does that do? __________________ 1967 C-10 Custom, 250, 3 on the tree.

10-19-2003, 01:33 PM	#9
Blue Beard Registered User Join Date: Jan 2003 Location: Washington State Posts: 8,834	Disconnect the yellow wire from the starter...........its not needed. __________________ 1970 Chev CST 2003 Harley Fatboy 1975 Chevrolet Step Van 1956 Chev Bel Air 1977 Blazer 2WD For Sale $3000.00 1978 Blazer 2WD For Sale $7000.00 1978 Silverado 2005 Monte Carlo

10-28-2003, 03:30 PM	#10
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	Thanks Bluebeard. That helped it some. It is running better after I removed that wire than it has in a long time. I don't know why, but it is. I may get her back to normal after some more timing and carb tweaks. She may be back to purring like a kitten soon __________________ 1967 C-10 Custom, 250, 3 on the tree.

10-28-2003, 04:15 PM	#11
CPNEJR Registered User Join Date: Sep 2003 Location: Franconia,N.H. Posts: 1,029	I know my 67 backfired because the carb was junk. as for the rest I have no clue __________________ (Only) Member of the CPNE Fan Club Aint that the truth 67 C-10 350 with 3-speed tranny and 308 gears swb step 82 Z28 Camaro 355 small block over 350 hp and a borg warner super t-10 (blown engine) 72 Chevy K20 Suburban (future project) 70 Chevy K20 now 68 GMC K2500 2003 Chevy 2500hd ccsb 4X4 (daily driver)

10-28-2003, 04:27 PM	#12
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	It is a prime suspect. The truck has only had 3 carbs in its lifetime. The original was not as good as the second. The second was the best carb I ever had. I had to replace it with the one I have on it now. It is a rebuilt model, like the second, but has never performed well, in my opinion. I hate to guess, pull it off, and replace it with another near $100 dollard rebuild. Unfortunately it is not like the old days when I had these things laying around everywhere and changed them out once a week (or weekend ) O 4 the good ol days __________________ 1967 C-10 Custom, 250, 3 on the tree.

10-28-2003, 06:13 PM	#13
69C10Atl Trying to stick with it Join Date: Jul 2002 Location: Atlanta,GA Posts: 115	How long ago did you change the HEI. If i'm not mistaken the position of #1 on a HEI is different than a point type distributor. At least it is on a V8. __________________ 69 Chevy C10 LWB somewhere in the pile of parts in my garage. 01 Cadillac STS as daily driver

10-29-2003, 09:47 AM	#14
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	2-3 months ago, maybe longer. I don't drive it too much, maybe once or twice a week, especially since it is not running as good as it has been, and drinking extra gas. Where can I find the firing order, if it is different than the stock distributor? Maybe I misidentified the distributor. The one I used was out of a 70 Nova 6 cylinder. It has an external coil, but no points. I was told that is what it was, but maybe that is not what it is. __________________ 1967 C-10 Custom, 250, 3 on the tree.

10-29-2003, 02:05 PM	#15
69C10Atl Trying to stick with it Join Date: Jul 2002 Location: Atlanta,GA Posts: 115	I'm not sure of the firing order for a 6cyl with an hei, but on a V8 the firing order is the same but the position for the #1 spark plug wire on the distributor moves over one plug clockwise. I found that out the hard way, just rebuilt Holley four barrel and rebuilt HEI, fired up truck it ran rough then backfired through carb blowing out my power valve. Once I found out the Hei is a little different I was so mad. Never did fix the Holley, just replaced it. __________________ 69 Chevy C10 LWB somewhere in the pile of parts in my garage. 01 Cadillac STS as daily driver



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10-29-2003, 02:37 PM	#16
67CHedges Registered User Join Date: May 2000 Location: Richmond, KY Posts: 13	That would explain why it seems like it is running on 5 or 5.5 cylinders. I think we have it timed right. We used a light and used the settings indicated in my manual. It does run better after I removed the yellow wire from the coil, but still is missing something. I would figure it would run pretty nasty if the wires are one out or order. Has anyone else had experience with this? Firing order that is. __________________ 1967 C-10 Custom, 250, 3 on the tree.

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