67 250 runs rough, backfires when shut down.

[67CHedges]

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]

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

[67CHedges]

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]

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]

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]

sounds like a timing problem to me also

[Blue Beard]

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]

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]

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

[67CHedges]

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]

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

[67CHedges]

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]

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]

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]

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]

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.