305E Backfire out Carb and Exhaust
 

My son and I recently bought a 1965 GMC C20 with a 305E. In an effort to troubleshoot a low compression issue on cylinder no. 1, I ended up adjusting the valve lash on no.1 and the rest of the cylinders. Turns out this didn't help no. 1 and it made things worse.

After the lash adjustment we now have a nasty backfire out of the right side exhaust and the carb. It idles fine but when I rev. it our try and drive the backfires begin.

I have double checked the lash many times and i confirmed that I have .012 on the intake and .018 on the exhaust. I checked this by manually turning the motor and waiting for the valve to fully close and the other one to start to open before I checked the lash.

If I pull the no. 6 plug wire the backfires go away so I am pretty sure the issue is with no. 6.

Do any of you guys have a suggestion on what I try next? The really frustrating thing is that we drove it around the block before we started down the valve adjustment road and it ran fine. I have been kicking myself all day as I am pretty sure I screwed something up but I can't figure out what.

Re: 305E Backfire out Carb and Exhaust
 

Wow, do you ever have one of those days when you are so focused what you know the issue is that the over look something very obvious that could cause the same issue? Turns out, all I had to do was step away from the truck, get a beer, eat some food, and watch some football.

After doing all that I came to the realization that we must have mixed up the plug wires during all out fussing with the truck. Sure enough after setting the right order we are back to a running truck.

Re: 305E Backfire out Carb and Exhaust
 

Chrisnaz Welcome! Thanks for checking back in and giving us the happy ending. Glad it all worked out. Photos of your truck?

Re: 305E Backfire out Carb and Exhaust
 

Here is a picture of the new addition to the family with the proud owner on the trailer heading home.

Re: 305E Backfire out Carb and Exhaust
 

:thumbsup: :coool:

Re: 305E Backfire out Carb and Exhaust
 

Minor correction.
It isn't a "C20"
It is a 1500 series (assuming it is, in fact, a 3/4 ton)

C20 would make it a Chevy.
1/2 ton - C10 - 1000
3/4 ton - C20 - 1500
1 ton - C30 - 2500

Otherwise... :)
Welcome to the group. One thing you will come to understand (and possibly hate) is that the parts stores either think GMC and Chevrolet use the same parts, or they will argue that a 305 is a v8, not a v6 (or my favorite "they didn't make a v6 in 1960,61,62,63,64,65,etc")
Motors manuals, if you can find them, are reasonably accurate with data. Chiltons (who now owns Motors) is decent as well.
Factory Manual will be X-6523 if you go looking for one.

What is most annoying with parts is that it can be the one part you are sure wouldn't be different... that is different. For instance on my 1963 2500 I had to replace the rear wheel brake cylinders. Those couldn't be different, right? Chevrolet lists the same part both sides, GMC has a right and a left and it does matter since the brake line won't line up with where it connects to the cylinder if you use the Chevrolet one (I do not remember which side that was).

Wiring harnesses are different, components under the hood are different... you will find all kinds of parts that don't cross over. It makes it more "fun" to work on!!

At least your '65 is in one piece. Mine is in several pieces (as in previous owner completely disassembled it... if it could be taken apart, they did. Even the leaf springs were taken apart)

Have fun with the truck though. They are solid, dependable, and reasonably easy to work on. Once running at its best, these are difficult to kill.

Re: 305E Backfire out Carb and Exhaust
 

Good looking truck.
Looks like a '62 or early '63 hood or turn signals (they appear to be clear lenses). Mid '63 they changed to the amber (orange) lenses (required by law).

Only a real GMC nut would even notice.

Re: 305E Backfire out Carb and Exhaust
 

My first car... several years back I did a tune up on it, replaced everything. Put it all back and it wouldn't run. I double and triple checked the firing order and the parts. A couple months later I look at it again and realize the order and parts are correct, but 180 degrees off. D'Oh!

Now I don't remove all at once, just one at a time. Especially since as I get older I know I am getting dumber... and I'd do it again...

Re: 305E Backfire out Carb and Exhaust
 

Wow, thanks for all the great info. David, you are correct, it is a 1500.

Both my son and I are really looking forward to working on this truck. We are still in the early "what should we do phase". I really want to move the gas tank, add power steering, power breaks and front discs to make it a little safer. Other than that my son is excited about leaving the 305 and three on the tree in there so his friends wont be able to drive it.

Re: 305E Backfire out Carb and Exhaust
 

What I'd like add in response to David's post is it looks like what has been mistaken is the designations used for Chevrolet trucks of "Fleetside" and "Stepside" rather than the correct GMC counterparts which are "Wideside" and "Fenderside".