Missing at cruising speed. Confused.

[YourBuddy'sTruck]

I posted a while back about this, but it was way to cold to work on the truck.
I've been experiencing missing, but only while cruising at 50MPH or so.
No trouble at 35MPH or while accelerating.

It started to backfire out of the carb on first start of the day. (scares the begeebuz outta my Wife, too!)
However, after that first start, it doesn't backfire at all.

So I started thinking Timing Chain?

I switched out the plug wires, thinkin that the originals couldn't handle the new HEI.
Then I got to thinking...yeah, I know...there I go thinking again...
Could I have the original Leaded Gas Heads?
I've only put about 2500 miles on the truck since I got'er runnin' last year.
And I only drive about 15-30 miles in a week doing deliveries with it.
Could it be that it took 2500 miles for the problem to show up?
I put in some Instead-O-Lead and it is greatly improved! Only missed once on me in the past 50+ miles.

So my questions are....
Can you tell a Leaded Head from the outside? Or do you have to pull it off and look at the valve seats?
If so, did I damage my engine? If not, am I damaging it with the Instead-O-Lead?
I'm just old enough to recall the Green and Red gas, but not to know what the symptoms were when you mixed'em up.

My truck is a '70 C2 with a 350ci and a TH350.
The engine block number is correct for the era, but I doubt it to be original, then again? It is 3970010

I bought it with a completely empty cab and obviously it was not "running".
Spent the spring getting the puzzle back together with the help of many threads here.
Don't really know much more about it other than it had a seriously bent steering linkage and other front end parts that had the wheel at 90* to go straight.
Like someone hit a ditch or something with it.

Anyway...what are the symptoms of Unleaded in a Leaded engine and vice versa?
Thanks for any help.

[geezer#99]

It's not the heads.
First thing to look at is timing. Usually backfire out of the carb is retarded timing. A miss at cruise is lack of timing.
Do you have a timing light?
Did you run 12v to your hei and eliminate the resistor wire?

[YourBuddy'sTruck]

I installed the HEI as instructions said to do.
I then timed it to spec with the timing tab.
It fired right up and ran great till winter.
I'll get a light on it to double check things though. (getting warmer outside)
If it is the timing, chain stretch or whatever, what should my timing be?
If I recall, I set it at 10* advance as I read specs calling for between 8*-12*.

[BB72CHEVKT]

Do you have fresh plugs?

[Lee H]

Don't get hung up on the heads, unless your running it hard or towing those "unleaded" heads will stand up to normal driving for a real long time, without additives. Check to see if the number 5 & 7 wires are touching anywhere. They are in line in the firing order and can cross fire. As geezer said it's time to dial in the timing. Make sure vacuum & mechanical advance are doing their job at the right RPMs.

[geezer#99]

And check true tdc on #1 to prove your timing marks are correct. Don't forget to plug off the vacuum line to the distributor while setting timing.

[YourBuddy'sTruck]

I pulled the plugs and they all looked good.
Like the golden hue of a perfectly roasted marshmallow...white with a hint of cook.
The driver's side had some loading, but I have an exhaust leak on that side and let it idle to warm up this cold winter.
I forgot about the 5-7 thing and I did have the 5 and 7 wires running side by side like a fool and I moved them.
Seems to be running fine now.
I won't know about the backfire until I've another cold start.

I cannot find my timing light.
Correction- My Dad can't find his timing light.
Looks like I'll finally have to grow up and get one of my own? 41 years was a good run, I guess. Ha!
Anyway...the 5-7 wire thing may have been my issue, but I don't recall having any trouble like this until it got really frigid out there this winter.
Regardless, it is running better and I bet if I dial it in a bit more, it'll be better than ever.

Thanks again for all the help and I'll let you all know if it happens again...or if it doesn't.

[CST10]

Sounds like ya running the carburator lean. That would cause start up backfire. Also will make it feel like a miss at higher speeds but not be there at lower speeds. I had this issue awhile back.

[Lee H]

Don't necessarily think that was your sole problem but a good thing to get out of the way.

[YourBuddy'sTruck]

I hear you fellas. I'll be finding more. I'll bet on that!

All I know about the carb is that it's a Quadrajet.
Not much else, other than they have two stages and are tricky to fine tune.

I much prefer the DellOrto on my motorbike.
Straight through and if you don't have a potato or something stuck in there, you're good to go.

[D.PASSMORE]

Originally Posted by CST10

Sounds like ya running the carburator lean. That would cause start up backfire. Also will make it feel like a miss at higher speeds but not be there at lower speeds. I had this issue awhile back.

x2

[BLK96]

I had this problem years ago with a 72 chevy I still have. I had put an HEI dist in place of the original points type. Went with a parts store reman. Long story short the distributor was defective, but ONLY at light throttle cruise where vac advance was being utilized. Didn't do it at idle (no ported vacuum) or under acceleration (very little vacuum there either). Only under vac advance conditions. Changed out distributor and problem solved. Good luck

[YourBuddy'sTruck]

Thanks for all the advice so far.
I still need to check the timing and learn about tuning the Q-jet.
Regardless...I took the truck on a 30 Mile delivery today and it did not miss once.
It is warmer out, but the other things are certain to contribute.

Also, I thought to share that today was my coolest delivery of all time.
Picked up 4 French Oak and 1 American Oak used barrels from a local winery to deliver to the Catawba Island Brewing Company for them to age some brews in!

I'd have to say that 5 used oak barrels are perfect for balancing out my suspension. Rode like a Cadilac on the way home!
Cheers!