402 backfiring hard start

[1972_C20]

So, I've been browsing the forum and haven't seen anything about the issue I'm having. I have a 72 c20 with the 402. All stock minus an HEI. Truck was running and driving just fine. Drove it a good 30 miles for thanksgiving, left to head back home and it idled normal, but when I put a heavier load on it the carb began to backfire. I took it slow the rest of the way home. Went to start it the next day and nothing. Absolutely no fuel was pumping. Replaced fuel pump. Got it started and idles good, but as soon as I put the gas pedal down it backfires again and dies. Then it's hard to start. I've checked for vacuum leaks and can't find any. Pulled the cap off of the dizzy to see if there was moisture and it's dry. Starting to pull my hair out with this one. Any help is appreciated. Thanks.

[bw46]

I'd try a new HEI module because they are cheap. Good to have on hand anyway.

Maybe clogged fuel filter or needle / seat float bowl issue leaning it out real bad.
That might explain why fresh fuel pump helped but didn't solve.

Check timing, distributor could have slipped although unlikely.

Check plug wires for cracks and aging. Unlikely they went bad abruptly but good to check anyway

Good luck.

[1972_C20]

I'll check the filter inside the carb tomorrow. And if that doesnt work I'll look into the control module. Thank you for the input. I'll keep you posted

[custom10nut]

x2 check to make sure your distributor is tight. Your symptoms sound like the hold down came loose and it advanced too far

[Coley]

I've also seen this with engines that have just scrubbed some of the teeth of the timing chain pulleys...thus resulting in timing chain slip. Not a good thing but fixable if figured out early before full part failure occurs.
As mentioned....heavily and thoroughly check your timing....TDC, #1 cylinder the whole deal...top to bottom because it does sound like a timing issue to me as well.
All good
Coley

[geezer#99]

Originally Posted by custom10nut

x2 check to make sure your distributor is tight. Your symptoms sound like the hold down came loose and it advanced too far

Actually a loose distributor retards the timing.
Rotor turns cw and that action induces the body to rotate with it.

[Coley]

Geezer....you are the king and grand prince of timing issues!....lol

[70STOVEBOLT]

My vote is for the fuel filter.

[1972_C20]

I'm going to look into it today. I'll check timing first. My tools are at the shop unfortunately, so i can only do so much. If timing is off it will most likely be advanced, so I'll try retarding the distributer a little and see if it makes it driveable. I'll keep everyone posted.

[57taskforce]

Originally Posted by 1972_C20

So, I've been browsing the forum and haven't seen anything about the issue I'm having. I have a 72 c20 with the 402. All stock minus an HEI. Truck was running and driving just fine. Drove it a good 30 miles for thanksgiving, left to head back home and it idled normal, but when I put a heavier load on it the carb began to backfire. I took it slow the rest of the way home. Went to start it the next day and nothing. Absolutely no fuel was pumping. Replaced fuel pump. Got it started and idles good, but as soon as I put the gas pedal down it backfires again and dies. Then it's hard to start. I've checked for vacuum leaks and can't find any. Pulled the cap off of the dizzy to see if there was moisture and it's dry. Starting to pull my hair out with this one. Any help is appreciated. Thanks.

I actually went thru something similar to this back during the summer. I started driving my K/20 around the house doing chores and such after it sat for an unknown amount of time. For the first few weeks it ran pretty good but eventually it started running worse and worse, back firing thru the carb under load, no power at all, eventually it got so bad that about 1 mile from my house I had to drop it into 4 low and limp home on the side of the road. I tore the carb down only to find rust and deposits and varnish gumming everything up. Rebuilt the carb, changed the filter and fuel pump and cleaned the tank out. I also added some initial timing in and now it runs very well.

[geezer#99]

Carb's don't backfire from advanced timing.
Retarded timing plus lean fuel vapor will make it backfire.
Being lean can be a result of a cold intake too.
What do you run for initial timing?

[davepl]

Originally Posted by 70STOVEBOLT

My vote is for the fuel filter.

I thought of that, but on a Q-jet the filter is somewhat ingeniously spring loaded so if you get lazy and never change it, it eventually gets pushed out of the way and still flows, even when completely plugged.

He could have an inline filter somewhere that's clogged, but from the description as soon as he blips the throttle he has an issue, and the fuel supply for that should be covered by the fuel bowls, not the supply line.

That was my reasoning, anyway!

[davepl]

Originally Posted by geezer#99

Carb's don't backfire from advanced timing.

If it's advanced enough that it's firing when the valve is still open it sure can. How you'd keep it running I have no idea, but on principal I guess if you were -right- on the verge of advancing it too far maybe one cylinder or another would pop back through the intake valve.

Not saying that's the problem here, doubt it is, just saying it's not impossible.

Extra retarded timing would be out the exhaust valve, extra advanced should be out the intake valve... no?

[HO455]

Check for carbon tracks or oil inside the distributor cap. Could be causing a cross fire problem. If your unable to see any signs of tracking, swap caps and see if the problem goes away, or clean the inside with electronic cleaner and let dry COMPLETELY before starting. It's not a good idea to fill your distributor cap with an explosive vapor and then try to start it. Trust me on this one.

[Grumpy old man]

The first question is how many miles are on the engine ? If it's new or a recent rebuild then you would consider ignition or fuel . If it's been some time and miles start at the timing chain . Easy check , pull off the distributor cap grab hold of the crank pulley and (looking at the engine from the front ) turn the crank pulley to the left BY HAND while watching the rotor , once the rotor moves turn the crank to the right and watch the rotor to see if it moves in concert with the crank or is slightly delayed . Any delay in movement of the rotor would suggest the timing chain has stretched or the old gears are worn . I'll bet a donut that's the problem and it takes 5 minutes to find out .

Many times a worn timing chain will slip with the added load of just pulling away from a traffic light .

[70STOVEBOLT]

Originally Posted by davepl

I thought of that, but on a Q-jet the filter is somewhat ingeniously spring loaded so if you get lazy and never change it, it eventually gets pushed out of the way and still flows, even when completely plugged.

He could have an inline filter somewhere that's clogged, but from the description as soon as he blips the throttle he has an issue, and the fuel supply for that should be covered by the fuel bowls, not the supply line.

That was my reasoning, anyway!

If a filter before the carb is clogged, the fuel bowls may not be filling up properly and could be causing a lean condition. On my 250 I had to change the fuel filter twice a year (was driving it daily then), and it was doing the same thing as the OP reported. I know, a Monojet is not a Q-jet, but the same principle should apply. I had a paper element fuel filter inside the inlet and an inline between the pump and carb and I could remove the one in the inlet and it would run ok until I was able to get it replaced.

[cleszkie]

Don't forget to check you accelerator pump in the carb. If you are not getting an extra shot of fuel when you try to accelerate or pull a load, the engine will bog down from a lean condition. This could lead to a backfire. A faulty accelerator pump can also make for hard starting as well. Just a thought.

[John H.]

My guess is a worn cam lobe. If motor is tired, the cam and timing chain could be the issue. especially if it is backfiring through the carb. They run ok and then under a load will backfire.

[1972_C20]

Ok, update. I was finally able to look at the truck today. It's been a hectic week. Found that the new fuel pump was now not pumping. Traced it back to the valve that switches my tanks back and forth. It was clogged up. I bypassed the valve and the truck is now running. I am still having backfiring issues, but I have not timed it yet. I'll take it to my work and time it there. What is the best timing to run on these 402's? I was going to do cap, rotor, plugs, and wires too just because I don't know when the last tuneup was done on it.

[geezer#99]

At your elevation I'd start at 14 initial.

But with that said, a half dozen other posts will come up saying you don't need that much. You live at high elevation. You need to start the burn way earlier than those at sea level.

[57taskforce]

Originally Posted by 1972_C20

Ok, update. I was finally able to look at the truck today. It's been a hectic week. Found that the new fuel pump was now not pumping. Traced it back to the valve that switches my tanks back and forth. It was clogged up. I bypassed the valve and the truck is now running. I am still having backfiring issues, but I have not timed it yet. I'll take it to my work and time it there. What is the best timing to run on these 402's? I was going to do cap, rotor, plugs, and wires too just because I don't know when the last tuneup was done on it.

I'm at about 5500-6000 ft and my truck runs real good at 16* initial timing. Just for reference.

[davepl]

Originally Posted by geezer#99

At your elevation I'd start at 14 initial.

But with that said, a half dozen other posts will come up saying you don't need that much. You live at high elevation. You need to start the burn way earlier than those at sea level.

If I were being cantankerous I'd point out that gasoline sold at higher altitudes, like that sold in cold climates, has a faster vaporization rate. You'd have a hard time burning San Diego summer gas in a Saskatchewan winter... but how much they adjust it for altitude I don't know.

So the amount of advance you need is really tied to the vaporization rate of the gasoline, and the elevation may already be accounted for in the fuel you've purchased.

But if not, if I were racing in Denver I'd try 4-6 degrees extra. Since I'm in Seattle and my 402 likes 12, I'd pick 16 if you had to pick something.

[1972_C20]

Ok, got my truck to my shop. I took Grumpy old man's advice and turned the crank while looking at the rotor. There was a delay in movement. The truck has about 110,000 miles on it. So looks like I need to get it apart and see what the problem is. Thanks for all of the help guys. I appreciate it.

[Grumpy old man]

Glad you have a direction on it , that's a old diagnostic trick I learned in the mid 1970's after my 68 Camaro had the same symptoms , I'd retime it and it run for a while then I'd retime it again ..... it's the only way to gauge timing chain slop without pulling it apart . If you would post some pictures of what you find once you open it up .I'm not sure what gears are in your engine but many had the nylon timing gears to reduce engine noise and after many miles (back then 100,000 was many miles ) or a few over heating episodes the nylon gears would wear allowing the stretched chain to slip and present the symptoms you had .had it been a fresh rebuild ignition or fuel would have been first on the checklist .

[1972_C20]

I will post pictures of it apart. I should get to it this weekend if all goes well.