1975 c20 backfire issues

[desmondpaulk]

Hi
Just purchased a 1975 C20, 3/4 ton, 454, duel tanks
The owner had it running (both tanks have a mix of 10 year old gas and new 1/4 in each) He ran it on the mix gas. When i went to pick it up, the only thing it needed was belts and fluids. I primed it to start it, but it wouldnt stay running only on the prime. After a few prime, it backfired through the exhaust big time. Not sure why it wont run. it will attempt to run with a couple of pumps on the throttle, but still backfires. my plan is to drain both tanks, new cap and wires/plugs. Any suggestions on what would cause the backfire, would bad gas cause it.
Looking forward to any help.
Desmond

 

[dsteelejr]

Check timing and plug wires for the correct firing order. 1-8-4-3-6-5-7-2

 

[PrairieDrifter]

I've found that exhaust leaks will cause backfires also, if the right conditions are there. Doesn't happen too often but it does happen, I've got an old grain truck that runs perfect but it literally sounds like a cannon. Mainly sounds like that because it has one of those big old 2 foot long 6 inch cylindrical mufflers that blown out its insides straight out the back lol.

One time a brand new muffler exploded like a small bomb, about as loud as an artillery firework shell, split the muffler in half. Was on an fuel injected Ford, just an exhaust leak and the right conditions.

 

[desmondpaulk]

Thanks for the suggestions. I'll check the firing order when I do the plugs cap and wires. Also thinking with the old gas possible water in it, would that cause the backfire out the exhaust. I can't give it any throttle or it is like a shotgun maybe???

 

[Gman73]

what carb is on it? Could have blown out the accelerator pump, I think it’s firing order related, with gas that old it’s crap full of varnish, you need a clean gas source

 

[Ellie Niner]

I have sticky valves in my 1962 Studebaker after running on rancid fuel for a bit... two exhaust valves will hang open just enough to cause some rather loud pops and bangs out of the exhaust. Possibility that's what's happening to you.

Since there's no chance of pistons and valves meeting in a Stude V8, I should be able to get those valves freed up by running it on fresh fuel for awhile (and/or maybe making it gulp down some type of solvent that will dissolve the nasty fuel deposits).

 

[desmondpaulk]

I did solve the issue of backfire. Pulled the cap and rotor, it was burned bad
New cap rotor and plugs she purrs
Thanks for the suggestions

 

[fast 99]

Get rid of that old fuel. It could stick valves. Ask me how I know that.

 

[Snoots]

SeaFoam!

 

[desmondpaulk]

Get rid of that old fuel. It could stick valves. Ask me how I know that.

I connected a hose and siphon to the line before the fuel pump and siphoned out the old gas. Turns out the tanks were almost empty fuel Guage is sticking showing 1/4 but both are empty
I'm guessing you had some valve damage from your old gas. With the new stuff and fresh gas she runs smoothly

 

[fast 99]

Yeah, was a while ago but had bad gas in a 74 Elcamino 454. It stuck an intake valve. Didn't do any damage other that having to remove the heads. If you've ever taken the heads off a 74 Elcamino big block with AC and air injection it will be burned in memory for life.

Although this was before ethanol fuel, today's fuel is even worse as far as storage life. It starts to degrade in 30 days especially on systems that are not sealed, like old trucks.