Engine making "popping" sound

[roger97338]

The motor is a 350, and has proven itself for many years to be reliable. It has a 750 Holley 3310 (vacuum secondaries), Holley 110 GPH mechanical fuel pump, Edelbrock Performer RPM intake, 14" x 3" K&N Filter, HEI distributor, Heddman headers, TH-400 trans. No air, no EGR. In fact, the only thing remotely resembling emissions equipment is the PCV valve in on valve cover and a K&N filter in the opposite valve cover.

My truck has been sitting in the garage since sometime last year. Every week (ok, sometimes two weeks.) I start it and let it run. When it gets low on gas, I get five gallons of 92 and let her have most of it. (The lawn mower gets the rest...just because it's handy. Not because I insist on high octane in the mower.) A few days ago, I added about 4 gallons or so. Yesterday, I started it and let it run for a while. Then last night, I was going to drive it around the block a few times. I started it and within about thirty seconds it started making a popping sort of noise. It wasn't running rough. In fact, I don't think the noise was coming out of the exhaust at all. It sounded like it was coming out of the carb and sorta echoing in the air filter housing.

I shut it off and called it a night. This morning I started it and it almost immediately started making the same popping sound. I checked all the plug wires, and all were secure and where they should be. I tried to jiggle the distributor and it didn't move. No fuel leaking. No odd smells. All vacuum lines were attached, and the unused vacuum ports had their caps on.

What do you guys think? Too much moisture in the gas tank, watering down the fuel? Moisture or oxidation on the distributor cap or rotor?

Collectively, there is more hands on knowledge of Chevy pickups and their motors here than any other place that I've found. And I respect your opinions. So, gentlemen, I would appreciate any opinion or advice that you have.

I'm going to be gone for the rest of the day, so it might be Sunday or Monday before I'm back. But, I leave you with a cartoon!

[Irish_Alley]

does/did it stall. what happens when you rev the engine. does it go away when warm? imo 750cfm is way too big for a stock engine with just a intake.

[roger97338]

The engine isn't stock. It isn't a racing motor either. It's been in it's current configuration for so incredibly long that I don't recall the cam or head specs...heck...I don't even remember my header pipe diameter. I couldn't even recall my fuel pump specs, so I logged on to my summitracing account and looked at my order history. If you want to know all the specs, I will surely look them up for you.

Before installing this 750, I tried running a 600 (also vacuum secondary) Holley, and the truck felt heavy and sluggish. I bought the 750, installed it, tuned it, and it's been one of the best choices I've ever made. I wish I had that sort of luck with the ladies, my employers, or adopted dogs.

The noise doesn't seem to occur more often or more loudly between the engine speeds of about 800 RPM and 2500 RPM. I didn't feel it wise to run it any longer nor any faster than that, since it was making an unfamiliar noise.

Which reminds me of an anecdote I heard about an Air Force fighter pilot. He kept insisting that his fighter was making an "unfamiliar noise", and continued to fill out repair requests for that noise. After many requests by the pilot, and many inspections by the fighter's mechanics, he was informed that his aircraft had been monitored while running for a total of 60 man-hours by four Air Force mechanics, trained to work on the Air Force's inventory of airplanes, and they all agreed that the noise was no longer unfamiliar to them.



Honestly, I don't think that story would happen like that. If four mechanics couldn't find a noise in a multi-million dollar aircraft, and they thought it was funny, their superiors would find four new men, and their current smart-mouthed crew would suddenly become civilians again.

[454Man]

Sounds like timing. Do you know what your timings set at?

[Jason S]

Pilot squawk:  "Number three engine missing"

Maintenance solution:  "Engine found under right wing after brief search"

How long had your truck been sitting? Ethanol fuel used?  Is there anything that has changed since the last time the engine ran and did not exhibit said issues?

I'm not the best a diagnosing issues without being right there.

You might pull the distributor cap and verify that there isn't any condensation buildup or carbon tracking on the inside of the cap.  While your there, clean the terminals and rotor and spray the inside with WD-40. You said the plug wires were in good shape, are they separate (not laying on each other) eliminating any arc over.

Along the lines of your original thinking:  A fuel tank that is not full and allowed to sit can build up condensation.  Ethanol gasoline will allow water to go into solution with the alcohol (ethanol).  Water does not burn very well.  Additionally, in an engine that hasn't run in a while, valves can stick and may not fully seat. Alternatively, the valve could stick closed and bend a pushrod when started again. As long as the valves are moving it may be a case of after being started that the engine may have to run for a while to bring everything back to normal operation.

I would look under the distributor cap, check wiring, check for any carb gasket leakage and then look into a fuel change (either let it idle and burn out the old or drain and replace).

I'm sure others will have more to offer.

[jrm93635]

Jason S is dead on with the D-cap maintenance. With that said. if the popping is coming from the carb, then its your timing. the popping sound you're hearing is the spark plug trying to ignite on the exhaust stroke of a particular cylinder. So either you're too far advanced or you're wires are crossed (its an easy mistake to make) check your wires, then set your timing. also i have included at pic of the firing order.

[werewolfx13]

Assuming timing is OK, I would start to point fingers at a rounded cam lobe...

[Irish_Alley]

make sure its coming out of the carb. is it a pop or ping or knock. how often? i dont see timing being a problem if it only been sitting and the bolt is tight.

[roger97338]

Thanks for the input, gentlemen!

I replaced the distributor cap and rotor, and the spark plugs, and that solved the problem. I also checked the timing, and it hadn't changed.