Loud popping noise after warm up

[Dan.Sev]

So i just rebuilt my atomic 4. I had her up and running smoothly on the bench before I installed her in the boat. Now that I have her in the boat, after she warms up she makes a loud popping noise and starts to run really rough. Oil pressure and engine temp are good. (oil is at 40 psi and temp is solid around 140)The only differences between now and when i ran it on the bench are that I added the alternator and the muffler. Any thoughts? The video is attached.

[YOUTUBE]8knLCTvJf5g[/YOUTUBE]

[sailbristol]

either you have your timing way off ,but it really sounds like a blown head gasket

[Mo]

Might be a broken valve spring (or springs) and subsequent valve slap...there's just not a whole lot of things going to sound as inconsistent and evident with the low rpm.

I'd take the side cover off and have a look at it.

All the best...hope it isn't much.

[ndutton]

This strikes me as an exercise of elimination, find out what it's not then focus on what's left. From the sound I think you can immediately eliminate deep drive train and fuel problems. That's a good thing. Now the hopefully educated guessing begins.

Inspecting the valves as Mo said is a good start. That is a static examination however and with the variable nature of the sound may reveal nothing but until it's done, who knows?. Run her in the dark too and when the popping starts see if there's a light show.

Following that I'd warm her up to the point of failure then immediately do a compression test.

[Marian Claire]

"Run her in the dark too and when the popping starts see if there's a light show." +1
I assume one other thing that was changed from garage to boat is you are now using the boats wiring harness. Heat caused connection failure?? arcing?? shorting out somewhere??
Dan S/V Marian Claire

[lat 64]

Sure sounds like metal clacking to me. Did you change from mechanical fuel pump to electric? Something loose left in the old fuel pump works?



Other than that, I'd sure look at the valve springs like Mo said.

Mine has much quieter clack in the Accessory drive that I'm trying to skull out.

Good video.

Russ

[edwardc]

I don't know if I'd buy valve/springs as an explanation. A failure there would take out only one cylinder, and the A4 runs way better than that on three cylinders.

To me, the rapid onset, plus the "on again, off again" nature suggests something in the ignition system. If it's an EI system, the electronics module could be failing under heat, or the magnet collar on the dist could be cracked or loose.

If it's a points system, the points could be sticking or loose.

On either type of system, check the advance weights and springs. If a spring has rusted or broken, it could be causing excessive amounts of spark advance and knocking, which could be the sound you're hearing.

Just be systematic, and eliminate one thing at a time.

[Mo]

Might be a broken valve spring (or springs) and subsequent valve slap...there's just not a whole lot of things going to sound as inconsistent and evident with the low rpm.

I'd take the side cover off and have a look at it.

All the best...hope it isn't much.

Hard to hear where that tick is coming from. You can use a rubber hose to and put one piece to your ear and move it around the engine to find the most pronouced noise area...fuel line, vaccum line etc will work as a stethescope. ???
Electronic fuel pump starving??? Are all your fuel line valves fully open...sometimes there are two valves.

[lat 64]

I don't know if I'd buy valve/springs as an explanation. A failure there would take out only one cylinder, and the A4 runs way better than that on three cylinders.

To me, the rapid onset, plus the "on again, off again" nature suggests something in the ignition system. If it's an EI system, the electronics module could be failing under heat, or the magnet collar on the dist could be cracked or loose.

If it's a points system, the points could be sticking or loose.

On either type of system, check the advance weights and springs. If a spring has rusted or broken, it could be causing excessive amounts of spark advance and knocking, which could be the sound you're hearing.

Just be systematic, and eliminate one thing at a time.

I had to go back and read in the OP the part about running rough.
So true, My engine ran on three cylinders for a week before I noticed

[ndutton]

Sounds more like a snap than a pop to me and the irregular frequency of the snap implies more than one cylinder is involved.. If so, I'd be thinking arcing off the coil secondary or downstream from there.

[Dan.Sev]

Update

Well I took some of your guys advise and I closed everything up and ran it. Unfortunately no light show. Compression tests solid at 80 psi across all cylinders after she's warmed up. I'm running a mechanical pump and an electronic ignition. I just had the cap and rotor apart and cleaned up. I'll pull it again and make sure I didn't fudge it up when I put it together. I also noticed that it is a little intermittent. It will stop for a few seconds and come back. I attached a pic of the new plugs. They maybe have an hour on them.

[Marian Claire]

How about a voltage reading at the coil + terminal during the popping. I'm reaching but I can not get away from the "what has changed since it ran fine" question.
Dan S/V Marian Claire

[Marian Claire]

Is there any way this could be a failing coil? Dan S/V Marian Claire

[Dan.Sev]

Ill check in to the coil, but I’m starting to think it might have to do with the gas. I mixed some old gas and some premium gas. I just noticed that if I choke it while its popping, it will recover and after a few seconds of releasing the choke it starts popping and struggling again. Going to pick up some regular gas and give it a try.

[Marian Claire]

I would be surprised if it was the type of gas. To little gas or to much air may be the issue. Lean condition. Is the popping coming from the carb air intake?
Dan S/V Marian Claire