Mercury 80hp pop/backfire out exhaust

[WahooJeff]

Hi all, been doing more work to my new to me boat. Recently finished removing the injection system and going to 50:1 premix.
After the removal I ran the motor to check for any issues after having the oil pump off. I’ve found that it’ll pop or backfire thru the exhaust occasionally at 2500-3000 rpm, while running on the muffs. Not sure if this is something new, or if it always did it.
I’ve read different things that maybe it could be a normal occurrence. And I don’t believe it to be a lean sneeze.
The motor fires right up and idles nicely and has no problems in the higher rev range.
I plan to do a compression check, rebuild fuel pump and rebuild carbs over the winter. Also made sure it has new plugs.
The motor is an 88 mercury 80hp 3 cylinder.
I attached a video, hopefully that is ok.
Anyone have any experience with this?
Thanks all for any advice.

 

[Chris1956]

I did not hear a backfire. Maybe it is lean sneeze.

However, It makes sense to test the idle pickup and max spark advance and assure they are correct.

 

[WahooJeff]

I did not hear a backfire. Maybe it is lean sneeze.

However, It makes sense to test the idle pickup and max spark advance and assure they are correct.

Right at the 9 second mark is when it pops out of the exhaust and sends a nice plume of smoke out thru the prop, and then doesn’t do it again till around the end of the video.
I’m gonna do compression first just to make sure I’m not working on a worn engine, then From there I was thinking about checking timing as well as the carbs and fuel pump.
As a side question, are the Sierra fuel pump and carb kits ok to use?

 

[WahooJeff]

Did compression check. Came up with 120 on each cylinder. So compression seems good. Got a service manual, will check carb sync, and adjust max and idle timing and max throttle.

 

[saltchuckmatt]

It's not a backfire, I'm wondering if because it has no back pressure on it it's just behaving normal. Motors are set up with a load on them. Have you tried it in the water yet? It may be fine.

 

[WahooJeff]

It's not a backfire, I'm wondering if because it has no back pressure on it it's just behaving normal. Motors are set up with a load on them. Have you tried it in the water yet? It may be fine.

I have not, and probably won’t get to for sometime.
I did try to check timing at idle yesterday just to see where it was and found the light I borrowed was working intermittently and would not provide any tach info. I’ll have to get my hands on another timing light. I also tried testing each cylinder individually by pulling plug wires off while running. I found that the motor will run on the bottom and middle cylinders alone, with the bottom running the best but will not stay running on only the top cylinder. I did notice that the bottom cylinder had a different plug wire, which I’m assuming is newer. I plan to swap wires and retry and swap coils if necessary and see what the results are.

 

[Dave1027]

Sounds like lean pop to me. The fact that you are getting different results on the top cylinder could mean the top carb is running lean.

 

[w2much]

Go with Dave. You need all three firing correctly and if not find out why.