Been trying to track down a problem with a backfire. The motor is an '84 Johnson 115 (J115TLCRD). I thought it was a lean sneeze. Here's what I know:

- Compression is good.
- Spark is good and all 4 plugs look to be firing the cylinders fine.
- I have flushed the tank and lines and replaced with fresh gas, pre-mixed properly.
- The carbs have been cleaned and rebuilt.
- The butterflies are fully closed and seated at idle.
- The butterflies are perfectly horizontal at WOT.
- The carbs are in sync and open/close at the same time.
- The plastic roller cam is in perfect condition.
- I can use the "warm up" lever and at a very fast idle the problem is gone. However, the boat won't idle or troll at low RPM's.
- At WOT, I don't believe there is any backfire, but I guess it's possible that it's there and I just don't hear/see/feel it. Seems to run perfectly fine at speed.

I don't know exactly what RPM's the engine is running at as the tachometer doesn't seem to be functioning properly (still working on it). I'm going to take a look at the fuel filter this evening. What else should I be looking at?

i'm leaning the other way, get it. lean the other way, rich, couple more things to check;
after an idle run, pull the plugs to see if there wet with fuel, if so, try readjusting the low end needles
with the air box lid removed at an idle, put a white peace of paper in front of the box to check for fuel spitting out, if so, it could be the reeds behind the carbs
check the timing linkage and timing itself, make sure it comes all the way back at idle

a back fire is from raw fuel in the exhaust that gets ignited, usually a rich condition or timing issue

Thanks spanky. This thing has got me confused and I'm about to bite the bullet and have a professional take a look at it.

Just finished checking the fuel pump and filter. The filter had just a little gunk, but not enough to impede flow through the filter. The pump looks perfect. No deposits and the diaphragms look good. Fuel lines also looked good and didn't appear to have any deterioration. No fuel leaks noticed anywhere. (primer bulb stays hard while running, too).

I pulled the plugs after an idle run today. They weren't fouled. All 4 had a slight coat on them, but I did notice that it looks like they have a little bit of black deposit after wiping off the fresh. Electrodes don't appear burn, pitted, or corroded. Basically, looked fine for what I would expect to see (in my little experience).

There isn't any low speed jet adjustments on this motor. From what I understand, butterflies closed all the way and the low speed orifices meter the air. I'm using #34 orifice plugs, as per default.

The linkages come all the way back at idle. I am able to slide a piece of paper between the cam and the roller so I know the butterflies are closing all the way. Pushing on the butterfly (motor off, of course) didn't budge so I'm sure they're seating/closed all the way.

When I did my idle test, I had the air box removed and was watching it closely. I even held my hand in front. Nothing sputtering out, though I didn't hold a paper in front of it.

I gave your suggestions a second thought spanky. Also dug through my manual again. I think my spark plugs are trying to tell me something? The first and third plugs I wiped off the black deposits. When I initially pulled them, they looked like #2 and #4.

i have no doubts its getting fuel, the backfire itself is raw fuel in the exhaust that gets ignited, the white or heck any colored paper will catch any fuel/oil mix being spit out the carbs, the oil is what spots the paper, to me it sounds like one of the two, running rich or timing issues.

explain again what you found on the plugs, 2 were dry, 2 were?

sorry, was talking on the phone with mom and dad, lol, those do look wet, i think i would start with the low end needle adjustment first, is transition from low rpm to higher rpms ok?

Thanks spanky. I'll try the paper test next time I pull it out and fire it up. How close do I need to have the paper?

None of them were dry. All 4 of them looked like the second from the left in my picture above. Not completely fouled (I don't think, anyway) but lightly wet. They all had the black carbon deposit that wipes off fairly easily. Gaps and electrodes otherwise look ok.

EDIT - You posted as I was posting.

Yep, plugs were wet. Transition is fine. No hesitation and no sudden changes in RPM's. I don't notice any sneeze after about 1/4 or maybe 1/2 throttle.

Oh, and there is no needle adjustment on these carbs (unfortunately).

i do believe it's rich, may just be a carb adjustment, what's your oil mix ratio?

I mixed at 50:1. Engine doesn't smoke much at all, if that means anything.

no smoke on start up? i kinda like in between 40;1 and 50;1 myself, smoke means oil and oil means lubrcation, this is a 2 stroke and they do smoke, whats the compression numbers? and use champion plugs in that engine, it does appear to getting lube from the pics

I wouldn't say NO smoke. Just not as much as my '83 Mariner used to spit out.

I have no doubt that it's getting lube in all 4 cylinders. Doesn't appear to be overheating at all.

Compression was 110-115 across all 4. The plugs are Champions and match the numbers on the sticker (can't remember exactly what they are, though).

i don't know why either and its kinda funny but these engines do like champion plugs, anyway, get a new set of plugs, cleaning works but i find it doesn't last long, the fuel is good and your positive its at 50;1, carbs cleaned and kitted, i got to say i would hit it with the timing light and readjust those carb needles

i hope this works out for you, i like the earlier 3 a 4 cylinder johnrudes, pretty solid engines

Thanks spanky. I'll try some fresh plugs and see what happens. If it's timing related, then it's off to the shop (I have never done anything timing).

Why not have Spanky come by and check it?