1969 Mercruiser 2.5 120, slight high rpm backfire.

[Starcraft Enterprise]

Had my 1969 Starcraft Starchief out last week for the last trip of the season on a week long Hunting / Camping trip. It has the original Mercruiser 120hp 2.5 engine and outdrive. It started and run perfect as usual but I noticed in the ski zones at 3/4 throttle and above a consistent light backfiring. It did it on the first day and seemed more noticable on the fifth day. When I would hear it I would just back the rpm's down until it went away. It was not lacking any in power. I don't think it was spark knock, but could have been. Gas from two different sources during the week and the air was the coolest I had ever run it in before. Seems if I would have had pinging issues they would have showed up during the summer. It almost seemed like maybe it was just firing a little bit late with exhaust valves slightly open at the end of combustion. I know it is due for a tune up but I still wanted some opinions. It sat for 8 years before I bought it towards the end of the summer. I changed the fuel filter, new gas tanks and ran it with all the same ignition components still in it from 8 years ago. I have new components but wanted to hold off until next spring before I used them so that they would be fresh for the season.
So what do you guy's think? Ignition? Resistance in the plug wires delaying ignition? Plugs shot? Points dirty? Running lean? (exhaust valves burn't from no lead in the gas, bad thoughts go away!)
Here is the history on the boat if you need more background info.
http://forums.iboats.com/showthread.php?t=347131

 

[Bronc Rider]

Re: 1969 Mercruiser 2.5 120, slight high rpm backfire.

Have you checked the fuel filters/water separator? Checked timing? I would definitely check the points also.

 

[Lodgepole]

Re: 1969 Mercruiser 2.5 120, slight high rpm backfire.

I have a 92 OMC 3.0. It gave me fits before I found the problem. Was backfireing at higher rpm and lost some power. Problem was an exhaust flapper valve fell off and stuck in exhaust passage. This might be your problem. Next, old egnition parts, which you plan to replace anyway.
GOOD LUCK!

 

[Starcraft Enterprise]

Re: 1969 Mercruiser 2.5 120, slight high rpm backfire.

I have a 92 OMC 3.0. It gave me fits before I found the problem. Was backfireing at higher rpm and lost some power. Problem was an exhaust flapper valve fell off and stuck in exhaust passage. This might be your problem. Next, old egnition parts, which you plan to replace anyway.
GOOD LUCK!

As old as mine is and the amount of rust chips that came out the first time I started it, something similar is a definite possibility. Thanks, never would of thought of that.

 

[ziggy]

Re: 1969 Mercruiser 2.5 120, slight high rpm backfire.

As old as mine is and the amount of rust chips that came out the first time I started it, something similar is a definite possibility

yeah it is. mine was similar. my boat came out of hibernation after an alleged 15 year slumber. i certainly had a fight with rust in my fuel tank. though mine was apparently surface rust. dust. not flakes. my symptom was low wot if i remember right. don't think it backfired much. anyways. it was a horse apiece. remove fuel tank and clean it, or keep replacing fuel filters. being lazy like i am, i chose the latter for a first attempt. 1st year, i went through several fuel filters. second year, a few less filters, this year, 1 filter. i also don't run a fuel/water separator, if i did, it would be suspect too. also check the screen at the carb inlet. i've had debris there too.
ya might run a minor tune up on it w/o parts. clean the plugs, check dwell and timing. maybe run a compression test just for kicks (or as to confirm nothing major has failed and for future reference as to what your compression is) so far, i've found that i need to check my dwell at least sometime in mid season. i've found it off more than once. when dwell changes it effects timing. my speculation as to why my dwell changes is that on my 165 there is supposed to be a little sponge that has lub in it. that sponge is supposed to ride on the dist. cam. my sponge is gone. so i think the block on the points that runs on the dist. cam wears down. thus effecting dwell which effects timing. the cure so far has been to R&R the points for me. once a season. spring. then check it out during the season to be sure nothing has changed.