04 Merc 115 classic, dropping cyl at low RPM's

[java230]

A few questions really, but that's the big one.

Details: 2004 merc 115 2 stroke classic (carb'ed), 2+2 motor

Runs great at high rpms, right at 5k at WOT, but sometimes it has a miss at low rpms(not idle), not like its dropping the 2 cylinders at idle, but it really shakes/misses and makes a lot of smoke. Bump the throttle up a little and it goes away. Any ideas where to start looking?

Also sometimes the tach stops working, when its doing this, Could it be the coil pack that the tach reads from thats missing?

Other questions:
Can the Trim pump be filled without tilting the motor all the way up? The directions say it must be in full up, but the way its set up on my jackplate and transom I can't get full trim. I think its low on fluid, symptoms are when you try to trim up, pump runs, ti goes up for a second or two then stops, pump still running, let go of the button, wait a few seconds and hit it again it will go up a bit more, repeat process. Low fluid or other issue?

 

[java230]

Anyone?

 

[racerone]

Report what the compression is on this motor.----------Running with a timing light hooked up may help you pinpoint which cylinder is acting up.

 

[Texasmark]

Reason they want the engine up is to ensure that the fluid reservoir is full. If you can't get all the way up it may be slighty low due to that. If it's very low, then you are low on fluid. Full is even with the threads....aka if you put more in it will just run out the hole.......color is the color of what you have in there....all per the service manual. With all that said, you need to have a leak somewhere...................look for oil residue.

You have 4 cyl and 4 carbs. One could have some crud in the bottom of the bowl that gets stirred around as fuel moves through the carb and at a certain point everything is in place to block the high speed jet.....located right in the bottom of the bowl....on mine anyway.

So now you have a fuel and ignition suggestion to help you to proceed in the right direction.

 

[java230]

Thanks! Ill look into the trim fluid.

As for crud in the carbs its definitely possible, but its intermittent. Doesn't always do it. I will check with the timing light to see if I can find the culprit cyl. But it doesn't do it at idle.... I will try to see if I can get it to do it on muffs.

 

[Texasmark]

Thanks! Ill look into the trim fluid.

As for crud in the carbs its definitely possible, but its intermittent. Doesn't always do it. .

Smoking gun for intermittent problems as you have. BTDT.

My most recent turned to be a tiny black piece of hose that had deteriorated after 10 years of engine age. I checked all 3 carbs on my 90 and all looked good till I got the air hose out and blew into the high speed jets buried in the bottom of the bowl. On one this tiny particle became visible. Removing it and reinstalling the bowls solved my problem. Apparently it was a piece of hose between the onboard engine fuel fuel filter and the carbs.

 

[java230]

Smoking gun for intermittent problems as you have. BTDT.

My most recent turned to be a tiny black piece of hose that had deteriorated after 10 years of engine age. I checked all 3 carbs on my 90 and all looked good till I got the air hose out and blew into the high speed jets buried in the bottom of the bowl. On one this tiny particle became visible. Removing it and reinstalling the bowls solved my problem. Apparently it was a piece of hose between the onboard engine fuel fuel filter and the carbs.

Good to know! I guess I figured if it was blocked it would always do it. I guess Ill look at the low speed circuits since it does it at low rpms before the high speed side gets used.

 

[Texasmark]

Good to know! I guess I figured if it was blocked it would always do it. I guess Ill look at the low speed circuits since it does it at low rpms before the high speed side gets used.

Not necessarily as I had happen to me. As new fuel enters the bowl, it stirs things up (apparently) and the suction of fuel by the venturi rearranges them.

Edit: While you at it, if your carbs were the same/similar series as mine....boat and manual are gone so I have no reference to numbers, on top of the carb is a Zinc plated plate with a couple of screws. Under it is an area that is part of the low speed fuel supply apparently. I had the high speed jet thing in one carb but another had some of the tiny black specks in this area so you might have a look there too.