115hp Mecury - Intermittent 3000rpm max

[convergent]

I have a '93 115hp 2 stroke 2+2. It starts on first crank when cold fine. Normally runs fine at hits max 5200 rpm. It's developed a problem, usually only when cold, that it won't go over 3000rpm at WOT. I can hold at WOT and no more. If I come back to idle a few times it will eventually go on up to normal and be fine for the rest of the day.

For anyone familiar with the 2+2, it usually has a big blast when you get to 2000rpms that is like a nitrous shot when the extra 2 cylinders come in. When it has this problem, that never happens. So I think it's whatever engages those 2 cylinders that is the problem. It is an abrupt there or not, and sometimes when it messes up at around 3000 it will cut in and out.

Can anyone give me a tip on where to start looking for what triggers the extra cylinders?

 

[Dave1027]

Re: 115hp Mecury - Intermittent 3000rpm max

There are fuel injectors for the lower two cylinders that work off a accelerator pump to help kick the bottom two in. Check those. When the lower carb butterflies are opened enough it begins spraying gas in. That gas is ignited by the spark plug which is already firing.

Have somebody squeeze the primer bulb while you hit it to see if it's a fuel delivery problem. You might want to rebuild the carbs.

 

[convergent]

Re: 115hp Mecury - Intermittent 3000rpm max

Hmm.. OK, I thought those direct injectors were for when you started the engine cold and pushed in the key. So are there two sets of direct injectors on these engines?

Its so abrupt when it goes in and out of this problem that it leads me to believe something electrical is making a bad connection. Is there anything electrical involved in the 2nd two cylinders cutting in that I could check? From the way you described it, it sounds like the cylinders are already sparking, right? I was assuming there was nothing I could check at idle since the problem only happens at higher RPMs, but if what you say is correct then I could check to see if I'm getting good spark on all four at idle, right?

Thanks for the help.

 

[Dave1027]

Re: 115hp Mecury - Intermittent 3000rpm max

Don't confuse the accelerator pump injectors for the choke enrichner. Those are total different things. The enrichment works off the key push and basically dribbles fuel into the intake manifolds. The accel injectors work off fuel pressures generated by the fuel pump and the accelerator pump and squirt fuel directly into the lower two intake ports in the block.

But even if the accel injectors were not working at all the lower two should eventually fire. It just would be delayed. So you may be looking at an overall lean condition.

There are two trigger coils, one for the top two and one for the bottoms. You may want to do the dva test to check those.

 

[convergent]

Re: 115hp Mecury - Intermittent 3000rpm max

Which two cylinders run at the lower rpms, the top two or bottom two? I'll look through the manual more and see if I can see what you are talking about. It still seems to me like its something electrical, but I could be wrong.

 

[79Merc80]

Re: 115hp Mecury - Intermittent 3000rpm max

Top 2 run all the time. Bottom 2 fire all the time, but the bottom carbs (As I understood) don't have idle jets. So even though some fuel/oil gets into the bottom 2 cylinders, there is enough to lube, but not enough to create power. I was told that the accelerator pump and injectors were for when you slamed the throttle down, that it would then kick off on all 4 for pulling up skiers, otherwise the bottom 2 come online around 2200 when there is enough vaccume for the high speed jets in the carbs to function.

I could be wrong, but that's how I understood it, and why I stayed with the inline 6

Craig

 

[convergent]

Re: 115hp Mecury - Intermittent 3000rpm max

I tried having my daughter pump the bulb when it was messing up yesterday and it had no affect. I talked to a mechanic that has done work for on the motor for the prior owner, and he was convinced the problem was the accelerator pump valves sticking and/or the accelerator pump itself. I have him scheduled to come in a week and make the change, but I'm not so sure that this is the problem. As you said, even if the accelerator pump didn't kick in, then it should eventually run up, right? It stays at 3000 rpms and it really seems like its only running on 2 cylinders and it will stay like that until I drop the throttle back down and try again.

 

[79Merc80]

Re: 115hp Mecury - Intermittent 3000rpm max

I really doubt that your engine would get up to 3000 RPM's if it was only running on 2. I have an inline 6 and with #3 dropping off at 1800 and #1 fireing intermittinly, I was stuck at about 2000 RPM's