Mercury outboard 100

[Swimming Minnow]

I have an 89 Merc 100 outboard question. It runs great, idles smooth but it has a rough transition from 2 to 4. How can I solve this problem? Any help would be much appreciated. Thanks

[boatfixer]

Has this been an ongoing issue or just happen?

There will normally be some roughness from about 1800-2200 in that transition

[Swimming Minnow]

Just bought the boat and have used it the last two weekends. But yeah the 1800-2200 range is about where it happens. Once it's up and running she is a dream and even when I'm putting around.

What would you recommend doing to fix this? And is this a common issue for this engine?

[aanderud]

There's something called an "accelerator pump" in the mercs that engages when you go from 2 to 4 cylinders. Mine does the same thing, and I've been thinking of replacing or rebuilding it just to see if it helps. Engine runs reasonably smooth at idle, and perfect at 3/4 throttle and above, but stutters when transitioning between 2 and 4 cylinder mode on acceleration.

[delcecchi]

Just bought the boat and have used it the last two weekends. But yeah the 1800-2200 range is about where it happens. Once it's up and running she is a dream and even when I'm putting around.

What would you recommend doing to fix this? And is this a common issue for this engine?

I had a 115 with the same "feature" and you just have to not try to stay in that rpm range. The carbs are set up so two cylinders don't get much fuel at low speed so it is nature of the beast. It is a long story about why it is that way and I'm not sure I have it right, so won't go into it.

Bottom line is I don't think it can be fixed. It should be a smooth transition with a "afterburner kick" as the rpms go through that range when you punch the throttle.

[Swimming Minnow]

There's something called an "accelerator pump" in the mercs that engages when you go from 2 to 4 cylinders. Mine does the same thing, and I've been thinking of replacing or rebuilding it just to see if it helps. Engine runs reasonably smooth at idle, and perfect at 3/4 throttle and above, but stutters when transitioning between 2 and 4 cylinder mode on acceleration.

So the accelerator pump is what help starts the fuel flow to the other 2 carbs then it transitions to a vacuum type function? I guess I'm a little confused of how the transition works.

What's all involved in rebuilding the accelerator pump and what it be wiser to replace it?

[Swimming Minnow]

I had a 115 with the same "feature" and you just have to not try to stay in that rpm range. The carbs are set up so two cylinders don't get much fuel at low speed so it is nature of the beast. It is a long story about why it is that way and I'm not sure I have it right, so won't go into it.

Bottom line is I don't think it can be fixed. It should be a smooth transition with a "afterburner kick" as the rpms go through that range when you punch the throttle.

Did your boat come out of the wrapper to where it had a rough transition? I feel like their has to be some way to solve this. It's kinda of annoying but not the end of the world type of an issue.

[delcecchi]

Bought it new. There is a definite transition and running in the vicinity of it, 2500 RPM or something like that was difficult. Accelerating through it was fine, just a definite transition when the second two cut in, like the secondaries opening on a 4 barrel carb, if you are old enough to remember that.

It is a little rough when running on two, but I wouldn't have called the transition "rough".

Could you explain what you are doing when it is rough?

[Swimming Minnow]

Bought it new. There is a definite transition and running in the vicinity of it, 2500 RPM or something like that was difficult. Accelerating through it was fine, just a definite transition when the second two cut in, like the secondaries opening on a 4 barrel carb, if you are old enough to remember that.

It is a little rough when running on two, but I wouldn't have called the transition "rough".

Could you explain what you are doing when it is rough?

I have a 4 barrel carb 454 sitting in my garage and she rolls real smooth. When I dump on the throttle man does she growl... But this is different with the two stroke. If I were to pop someone up skiing they would not make it on one and would struggle on 2. Even on the the decline when throttling down it feels terrible. I'd understand if this was a minor hiccup, but this to me seems a little extreme.

[delcecchi]

I have a 4 barrel carb 454 sitting in my garage and she rolls real smooth. When I dump on the throttle man does she growl... But this is different with the two stroke. If I were to pop someone up skiing they would not make it on one and would struggle on 2. Even on the the decline when throttling down it feels terrible. I'd understand if this was a minor hiccup, but this to me seems a little extreme.

That does sound worse than the normal transition. Would a service manual help? If so, PM me your email. I think I have one that covers that motor.

[aanderud]

I am sure that I don't know exactly what the accelerator pump actually DOES, but I have read that this is what starts getting the fuel to the other two cylinders when you punch the throttle. What's involved in a rebuild? I don't know, but the part that I'd guess you/we need is number 40 on the diagram here -- well you don't need the pump assembly itself, just what is described as "PUMP ASSEMBLY, Accelerator, Repair Kit, 1395-811287". I'm guessing it can't be rocket science to repair that puppy. I called the shop here and went and picked up a repair kit for 21 bucks. I'll let you know how it works when I get around to actually installing it. Worth a shot.

http://www.boats.net/parts/search/Merc2/Mercury/125%20(4%20CYL.)/9793577%20THRU%200P016999/FUEL%20PUMP/parts.html

One guy on some forum describes the accelerator pump as follows:

Quote:

That motor idles on the top two cylinders, If you look you will see that the bottom two do not even have idle mixture screws, just enough fuel gets by to lube the bottom two cylinders at idle. The bottom two cylinders get extra fuel upon acceleration from a accelerator pump, this pump has fuel pump pressure on it when the motor is running, when the throttle is raised, the plunger is pushed down against a diaphram, this causes greater than fuel pump pressure to be exerted on two check valves mounted on the starboard side in the intake ports of number 3 and four. The fuel squirts into the intake port to give the motor enough fuel to accelerate.

[aanderud]

I have a 4 barrel carb 454 sitting in my garage and she rolls real smooth. When I dump on the throttle man does she growl... But this is different with the two stroke. If I were to pop someone up skiing they would not make it on one and would struggle on 2. Even on the the decline when throttling down it feels terrible. I'd understand if this was a minor hiccup, but this to me seems a little extreme.

[delcecchi]

Glad to help. I learned something from your experience. I no longer have mine, but a buddy here has a 115.

[aanderud]

Now, for full disclosure, I haven't driven it since I "fixed" it. It was getting dark by the time I finished. However, I am 99.9% sure this was the problem. A broken off accelerator pump actuator button can't lead to good things.

Drove it tonight after work. It's sweet. Night and day difference. Best 21 bucks I ever spent.

[TrollnDrift]

Sounds great, however, the accelerator pump will not stop you from reaching full throttle, it just helps you get there faster.

[aanderud]

Sounds great, however, the accelerator pump will not stop you from reaching full throttle, it just helps you get there faster.

I never said I couldn't get to full throttle, just that there was a crazy jump from "idle" to "1/2" or "3/4" throttle, and the transition was not smooth. Well I guess I said I was a bit surprised that I could get WOT with the broken pump, but I've always been able to get there, it was just a rough transition. But anyway, it's better now and I'm happy about that.