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1984 15hp won’t idle unless knob fully in

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bigtime_mcalpine

Regular Contributor
Morning!
we upgraded my sons fishing boat to a 15hp this week. Thing has some serious umph vs the Mercury 9.8 we had on there. Most noticeable power increase with more than one person on boat! Only one little challenge. Motor will only idle with the idle knob on side of cowling fully extended arguably beyond its range. This severely limits the throw of the throttle. At the same time I’m chasing what I think may be a related issue: while motor runs beautifully at full throttle in mid range it almost sounds like timing is off (it sounds bumpy if that makes sense). Lastly if you roll the throttle smoothly there is a whole middle zone where it sounds like mix or timing is set wrong. At idle it’s smooth, at full throttle it’s also smooth. Motor looks like it’s had some real TLC but I’m still in honking maybe carb cleaning is up first. Thoughts? Anyone happen to have a screen shot of the set-up procedure for throttle roller handy?
 
That idle knob does not work the greatest pretty normal to have to extend it that far. As far as the burble goes make sure the roller is in one piece. Most of those will have a clear sleeve on it that breaks down and falls off also causing idle problems. If the roller is good try adjusting the low speed knob on the front of the motor and see if acceleration improves. As far as setting the roller it should start opening the carb at the mark on the cam. https://www.leeroysramblings.com/OMC_info/OMCinfo.htm this a good website with a lot of info
 
bigtime_mcalpine said:
Morning!
while motor runs beautifully at full throttle in mid range it almost sounds like timing is off (it sounds bumpy if that makes sense). Lastly if you roll the throttle smoothly there is a whole middle zone where it sounds like mix or timing is set wrong. At idle it’s smooth, at full throttle it’s also smooth.
Click to expand...

When I went to the round meg tuner from the wet leg design mine did the same mid range off song thing. I think your '84 has the round meg tuner. Its not mix or timing issue. Asymmetrical exhaust coupled with a strong return wave might be the reason..if ya want that top end ya gotta put up with that mid range "confusion". imho..
 
Sunkinmess said:
When I went to the round meg tuner from the wet leg design mine did the same mid range off song thing. I think your '84 has the round meg tuner. Its not mix or timing issue. Asymmetrical exhaust coupled with a strong return wave might be the reason..if ya want that top end ya gotta put up with that mid range "confusion". imho..
Click to expand...

interesting. Thank you!
 
flyingscott said:
That idle knob does not work the greatest pretty normal to have to extend it that far. As far as the burble goes make sure the roller is in one piece. Most of those will have a clear sleeve on it that breaks down and falls off also causing idle problems. If the roller is good try adjusting the low speed knob on the front of the motor and see if acceleration improves. As far as setting the roller it should start opening the carb at the mark on the cam. https://www.leeroysramblings.com/OMC_info/OMCinfo.htm this a good website with a lot of info
Click to expand...

I took a closer look at the cam roller on the carb. Seems like previous owner put a plastic sleeve over top of the OEM cam roller to address wear. Clever idea but I think that also messed with its swing specs. I’m going to simply start by replacing the roller. Stay tuned.
 
bigtime_mcalpine said:
I took a closer look at the cam roller on the carb. Seems like previous owner put a plastic sleeve over top of the OEM cam roller to address wear. Clever idea but I think that also messed with its swing specs. I’m going to simply start by replacing the roller. Stay tuned.
Click to expand...

Dug a bit deeper. So here's what I think happened:
1) a previous owner lost the little washer that sits between carb and plastic cam follower arm.
2) With that washer missing, the white plastic cam arm sits too close to edge of carb. As a result, at full throttle the cam arm takes the full brunt of the twisted throttle action and the plastic roller gets unnecessarily squeezed by the ignition timing wedge (not correct term).
3) Above leads to premature wear of the black plastic roller cam
4) Previous owner attempted to fix by making a thicker cam roller - he put a plastic sleeve over top of the worn OEM cam roller.
5) Larger cam roller doesn't quite fit in the space like it should and also results in bits being at their limit of movement when they should not be.

New cam AND washer on order. Will report back.
 
Sunkinmess said:
When I went to the round meg tuner from the wet leg design mine did the same mid range off song thing. I think your '84 has the round meg tuner. Its not mix or timing issue. Asymmetrical exhaust coupled with a strong return wave might be the reason..if ya want that top end ya gotta put up with that mid range "confusion". imho..
Click to expand...
I am sorry wet leg design? I did not know there were 2 different midsections on those motors? Geez Evinrude only shows one midsection. Is yours a military motor or racing motor?
 
flyingscott said:
I am sorry wet leg design? I did not know there were 2 different midsections on those motors? Geez Evinrude only shows one midsection. Is yours a military motor or racing motor?
Click to expand...

The mid on my 76'...and as per service manual...the mid fills with the dispelled cooling water and overflows to the small passage in the mid and out the holes just above the vent plate. When not in use it drains out the holes on each side of the water tube. The meg tuner did away with that feature....and at least on my motor changed the midrange tuning. There have been many changes on those mid sections that are not readily evident. If ya go on ebay and look at all the mids for sale you will see many different designs where the mid meets the lower.

..just off the top of my head...

mids with open hole at the bottom for the wet leg.
mids with open hole for wet log and that baffle that connects that small back channel to the thru-prop area.
mids with 3 holes at the bottom for the meg tuner for a bounce plate (return wave) and the added rear baffle to the thru-prop area.
mids with 2 holes at the bottom for the meg tuner and a baffle with an extra addition that blocks off the lower.

The newest lowers..post 92' had no holes at the bottom above the vent plate..all water is diverted to the thru-prop area.
 
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Sunkinmess said:
The mid on my 76'...and as per service manual...the mid fills with the dispelled cooling water and overflows to the small passage in the mid and out the holes just above the vent plate. When not in use it drains out the holes on each side of the water tube. The meg tuner did away with that feature....and at least on my motor changed the midrange tuning. There have been many changes on those mid sections that are not readily evident. If ya go on ebay and look at all the mids for sale you will see many different designs where the mid meets the lower.
Click to expand...
Every one of those motors is a wet leg. They ALL have cooling water in the exhaust housing there are no exceptions to that. All those changes were to make the motor quieter. But you still have a wet leg motor no matter the tuner.
 
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flyingscott said:
Every one of those motors is a wet leg. They ALL have cooling water in the exhaust housing there are no exceptions to that. All those changes were to make the motor quieter. But you still have a wet leg motor no matter the tuner.
Click to expand...

The wet leg in my service manual refers to the leg filling up with water and dumping over the small passage and out the holes in the lower. With the meg tuner there is another 3/8 tube going right into that small passage...like an attempt at dry stacking I presume. And the tuner is short of the mid bottom so it won't seal for the leg to fill with water....if that's what you are saying.
 
Sunkinmess said:
The wet leg in my service manual refers to the leg filling up with water and dumping over the small passage and out the holes in the lower. With the meg tuner there is another 3/8 tube going right into that small passage...like an attempt at dry stacking I presume. And the tuner is short of the mid bottom so it won't seal for the leg to fill with water....if that's what you are saying.
Click to expand...
No what I am saying is it doesn't matter the tuner they are all wet legs.
 
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