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M18E2 Carb adjustment question

Jupiter Dinghy

Jupiter Dinghy

Regular Contributor
Tohatsu 18 hp 2 stroke.
I just got a hardline tach to check RPM's for adjustments and other purposes, and I was checking the idle specs on the service manual, it says that idle neutral should be 950 RPM and 800 RPM in gear.
I'm right on target in neutral but when engaging into gear, if falls to 650 / 700 RPM.
The motor sounds good mind you, and it doesn't feel as if it would be about to stall or anything, but I was wondering:

A/ Should I be concerned with the fact that there is more of a drop in RPM between neutral and gear, than the service manual calls for?
B/ Is there a way to adjust RPM setting in gear without disturbing the RPM setting in neutral?
 
First, VERIFY that your tach is setup properly. Easy to get that wrong. One fire per revolution on a 2-stroke.
Then, understand that if the mixture is off some (pilot screw), then a load may lower the idle-in-gear speed some from the rated speed.
Also understand that you MUST have the silencer on the carb; it is part of the mixture setting, because it induces drag on the inlet air.
Also be sure that you are running 50:1 fuel, and are at full operating temperature.
Once you are certain that everything is OK, I would not worry too much about the idle-in-gear being a problem, as long as it doesn't stall.
The pilot screw and the idle stop interact with each other to get a smooth idle of correct speed.
IF the idle fuel circuit has any varnishing, that could cause your issue. You DO run the carb dry each and every time you shut down, right?
When was the last time the carb was PROPERLY cleaned?

However, you could check the number of turns on the pilot screw. Typically around 1-3/4 to 2 turns out. You can adjust it (usually a little richer) for better stability in gear, while watching the tach, and listening VERY carefully. You will need to reset the idle stop and the mixture several times to hit the sweet spot. The engine must be on the boat, and in the water to make those settings come out right. Understand that you can get the highest low idle with the mixture, and then reset the idle stop, then readjust the mixture. When complete, richen about 1/8 to 1/4 turn for stability, then do a final idle stop adjustment.

Again, if it doesn't stall, don't worry too much.
 
pvanv said:
First, VERIFY that your tach is setup properly. Easy to get that wrong. One fire per revolution on a 2-stroke.
Then, understand that if the mixture is off some (pilot screw), then a load may lower the idle-in-gear speed some from the rated speed.
Also understand that you MUST have the silencer on the carb; it is part of the mixture setting, because it induces drag on the inlet air.
Also be sure that you are running 50:1 fuel, and are at full operating temperature.
Once you are certain that everything is OK, I would not worry too much about the idle-in-gear being a problem, as long as it doesn't stall.
The pilot screw and the idle stop interact with each other to get a smooth idle of correct speed.
IF the idle fuel circuit has any varnishing, that could cause your issue. You DO run the carb dry each and every time you shut down, right?
When was the last time the carb was PROPERLY cleaned?

However, you could check the number of turns on the pilot screw. Typically around 1-3/4 to 2 turns out. You can adjust it (usually a little richer) for better stability in gear, while watching the tach, and listening VERY carefully. You will need to reset the idle stop and the mixture several times to hit the sweet spot. The engine must be on the boat, and in the water to make those settings come out right. Understand that you can get the highest low idle with the mixture, and then reset the idle stop, then readjust the mixture. When complete, richen about 1/8 to 1/4 turn for stability, then do a final idle stop adjustment.

Again, if it doesn't stall, don't worry too much.
Click to expand...

Thanks a lot for that Paul.

To clarify and answer some of your questions, this motor had a brand new carb installed (by a shop) a couple of months before I bought it from its previous owner (2 weeks ago), so hopefully the shop knew what they were doing, and I'm gonna go with your "don't worry too much" advice... especially since after my first outing this past weekend, I was extremely happy with how it runs. Like a brand new motor basically.

Yes the carb has the silencer on it, I run 50:1 with C90 marine fuel (ethanol free), and since I started reading about Tohatsu on several boards including this one, yes, I am running the carb dry after flushing the motor with fresh water after use.
One question about this: Is there no risk to rings/cylinder walls by running the carb dry since there will be no lube for the few seconds when fuel stops getting in?

Now to be sure, since unfortunately the manual is very light on pictures, the "pilot screw" is that vertical screw on the left side of carb when facing front of motor, that adjusts the throttle stop correct? Or is this just an idle setting screw?

I'd love to find a pic of this carb, or an exploded view, with all the screws / jets labelled properly...
 
Last edited:
Here ya go:
002-21043-4_FIG04.jpg
1-8 is the stop screw; It controls how far the throttle plate closes.
1-10 is the pilot screw; It adjusts the low-speed mixture.
 
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