Nissan/Tohatsu 9.8 high to low altitude

waltsn · #1 (**permalink**) 12-10-2009

Since the Tohatsu guru is visiting here.. I have another question.

I will normally use this engine for a couple months at 5000 feet el, most of the summer it will be used at 8600 feet el

But for a week, I will use it about about 3000 ft el. At at the 3000 foot elevation, the motor is run a LOT. If Im lucky on time (not likely), I could even use the outboard at lower elevations.

Because of the normal use mostly between 5000 to 8600 feet, Im getting a "high altitude jet". Im hoping this makes the engine start easy at 8600 feet as Ive had lots of trouble in the past (part of the reason I also want electric start).

But if I leave the high altitude jet in the motor and use it at 3000 foot where it might be run for 8 hours at a time, am I at risk for overheating and damaging the engine? Im going to get both jets and learn how to swap but am wondering what the risk is..

pedcab · #2 (**permalink**) 12-10-2009

Quote:

Originally Posted by waltsn

But if I leave the high altitude jet in the motor and use it at 3000 foot where it might be run for 8 hours at a time, am I at risk for overheating and damaging the engine? Im going to get both jets and learn how to swap but am wondering what the risk is..

In my opinion, probably not. I suppose that what the "high altitude jet " does is to provide a leaner mixture for the engine to be able to cope with the thin air at altitude.

Your moving to a lower altitute (higher pressure, "thicker" air) place while maintaining a high altitude jet will result in a leaner mixture and I think that will not cause your motor to overheat... It may cause it to have to run with the choke actuated for larger periods though...

In fact if you learn how to tune a carburettor you won't even need to change jets, having just to slightly tweak the mix everytime you move up or down in the barometer...

tommays · #3 (**permalink**) 12-10-2009

The motors are already lean and theres nothing you can tweak its all fixed jets

IMHP if your jet is correct at 8600 it will be to lean at 3000 and lower and make a lot of heat

pedcab · #4 (**permalink**) 12-10-2009

Quote:

Originally Posted by tommays

The motors are already lean and theres nothing you can tweak its all fixed jets
t

Are they? You can't lean or enrich the mixture?

So whas the purpose of the 1-17 screw?

tommays · #5 (**permalink**) 12-10-2009

Well in the good old days my Johnson motor had adjustable low and high speed jets RIGHT on the front of the motor

The good old days are gone on USA motors

pedcab · #6 (**permalink**) 12-10-2009

In fact they did. My father still has one in perfect running condition. Here´s one just like it

But the picture I've attached is, in fact, of a Tohatsu M5 engine carburettor and the 1-17 part sure seems to me like a mixture adjustment screw, being 1-15 the throtlle stop which acts as an idle speed adjustment point...

So do they or do they not have an adjustable mixture rate?

tommays · #7 (**permalink**) 12-10-2009

I never seen anything other than idle speed on a newer USA motor

Even the inboards that still have carbs have the idle mixture screw capped off and you have to break it and replace it with one from and older motor model if you want to adjust it

pedcab · #8 (**permalink**) 12-10-2009

Well, in that case, I'm sorry for you guys...

Arround here every 2 stroke carburettor fed outboard I've ever seen features an acessible mix adjustment...

As I've said before in other posts it amazes me the American government policy of being so environmentaly conscious about outboard engine's emissions and other minor polluters and being so permisive about motor sports and absurdely oversized car engines of any kind. Maybe one of those jet boats of yours running on Chinook reactors burns more fuel in a qualifying run than 1000 small sailboat outboards in a year...

PS: What do you mean by "newer"?

tommays · #9 (**permalink**) 12-10-2009

There is 1,2 and 3 star motors mine is a 2 star about 2004 in age

You gotta see whats coming gas inboards are going to catalytic converters and diesel is next on the list

pedcab · #10 (**permalink**) 12-10-2009

Quote:

Originally Posted by tommays

There is 1,2 and 3 star motors mine is a 2 star about 2004 in age

You gotta see whats coming gas inboards are going to catalytic converters and diesel is next on the list

We have a saying here that sure fits you like a glove: The worse part of any sailing boat is its motor

Good luck then...