outboard - high altitude jets at low altitude

[walt]

Ive got a Tohatsu/Nissan 9.8 four stroke. I put the "high altitude" 3300 to 6600 el. jet set in the carb - it ran nicely this summer used between around 3600 to 8600 feet el.

But next trip will be to about 700 foot elevation.

I'm inclined to just leave the high altitude jets in the carb. I understand the carb could run lean and possibly run hot but how big a problem is this???? Is it worth all the hassle to change the jets for just a weeks use?

 

[Ross]

As long as you don't burn a hole in a piston you shouldn't hve a problem. When it runs very lean it burns a hotter flame in the combustion chamber. But if you don't run full throttle for long periods. .............

 

[kloudie1]

Ross is correct.. .. four strokes are a lot more tolerant than two strokes.. A two stroke would probably burn a hole thru a piston.. a four stroke may not, but it will run very poorly and not make power.. If you want an engine to depend on, definitely put in the normal jets before the trip..

 

[walt]

Thanks! I'm inclined to leave the 3300/6600 el. altitude jets in the carb and see how it runs (get the "data point") - and in general go easy on running the outboard. Ill have the low altitude jets with me and carry all the tools for changing the jets on the boat (since I have already had to do this because of a clogged jet).

Down side to waiting is some chance Id end up changing the jets on the water.. I bet the carb makes a fairly soft "ker-plunk" splash noise when dropped from only a few feet above the water.

 

[timebandit]

Walt
Do yourself and your engine a favor and change the jets before you leave.
If a high pressure system moves over you, it might produce corrected alt of -1,000 feet.

 

[walt]

I did somewhat of a search for info on this and didn't come up with much. There are discussion which say you need to change jets - but.. you don't know if this internet advice is regarding a 2 stroke trying to pull a skier or a small 4 stroke on a displacement hull using generally less than 1/2 its rated HP.

My experience.. I have a 4 stroke 4 HP Tohatsu - bought it new from a dealer in Denver around 2006. I don't know what jets are in this outboard, I assume the dealer would have put the proper jets in - but don't know. Ive used this motor a LOT at 8600 feet and some at just over 5000 foot. The outboard is harder to start at 8600 feet - but runs just fine with some power loss. I recently used this same outboard for many hours at about 600 foot el. Didn't even think about the jets - but it started easy and ran great the whole time. In general, this outboard has run fine at all these elevations, has lower hp and harder to start at the high elevation. If there was a real issue with elevation, I didn't notice it.

My old Honda 8 HP 4 stroke - used it between 8600 feet, just over 5000 foot and about 3400 feet. Never adjusted anything, it ran good at all elevations but once again was sometimes hard to start at 8600 feet and lower power at high el.

Nissan 9.8 4 stroke - has the 3300 to 6600 jets - used between 3400 and 8600 feet, never changed the jets - works and starts great at all altitudes, some power loss at 8600 feet.

 

[flynfol]

In my experience, a 4 stroke will probably run okay at lower power settings, but can detonate, backfire,run hot at high power settings with a lean mixture. A 2 stroke will melt down for sure at high power settings(use to, and still do race vintage snomobiles). Can you adjust the low idle circuit? or is that fixed?
Try taking off in an airplane with a lean mixture, and you'll KNOW right away.
Sorry, but the Boy Scout in me is saying "juggle the jets".

J

 

[walt]

There are no screw adjustments for gas/air mixture. The picture shows the location of the two jets (under the brass colored screws). You have to remove the carb, take it apart, use a special screw driver to get the jets out, etc - ie, not a huge hassle - but certainly a significant one - especially to do twice - once before the trip and then once again after the trip.

 

[walt]

well... dang... after pondering this for a while, I probably will change the jets. I guess its the price I pay for using the boat over an 8000 foot elevation range (8600 feet to about 600 feet) and the simplicity of a carb engine.

Im thinking there is too good of chance that my Tohatsu 4 hp 4 stroke that Ive been using all these years has always been set up with low altitude jets. It seems that the overheating danger comes from using high altitude jets at low altitude. I understand the opposite - using low altitude jets at high altitude - will cause the fuel mixture to run rich but doesn't have the overheat issue.

Assuming I have the low altitude jets in the 4 hp, the only thing Ive really noticed is that it is harder to start at high altitude and of course has less power. Is this about what would be expected?

 

[txjim]

Down side to waiting is some chance Id end up changing the jets on the water.. I bet the carb makes a fairly soft "ker-plunk" splash noise when dropped from only a few feet above the water.

I know for a fact that spark plugs are the same way!

 

[walt]

This could be somewhat incorrect (from internet forums) but comparing a 2 stroke to 4 stroke, as other posters in this forum have already mentioned, seems the 2 stroke would be a more sensitive to heat damage from running lean than the four stroke. The heat from the lean burn might be similar for the 2 and 4 stroke but the lean mixture on a 2 stroke also means less lubricant for the piston. 4 stroke gets its piston lub from the crankcase sump. Also, somehow the richer mixture on a 2 stroke is better at removing heat from the engine as the gas exits. Four stroke is also much less sensitive as the sump oil aids in cooling as well as the extra stroke.