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55hp 2 stroke jetting

Steve Allard

New member
Hi, Im new here, I use my boat at approx 3500ft and I was wondering if it would be worth re jetting the carbs. It EATS fuel normally, I have played with the props, trim, alignment etc and I cant think of anything else to help. Any suggestions ???
 
Not sure about re-jetting carbs at 3,500FT as I run at sea level but it could be running a bit rich. Lower ambient air pressure = less oxygen entering the cylinder on each stroke so you may ask someone about adjusting rich/lean screws on the carbs to run a bit more lean so the fuel air mix is optimal at that altitude.
 
Was engine previously run at 3500 feet,if not rejetting is recommended,smaller jets.contact local dealer in your area for proper ones for your elevation.
 
The engines were bought new (sea level) and fitted here in Entebbe which is about 3500ft. No dealers here. a good days fishing say 7 hours and 35 miles will use about 30 gallons or so

The rich/ lean screws are only for idle though, is that right??
 
I'd wait and see if you get any responses from a service tech here or find a service center and ask them. Playing with the rich/lean settings is a very touchy subject. If you run too lean some signs may be backfiring at high RPM, failure to accelerate well, etc. but most worrysome is running a 2-stroke too lean means not enough fuel (that contains the oil) entering the cylinder. That can lead to burned rings, a melted cylinder head, scored cylinder walls and one heck of alot of expense re-boring and re-ringing if neccessary. Far better to be introducing more fuel than necessary than not enough in a 2-stroke. When you say "engines" in plural depending on what they are and what they a pushing a little over 1 MPG may not be that unreasonable. Is the boat running fine other than seeming to be gas hog it may just be the nature of twin 2-strokes pushing a heavy boat.
 
That fuel consumption seems normal.10% of displacement of engine is the ball park figure of fuel used, Computes to 11 gallons a hour at 3/4 throttle.Steve pull plugs and inspect,are they fouling?They should be light tan in color.
 
Hi,
The plugs always seem fine, light tan as you say, perhaps I am just being too tight and will have to accept what it is....... 4 strokes here we come!

I guess that my main query was that considering the altitude, could the jet size be reduced to compensate for the slightly lower oxygen at 3500ft

but thanks so far for the info
 
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Remember one thing about 2-strokes, at low RPM they are pretty much just as efficient as 4 strokes. As RPM increases you get more blow- through of the fuel/air charge being introduced. A 2-stroke fires on every stroke... on the downward stroke ports on the cylinder walls are exposed - one is exhaust and the other is the intake where a pressurized fuel/air charge enters the cylinder slightly later than the exhaust port was exposed (but hasn't fully closed yet). The higher the RPM, the more fresh fuel/air mix that blows out with the exhaust just before the exhaust port is fully closed by the cylinder travel. If you want better gas mileage - cut back on the speed and run at just enough RPMs to maintain the boat on level plane. At high rpm's the amount of blow-through can get pretty substantial. I can't remember the percentages but at WOT I thought it could be about 20% of the fuel is going out with the exhaust instead of being combusted.
 
rule of thump for a properly running 2 stroke at wot...10% of horse power per hour...a 55 should use 5.5 gallons per hour..

forget saving any gas money going to a 4 stroke....a 4 stroke rated at 6k rpm and run at 5.5k rpm is a gas hog..to get any kind of efficiency out of a 4 stroke it needs to be run at 4k or 4.5k...and then its no better than a 2 stroke..salesmen lie...

keep in mind another thing..a 2 stroke engine is built to run at top end or near it..and if it is maintained it will do it for many years...I am currently running a 4stroke and if I did that I would have a pile of expensive junk in 3 years..just my opinion on this last statement...
 
I will say... I had my 2-stroke days with the most HP I could hang on the back of the smallest, fastest boat I could find. That fun but these days I'm happy with my 4-stroke F115 on a 19' boat rated for 150 Max HP. I cruise at 4200-4400 RPM (in the sweet spot around 24-25 MPH) and enjoy almost 5 MPG. At WOT it will do about 38-39 MPH at 5900-6000 RPM depending on load but,let's be serious, how often can you find water that you really want to be running 40MPH+ unless you boat on a distant inland lake or early on a sunday morning?

Furthermore, I don't care that my Yamaha Tech claims a 4-stroke can run all day at 5000-5500 and "loves it". I can HEAR how hard that engine is running when all those valves in the OHC are screaming. Outboard engines are expensive and they are under great stress ALL THE TIME when running. Unlike a 4-CYL car rolling down the highway at 60MPH turning 3,000 RPM and getting 30 MPG, at WOT I'll get maybe 3.5 MPG and see 38 MPH. Running a boat is like driving a car up Mount Everest ALL THE TIME as far as the engine is concerned. Lift you foot from the gas peddal in your car and you can coast another 3 miles... pull the throttle back on your boat and you come to a STOP. Long story short... BABY YOUR OUTBOARD IF YOU WANT TO SEE THE SAME HOURS THE COMMERCIAL FISHERMAN GET OUT OF THEIRS! 4-strokes will last for 1000's of hours if you don't abuse them the way you did the old 2-strokes that could handle those high RPM's all day long.
 
This is all good and interesting stuff, it looks like I will stay as I am for now
My boat is a 21ft glass "canoe", made by Panga possibly from Phillipines? A bunch of them were imported here just after the Asian tsunami for whatever reason.........
But its a good machine and it takes us fishing which is dwindling due to overfishing by so called local fishermen who hunt with undersized nets and take everything that they can find. We trawl for Nile Perch and nowadays we are lucky to come back with 2 or 3 fish around 17 pounds.
The old boys used to stop fishing when the boat was gunwhale low in the water!

Happy days!
 
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