aircooled outboard - reduction in top speed

[pescadero]

Some of you may remember my previous thread regarding this motor:

http://www.michigan-sportsman.com/forum/showthread.php?t=145748

Well, that problem was solved by cleaning the petcock and the float bowl/needle valve. Now there is a new issue...

A little background: Sears Gamefisher 1.2HP aircooled outboard, older than dirt.

After getting the issue fixed with the fuel supply problem I took the boat out twice. Both times the boat ran well, started within a couple pulls, etc. No real issues to speak of.

Finished off my gas can of premix before the third time out. Mixed up new can of gas at ~25:1 - this may be a bit heavy on the oil, but it depends on which source you use to determine it.

The original sticker on the motor says 30:1.
The manual for the motor says 32:1.
The manufacturer website (Tanaka) says the manual is wrong, run 50:1.

My initial mixture was probably 35:1 (it was what I had laying around), the boat seemed to run fine, and top speed was about 5mph according to my GPS.

With the new fuel the boat ran about 5mph for about 2-3 minutes (about the time it takes to empty the float bowl based on my observations from the previous problem) then slacked down to about 3mph at full throttle for the rest of the trip.

The motor doesn't seem to be missing (well, anymore than is normal for a small aircooled engine), it starts fine - but it seems to be lacking in power: the top speed is significantly slower, and running through weeds etc. seems to cause it to bog where before it would chop right through.

It seems obvious that it would be the fuel mixture, based on it running like normal just about long enough to empty the old fuel from the system - but I've never seen such a drastic change in power/speed from a little extra oil. Unless it was so much oil it was fouling the plug, and that doesn't seem to be the case.I'm a big believer with two strokes in "If it ain't smoking, it's burning up" and have never seen anything quite like this.

My debug plans are as follows:

1) open the carb up and make sure the throttle is really opening all the way.

2) throwing a tachometer on and see if it getting to full throttle RPM's. It is supposed to be 8000rpm, but I'm pretty sure it isn't hitting that as the boat has a direct drive and if it was, I don't see how it would be going any slower.

3) I guess I'll empty the tank and start trying fuel mixtures with successively less oil - although this and #2 above are difficult as I need the boat in the water to have any sort of realistic load.

Any one have any other ideas? Suggestions? Recommendation for fuel mixture?


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lp

[tommytubular]

Probably hauling around to much welfare cheese.......:lol:

I'm kidding of course...

with todays oils.....25:1 is rich....real RICH

50:1 is what I run my old Johnson at and it seem to run good

I would bet you putt around a little with 25:1 you would certainly foul the plug and would leave a good oil slick in your wake

Does it have a fuel rich/lean mixture control.....try to adjust it while cruising and see what happens

[pescadero]

Probably hauling around to much welfare cheese.......:lol:

I'm kidding of course...
You'd better watch it... my wife was with me, and when I tell her what you called her... :lol:


with todays oils.....25:1 is rich....real RICH

50:1 is what I run my old Johnson at and it seem to run good
Hmmm... I wonder why all my other 2 stroke equipment is still spec'ed in that 30:1 - 40:1 range... I guess it is all newer, and probably designed with modern oils in mind.

I would bet you putt around a little with 25:1 you would certainly foul the plug and would leave a good oil slick in your wake
In a boat that only does 5mph at best on calm water when running good you don't do a whole lot of "putting around" - it's pretty much wide open all the time :D

Does it have a fuel rich/lean mixture control.....try to adjust it while cruising and see what happens
Not that I can see - as far as I know the only carb adjustment is the idle speed adjustment.

I'm pretty sure at this point that it must be the mix... but the quote from the Tanaka FAQ about it doesn't exactly inspire confidence:

Q. What gas / oil mix do I need to use for my outboard?
A. 50:1 gas / oil. Be sure and use oil that is designed for air cooled motors not water cooled motors. Owner's manual says 25:1. This is incorrect for today's gas and oil formulas?

Question mark? That really inspires confidence. As does the fact that neither the manual for the motor, nor the sticker on the motor specify 25:1.

You think 50:1 would be ok, or should I go a bit richer than that?

--
lp

[jmoser]

I once 'broke in' an older 4 hp Evinrude with 25:1 and I distinctly remember problems running especially at low speeds. Could be a fouled spark plug with the oil-rich fuel mix. Try changing to a fresh plug and see. Cleaning the plug will not always help as the ceramic insulator can get permeated with oil/fuel/carbon and effectively 'short out' the plug.

Too much oil could also be preventing the air/fuel emulsion from 'vaporizing' properly in the carb main jet/nozzle. [This would make the plug wet as if it was way rich or choke on.]

Your spark plug is the best diagnostic available on that engine: check it for color. White = too hot / too lean; Black = too cool / too rich. You want a light tan [cardboard] color ideally. If the plug is wet you are either way too rich or you are losing ignition. There is a trick to this: run at full throttle and IMMEDIATELY shut down engine before it can idle for even a few seconds if possible. This preserves the plug condition at full throttle operation - the pilot jet in the carb takes over at low speed and can give very different results.

Question: Oil specs are different for air and water cooled engines. [You don't use the same oil for a chain saw as an outboard.] Are you using the proper premix oil for an AIR COOLED 2 cycle engine? You could be seeing some overheating issues, this will rob power. The marine 2-cycle oils are made for water cooled engines - check your book for proper oil spec as they are NOT the same! Air cooled engines require different oil additives, etc. due to higher cylinder temps.

General learnings on 2 cycle oil ratio: Many '50:1' engines are quite capable of running at leaner mixes of 100:1 or more. The issue is leaving an oil film on internal parts for corrosion protection during storage. Modern oil injected 2 strokes use much less oil than the 'standard' 50:1 premix. I would be willing to bet that your engine will run fine with 40:1 or 50:1 but you may want to fog it before each storage. [If I lose that bet just remember that it is YOUR engine! ;) ]

P.S. skip the tach for now - if it is as bad as you say then you are surely well below top RPMs. As you say these are direct drive: engine RPM => Prop speed => Boat Speed.

Since you know it runs good the only issue I can see with staying at 35:1 mixture is that the plug may foul earlier than normal.

[pescadero]

I once 'broke in' an older 4 hp Evinrude with 25:1 and I distinctly remember problems running especially at low speeds. Could be a fouled spark plug with the oil-rich fuel mix. Try changing to a fresh plug and see. Cleaning the plug will not always help as the ceramic insulator can get permeated with oil/fuel/carbon and effectively 'short out' the plug.

Too much oil could also be preventing the air/fuel emulsion from 'vaporizing' properly in the carb main jet/nozzle. [This would make the plug wet as if it was way rich or choke on.]

Your spark plug is the best diagnostic available on that engine: check it for color. White = too hot / too lean; Black = too cool / too rich. You want a light tan [cardboard] color ideally. If the plug is wet you are either way too rich or you are losing ignition. There is a trick to this: run at full throttle and IMMEDIATELY shut down engine before it can idle for even a few seconds if possible. This preserves the plug condition at full throttle operation - the pilot jet in the carb takes over at low speed and can give very different results.

I'm definitely going to take a look at the plug and see if it looks carboned up - It isn't fouled out, as it will run, but it might be really close to it. Interesting idea on the fuel not atomizing correctly also...

Question: Oil specs are different for air and water cooled engines. [You don't use the same oil for a chain saw as an outboard.] Are you using the proper premix oil for an AIR COOLED 2 cycle engine?

Yep... with all the aircooled 2 stroke crap I seem to accumulate (string trimmer, cultivator, lawnmower, etc.) I always have 2 stroke air cooled oil laying about - it isn't specifically for marine air cooled applications, but I doubt that would be an issue.

You could be seeing some overheating issues, this will rob power. The marine 2-cycle oils are made for water cooled engines - check your book for proper oil spec as they are NOT the same! Air cooled engines require different oil additives, etc. due to higher cylinder temps.

Check the book... ha, very funny. The book specs are "air cooled 2 stroke oil". I'm pretty sure it isn't overheating, as the problem would have been worse with the "leaner" mixture - and it seemed to run fine with that.

General learnings on 2 cycle oil ratio: Many '50:1' engines are quite capable of running at leaner mixes of 100:1 or more. The issue is leaving an oil film on internal parts for corrosion protection during storage. Modern oil injected 2 strokes use much less oil than the 'standard' 50:1 premix. I would be willing to bet that your engine will run fine with 40:1 or 50:1 but you may want to fog it before each storage. [If I lose that bet just remember that it is YOUR engine! ;) ]

Part of my issue is that the results of running too "rich" (I hate that terminology... rich and lean apply to air-fuel mixture, but I can't think of anything better) usuaully just fouls plugs - running too lean has a bad tendency to melt a hole in the top of the piston.

P.S. skip the tach for now - if it is as bad as you say then you are surely well below top RPMs. As you say these are direct drive: engine RPM => Prop speed => Boat Speed.

Since you know it runs good the only issue I can see with staying at 35:1 mixture is that the plug may foul earlier than normal.

I think I'll just assume this is the problem and "lean" out my fuel back to where it was for the previous tank. We'll see if it works next time I'm out...

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lp

[pescadero]

Final Answer:

Yep - it was too much oil in the mix. I'm going to run it 40:1 from here on out.

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lp

[Burksee]

Final Answer:

Yep - it was too much oil in the mix. I'm going to run it 40:1 from here on out.

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lpGlad to hear you got'r going again! Once I read Jmoser's post there wasnt to much to add and figured you might have a fouling issue from previous mix. Aircooled two strokes run much hotter than watercooled ones, a 40:1 mix v/s a 50:1 for an air cooled engine is a good call! ;)

I've played around with different oils and mix's and found a good quality oil (Klotz) at a 40:1 mix (*regarDless what the manufactures recommendations are) gives me the best all around coverage for all my air cooled two strokers. And I can run them all outa the same can! :D