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Author Topic:   Two-Cycle Oil Consumption Rate

btb posted 07-14-2008 09:15 PM ET (US)
I measure the two-cycle oil consumption rate for my 1991 Yamaha 90-HP. I note the amount of oil and gasoline I put in every trip and calculate the ratio of oil to gas, to check if it seems reasonable, and if it varies, which could be a sign something is wrong. Last year I posted here that the usage was around 110:1 of gas to oil, and someone said that didn't seem like enough. So I adjusted the arm [perhaps a control arm on the engine which regulates the oil consumption rate] and have been getting around 85:1 since, which seems about right.
The engine runs fine. It smokes a bit when starting from cold, but other than that everything seems OK.
Has anyone else similar consumption figures I can compare?
Or do most people not bother measuring consumption and just top up the oil tank each trip?
For example, my last trip was 40 nm, and I used 21 ounces of oil and around 12 gallons of gas. Does this sound about right? I get 3.2 NMPG
Sal DiMercurio posted 07-14-2008 09:40 PM ET (US)
It all depends on how hard you run the engine. If you troll 5 hours a day on it, you will only use about 150:1, but if you run wide open, you will use 50:1. It all lays on how far you push that throttle,how big of a boat your running, so there's no way you can compare with someone else. If your running a Montauk 17 with a 90-HP two-cycle Yamaha, and I'm running a Revenge 22 with the same engine, I'm most certainly going to use far more oil and gas. The only true way to compare would be identical boats, run at identical speed and RPM, in the same air temp, on the same water condition.
Sal
jimh posted 07-14-2008 11:20 PM ET (US)
I am not certain about the Yamaha oil mixing system, but, as Sal notes, usually the oil mixing ratio is varied as a function of throttle setting. The overall average mix ratio would be highly dependent on what throttle settings were used and how much fuel was burned at those settings. Note that the amount of time is not a factor. It is the amount of fuel burned at each throttle setting.
btb posted 07-14-2008 11:31 PM ET (US)
aha, I see. OK, well so apart from the odd bit of slow manouvering, I would say most of my engine use is at say 3500 revs, maybe up to 4200 if I am lucky (depends on sea conditions), getting to where I fish and back. Almost never WOT. I hardly ever troll (I have never caught anything trolling, I must be doing something wrong!)
Yes, this motor varies oil delivery with throttle - its via the rod which connects the two that the adjustments are made.
So if I am running at 2/3 throttle 90 to 1 overall sounds at least reasonable?
Bill
jimh posted 07-14-2008 11:59 PM ET (US)
Let me give an example of what I mean about the amount of fuel not the time being important. A motor burns oil at 1:100 at idle and increases to 1:50 at full throttle. The boater spends an equal amount of time at both speeds. However, he burns ten times as much fuel at full throttle than at idle. So our average oil mix ratio is as follows:
1-gallon of fuel at idle implies 1/100-gallon of oil (0.01)
10-gallons of fuel at full throttle implies 1/5-gallon of oil (0.20)
Total Fuel is 11 gallons. Total oil is 0.21
Our average mix ratio is therefore 0.21/11 = 1:52
We see that the amount of fuel burned is the weighting factor, not the time.

btb posted 07-17-2008 01:27 PM ET (US)
Jim, thanks for your responses.
So I gather there is no practical way for me - or anyone else - to determine if their 2-stroke is using an adequate amount of oil?
Can I also gather that most 2-stroke owners simply refill the oil tank after an outing and hope for the best regarding adequate lubrication?
Bill
seahorse posted 07-17-2008 01:53 PM ET (US)

The Yamaha service manual describes an oil delivery test that you can perform, plus refer to it to set the oil link rod adjustment for correct oil flow.
jimh posted 07-17-2008 01:59 PM ET (US)
Bill--A very practical way to establish the ratio of oil to gasoline being used by your engine is to make careful measurements of how much oil and how much gasoline is used. That result is your average oil use ratio.
Tohsgib posted 07-17-2008 02:19 PM ET (US)
Also look at your plug's ceramic anode thingie. If they are sooty or dark then you might be too rich. If they are still white you are too lean.
btb posted 07-17-2008 03:16 PM ET (US)
Jim,
hurray - we go full circle - I have been doing what you suggested ever sine I took posession of my boat - see my first post in the thread. I was wondering if others took the same careful measurements (that we could compare). The question is - now I know what my consumption is - is it ok (or too little or too much?)
I do recall seeing a test somewhere in the manual - thanks for the reminder, I will go back there.
My spark plugs generally look ok.
Bill
Blackduck posted 07-17-2008 04:41 PM ET (US)
Your oil usage seems about right to me, unless you are operating at WOT, a lot, then your usage is to light.
jimh posted 07-17-2008 08:52 PM ET (US)
The figure you initially reported (110:1) seemed like a very high ratio. The figure you reported after making an adjustment (85:1) seems more reasonable.

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Author	Topic: Two-Cycle Oil Consumption Rate
btb	posted 07-14-2008 09:15 PM ET (US) I measure the two-cycle oil consumption rate for my 1991 Yamaha 90-HP. I note the amount of oil and gasoline I put in every trip and calculate the ratio of oil to gas, to check if it seems reasonable, and if it varies, which could be a sign something is wrong. Last year I posted here that the usage was around 110:1 of gas to oil, and someone said that didn't seem like enough. So I adjusted the arm [perhaps a control arm on the engine which regulates the oil consumption rate] and have been getting around 85:1 since, which seems about right. The engine runs fine. It smokes a bit when starting from cold, but other than that everything seems OK. Has anyone else similar consumption figures I can compare? Or do most people not bother measuring consumption and just top up the oil tank each trip? For example, my last trip was 40 nm, and I used 21 ounces of oil and around 12 gallons of gas. Does this sound about right? I get 3.2 NMPG
Sal DiMercurio	posted 07-14-2008 09:40 PM ET (US) It all depends on how hard you run the engine. If you troll 5 hours a day on it, you will only use about 150:1, but if you run wide open, you will use 50:1. It all lays on how far you push that throttle,how big of a boat your running, so there's no way you can compare with someone else. If your running a Montauk 17 with a 90-HP two-cycle Yamaha, and I'm running a Revenge 22 with the same engine, I'm most certainly going to use far more oil and gas. The only true way to compare would be identical boats, run at identical speed and RPM, in the same air temp, on the same water condition. Sal
jimh	posted 07-14-2008 11:20 PM ET (US) I am not certain about the Yamaha oil mixing system, but, as Sal notes, usually the oil mixing ratio is varied as a function of throttle setting. The overall average mix ratio would be highly dependent on what throttle settings were used and how much fuel was burned at those settings. Note that the amount of time is not a factor. It is the amount of fuel burned at each throttle setting.
btb	posted 07-14-2008 11:31 PM ET (US) aha, I see. OK, well so apart from the odd bit of slow manouvering, I would say most of my engine use is at say 3500 revs, maybe up to 4200 if I am lucky (depends on sea conditions), getting to where I fish and back. Almost never WOT. I hardly ever troll (I have never caught anything trolling, I must be doing something wrong!) Yes, this motor varies oil delivery with throttle - its via the rod which connects the two that the adjustments are made. So if I am running at 2/3 throttle 90 to 1 overall sounds at least reasonable? Bill
jimh	posted 07-14-2008 11:59 PM ET (US) Let me give an example of what I mean about the amount of fuel not the time being important. A motor burns oil at 1:100 at idle and increases to 1:50 at full throttle. The boater spends an equal amount of time at both speeds. However, he burns ten times as much fuel at full throttle than at idle. So our average oil mix ratio is as follows: 1-gallon of fuel at idle implies 1/100-gallon of oil (0.01) 10-gallons of fuel at full throttle implies 1/5-gallon of oil (0.20) Total Fuel is 11 gallons. Total oil is 0.21 Our average mix ratio is therefore 0.21/11 = 1:52 We see that the amount of fuel burned is the weighting factor, not the time.
btb	posted 07-17-2008 01:27 PM ET (US) Jim, thanks for your responses. So I gather there is no practical way for me - or anyone else - to determine if their 2-stroke is using an adequate amount of oil? Can I also gather that most 2-stroke owners simply refill the oil tank after an outing and hope for the best regarding adequate lubrication? Bill
seahorse	posted 07-17-2008 01:53 PM ET (US) The Yamaha service manual describes an oil delivery test that you can perform, plus refer to it to set the oil link rod adjustment for correct oil flow.
jimh	posted 07-17-2008 01:59 PM ET (US) Bill--A very practical way to establish the ratio of oil to gasoline being used by your engine is to make careful measurements of how much oil and how much gasoline is used. That result is your average oil use ratio.
Tohsgib	posted 07-17-2008 02:19 PM ET (US) Also look at your plug's ceramic anode thingie. If they are sooty or dark then you might be too rich. If they are still white you are too lean.
btb	posted 07-17-2008 03:16 PM ET (US) Jim, hurray - we go full circle - I have been doing what you suggested ever sine I took posession of my boat - see my first post in the thread. I was wondering if others took the same careful measurements (that we could compare). The question is - now I know what my consumption is - is it ok (or too little or too much?) I do recall seeing a test somewhere in the manual - thanks for the reminder, I will go back there. My spark plugs generally look ok. Bill
Blackduck	posted 07-17-2008 04:41 PM ET (US) Your oil usage seems about right to me, unless you are operating at WOT, a lot, then your usage is to light.
jimh	posted 07-17-2008 08:52 PM ET (US) The figure you initially reported (110:1) seemed like a very high ratio. The figure you reported after making an adjustment (85:1) seems more reasonable.