2 Cycle Fuel Mix Revisited

"You might be surprised to know that testing shows richer than necessary oil mixture ratios cause the engine to run hotter".

Is this true? I know if I lean out my saw too much with the high speed mixture screw, it gets hot enough to shut 'er down. The electronic module has to cool off before I can start it again.

Am I missing something somewhere or do I have a module starting to go bad?

Thanks,
PoulanMan

• Al's Post

Ken Dunn's one of the race saw builders. I don't know his email, but he's over on Arboristsite.com.

Based on how I read your post we are talking about two different subjects.

As you have apparently discovered, a lean air/fuel ratio can cause an engine(especially a 2-cycle) to run hot. A lean air/fuel ratio could, as you described, be caused by an improper carb needle adjustment.

Oil to fuel ratio is another matter. As I indicated, more oil than needed will cause a 2-cycle engine to run hot. Although people who should know have told me this in the past, I didn't truly believe them until I saw data proving it.

So to summarize, too lean an air/fuel ratio is bad....and excessively rich oil/fuel ratio is bad.

If the engine is running on a correct oil/fuel mixture, and it stopping because it is getting tight/seizing, it is most definitely running hot. If it is stopping due to an ignition problem, that's a condition which is often aggravated by heat.

I hope this answers your question/comment...Al English

Thanks

I think the temperature(engine metal) differences were about 40 degrees. I don't remember what oil mixture ratios were being compared. If any explanations for this are given, I don't remember what they are.

Two-cycle engines rely heavily on the cooling affect caused by the evaporating fuel charge. I suspect the presence of oil diminishes this cooling affect. It would make sense then that the more oil, the less cooling the engine would receive. Also, the octane rating of gasoline is lowered by the presence of oil. Possibly excessive oil leads to detonation, and the overheating detonation causes.

Sorry I can't provide more specific information...Al English

I've used two-strokes for years and have never had one fuel-related problem. I had a summer job one year mowing grass at a power plant. The guy, or should I say idiot, in charge told me to mix the Lawn-Boy fuel as 2 1/2 gallons gas to one can of oil "to keep the spark plug from fouling out." If you use a Lawn-Boy, you know that such a mix is too lean. I cleaned the spark plug, the muffler, and the filter, put in a correct mix of gas/oil, and she ran like a champ all summer. I took the lean mix and dumped it into the company truck(which then had a little blue cloud following it for a week, lol).

Get that mix right, and they run great.

Kerosene has a higher octane than gasoline, and it seems that oil would too. But consider this, every precaution is taken to keep oil out of the combustion chamber of race engines. That is because they are ideally already on the verge of detonation, and any oil entering the chamber can and will cause detonation and quickly destroy the engine. I don't know the reason oil causes detonation, but there is no disputing that it does. Although oil has a high flash point, apparently its octane rating is low?. Two-cycle engines are not able(due to detonation) to run compression ratios as high as would be possible using the same fuel in a 4-cycle engine. My guess is that the oil/octane situation is at least a good part of the reason why...Al English

I'm humbled that you are humbled. And even worse, you are right. Because of the octane difference a diesel engine flat won't run in any fashion with gasoline in the tank. I am very careful to give the best answers I can, and to provide only information that is documentable. But just to add some humanity to the widespread and well known belief that I never make mistakes(you do believe that don't you?), every once in a while I throw something in to see who's paying attention. Seriously, I apologize for the error, and hope no one was misled. Thanks for catching it...Al English

Gasoline has about 122000btu/gal
Kerosine has about 128000btu/gal
#2Diesel has about 132000btu/gal

So from that it would explain why a oil/gas mix would have a higher running temperature than straight gas.

T_Bone

I thought the same thing as you T_Bone, but I have read that much of the oil in the mix doesn't actually burn. However, I don't know how informed the author really was. His contention/evidence was that unburned oil is the source of the smoke and carbon deposits that 2-cycle engines produce. I know smoke consists, at least in part, of unburned particles. So, he may be right. But on the other hand, some of the oil must be burning, what you say makes sense, and could partially or completely explain what's going on...Al English

The smoke would be burned hydrocarbons, oil turning from a liquid to a vapor state but would also contain saturated oil vapor seen as oil leaking from the exhaust. By adding an excessive amount of oil to the gasoline then the engine temperature would rise as heavy oil has more BTU/gallon than gasoline. By how many degrees I don't know a I never seen any data on cylinder temperatures. You would almost expect pure gasoline to have a higer cylinder tempature as without oil then cylinder scuffing would happen and it would seam friction would add more heat to the cylinder than a over rich gas/oil mix and if that were the case then a lean mixture would run hotter than a over rich mixture. To bad someone has not published an on-line study of cylinder temperatures per oil mix.

I've never seen an offical break down on just how much of an oil mix is burned vs how much is acutally is being used to lube the engine. The EPA would probably have that info but finding it with-in the EPA may be another problem.

T_Bone