can someone please explain this (non-motorcycle)

[XV1100SE]

Someone more knowledgable on engines can comment but on the Venture (1st and 2nd Gen) there is the normal ignition point at or near the compression point or top dead center on the ignition stroke but also an ignition at the end of the exhaust stroke (wasted spark).

 

If there is unspent fuel in the cylinder during the exhaust stroke, wouldn't this burn it off ? So...debate on whether high octane is better or not continues.

[Kretz]

Ok to clarify,

My only reason for using higher octane gas was that 91 & above is the only gas available around here that does not have ethanol in it.

Like I said a yard machinery mech told me some years ago to run all my yard equip't engines on ethanol free fuel to avoid carb problems.

Around here Costco 91 & Chevron 93 are the only ethanol free fuels I know of. (there are a few others but fnot in my area)

My other machinery runs fine on 91 (but some of it is also 2 stroke with oil added) but the lawnmower didn't

Maybe 2 stroke machinery (trimmers, brushcutters, chainsaws etc have higher compression ratios than the lawnmower?) So maybe that's why I was advised to run them on higher octane gas.

Anyway thank you all for your input & thoughts.

[Kretz]

Back to the original poster. I sold SEARS craftsman mowers for years. I eventualy ran the lawn and garden department. Every time I heard a salesman make the statement "use Premium fuel its better" I'd send him upstairs to the training room and have him reread the module on Mower care. They'd always come back and ask why, I'd explain what I'd heard. They would look sheepish and say Okay I understand now.

 

The slower burn rate of Premium and semi premium fuels are especially apparent on engines with out adjustable timing. The biggest problem is that some unburnt fuel is trapped after the power stroke and before the exhaust valve fully opens leaving SOOT on the valves and cylinder head. I've actually pulled motors apart that the exhaust valve wouldn't close completely allowing oil to drip into the cylinder during storage.

 

The problem is similar in bikes. Most motorcycles engines are "static" timed and not adjustable. Now higher performance cars with Higher compression ratios often require premium fuel to prevent detonation during the compression stroke, and to allow for more initial advance in the timing signal to the plug. meaning the explosions at maximum power on the down stroke of the piston. to accomplish this the "burn must start sooner in the up stroke.

 

To dispel another myth Premium Fuels do contain Ethanol in most places ( two or three states ignore the federal mandate and don't require ethanol) in the US unless labeled Ethanol free. I understand the in Canada it varies by province, in some Premium does not have ethanol in others it does.

 

Thanks baylensman that's pretty understandable explanation & reasoning.

Up here our gas is usually marked at the pump "may contain up to 10% ethanol" or "Contains no ethanol" (around here Costco 91 & Chevron 93 is the latter) afaik there is no regular 87 oct ethanol free here in BC.

[MiCarl]

That must be the same small engine mechanic that told my father-in-law not to use the electric starter on his snow blower because it spins the engine too fast.

 

The change in octane isn't what cleared up the problem, in fact it didn't completely clear up right away. You either had a fuel problem or shook something (crud) loose in the process of draining the 91 out.

 

One thing on octane: I bought a new Stihl chainsaw a few years ago. They say specifically that the saw requires 87 octane fuel but to use 89 or higher if the fuel contains ethanol. Their explanation is that the 10% ethanol fuel burns hotter than pure gasoline and the higher octane compensates for that. Their engine can overheat with 87 ethanol fuel.

 

I've gone to using 89 in all my little air cooled 2-stroke engines. Partly because it's easier to have one fuel mix and also to help those engines stay a bit cooler.

[Kretz]

Thought I'd just update you guys, appears it's probably nothing to do with the "type" of fuel & changing it did as someone said dislodge some crap & & was merely a coincidence that it ran afterwards.

I went out to use the lawnmower today, it has regular 87 oct in from last fill, same thing... starts after being primed, runs for maybe 5-10 seconds then cuts out. I did eventually get it to run, & was able to cut the grass.

Anyway trying again later I got the same results (won't run reliably) So I got some Seafoam & I'm trying that, so far no change so I'm just letting it sit for a few days now after adding Seafoam to the gas (starting it a number of times) & also squirting it into the air intake & running it.

If that doesn't work I may try a carb service kit but at 15+years old I don't think it owes me anything. Maybe a new lawnmower is in the stars, but I would like to know what the problem is. Maybe like me, it's just old age! lol

[MikeWa]

Premium high octane fuel burns slower than regular fuel. It is also harder to ignite (anti knock). Many small engines like lawn mowers and chain saws will not run on high octane fuel. Just use regular and don't worry about it. I also do not like very high alcohol fuel because of possible damage to rubber components but ordinary regular is generally ok.

 

Be sure to use fresh fuel at the start of the season. A lot of water can be in the tank after sitting over the winter.

 

Mike

[BlueSky]

Thought I'd just update you guys, appears it's probably nothing to do with the "type" of fuel & changing it did as someone said dislodge some crap & & was merely a coincidence that it ran afterwards.

I went out to use the lawnmower today, it has regular 87 oct in from last fill, same thing... starts after being primed, runs for maybe 5-10 seconds then cuts out. I did eventually get it to run, & was able to cut the grass.

Anyway trying again later I got the same results (won't run reliably) So I got some Seafoam & I'm trying that, so far no change so I'm just letting it sit for a few days now after adding Seafoam to the gas (starting it a number of times) & also squirting it into the air intake & running it.

If that doesn't work I may try a carb service kit but at 15+years old I don't think it owes me anything. Maybe a new lawnmower is in the stars, but I would like to know what the problem is. Maybe like me, it's just old age! lol

 

if the seafoam doesn't work maybe change the spark plug and/or check the compression.

[revks]

In the late 1980's I worked for a mechanic shop that also sold and serviced small engines, lawn equipment, and chain saws. The small engine manufacturers changed the aluminum alloy in the carbs in the mid 80's. The reason for the change was that the ethanol would draw enough moisture and cause the fuel to become acidic. The old aluminum couldn't handle it and became porous causing the carbs to fail. Same thing happened to several cars in the 70's. The gist of this is that ethanol in small engines built in the late 1980's on can use ethanol with no serious detriment to the carbs.

The next problem is that ethanol does not keep well so additives are recommended when storing your small engines for more than a week or two.

The higher the octane rating the slower the burn of the fuel. Octane that is too high will cause running problems with some small engines as they do not produce a hot enough spark to ignite the air/fuel mix properly. If you want to keep using higher octane I would recommend a hotter plug and maybe checking ignition components to ensure the best possible spark.

 

Check the carb gaskets as well, I had a similar experience and found a leaky gasket. simple fix