PRE-MIX, Four Stroke Engine!!

[TownsendsFJR1300]

For curious gear heads..

I recently worked on two Stihl commercial weed eaters.. (one had blown up, I used the coil off that for the other engine that was loosing spark once hot)

These engines have 2 over head valves, a cam, push rods, rockers, but NO
CRANKCASE OIL.

Sounds like a 4 stroke but the OHV and lower end (crankshaft, lower rod) is lubed with Pre-mix (oil-fuel) mixture thru the carb...

See the link and as for oiling, there's a short video (on the same page) how the rest of the engine is lubed, kinda interesting...

4-Mix engine:

4-MIX engine: Lightweight with good lugging power | STIHL

BTW, valves need to be checked/adjusted every 139 hours to .004" BUT with a special feeler gauge (just ordered one), OR hack up a .004' feeler gauge(must be this in width to fit between the valve and rocker).

[fairdeal]

had no idea this existed

interesting that "Fewer emissions: The fuel burns with very little in the way of residues and thus complies with the strict European exhaust emission limits, Stage II. "

so it would seem that even with 50-1 premix, a four stroke has inherently "cleaner" exhaust than a 2-stroke.

[TownsendsFJR1300]

had no idea this existed

interesting that "Fewer emissions: The fuel burns with very little in the way of residues and thus complies with the strict European exhaust emission limits, Stage II. "

so it would seem that even with 50-1 premix, a four stroke has inherently "cleaner" exhaust than a 2-stroke.

Perhaps as it's firing/burning fuel HALF the time of a two stroke.

So you get the weight of a four stroke, performance of a four stroke, more moving parts, BUT still have to pre-mix fuel...

IMO, the worst of both engine styles...

Cam set up, timing and valve clearance checks are pretty easy thou..

[ausnoelm]

It could burn premix with fewer emissions simply because it doesn't have exhaust ports cut into the cylinder wall, in a two stroke, some and the fuel/air mix is forced out the exhaust before the piston covers it, making it a "dirty" engine. It's often touted how a two stroke has so much more power than a four, because of the power stroke every revolution, but it needs to be remembered that this power stroke is only for half the time, because the piston uncovers the port half way down, the longer "full" power stroke of the four stroke is what gives a four stroke outboard a much wider power band, and conversely what gives the short power Stoke on a two, the distinct narrow power band, but in this engine, I am not exactly sure how the lower end gets adequately lubricated, but it must, but then, if it was so good, there would be more of them made (perhaps)

[99yam40]

Not sure how the 4 stroke has the same or more torque than the 2 stroke being the 2 fires on every down stroke and the 4 fires on every other down stroke.
if both are the same displacment the 2 should have more torque.
any thoughts on this??

More compression/ longer power stroke maybe

I see comments were posted on this as I was typing

[TownsendsFJR1300]

It could burn premix with fewer emissions simply because it doesn't have exhaust ports cut into the cylinder wall, in a two stroke, some and the fuel/air mix is forced out the exhaust before the piston covers it, making it a "dirty" engine. It's often touted how a two stroke has so much more power than a four, because of the power stroke every revolution, but it needs to be remembered that this power stroke is only for half the time, because the piston uncovers the port half way down, the longer "full" power stroke of the four stroke is what gives a four stroke outboard a much wider power band, and conversely what gives the short power Stoke on a two, the distinct narrow power band, but in this engine, I am not exactly sure how the lower end gets adequately lubricated, but it must, but then, if it was so good, there would be more of them made (perhaps)

Stihl is about the MOST POPULAR engine (expensive too) for commercial lawn companies down here in Florida (where grass is cut 365 days a year).

Look on any "lawn trailer", Stihl's all over it.

Biggest complaint I have, you CANNOT quickly swap "heads"(cutting, blower, hedge trimmer, etc) quickly (like the cheaper residential machines)...

You need a separate machine for each. Now the "working end" of the machines, (from what I've seen), are simply clamped on (HD cast aluminum) to the end.

Agreed too, with "real valves", you don't have the overlap of fumes from cylinder ports mixing...

[pstephens46]

And the info suggested it has a pleasant sound. Pleasant mind you....

I like the rip scream of the 2 stroke lawn stuff I have. But I wear ear muffs now. I sacrificed too much earlier in life to gunfire and such..

[ausnoelm]

Stihl is about the MOST POPULAR engine (expensive too) for commercial lawn companies down here in Florida (where grass is cut 365 days a year).

Look on any "lawn trailer", Stihl's all over it.

Biggest complaint I have, you CANNOT quickly swap "heads"(cutting, blower, hedge trimmer, etc) quickly (like the cheaper residential machines)...

You need a separate machine for each. Now the "working end" of the machines, (from what I've seen), are simply clamped on (HD cast aluminum) to the end.

Agreed too, with "real valves", you don't have the overlap of fumes from cylinder ports mixing...

Didn't mean more Stihl made, just more of that operating principal

[TownsendsFJR1300]

Didn't mean more Stihl made, just more of that operating principal

Ok.. Yep, I haven't come across any others machines designed like this.

They are definitely the most popular for the hard core lawn guys, Echo's too (normal 2 strokes).

They may have a patent on it still(no pun intended), till that expires...


And as Pat said, I like the sound of my 2 strokes, especially a great running one.

Certainly more obtrusive (noise wise) than the 4 mix, but I wear ear plugs with every machine short of the lawn mower..