yamaha 1977 15hp hard to pull start

[boscoe99]

Yamaha uses one in their single cylinder 2 HP two stroke model.

50 plus years ago my Cox .049 model airplane motor used a reed valve.

Still trying to wrap my mind around a piston drawing air into the crankcase on the upward intake stroke and then there not being a valve of some kind to keep the air from flowing back out the intake as the piston descends.

[TownsendsFJR1300]

There is no need for reeds on a single cylinder two stroke with the carb on a manifold on the side of the cylinder.

Agreed for these little engines that are run often at only WOT.

For larger, engines, actually there is...

During my younger years, growing up on Yamaha's (1970 and newer), Reeds came out on the RD250 and 350's in 1975(among other machines). I had an RD 350 as my first street bike. (this was a twin cylinder-in-line)

With the reeds, those engines had more low end power vs a non reed engine which tended to NOT have the throttle response on the lower end of the RPM spectrum.

Boscoe, it was very common for the rear (intake side) skirt of the piston (MX racing bikes) to have two or four larger holes cut into the skirt to allow more air/fuel into the engine.

Thus the PP, depending on the application is NOT needed on certain applications BUT the reeds are MUCH MORE EFFICIENT..

[pstephens46]

No expert here. What is referred to as PP?

[TownsendsFJR1300]

No expert here. What is referred to as PP?

Read post #12, Piston Port vs reed valve intake systems..

[ausnoelm]

The reeds are not needed on simple engines for things like mowers, however, fitting reeds to single cylinder 2 strokes improves efficiency, that's why they were fitted to bikes and stuff, in a simple piston ported motor, there is a lot of "over lap" where intake and exhaust kind of get mixed, but for simplicity, this was over looked, the common lawn mower has very few moving parts, no valves or any of that kind of thing, making them easy to maintain for even the most lame home user.

[ausnoelm]

OH, and the reed Assy in that diagram is needed because the carb is bolted to the rear and of the crankcase, not the cylinder side, I can explain in very basic terms how it works for those who can't "get their head around it" I was involved in these engines for decades, from manufacture to end user support, and know them backwards, frontwards and inside out.

[boscoe99]

I have not heard of a two stroke lawn mower in about fifty years. My Honda mowers have all been four stroke models with valves. Over head valves actually. The last Briggs motor I had about a million years ago was a flat head.

I gots to go and google Piston Porting. Never too old to be learning something new.

[ausnoelm]

You need to remember that a simple 2 stroke has passages (ports) cast into the cylinder walls for the transfer of fuel from the crankcase, the exhaust was just a simple "hole" in the cylinder wall that exhaust gas just went out, with no control as such, hence the typical 2 stroke "cackle" sound.

[ausnoelm]

We still have a very common mower, designed and manufactured here, and it's a 160cc 2 stroke, of course there is lots of Hondas and cheap Chinese copies around, but the simple old 2 stroke reigns supreme on our lawns.

[pstephens46]

I bought one of the last 2 stroke Lawn Boys in 2000. To replace another old 2 stroke Lawn Boy. The deck rusted through. Was a great mower

The plastic carb was a POS. Tended to leak fuel.

[99yam40]

Last one I had(Lawn Boy) deck was made of aluminum I think.

I have seen chainsaws with reeds and compression reliefs.
that is one reason I asked about if this old motor might have had one. if they do not work correctly pulling the start rope does not work well

[pstephens46]

I bought one of the last 2 stroke Lawn Boys in 2000. To replace another old 2 stroke Lawn Boy. The deck rusted through. Was a great mower

The plastic carb was a POS. Tended to leak fuel.

Deck rusted and the front wheel fell off.

That old 2 stroke would still be running today. Carb had a funny "wind vane" sort of thing that reacted to the flow of cooling air. Con*****ed rpm as it had no throttle. Blasted right through the heavy Bermuda grass.

Sorry for the derail. Reminiscing....

[TownsendsFJR1300]

I have not heard of a two stroke lawn mower in about fifty years. My Honda mowers have all been four stroke models with valves. Over head valves actually. The last Briggs motor I had about a million years ago was a flat head.

I gots to go and google Piston Porting. Never too old to be learning something new.

I have two generators, one being a Briggs 8 HP "flat head", the other a 10HP Birggs OHV. My Techumseh chipper / shredder is also a flat head (both are older units).

The overlap of the non reeded motorcycles plus burning oil (2 stroke) was the demise of them with the EPA...

At an old engine show I went to down here years ago, (middle of the state),
they had a "push" lawn mower (it was in parts) with NO wheels. The blade generated enough upward "draft" that it operated like a hovercraft-very cool!!!

A design most probably haven't heard of is the rotary valve:
Kawasaki Motorcycle Parts 1973 F6-B CRANKSHAFT-PISTON-ROTARY VALVE Diagram

This is what I referred to earlier I had to tear down and replace the main cases. The carb is on the side of the crankcase, the disc replaces any reeds, PP, etc.

[99yam40]

I am curious as to what the rotary valve part this refers to.
I do not think it is in the diagram you posted.

I saw some of those hover mowers in action many years ago.
guys were mowing weeds in a ditch that had water in it

[TownsendsFJR1300]

I am curious as to what the rotary valve part this refers to.
I do not think it is in the diagram you posted.

Part #21 is the actual rotary valve.

It spins (attached and clocked) to the crankshaft. It opens up the lower crankcase to the carb (as a reed valve would) BUT is strictly set/mechanical and obviously doesn't work on vacuum to open(as a reed valve does).

Click on the link I posted, it shows all the parts and names. You could say it works like an intake valve...