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View Full Version : OT- Chainsaw reed valve



Bill736
09-02-2011, 12:46 AM
I've had a $99 Wally World small chain saw for about 5 years . It's been handy for overhead trimming that is hard to do with my heavier chain saw. The small saw is a Patriot 1900 model ( made by Poulan) with a 12 inch bar.
The carburetor seemed to be gummed up, so I removed it and a few other parts, such as the fuel lines, to service the saw. Looking over my can of parts, I notice that there's no reed valve in sight. Most small 2 cycle engines, including a larger Poulan I worked on, do have a reed valve next to the carburetor. Sears has a poorly illustrated exploded parts view of the engine, but it's still not clear whether there is a reed valve or not, and it's not mentioned by name.
The larger Poulan I worked on had a reed valve that was simply a flap of plastic that was part of a larger plastic housing assembly, and looked to be rather flimsy. So, I have a conundrum; do I reassemble the motor without a reed valve ( going to take some time , including cleaning the carb and replacing the fuel lines) or do I assume that I lost or broke it, and it's somewhere out in the grass ( yeah, shade tree mechanic on these summer days) ? Maybe I should just consider that I got my money's worth from the saw, and retire it.

Black_Moons
09-02-2011, 12:59 AM
Lots of 2 stroke motors don't have reed valves, they use the piston as all the valves.

a reed valve does definately improve proformance compaired to piston valving, but most cheap motors won't have one. They are also mildly prone to snaping and getting sucked into the engine, ruining it.

If you can see the piston block the intake port, its likey not a reed valve motor. Reed valve motors have the intake go right into the crankcase. Non reed valve motors have the intake go into the side of the cylinder.

Bill736
09-02-2011, 01:44 AM
That's quite helpful, Black Moons, since I can indeed see the piston just past the carburetor mounting flange. There probably was never a reed valve in this engine, so I may go ahead and reassemble it.

macona
09-02-2011, 02:19 AM
Lot of high end chainsaws have piston porting like Stihl and Shindaiwa.

Black_Moons
09-02-2011, 02:41 AM
Lot of high end chainsaws have piston porting like Stihl and Shindaiwa.

I guess its a little less required in a chainsaw where it will only be used at high RPM, But in any kinda mover motor where power is required thoughout the RPM range, reed valves greatly improve low end power by allowing asymetrical intake timing that basicly self tunes to the engines need of intake air, as opposed to being fixed, symetrical timing around TDC (although the reed itself does clearly offer some resistance to airflow, I have not read anything about reeds reducing high RPM hp however.)

A.K. Boomer
09-02-2011, 08:31 AM
Yes and some (like my old suzuki 90) have a rotary valve off to the side of the crankcase, it offers a timed port without the restriction of the reed - but it does not compensate for the airflow timing changes at various RPM's.

gbritnell
09-02-2011, 09:23 AM
Actually most all 2 cycle motorcycle (dirt bike) engines from many years back have reed valves. This is even with the carbs mounted on the cylinders. The reason for them is because of the port overlapping. There is still some back pressure applied to the intake porting so the reed valves were added to stop the flow inversion from the carb. Boyesen made their living supplying aftermarket reed valves for every piston ported engine made (dirt bike).
gbritnell

gary350
09-02-2011, 09:58 AM
I had a Polan Chain saw it ran good for 2 hours then it suddenly stopped running like I turned it off. I had just filled the tank with fuel so it was full. Spark plug still worked too but swapped it for new one anyway. It won't even try to start even if I put gas in the carburetor. I took it apart it has NO reed valves. It has excellent compression but after spending 2 weeks an hour or 2 each day trying to fix it I finally threw it in the garbage can. I bought an electric chain saw with 14" bar WOW I should have bought one of these 10 years ago I love it. The electric cord is a bit of a problem to deal with but it is a lot less hastle that gasoline and oil mix, this thing is light weight, cuts fast, easy to use too. I cut down 48 trees in my yard with it and made firewood out of all the trees.

Don Young
09-02-2011, 10:43 PM
I had a Polan Chain saw it ran good for 2 hours then it suddenly stopped running like I turned it off. I had just filled the tank with fuel so it was full. Spark plug still worked too but swapped it for new one anyway. It won't even try to start even if I put gas in the carburetor. I took it apart it has NO reed valves. It has excellent compression but after spending 2 weeks an hour or 2 each day trying to fix it I finally threw it in the garbage can. I bought an electric chain saw with 14" bar WOW I should have bought one of these 10 years ago I love it. The electric cord is a bit of a problem to deal with but it is a lot less hastle that gasoline and oil mix, this thing is light weight, cuts fast, easy to use too. I cut down 48 trees in my yard with it and made firewood out of all the trees.

An easily overlooked cause of an engine not to run is a blocked exhaust. This is especially common on small two-stroke engines. I have had them fail to start because of insect nests built in the exhaust openings. That is not very likely the cause of sudden stoppages, however.

Iraiam
09-03-2011, 12:50 AM
Most likely a piston ported engine, I tear down alot of these little engines and machine them into model airplane engines, few of them have reed valves. Some of the rear carb engines have them (like the Ryobi 31cc weedeater), but I haven't seen a reed valve engine in a chainsaw in quite a while, I did lots of Poulan and Echo. (Poulan engines are in many brands of chainsaws)

macona
09-03-2011, 01:28 AM
An easily overlooked cause of an engine not to run is a blocked exhaust. This is especially common on small two-stroke engines. I have had them fail to start because of insect nests built in the exhaust openings. That is not very likely the cause of sudden stoppages, however.

That and the spark arrestor screen gets totally plugged up with carbon. I repaired many chainsaws by cleaning out the muffler. Best way I found was with an oxidizing flame with a torch. Burns the carbon out real nice.

lakeside53
09-03-2011, 02:03 AM
I heat them red-hot with propane, then blast with compressed air. Quick, and like new. Correct fuel mix and mixture, and they don't block, but that's another entire thread :)

macona
09-03-2011, 02:40 AM
This was way back when I used to work as a mechanic for farm equipment.

I also did something similar on my old 57 chevy pickup. I had rebuilt the engine (235) and the crankcase breather that went from the left side down weighed a ton. Full of tar and crud in general. Took it out to the gravel drive way and my dad took the oxyacet set and heated it up. Then shot oxygen down the end. Darned near turned into a rocket engine! Didn't go anywhere but it was really cool looking. I would say a nice bluish flame about a foot long!

Really not much different than the solid fuel rocket engines using acrylic with o2 for propellant.

dp
09-03-2011, 03:06 AM
I had a '65 Bultaco Matador 250cc that was originally piston ported. I nine-ported it and added a reed valve, a McCulloch 32mm pumper carb and milled the head. Amazing how strong that engine became over time. The Matador was originally a Trials/Scrambler before the uber specialized trials bikes came on the scene.

http://community.motozania.com/_1969-Bultaco-Matador-250cc/photo/10999471/29029.html

Fun scoot. My Yamaha YZ 360 had a reed from the factory. Lordy that was a handful.