Old reed valve questions

ol'homey · Jan 30, 2005

The HL-141D carb. on my old C-72 Homelite (circa 1970) was flooding and dripping gas out the intake when idling. Bought a kit and tore into it today. The needle and seat were fine as I couldn't blow any air though it when closed. The diaphram looked like it was out of shape though so that the button was pressing on the fulcrum and allowing the needle to stay open all the time. Anyone ever seen this before? I bet that diaphram was at least 20 yrs. old. The carb. cleaned up and went back together fine.
Before I put the carb. back on I just couldn't resist pulling the intake elbow off to see what the reed valve looked like. It is a single steel flapper valve about one inch in diameter. It was nice and clean and looked like it was seating like it should. I put the elbow and the carb back on. Pulled it 3 times and it fired right up. A couple min. later I had the carb dialed in and it was running like a champ slightly blubbering at about 8500 rpm chain on, no load and idling at about 2200 rpm. No gas dripping out the intake anymore!
Questions:
1. How long do these old steel reed valves last?
2. How do you know when they need to be replaced? Is fuel/air mix blowing back out the intake the only symptom? Or are there other signs to look for?
3. Do the rpm numbers above sound about right for this old saw?
Any tips re. care and feeding of these old reed valve saws are greatly appreciated. Thanks, Lee

Al Smith · Jan 30, 2005

That mis-shaped diaphragm,is just a typical thing.I've replaced a bunch of them .I've only had to replace the reeds on one saw,a McCulloch sp 125 .As I understand it,the modern gasoline has deteriorating affect on diaphragms,more so then than the gas of years ago .I am guilty of not running them out of gas,when I'm done using them,for a period of time,but it is a good idea,and helps ,a great deal,in preventing diaphragm warpage.

oakman · Jan 30, 2005

there's no sure way of telling when or if a reed valve will fail. one thing is sure. put enough hours on it, and it will fail, eventually. i've used a scope to inspect 'em on occasion. sometimes you can see fatigue lines. i have several saws that use an epoxy or fibreglass material for the reed. these are the best, as i'm sure you can surmise what happens when a chunk of steel goes thru the motor at speed. but i have seen it happen with no damage. i've also seen a couple smaller 2-stroke motors that have the reed on a hinge/flapper type device so the reed itself isn't flexing. theoretically this should last a good deal longer than the other type. most modern motors using reeds are carbon fibre type. but they still break. someone posted a while back a website for reed material to make your own. i searched but couldn't find it. but theres lots of info about reeds on the net. i have made several replacements of my own. i currently have a P-62 running very strong on a homemade reed plate made from a chunk of hi impact plastic salvaged off an air compressor flywheel guard. i thinned it down to where i thought it had the right stiffness, and voila. this saw had an epoxy glass type reed on it that had broke and went through the motor, even though the saw had very few hours on it. i am gonna be freshening up my SP125 soon with a new block/cylinder and piston, and i am gonna put some carbon fibre reeds in there for sure.

jimbo1490 · Jan 30, 2005

Accurate Plastics make the brand name Acculam laminated fiber reinforced plastic material. You can get it in any thickness you want from .005" to 3.00" and made with a variety of matrix and reinforcements. You don't really need anything exotic for a chainsaw reed valve, just the G10/FR4 style epoxy/glass material. This is the same grade of material they use to make circuit boards. Somewhere between .010 and .020" thickness should do the trick. I use to make my own reeds for outboards from this stuff. It looks like they are selling directly to the public now since they started publishing pricing info on their website. I used to buy it through MSC supply. Here's a link:

http://www.acculam.com/s4.htm

Jimbo

ol'homey · Jan 30, 2005

Well I put the ol' homey in some 30" oak this morning and it ran real strong, probably better than it ever has. I'm running a hard nose 21" .063 bar, a GB rim drive sprocket with a 3/8x8T rim and Oregon 75DP semi-chisel chain. The semi-chisel seems to stay sharp longer in hardwood than the chisel. The saw will run 6-7k rpm with the bar buried. The chain still has enough bite that if I push down on it hard while it's buried it will drop the rpm down to 4-5k where the saw will start to lug. I have a 3/8x7T rim but I think I'll save it for use with a longer bar as it seems to have plenty of power with the 21" bar and 3/8x8T.
After I got it back to the shop and cleaned it up, I fired it up with the filter off so I could see the carb. There was a film of fuel mix in the carb box, but no raw fuel like there was before when it was dripping out the carb intake. At idle and part throttle there is nothing visible coming out of the carb intake. But at full throttle there is a slight and very fine vapor or fog coming out the intake. No drops of fuel just kind of like a slight fog. This is what put the film in the carb box. Is this normal or is it a sign of a leaky reed valve? It sounds like the reed valves in these old saws aren't normally replaced until they break and get eaten right?

oakman · Jan 30, 2005

thanx for the heads up on the acculam. i'll give em a try. i've considered using pc board in the past, but couldn't find a chunk big enough without holes and wasn't too sure about thinning it down, anyways.

oakman · Jan 30, 2005

homey, some blowback is normal. i think its the overlap period between positive and negative crankcase pressure. on my old yamaha gp433 with twin carbs and velocity stacks you got a pretty good blast of fuel mix on ya after a days ride.

Justin Garrison · Jan 31, 2005

My dirtbike uses carbon fiber reeds. Run them untill the bike dosen't run worth a darn. They go right out the silencer no harm done.

Pioneer · Feb 2, 2005

I have some old saws with the original metal reeds and they all look and function 100 percent. Of course everything that has to go under a bending cycle will fail in time because of fatigue, but I don't think anything less than a pro user who puts many hours on it has to worry. Carbon and fiberglass seem to be the way to go nowadays, but has anybody tried mylar? Would it have the correct combination of properties to be used as a reed?

jimbo1490 · Feb 3, 2005

Mylar is not a thermoset plastic like epoxy is. What's more it has low Tg and HDT numbers so would soften and deform with the temperatures it will likely encounter. Questionable solvent resistance as well. Mylar is a brand name for non-woven polyester sheet goods. Lexan HT sheet would be OK for the temperature and stiffness but has virtually NO solvent resistance; freakin stuff melts in mineral spirits! Glass, carbon or Kevlar in epoxy matrix is definitely the way to go.

Jimbo

ray benson · Feb 3, 2005

I purchased 10 (NOS)steel reeds for 2 old Homelite saws about a year ago.
6 @ $2.21 and 4 @ $1.86. So for about 20 bucks the old saws have new reeds.
Ray

Lewis Brander · Feb 3, 2005

Reed Valves:

Hello to all. I have a 55 year old saw that still has her original (metal) reed valve and it's in fine shape. All the rest of the saws I have are in fine shape with the oribinal reed valve also and don't expect to repalce them soon. These new (plastic) reed valves may be more advantagious as far as performance, but looks like they have to be replaced periodically to insure that they aren't ingested into the engine from breakage. This might be fine for a dirt biker who goes through his bike and wants to replace the reed valves before a race as part of his normal routine. Just looks to me to be an addid expence and someting else to worry about as far as saw maintinance goes. I'll stick with the old metal reed valves for now. JM0. Lewis.

jimbo1490 · Feb 3, 2005

It's really the opposite. A failed steel reed often destroys the engine completely. A failed plastic reed goes through harmlessly. The fatigue life of fiber reinforced plastics is much longer than spring steel. And if the steel has ANY corrosion, all bets are off since the hydrogen embrittlement from the corrosion just makes surface cracks more likely. The only reason reeds fail on dirt bikes now is that designers push the reeds that much harder in terms of max allowable open deflection since failure is not that catastrophic. Back in the days when we replaced our steel reeds in our dirt bikes with the new Boysen glass reeds, the new reeds NEVER failed since the reed blocks were engineered for the old steel reeds. If you replace an existing steel reed with glass, it will probably NEVER fail.

Jimbo

jimbo1490 · Feb 3, 2005

I should add that we did replace the Boysens periodically since the plastic does 'creep' in that it begins to slowly bend permanently in the direction of flex. Steel does not do this. After a while the reeds don't close completely at rest making the engine much harder to start. The low end throttle response suffers a bit too when the reeds get like this. But I never saw one fail completely. Only later when I had a late 80's bike which came with plastic reeds from the factory. You can inspect a plastic reed and see this creep, too whereas the steel reed gives no indication of impending fatigue failure, at least not to the naked eye.

Jimbo

oakman · Feb 3, 2005

i agree totally with jimbo. i have lost perfectly good saws, dirt bike, and snowmobile motors in the past due to failed steel reeds. as far as vintage saws go, if ya just wanna look at them or run 'em a couple times a year, then leave the steel ones in. i myself prefer to run my old saws a lot. and i run 'em hard. in big wood, not little pecker poles. making and replacing reeds is relatively easy. ya can't just trot down to the mcculloch store and buy a new block and piston for a 125 anymore. or any other saw over about 20 years old. it took me awhile to find this one, so you bet i ain't riskin it with no stock steel reeds.

Justin Garrison · Feb 4, 2005

I have a magazine with a boyesen reed ad that says "the reeds in your 2 stoke engine take an incredible amount of abuse at 8000 rpms they"ll open and close 133 times per second. If the reeds sit more than 10 thousandths of an inch off the block it is time too replce your reeds".

ol'homey · Feb 4, 2005

When I looked at the steel reed on my old C-72 it looked like it sat flat down on its seat. I put a flashlight on the other side of the reed and couldn't see any light from under the reed. Then I turned the reed over poured a little fuel mix on the crank side side of the reed. It leaked right through. I guess it needs the pressure from the crankcase against it to seal right? I put it back together and it runs great. So what should i do? Leave well enough alone, or try to find a NOS reed or maybe the whole elbow assembly and replace it? I don't think it would do any good to keep a spare reed because if the one in the engine ever fails the engine will be junk and I won't need the spare reed then. I wonder how common is it for one of these old saws to eat a reed and destroy themselves? If it pretty uncommon I might just take my chances and leave it alone. Anybody know the max. recommended RPM and operating RPM range for the C-72? I don't want to damage the old reed by over reving the saw. I've got it slightly blubbering with the chain on at about 8500 no load and in the wood at around 6500-7000 rpm. Am I way off? Thanks

jimbo1490 · Feb 5, 2005

This is exactly what we would expect since steel does not creep the ways plastics do. As I said earlier, there is no reliable way to asses an old steel reed with the naked eye. I mean, you can look for the obvious like corrosion or chips and the like. But even if it looks perfect, it may be beyond its fatigue life and ready to fail any time. That's the 8itch about steel reeds. Plastic creeps, but has longer fatigue life. So you will wind up replacing plastic once it creeps since your motor will be hard to start. On a chainsaw, where the reed block was designed for steel reeds anyway, you might not get any creep since the reeds don't deflect that much. It's different for a dirt bike or other race motor where the reeds are pushed to the max. They definitely do creep in that case.

Jimbo

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