A Reed Valve on an ETZ251

[Puffs]

As some will remember, I had reeds on my 251 for a couple of years (see viewtopic.php?f=5&t=9224&p=47843#p47821 ). While the top speed (or maximum power) did probably not increase, it made the engine's power curve smoother. More torque at the lower rev range, requiring less gear shifting and actually a more pleasant riding. Going fast was not the objective, just a wide power band. I used it for a couple of years, 3 or 4, for a daily commute.

I removed the reed valve gizmo because my home-made reeds were showing their age.

Original kevlar reeds as fitted

01 - Original kevlar reeds as fitted.jpg (33.01 KiB) Viewed 60 times

I had made those reeds from Aramide (=Kevlar) fibre impregnated with epoxy, and I had chosen that fibre because of its resilience. Those kevlar strands are very difficult to break, and consequently, the reeds are quite stable. They shouldn't break, and therefore pose very little risk to the engine.

Nevertheless, with time a crack appeared in both of them, at the location where they bend strongest:

Dark line in reeds indicates crack (kevlar - ragged edges)

02 - Dark line in reeds indicates crack (kevlar - ragged edges).jpg (27.3 KiB) Viewed 60 times

Note that the ragged edges are not necessarily a sign of wear, this material is quite difficult to work & shape. Tools (files/grinders/knives/drills) remove the binding epoxy fine, but fail to properly cut the aramide strands. A good example are the holes I drilled to fit the reeds; obviously there is no wear there (no movement), but you just cannot drill a decent hole in this stuff with normal tools.

But that's not the issue, the problem is that both reeds are more or less cracked, about 6mm upwards from the holes. A darkened line indicates that crack: some fluid has impregnated there. Maybe oil, although in those days I used the oil pump, yet some mixture will blow back. Those cracks have caused the reeds to bend:

Aramid reeds are bent @ crack

03 - Kevlar reeds are bent @ crack.jpg (11.85 KiB) Viewed 60 times

And that causes the whole reed valve to stay open in its neutral state:

[Puffs]

Reeds stay open due to crack

04 - Reeds stay open due to crack.jpg (31.81 KiB) Viewed 60 times

So at the time I figured: lets go back to original. The benefit of these reeds was not enormous anyway, so after that I continued to use the bike without. At a later stage (a little over a year back) I tried the reed valve again, and it still works, but it clatters a bit. I do not have the material anymore to make new reeds, so I put the thing aside.

But a few weeks back I ran into an offer for Carbon Fibre plate, and having a bit more time nowadays, I thought: lets give that a try. And bought some. Of course I had forgotten what I made at the time, but those original reeds were 0.70mm:

Original reeds are 0.70mm Aramide = Kevlar

05 - Original reeds are 0.70mm Aramide = Kevlar.jpg (32.25 KiB) Viewed 60 times

Hence I bought a sheet of 0.50mm Carbon Fibre (CF) from China. I measured it on all the corners, and the the thickness varies a bit:

CF plate (245 x 300mm), thickness varies 0.51 - 0.59mm

07 - CF plate (245 x 300mm), thickness varies 0.51 - 0.59mm.jpg (37.35 KiB) Viewed 60 times

But within acceptable limits (stated by the seller), and good enough for my intended use (I also had another non-bike related purpose for it).

[Puffs]

So I selected a location and made 2 new reeds out of it:

CF reeds fitted

07 - CF reeds fitted.jpg (31.97 KiB) Viewed 60 times

And assembled the valve again, with the 8 screws (which once were of plain steel, hence the corrosion traces on the gasket).

CF reeds installed (seen from carb side)

08 - CF reeds installed (seen from carb side).jpg (32.04 KiB) Viewed 60 times

CF reeds installed (seen from engine side)

09 - CF reeds installed (seen from engine side).jpg (26.41 KiB) Viewed 60 times

Now one thing I noticed, is that this CF material is a lot easier to work than the kevlar. Cutting with a jigsaw (blade for SS) works well, as does shaping on a belt grinder. Apparently these carbon strands are cut off quite easily. That makes it easier to work, but in my mind it also raises a risk: what if one of the reeds breaks (same as the kevlar ones, but then not only crack, but really break off), and gets sucked into the engine? Now I know that nowadays CF is a common material used for reeds, and in the past KTM used glass, which also shears easily. But still, I'm a bit apprehensive of the risk, and I think I'll make something like a bar or steel wire which should prevent the broken off shard to pass. The reeds are a bit wider than the opening on the engine side, so just a single length-wise restriction should be adequate. But I'm still thinking about that, so this is as far as I am for now. Any input would be nice.

[Blurredman]

Now I have nothing to add- but I am entruiged as to the long term use this will have esspecially compared with the old stuff..


Does it feel like it used to?

[Puffs]

Thanks for showing interest.

I do not know how long these will last, but nowadays CF is commonly used in aftermarket RVs. Metal reeds like in my YZ and your TS last a long time, and I mentioned how long the home-made kevlar ones lasted. In the past I have worn out glass reeds on the KTM500:

Old KTM reed (TH=0.51mm), delaminated @ upper LH side

Old KTM reed (0.51mm).jpg (27.46 KiB) Viewed 60 times

Of course I wouldn't know after how much time or miles.

But it is obvious: for longevity & reliability you shouldn't fit anything like this. Anything like this is a risk, which you do not have in MZ's original construction. So if I take my MZ on a long trip, like a pilgrimage to Tavulia, it wouldn't be with reeds, yet for the trips I make here in the Ardennes it's a risk I'll run. Well, dependent of how the engine feels with them, obviously. I have not fitted these new CF ones yet, but I expect very much the same performance as from the previous ones.

[Puffs]

So my main worry was to prevent that if a reed broke, roughly along the line where the original ones cracked, the shard would enter into the engine. I decided to fit a single bar in the length direction. That should be adequate in preventing damage, and while it will cause some extra drag, the performance reduction should be minor. I choose to use a D=2.5mm nail.

10 - Make slit for 2.5mm nail.jpg (27.23 KiB) Viewed 60 times

And fitted it:

11 - Fit nail & adapt reed.jpg (29.13 KiB) Viewed 60 times

Because of the angles, the LH reed would hit the nail when fully opened, but I could solve this by shortening the reed a bit.

I'll try it shortly, when hopefully the torrential rains finally stop.

[DAVID THOMPSON]

did some pulse jet engine stuff in the early 1960 with them we had a NO MANS LAND SAFETY AREA
behind the exhaust when the metal reeds blew it was very dangerous out there in the back yard

also tested go kart motors at very high rpm no one allowed in room with motor running

dave

[Kruh]

Pretty cool. But to take full advantage of the reeds, you would need to have windows in the piston or at least, a shorter piston skirt. So that the intake is controlled by the crankcase pressures, not the piston. Otherwise its the same thing, but with less blowback at the carb.

[Puffs]

Thanks guys.

Well Dave, my concerns are not in that order. I just fear that such a shard may cause damage to the engine internals.

Kruh: you are completely correct. I think I showed it elsewhere, in the past I used a piston with a shortened skirt at the intake side. Shortening the skirt, however, can have a negative effect on piston wear & enhance piston tilt, so I would only take a bit off.
In some engines you see the RV directly on the crankcase, so without any flow control by the piston, but I get the impression that serious designs always use some piston control, probably to limit the pressure on the reeds (and because the extra benefit of full RV control is small).

For me the situation currently is even worse: I placed a 1mm sheet of aluminium below the barrel to raise the inlet 1mm (I have the 1989 EM250 barrel with the deepest intake channel, also in an earlier post for pics of that piston), so the plan of action is to get rid of that. Of course a deep intake port increases the time the intake is open, which is good for an RV. And of course that 1mm also raises the overflow & exhaust ports by 1 mm, which enhances high RPM focus, which is a bit counteractive for my current objectives.

But just reducing the blow-back, particularly at low RPM/full throttle, is already quite something & will improve the engine's character (for normal street use). I'll try it first as-is, and if it works well I'll remove that 1mm gasket/sheet & maybe even take off a bit from the skirt, as before.

I fitted the lot yesterday evening:

12 - Fitted on the bike.jpg (25.48 KiB) Viewed 60 times

Of course started it, and noticed the apparently complete absence of any reed clatter. This is presumably because these reeds are closed in their neutral position; the reeds on my YZ don't clatter either. I have not ridden it yet - more to follow.

[Kruh]

True, that does increase piston wear, l hence the windows. Although the concerne there is the strenght of the skirt. MZs have pretty high pistons as is, so I don't think it would increase wear by much.

Yes, most modern bikes with reeds feed directly into the crankcase. Which offers the bet filling potentional. I guess, thats not something that matters much in a low revving machine like the MZ.

Raising the barrel, and shortening the skirt is good for achiveing higher rpms. But, unless you shaved the top of the barrel or head, you deacrease the compression and more imporatantly increase the squish gap. So you loose more than you gained with more port timing.

In any case, seems like something that could work pretty well for street use. Since blowback is most noticable below 4000 rpm. Which is where these engines spend most of there time

[Puffs]

Well, yesterday I did some 150km with it. The bike ran well; maybe slightly less surging & 4t'ing on the overrun. More importantly: slightly stronger in the low & mid revs, but a bit less potent in the high rev + full throttle (and a bit lower top speed). In all it ran well, but as I remember I previously had more benefit of it. Of course then it did not have the nail, which causes extra drag, and of course that was without that 1mm spacer (and don't worry, I meticulously manage the CR), and with a piston with a shortened skirt:

Megu skirt trimmed.jpg (16.34 KiB) Viewed 60 times

That's the one on the right:

ETZ pistons, trimmed Megu on the R.jpg (12.27 KiB) Viewed 60 times

Not sure if I actually posted these before, now I have. In that setting, the intake opened some 3mm earlier.

Coming back from that trip, I took the RV off to inspect it, and immediately fitted back the original manifold & did a short spin to compare. That confirmed that in the current setting the benefits of the RV are quite marginal, so for now I'll leave it as-is, with the original manifold.

Looking at the RV, I haven't seen any signs of wear, so in that sense it was a good test. I had no experience with this CF, and it is not specifically made for use as a reed, but it seems to perform well. And, as said, these reeds don't clatter.
I did see signs of leaking some oil though, at the junction with the barrel. Now it builds-up pressure there, forcing out some oil. So that needs looking at.

Anyway, when I pull the barrel off I'll try the RV again, together with the other mods for longer opening of the intake. And maybe I'll dare remove the nail; not only does it cause extra drag, it also reduces the length of that reed, thus causing it to open less far (torque = force x arm) & give more resistance. That too might be a reason for the lower high rev/full throttle performance.