Reed valve


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Supporter 2020

I don’t think I’m following you on the intake over exhaust fix. Can you explain?

Your reed cage looks massive for the carburetor and engine size. Do I see a volume stuffer?

As far as my challenges, it’s starts with a size constraint. The size constraint makes total flow a challenge, forcing an exploration of lower tension reeds. This lower tension will only be able to perform well under the lower rpm range due to the lower frequency response, throwing the 360 degree intake timing option out the window.

For those that don’t know… the lighter the spring, the less force it takes to move but also the slower the oscillation. This holds true for all springs.

I have purchased .25mm carbon fiber to test against the .40mm resin reeds on the KX 85-100cc reed cage. I hope to have a little more to show next week.
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I don’t think I’m following you on the intake over exhaust fix. Can you explain?

Your reed cage looks massive for the carburetor and engine size. Do I see a volume stuffer?
I was referring to the early 4 strokes with an exhaust cam operating a side-valve setup, and the intake being a lightly sprung suction-to-open setup. Reductio ad absurdum or sumn'

Reed cage is from a 485cc Yamaha twin, so my 250cc single, albeit with slightly less aggressive porting, should be fine.
The 32.5mm venturi Tilly HR is a little small, but i have 38-40mm HD versions as well. No stuffer for now. I have more aggressive piston-port aluminum cylinders from Polaris ATV and sleds, all the same Fuji lineage. This is an interim step before milling the inlet side off an ATV jug and welding the whole Yamaha reed box in place, adding rear booster and tunnels to the transfers. Moving, and the force behind it, have slowed most of my projects to the moss-gathering stage.

John aged-out and stopped making his, but some interesting finds, IMO
I'd think that even fairly short adapter/spacer would allow a larger cage to fit.


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Supporter 2020
I am proud to announce my homemade flow bench. For about $275 I was able to put together what should be an accurate and easy to use platform that is capable of flowing up to 224 CFM. Knowing that these heads will max out at 230 CFM, it didn’t make sense to measure past that.

The structure is simply melamine particle board shelving from homedepot. Two 12” x 8’ boards cost $32. I needed something non porous and cheap. Look no further.

Using a head gasket as a template the head bolt locations can be drilled in the top, making it easy to properly align the head over the test bore. If the head gasket has a metal ring around the bore opening, a router with a flush cut top bearing bit can be used to get a perfect circle.

The motors are Ametek 116309-00. For $60 a piece they are capable of flowing 112 CFM. Hooked up in parallel, they hit 224 CFM. What really sold me on these motors is the published bulletin detailing their performance. Ametek provided a very nice chart and graph for vacuum depression per flow making these a perfect candidate for a measuring device. It doesn’t get easier.

Klein makes a digital manometer for $115 (ET180) that will read the range of vacuum these motors will create. If I put the signal tube in a low turbulence area it should be very accurate.

I just need to put the ear muffs on and get testing. This thing draws 14 amps and is loud! My only regret is not building this before I ported a head.


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I am working through some calibration issues. This bench was very accurate until the upper flow range. After spending some time with a probe reading pressure differential, I believe turbulence is affecting the readings. By moving the signal tube directly under the bore and providing a shield, the readings are much more accurate. I’ll have to play with it a little more, but it’s accurate enough to tell me what I need to know.

The KX 85-100 reed cage with no reeds is flowing about 184CFM. The Ktm 65 reed cage with no reeds is flowing about 160CFM. After I get the bench more accurate, I’ll begin porting the cages.

Also, I got the chance to speak with a gentleman who spearheaded a similar project in the late 70’s. He’s given me more hope that there might be something here.


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I’ve worked with this bench from 4-9 every night this week. I’ve tried baffles, flow straightest, expansion cambers, signal locations and every combination. I’ve come to terms that the manometer readings will always swing. Dead center of the bore with a setting chamber seems to work the best. Accuracy is looking like plus or minus 8cfm. For 270 bucks it’s still money well spent.

The ktm 65 reed cage is the more practical option being that it fits in the intake port the best. Here are the results.

Stock (.34mm reeds) 103cfm
No reeds. 144cfm
.27mm reeds (non split) 101cfm
Ported/ no reeds 184cfm
Ported/ .27mm reeds. 132cfm

Im beginning to think that efforts might be better spent on other areas. I can’t do too much with 132cfm. For reference the photo showing the reeds opened while fixed in the bench is under 25in.h20.

Edit: flipping the orientation of the carbon fiber weave picked up 20cfm, coming out at 152cfm.


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Supporter 2020
Before putting this to bed I performed the same tests with the Kx 100 valve. Results were:

stock (.40mm reeds) 129cfm
.27mm cf reeds. 160cfm
Ported with cf reeds. 182cfm

Better, but I’ll spend my efforts elsewhere. The only use I can see for this is preventing the “choke cough die” or “flame out” traits that happen with large cams at low rpm, which was Honda’s intent. A more appropriate cam would likely yield better results… which is probably what Honda figured out. Case closed.