Can a reed surface be too large?

Dear 2stroke enthusiasts,

I am tunig a small 70ccm 1 cyl. outboard just for fun. I did change and test a lot of different things using my DIY hydraulic dyno. I'm happy with my progress till now. Certainly it becomes more difficult to raise the power any further without having the top peak at a to high rpm range.

Attachment 69413


One of my last tests was about using different reed cages.

Attachment 69414

Left is stock with 3 pcs steel reeds and a small surface. Middle is from a scooter with 2 glass fiber reeds and a larger surface. Right is for tuning cylinders for small semi road racers with 8 carbon fiber reeds and a huge surface.

I tried to keep the crank volume for the right reed as small as i could but the throttle reponds compared between left and right reed cases was huge. The small one works agressive direct and the right one was more week and the peak power less. Due to that i had smaller ignition problems (as i found out later) i have to repeat the test runs to confirm.

But my question in general: Can a reed surface be too large? What do you think?

Oliver

YES.

Years ago I was racing Kawasaki 125 based engines. I had the brilliant idea to use their 250 sized reed cage in my next motor. (Six pedal verses 8 pedal) Built a new billet crankcase and some real trick welded up aluminum intake manifolds over the winter. The motor was a dog. Tried all kinds of stuff and it was several mph off my good motor. Ended up filling in the crankcase with Marinetex epoxy and put a set of 125 reed cages in it with real short intake manifolds and it ran real well.

Looking at your three reed cages, the stock one and the scooter one have way too much reed stop area. The one on the right looks like a 125 motocross sized cage. Huge jump from the two on the left. My vote would be to find a normal pyramid reed cage out of a 70 or 80 cc motorcycle, not a V Force and test it. Other option is to close off some of the reeds in your existing V Force cage.

Years ago we did a lot of dyno work on 100cc Yamaha kart motors. We found that you need to get the carburetor as close to the reed cage as possible. It improves the "signal" to the carburetor.

Good luck

Michael

Thank you for sharing your experience!

There is a 10mm adapter only between carb and reed.

Attachment 69415

Attachment 69416

A very good idea to double check the surface influence by blocking reed by reed. I'll put this on my to do list.

Oliver

Reed casing made from nylon platic for a quick test.

Attachment 69417

Quote:

---

Originally Posted by Mike Schmidt

YES.

Years ago I was racing Kawasaki 125 based engines. I had the brilliant idea to use their 250 sized reed cage in my next motor. (Six pedal verses 8 pedal) Built a new billet crankcase and some real trick welded up aluminum intake manifolds over the winter. The motor was a dog. Tried all kinds of stuff and it was several mph off my good motor. Ended up filling in the crankcase with Marinetex epoxy and put a set of 125 reed cages in it with real short intake manifolds and it ran real well.

Looking at your three reed cages, the stock one and the scooter one have way too much reed stop area. The one on the right looks like a 125 motocross sized cage. Huge jump from the two on the left. My vote would be to find a normal pyramid reed cage out of a 70 or 80 cc motorcycle, not a V Force and test it. Other option is to close off some of the reeds in your existing V Force cage.

Years ago we did a lot of dyno work on 100cc Yamaha kart motors. We found that you need to get the carburetor as close to the reed cage as possible. It improves the "signal" to the carburetor.

Good luck

Michael

---

was the engine a dog due to the larger crankcase volume?

Crankcase doesn't see the exact same pressure or vacuum everywhere so distance to the source is the driver...………...

When the reeds are too big fuel puddles so for carby engine they idle bad.
You can see on e-tec engine some models have doulbe the reed area than when it was a carby engine or engine of same hp.
Then there has to be some relationship between the volume between the carb and reed that will effect acceleration

I've often wondered about this sort of thing epecially as powerabout has mentioned the etec's have large reed cages and more reeds in them vs the equivalent power carbed motor. I know the 20-35hp etec reeds/cages are around the same size as a carbed 70hp triple. The etec 75-90hp triple has reed cages more similar to the old v4 omc engines. I've been meaning to experiment with the 25hp etec reeds/cages on a carbed 70hp triple they are about the same size cages but they they have larger rectangular passages compared to the stock carbed triple reed cages.

Quote:

---

Originally Posted by ferdthe4

I've often wondered about this sort of thing epecially as powerabout has mentioned the etec's have large reed cages and more reeds in them vs the equivalent power carbed motor. I know the 20-35hp etec reeds/cages are around the same size as a carbed 70hp triple. The etec 75-90hp triple has reed cages more similar to the old v4 omc engines. I've been meaning to experiment with the 25hp etec reeds/cages on a carbed 70hp triple they are about the same size cages but they they have larger rectangular passages compared to the stock carbed triple reed cages.

---

IMHO lots of carby OMC's are on the limit re reeds hence if you mod them to produce more power increasing the reed area will help.
I think of it as the low pressure from the piston going up happens so fast its all about the volume that can get through the reeds in an instant not what can get through for the whole duration plus ram effect ( which still happens of course)
Many engines have the reeds looking at a dead end but appear that lining them up with the carb was more inportant
Look at the top reed on a xflow, straight into the front half, I wonder if you put a 3 over 3 in there you could notice? Would that test prove my theory?
Then we have the issue of big leaves versus more small ones?
120 omc 5 over 5
140 6 over 6, thats the only difference, mounting screw holes same distance apart
Great subject as very difficult to find out what is happening, high speed camera, CFD etc, air speed measurements etc?