GEM or not

[tmc3c]

I have a 74 250 GS motor that I need to do a top end job on. I might have to buy a piston for this job and I was wondering if I should pull off the GEM set up. My question is what are the pro's and con's ? Thanks guys!!



Thomas Carmichael


1970 125 Six Day
1976 250 Hare Scrambler

[dirtbike]

Sorry for not beeing all too smart but what is a "GEM" setup?

[tmc3c]

Dirtbike,
   GEM is a reed valve set up for the Pentons.



Thomas Carmichael


1970 125 Six Day
1976 250 Hare Scrambler

[G Ellis]

Thomas give me a call I can give you some help. 217 260 0237  Later Gary

[rob w]

TMC,
Yes on the reed valve.....it's the wave of the future.

RAW

[dirtbike]

Do they do any good?

[brian kirby]

I only have experience with one GEM reed valve bike and its SLOOOOOOOW. Its a Jackpiner and it feels like my 100 is faster. John D told me that those GEM kits had reed valves that were way too small for the size engine they went on. When he was looking at the bike he said the 175 reed valve was too small for a Hodaka 100, much less for a Jackpiner. From my experience with this Jackpiner I recommend sticking with the piston port setup unless you want someone to do it right and weld on a modern reed assembly.

Brian

'73 Berkshire

[Big Mac]

GEM made a 250/400 reed and a smaller one for smaller bore bikes. The larger GEM appears more restrictive than the stock intake but it doesn't seem to hurt any on the top end and the reed makes the power more tractable when it comes on the pipe, and gives more grunt. I have two of the 250/400 GEMs on 400 motors and they rev to the top just fine--yours should breathe fine.

See the GEM reed before/after test by some cycle mag (DB?)in the Penton Technical site and Knute's race bike setup article in SKT.

[brian kirby]

That is good to know that the 250/400 kit had a larger reed more appropriate to the engine size. Like I said, the only thing I know about GEM reeds is on the 175. The bike does run really well, it runs cleaner on the bottom and its very smooth, no hit like with the piston port setup, but when you ride them back to back it is very obvious the reed bike is much slower.

Brian

'73 Berkshire

[Larry Perkins]

Thomas,

The GEM reed will lose some on top but response off the bottom is far improved.  We are generally beat more out of corners than on straights.  All in all you don't want the reed so take it off and send it to me.   LOL!  No-Just kidding.  I think it improves the overall performance of the KTM motor in most situations.  The Red Rocket I won on at Mid-Ohio a few years ago was a Mikuni with a GEM reed.  I loved that bike and if I could ever get it to leave Oklahoma I would try to buy it back.

Larry P

[tooclose racing]

Larry - do you have any pictures of the Red Rocket you could post? It just sounds like this bike had "all the tricks".  I'm curious about the red tank - was that your idea, or did some 76/77 250's come with red tanks (I know I've seen 75s with red tanks)?

I'm the proud new owner of a '76 MC-5 250 with Ceriani forks, and I've followed (with great interest) your posts regarding racing the MC-5s.

Thanks for all of the "intel" you've posted on these bikes!

[Larry Perkins]

The Red Rocket was a true 74 and was a Vintage bike.  I do have some racing pics of it but I would have to dig them out and learn how to post them here.  It was redone by Vintage Iron and I purchased it from Lynn Williams.  It had  porting, reed, and Jemco pipe.  I sorted the suspension out and dialed it in.  She were fast and one of the stipulations when I sold it to Larry Shope in Oklahoma was if he sold it was that I got first shot.  Unfortunately he forgot that and sold it to a POG member who also lives in Oklahoma and it is a CC bike now and not in it's original form.

An off topic side note is that I have sold several bikes where upon sale the new owner promises me a chance at them when they get ready to sell and only 2 people have ever kept that deal.  My Dearest friend and Brother Ron Carbaugh with my Astrodome bike and Tom Flores with the MC5 Penton that I am greatfully racing again.  Both of these guys are top notch and good men of their word from TEXAS!  Thanks guys!  

By the way Tom was wailing on his Berkshire this weekend at Diamond Don's winning 100 Expert Cross Country and 2nd in 100 Expert MX.  A stock bike on standard bore.  Gotta love those Berkies.

Larry P

[tmc3c]

What about different cylinders ? Will the reed valve set up run better with some and worse with others? I am not sure which cylinder I have but I think it is on it's last leg.
       Thanks Guys!


Thank You Gary for your wisdom ! I still have what you said in my mind.



Thomas Carmichael


1970 125 Six Day
1976 250 Hare Scrambler

[tmc3c]

I guess I should first ask is there several cylinders that work for the GEM on the 250? I am to a point where I am going to have to sleeve the one I have or go to plan "B". Thanks Guys for taking time with me on this, as you can tell I am flying blind on this.[8]



Thomas Carmichael


1970 125 Six Day
1976 250 Hare Scrambler

[454MRW]

Thomas,
If you just want to go larger on your existing cylinder, I have a new Wiseco .080 piston with rings, but you would have to get a pin from AL. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S