GEM Reed---add booster ports, or no?

[Big Mac]

I'm adding a GEM reed to a '75 Mint 400. The original install instructions call for only cutting in 2 windows in the piston skirt. But the bike machinist expert I rely on for motor work says the right setup should include drilling a couple of "booster ports" in the ceiling of the intake tract into the bypass port.

He says otherwise the intake tract charge that's created on the downstroke has nowhere to go, and the transfer holes allowing the charge to flow up were the standard setup on later-designed factory reed valve systems.

Anyone ever add these booster ports in a KTM cylinder with reed added on? Or run one that had the ports added? I've owned a Husky and a CZ that had aftermarket reeds and the boosters had been cut in, so makes sense, but unsure why GEM never mentions them in the original instructions.... Thanks,

Jon McLean
Lake Grove, OR

[Merlin]

Do yourself a favor, put the engine back stock and leave the too small G.E.M. reed off of it (that reed may be large enough for a 125/175), lower the compression to around 125/135 psi cranking pressure and get the carb right, you'll love it!

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[john durrill]

Jon,
You could send Kent a note and see how he liked the reed setup. They ran it on a 250 and did well. We did 2 175 KTM's with the Gem kits and found what Merlin has said to be true on the Gem kits. The 250 kits were 6 petal but the cylinder intake track is too small to get the kit working right. Because the Gem kit was a bolt on it is limited in size and will probably not pass enough air.
 We found you needed 4 holes in the piston to see any improvement on the 175 engines and thats is a sign of a restrictive reed. I think your Machinist is right on opening up the bottom of the V ports. It does work on other engines like the CW Hodaka.
Are you looking for a better enduro , trail type engine? If you are then the changes KTM did in the 79 400 would be the best. They added some weight to the crank halves with bolt on plates , did the cylinder porting a bit different and came up with a pipe that delivered power smoother.
 I had one in the early 90's and wish i had never sold it. I am a small bore rider with VERY limited skills and that 400 was a dream in the woods. Even the tight stuff it was easy to handle.



 You could use a Super Trapp to get the pipe you have now working better , drop the compression some like Merlin suggested and add the weight to the crank halves. I bet you can find a bad crank with the plates or a rebuild able one you could use.
 If you get the Bing carb like new and jetted right it will be a one kick starter cold. We did that on my 400 and it would start for me cold after a trailer ride on one. I weighed about 175 then so you know it was easy to start chuckle chuckle!

 

[Big Mac]

I have a '77 400 that I bought from original owner, was set up with a 6-petal GEM 250-400 reed by the dealer when bought new, no boost ports, minor re-jetting, stock mx pipe, deck height per the book. It works extremely well from bottom to top, very linear power with no arm-jerking hit, runs very hard on top. Also starts very easy when warm, the only left-side start open bike I've ever owned that I can kick alive with my left foot. Mostly have ridden mx with it, but also have run GPs and trail ridden the woods and it works great in all.

I'd be tickled to have the '75 400 I'm setting up for mx run as well...thought maybe the addition of booster ports would make it run even better. No going backward once ports are drilled so prefer to hear from someone who's already experimented. I tried to send a note to Kent Knudsen, don't think his e-address in the register is right. Kip K suggested I talk to Forest Stahl who said he didn't think the booster ports were necessary. Have a mssg out to Larry Perkins too.

And agree with you John about the '79s--have a stock 420 that runs like a champ, very fast and a joy to ride in anything. Except maybe trials. [^]

[Merlin]

Jon, if you really want a reed, put a period six petal Yamaha MX reed cage on it (construct a box to weld to the cylinder) and do open the back "V" ports. You will need to cut the back of the piston like a Yamaha (the big half moon skirt configuration) or put some slots in the back of the piston to take adavantage of the reed, the pipe will need to get a bit more volume to work real well overall, very impressive power will result but I ask, what for? I'm a pretty fast expert rider and top five or better finisher at nationals, I still ask what for?

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[Big Mac]

The impressive power that works may not be more, just better...less pipey, more linear power with less hit, and able to carburete clean and torquey from down low in the rpms even if you're in too tall a gear by mistake. That's how my '77 with a GEM works. Small bores might need all the breathing they can get, while restrictive & controlled breathing with a reed on a big bore/large carb makes for better lap times?

The before/after test and results done by the magazine testers back in the day on a 250 is pretty convincing (see under Penton Tech pgs). And using stock cylinder with no welding or grinding is more appealing to me, in case anyone wants to go back to stock later. My view anyway.

[john durrill]

Jon,
 A Lectron will do that for you with the 400. If you would like someone to talk too about it Speedy is a world of knowledge on them. He has had them on all his bike but the his latest's KTM's  for many years.
We have 2 on 175's and they make a big difference in smoothing out the power hit and you get better traction .
 It would put you in historic AHRMA class though I think.
 I understand what you looking for now. It would be the only way i could ride an open class bike in MX chuckle chuckle!!!
John D.

[john durrill]

Jon,
 We are working Bing to Reed adapter now for another bike ( its a not quite a rodent , well maybe its an Australian rodent! [8D] )
 The pictures  shows how you would adapt a reed to a 125 Sachs engine and what kinds of flow problems you run into. The same would be true for a KTM engine but you would need to make up a different adapter.
 Gem use to make a kit that had this type adapter for the 125 Sachs engines. It had a clamp on both sides so as a bolt on kit it was very restrictive.
















You can see where we had to blend the 27 mm hole the carb flows through to the odd shape at the back of the reed cage. There is not enough distance to really do that right I think.
 Merlin , any suggestions by you would be greatly appreciated on how we could do a better job at that.
 Anyone else that can see a mistake please let me know. I have a lot to learn still and not as many years to learn it in anymore
 John D.

[john durrill]

Jon,
 We are working Bing to Reed adapter now for another bike ( its a not quite a rodent , well maybe its an Australian rodent! [8D] )
 The pictures  shows how you would adapt a reed to a 125 Sachs engine and what kinds of flow problems you run into. The same would be true for a KTM engine but you would need to make up a different adapter.
 Gem use to make a kit that had this type adapter for the 125 Sachs engines. It had a clamp on both sides so as a bolt on kit it was very restrictive.
















You can see where we had to blend the 27 mm hole the carb flows through to the odd shape at the back of the reed cage. There is not enough distance to really do that right I think.
 Merlin , any suggestions by you would be greatly appreciated on how we could do a better job at that.
 Anyone else that can see a mistake please let me know. I have a lot to learn still and not as many years to learn it in anymore
 John D.