Gem Reed Setup

[imported_n/a]

Due to the large investment necessary for developing a mold for these items, would there be any value to create a "blank" part that had applicability to more than just the KTM engine; for example, with final machining, could fit Buls or Huskys or Maicos, etc., so a larger market would be out there to justify the cost, time and effort?

Glenn

[tanz]

My brother in-law owns a small aluminum foundry, he does a lot of small runs and one offs. If someone comes up with some designs I could have him run them by his moldmaker and get some prices or if someone comes up with a mold I'm sure I could get a small quantity made for cost of materials.
Steve

[Big Mac]

I'm thinking the same thing. The other vintage MX brands that are plentiful and non-reeded are Honda Elsinores, CZ and Bultaco. Seems to be a bunch of Elsinore reed setups already being produced from something similar. But CZ's are like Pentons--everyone's chasing aftermarket DH reeds for the 250s and 400s and the demand could be strong enough to warrant. Not sure about the Bul's. Husky's went to reed in '74 and earlier are probably not valuable enough bikes when made race-ready to warrant cost of reed setup, to still get smoked by a Husky Mag.

I believe Charlie Richardson of CR Highperformance uses a later model Honda reedvalve setup and machines CZ's to match, for around $300. Haven't seen too many in action.

[imported_n/a]

Thanks for the info, Mac.

Are there any AHRMA restrictions on reproducing to correct era specs that we need to worry about?

Glenn

[Knute]

Hey guys,

The GEM 250/400 reed valve kit works well on the 250's but we have not gotten around to trying one on the 400.  With this setup on a stock 1974 250 engine, Kevin Brown won the AHRMA Sportsman 250 Expert National Championship both times we contested it - 2001 and 2002.  Each weekend this engine outran the healthiest CR's, YZ's, CZ's, Bultaco's, and Husky's around and had 0 DNF's while doing so.  

A properly designed reed valve setup is almost always an enhancement to a 2-stroke engine.  A reed valve is, in essence, a check valve that allows the air/fuel mixture to flow into the engine but prevents it from flowing back out (reversion).  When retrofitted to a stock piston port engine they can be used to increase low end and mid-range grunt, while sacrificing some airflow on the top end.  In our case, the Gem reed made our bike's powerband more linear, smooth, and forgiving, which, with Kevin's skill and throttle control, allows him to really maximize speed in the corners and rough stuff.  Realistically though, the 250 has an excellent powerband to begin with, so I don't think the reed setup is that important on it.  

More frequently, reed valves are used by engine designers to enhance mid-range and top end power because they allow more radical cylinder/port designs that would otherwise be flat on the bottom end and too pipey.  This is the reason you see reed valves on all modern 2-stroke's.  Reed valves are also most helpful to smaller, higher revving engines like 125's and, conversely, less helpful to torquey open class bikes.

While you've got me going, I need to preach a little bit about horsepower.  Peak horsepower wins when you have ideal conditions i.e. drag racing, where you have excellent traction and no turns.  For anything else I would gladly sacrifice a few peak horsepower for a smooth, linear, predictable spread of power.  I've witnessed hundreds of dyno runs in my life and can easily say that the engine that has the most peak horsepower doesn't always win.  Engines tuned only for peak power typically give away some lower end power and are more prone to funky power curves as well.  A good race engine designer first considers the engine's application and then attempts to yield the ideal combination of horsepower, torque, "area under the curve", and the shape of the curve.

As far as the KTM 250 goes, I personally think it was KTM's best design from that period.  With a fairly elaborate porting scheme and good crankshaft flywheel weight, the 250 made tons of power yet was very smooth and tractable.  The 250 is such a well designed engine that it can win races in its stock configuration.  If you really need more power, then the Cranke porting and a better pipe are worth considering.  Ultimately, a reed valve can be added but I don't think it's really necessary.

In contrast, the early 400's had fairly radical port timing and compression ratio, and too little crankshaft weight.  The result was a pipey, explosive powerband like a 125 on steroids.  As a result, on a tight or slippery track our 250 will beat our 400 any day.  To improve the 400 you should use the heaviest ignition flywheel possible, reshape the combustion chamber like a later 400 (or use a later head), use a better pipe, or even use a bottom end from a later 400, which had extra weights bolted to the crankshaft flywheels.  This engine would also be a good candidate for a reed valve.  The GEM 250/400 kit (not to be confused with the small 175 kit) will work but may prove too restrictive based on its design.  To design a reed kit that replaces the stock intake manifold you must work within the confines of the existing 4 bolt pattern on the cylinder, which doesn't leave much room for the added area of a reed cage and petals.  Unfortunately, I have not used the GEM kit on a 400 so I can't really say how it would perform.  The ultimate way to add a reed valve to the 400 would be by fabricating a large reed block and welding it to the cylinder.  This would allow a larger cross sectional area that could house a reed cage and still maintain adequate flow.  I have heard this is what J.P. Morgen does with his KTM 400's and I know that some of our competitors do this to their Maico's and CZ's.

For more info on this type of modification go to Replika Maschinen's web site and look at the article entitled "Big Reed Maico".  They discuss their modifications and the related AHRMA rules.  Also, at the top of the article there is a link to an AHRMA article with pictures of the conversion.

Whew!  That's my .02 worth.  Let me know what you guys end up doing.

Kent Knudson

[john durrill]

Kent,
 i agree with  you on reed kitting that generation KTM motor.
 the Gemm 175 kit is too small for the 175, its too restrictive. the ones i did for customers i did not like at all.
 the early 400's power stock was like pulling the trigger on a gun. the 79 400 GS i had was a dream to ride in the tight woods with a stock Bing carb. the early 400 would probly work  much better with  weight added to the flywheel and some work on the pipe.
 the key i believe to adding the reed kit and not looseing reliability is how much you modify the piston and ports.
 where you cut the holes in the piston and any changes to the intake boost port area of the cylinder..
  the way the piston is designed  would limit where and how large you could go with the holes. same with the intake area where the bridge in the intake port is located. it would be better for anyone doing it to get the measurements from someone like you who has done it and tested it.
 we  reed kitted a lot of engines back then with Gemm kits.some designs did not work well at all with the gemm kits  designed for that specific engine .
just my 2 cents chuckle chuckle!
JOhn D.

[Knute]

Hey John,

That's great info.  I always enjoy hearing what dealers did back when the bikes were new.  Were you ever able to tame down the 400 back then?

Well, I'm off to the Dealer Show in Indy.  I'll catch you guys Monday.

Kent

[firstturn]

Kent,
  On taming the 400, we put a smaller carburetor and retarded the timing.  This was mainly to help out of the turns so that the explosive power was more trackable.

Ron Carbaugh

[imported_n/a]

Ron,

Did you guys tame the 400 by using a 36mm Bing from the 250 or did you use Mikunis?

Glenn

[firstturn]

Glenn,
  You can use either, but I used a Bing(36mm).  I normally used carburetors that I had the most selection of needles and jets.  Again this was out personal preference for keeping horsepower to the ground on tight tracks.  I changed gearing for the long tracks.  I don't consider myself a expert on the big engines since my real love was the 100/125 class.

Ron Carbaugh

[tlanders]

Ron,

On the old 400, what did you retard the timing to? If you used a Mikuni, would you use the 38mm or 36mm? I am preparing two 400s for this race season. The first is a rebuild of my Mint 400 vintage bike which broke a cylider stud and sucked in enough dirt during a national to completely wear it out. THe cases were never very good so I have two lower ends to replace it, a '74 and a '77. I thought I had decided to use the '74 because it was supposed to be hairier than the '77 figuring that the people who didn't like the '74 were woods riders, and for MX it might be just great. I do have an aftermarket torqueier pipe for it and I'm putting on a PVL ignition so the most flywheel I can get is 17 oz. I wouldn't mind going to a Mikuni, they seem to work so much more reliably and parts are so much more available and cheaper. Should I use the '77 lower end or do you think I'll be happier with the '74 with pipe, timing and Mikuni changes?

The second 400 is going in the 1980 MC80 I'm building. I wanted a 420, but there just isn't enough parts available to race them I believe. So I opted for sending a '77 400 clyinder to J.P. Morgen to do his magic reed valve conversion and porting work on. I got it back and I am still trying to get everything else together. I can't wait to try it out. His porting work was beautiful. It uses a late '70s 6 pedal Yamaha reed cage which is then AHRMA legal. He supplied a 38mm Mikuni with it. I have less than 2 weeks before we take off for Florida for bike week to get both these ready. It has been so hard working in an unheated barn in sub 20 degree weather. I can't really try them out anyway because of the snow on the ground!!! I hoping I get one running by the end of this weekend and that all the snow melts!!!!

Thanks for your input.

Teddy

[firstturn]

Teddy,
  The Mikuni is probably a better choice, but I just had so many Bing parts(and it looked stock) is why I used a Bing.  On the timing I don't have my original notes(went with the bike when my friend sold it), but it was very little.  I retarded it to stop such explosive hits on coming out of slow turns or sweepers.
  The pipe sounds interesting especially if it is a real torquer.
  The other area to address (and I was visiting with Larry Perkins about this subject just the other day) is the flywheel.  With a regular flywwheel we were discussing how we used to glue weights to get it under control.
  This all may sound funny to some that read this as if we are trying to slow this bike down.  But my point is trackable or useable horsepower.  
  The second engine may also be a candidate for trying the torque pipe on after carburetion is sorted out.
  I guess the last thing after I sorted out the engines was to tune the gearing for the rider to get the most of bottom to mid range out of the bike.  Any one can go fast down the straights according to Kenny Roberts[].
  I final analysis I again state I am not a expert on these big engines in that there are too many years gone by since played with them.  Now I won't sleep until I remember the setting for the timing.
  TEDDY may I ask you for the frame number off you 125 Sachs with the lay down suspention.  I'm trying to keep up with these bikes for hopefully  future POG project;).

Keep them in line Teddy,

Ron Carbaugh

[tlanders]

Ron,

I wrote a note to myself to remember to get the frame number for you. Thanks for your input. J.P. Morgen recommended 2.2mm advanced for the new set up. I was using 2.4mm for the Mint 400. Last year, I took off the 17oz weight on the PVL because I wanted more accelleration. I think the bike was just not running right all year long and I think the carb was a major part of it. I will order a Mikuni.

Teddy

[firstturn]

Teddy,
  One last comment before I'm banned from the message board for using up too much space.  Timming is a good and bad thing for making horsepower.  It is like Nitro for our big engines if a little helps, a little more should help it that much more[:0].  I have a set of $1500 pistons that prove that theroy wrong.  What I am saying is each engine has a sweet spot and when I build a strong engine, with tricks some people will never know about(I'm not being funny),  I start stepping it back to save it..and timing is one way to keep from burning up pistons(heat).  I believe in the old numbers of 3600 ft. per second(piston speed) is when normal pistons start melting or go into stage one of fatigue.  Understand these are just my thoughts and are probably worth what you are paying for them:).
When it comes to ignition and timing Dane is THE EXPERT period.  I am honored he takes the time to contribute to this Forum.
  Teddy when I grow up I hope to be a racer like you[8D].

Ron Carbaugh