Reed valve discussion and jetting-Brain picking

[Larry Perkins]

We have some good brains here on motor and horsepower theory.  If you had a Penton 250 with a reed valve and you used the same Bing it was already jetted for in piston port form, how much different would the reed effect the jetting?  Would it effect all ranges of jets or just in some places.  I have a reed 250 and know of some others but all are set up with Mikunis.  I like and understand the Bing better and know it flows more than a Mikuni in equal size.  If you have the thought for this science tell me your thoughts.  Thanks science engineer types!

Larry P

[firstturn]

Larry since I have not worked with KTM engines for over 10 years I am not much help, but I do know that we used to start with the same carburetor and work with looking at the spark plug reading and moving on with our changes.


Ron Carbaugh

[Larry Perkins]

No thoughts on how much adding a reed changes jetting from anyone?  It is a theoretical question.

Larry P

[DKWRACER]

Larry, some good info here:
http://compotec.pl/en/index.php/reed-valve-two-stroke-engine/
Tom Brosius

[firstturn]

When I get Home I will see if I have any notes on the subject, but mine would be for Sachs and Honda early CR bikes.  I will check as I just do not remember.

Ron Carbaugh

[Larry Perkins]

Thanks for info Tom.  Thanks Ron, I was just hoping to hear from someone that might know if the reed actually does something so that jetting is different.  I know the way it makes the engine perform is different but not sure if actual jetting needs change because of reed.  I am not actually needing to jet this bike just curious if it had the same carb and was not reed what would be different in jetting if anything.  Just a thinking process of a curious mind trying to understand the whole picture of motor dynamics.  As always thanks for your help and friendship Brother!

Larry P

[Daniel P. McEntee]

I have never had a Penton with a reed kit on it that ran, but when I acquired two project bikes from my younger brother's estate that had GEM kits on the engine cores, I did some checking. One bike was allegedly a bike owned by Carl Crank. I was able to contact Mr. Crank about this matter and he said that he felt a reed was totally unnecessary on these engines and he never used them. The only reed set up he had anything to do with was trying to improve the KTM 125 engine when it made the scene.
   In thinking about the subject matter, I'll offer my 2-cents. I think an engine in good working order, in a certain atmospheric condition, needs a specific amount of fuel/air mixture to operate normally. That is what is controlled by the jets. The reed valve just does a better job, at certain speeds, of metering that amount of fuel. If you have the main jet a bit to big and it is running rich, you still get that amount of fuel into the crank case when the window in the piston is in the proper position in front of the reed, correct? In a way, the engine is still a piston/port timed fuel delivery. The reed won't make any additions or deletions in any amount of fuel. The same if the main is too lean.
   I agree with your thoughts on the Bing vs Mikuni and flow rate. I have a '77 Hercules 250 with a Sachs 7-speed and Bing carb and one of the things I worked with in getting used to this bike was getting enough air to the engine. It has a pretty big air box, but a very small opening on top to let air in. I was seeing evidence that there was a partial vacuum in the air box when running. I drill a bunch of holes in a specific spot on the top of the box and greatly relieved that condition. Even after that, the bike would, what I call, "out run the float bowl." At high speed in top gear it hesitate a second or two on occasion. I did some reading on this in some old magazines and found out that desert racers back in the day use to just drill through the carb body under the fuel line fitting, and just control the fuel during pit stops and such with the petcocks. I had thoughts of trying a Mikuni on my bike just to see what difference it would make on bottom and midrange but was cautioned against it just for the flow rate reasons. I would have had to go to a much bigger carb and spent a lot of time and money and potential engine damage trying to work out jetting. Some one already mentioned that as long as your spark plug tells you good news, you are about as good as you could get. I'm no great engine tuner/rebuilder, but this is what I have learned and experienced through the years.
  Type at you later,
   Dan McEntee

[Larry Perkins]

Thanks Dan.  What you are saying is what I suspected.  I t5hink the main thing the reed does for the Penton is it pulls lower and is smoother in transition but Pentons are real healthy in those areas to start with for a piston/port motor.  I was wondering if the jetting would really be that much different with or without a reed and you echo my suspicions.  Thanks for your thoughts.

Larry P

[JP Morgen]

Generally you will need a richer mid range, leaner bottom and top.

[Larry Perkins]

Thanks JP!

Larry P

[G Ellis]

Cut .040 of the intake skirt, does more for the bottom end than a Reed system.

[Larry Perkins]

Good tip Gary!  We used to do that and raise the exhaust a bit for MX back in "The Day".


Larry P

[G Ellis]

I do this on all my race motors. There are some other tricks to do to the motor, to make the power better. I just don't give out all the secrets.

[Larry Perkins]

Carl Cranke's specs done properly, including the pipe make for a pretty fast mount.  The one I have is the fastest 250 Penton I ever rode and there were a bunch of them.  It is definitely a drag race winner.  Now if the pilot could recapture some youth it would be unbeatable.

Larry P