Cylinder mods - "DE" Bore

[Dan105]

Now I know everyone else is going big these days but Id like to hear what eric or some of you other mechanics think of deboreing my engine on my 01 cr 250. When I have the cash I would like to get a extra cylinder and have it setup as a 200 or 220 or whatever is reasonable, obviously a piston to fit it and carb mods. What are the disadvantages to doing this, I should be able to swap jugs back and forth as I wish right? Im looking for a cylinder setup that will mimick a kdx220 after its been ported for woods racing. I know this sounds dumb but its something ive been thinking about doing for along time. I would like to start the season with a less powerful engine and then swap back to the original bore. Besides the smaller sleeve and porting what else has to be done for the engine to perform correctly with the smaller bore? jetting? Thanks

 

[Dan105]

Sorry I mean sleeving it down, theres no such thing as deboring. What would the power characteristics be without re strokeing it. A long strokea and a small piston

 

[fastwes]

Dan,
If you just want less peak power there are lots of cheaper ways to get it. The simplest is a tighter (smaller inside) exhaust manifold. It could also make more bottom end power and take all the hit away from it. It would save you about $500.
Wes Gilbert
Boyesen Precision Porting

 

[Dan105]

Fastwes, Im willing to try that. Sounds good. Where would I find a smaller exaust port manifold?

 

[cujet]

That is probably somthing you would have to cobble up on your own. There have been smaller exhaust flanges available from time to time. But, most of the time, they are just a little smaller than stock and change the power slightly. You could try a washer or other restriction in the headpipe area in an experiment.

I think your smaller bore engine would need serious mods to function as you would like. Probably a custom set of powervalves too. There are many ways to get what you are looking for, including machining the bottom of the cylinder to lower the port timing, reducing compression to lower the total output, welding ports up and then machining them to a smaller size vs. stock. A smaller carb also can help with throttle control and will help to reduce output. Ignition timing can be adjusted for less power also. As a matter of fact, outboards used to adjust the timing to control the power output! It was directly connected to the throttle!

Good Luck!

Chris

 

[Dan105]

thanks cujet. I will start experimenting and see what I can come up with

 

[Rider 007]

Fastwes,
Is that something that Boyesen does/can/will/might offer? I've been chasing that grail for some time myself.

 

[Dan105]

ok guys I picked up 2 different size washers and got spares of each incase i want to double up on the thickness, which i doubt ill need too. The stock ring that is in their that goes inbetween the pipe and engine is a little less than 1/32 thick. The two size new ones that i got are both 1/16 thick. The oem washer has 1in 6/8 of daylight for exaust flow. The first washer im going to try has a 1in 1/4 daylight hole. And the 2nd washer has a little more than 6/8 daylight. I think the 2nd washer will have way too small of a hole so im going to try to get another washer that is inbetween the two i picked up say around a inch of daylight.

 

[Rider 007]

Dan,
please post the results of your testing as I'm curious to what effect it has. I have an 03 model that I plan to test sometime but I'm spending what little riding time I have tuning my CRF.
Have you thought about disabling the RC valve so it doesn't open? I've never personally done this but heard it can be effective in smoothing out the power since the valve stays closed, therefore the exhaust port stays small.

Regards

 

[Dan105]

Ok I rode with the oem washer and the first washer i picked up that had a daylight hole of 1in 1/4 hole. These were both in there for this ride with the oem up against the cylinder. What i didnt realize until i got in there was that the oem washer doesnt block off any of the exaust, i just assumed that it did. Anyways the bike seemed to be alittle less abrupt and felt as though it gained some bottom power but lost on top. It wasnt the drastic difference that Im looking for but its a start. I only got in about 30 min then I broke my shifter off in a bad rock section. I have the bike apart cleaning it and doing some little stuff that wont alter the testing so once i get it back together I will put the 6/8 hole washer in and see how the bike reacts. I think this will prob be what im looking for. My purpose of doing this isnt to run the bike like this all year, just for the first few races when I know im going to be in survival mode halfway through the race. By the way i dont think my 01 has the same powervalve as the 02 or 03

 

[Rider 007]

Dan,
You're correct that the powervalve in the CR was changed in 02. I believe the design that you have was introduced in 92 after the HPP design (which debuted in 86?) and ran through 01. The principle is the same though. If you disconnect the actuating member then the exhaust valve should remain closed and "detune" the bike, since the exhaust port is smaller then it would be if it opened. Just thought it might assist you in your efforts.
Let us know how it goes!

Regards

 

[pyromaniac]

That was the most expensive way to get less power i have heard of. A free and easy way would be to make slack in the throttle-cable so it doesnt open completly.

The only "degrade" i would like to know about is turning a cr/kx500 to like 350cc or something to make a cheap fast engine that doesnt vibrate too much. Any ideas how this could be done?

 

[john stu]

what about a big flywheel weight?

 

[MikeeeP]

Here are a few inexpensive mods...

Take your exhaust manifold and have a machine shop make a billet aluminum insert so that it is smaller. This would taper from the same size as the exhaust port to a smaller diameter.

You can do the same thing with the carb at the manifold. We have an insert in our KTM125/144 that drops it down to a 36mm carb from 39mm. Way more throttle response and mid-range. Only had to drop a couple jet sizes.

You can also adjust your timing for more mid.

Regarding putting a smaller cylinder, that is something that will work but it's expensive. Some stated it can't be done but one of the most popular classes was the 100cc class at one time back in the 70s/1980s, and all they did was sleeve down 125cc bikes and make them into 100cc bikes most of the time - though Yamaha and others made 100cc bikes they weren't as well suspended as the 125cc chassis bikes.

Inexpensive porting can mellow your power, drop your powerband lower, tame the hit and just about anything you'd like - and it'd be less expensive than resleeving your cylinder - the catch is, it would probably be hard to find a 250 to 200cc style sleeve. You'd have to make something work.

Just a few thoughts.
MP