dirt bike hotsaw

[troutfisher]

i have a project/toy maybe racer. I bought a 1993 Suzuki RM125 motorcycle, a hotsaw project. I have removed the engine, cut the transmission off, and purchased my aluminum side plates (1/4 inch plate). I am going through the engine now.

now to my question: The dirt bike engine has a "power valve", the exhaust port can be opened or restricted via a mecanical valve in the port.

Can I put this valve in one position and leave it? or does it have to work? should it be made open or closed or partly open? thanks for any help.

 

[blis]

you can open it up and leave it that way for best top-end power, but it will mean a big loss in low revs or leave it down for better torque and lousy top-end...

If possible, id leave the powervalve on and make it so that it works..

 

[romeo]

Weld it open permanently. They only close to give bottom end torque, great on a dirt bike but crap on a saw.

 

[romeo]

Both of these have the powervalves fixed open. The Yamaha is held open with a bracket, the Kawasaki is welded at the cogs that actuate the valves.

 

[troutfisher]

nice saws romeo! my plan was to open the valve permanently. Thanks for the info.

 

[mattinky]

you can open it up and leave it that way for best top-end power, but it will mean a big loss in low revs or leave it down for better torque and lousy top-end...

If possible, id leave the powervalve on and make it so that it works..

I don't mean to start an argument here ..... and I have to admit I know very little about "Hot Saws" but I've been a motorcycle mechanic specializing in 2-stroke dirt bikes for over 25 years and I'm very familiar with power valves. There is no comparison in the old non-power valve motors and the newer ones with power valves. I can't see why in the world you would want to rig the power valves to stay open all the time, they open fully when the engine is revved up anyway - why not leave them alone and get the MAJOR low-end benefits as well as the top-end too? I'd almost guarantee the saw would rev smoother and quicker and run just as well on top with no flat spots in the powerband, and have more torque to boot. I don't get it.... Sticking them open makes absolutely no sense to me at all. Also, just to satisfy my curiosity how do you keep these liquid cooled motors from overheating with no radiators or water pumps on them?
Matt

 

[Freakingstang]

I don't mean to start an argument here ..... and I have to admit I know very little about "Hot Saws" but I've been a motorcycle mechanic specializing in 2-stroke dirt bikes for over 25 years and I'm very familiar with power valves. There is no comparison in the old non-power valve motors and the newer ones with power valves. I can't see why in the world you would want to rig the power valves to stay open all the time, they open fully when the engine is revved up anyway - why not leave them alone and get the MAJOR low-end benefits as well as the top-end too? I'd almost guarantee the saw would rev smoother and quicker and run just as well on top with no flat spots in the powerband, and have more torque to boot. I don't get it.... Sticking them open makes absolutely no sense to me at all. Also, just to satisfy my curiosity how do you keep these liquid cooled motors from overheating with no radiators or water pumps on them?
Matt

To answer most of your questions....Most of these run WOT and cuts in the wood in under 5 seconds. Most are half of that time....

 

[troutfisher]

I watch the saws race in Tupper Lake every year, and I try to get as close as I can and talk to everyone I can. Most are freindly and are willing to share info. I haven't seen any kind of cooling system, I guess they don't run long enough. I have watched the guys check the temp of the pipe by touching it before a run. They seem to warm the saw up until the pipe is too hot to touch, just before the start. I also see the chain moving as they pull the starter rope into position, I think they are direct drive, no clutch. I have learned just enough to attempt to build a toy, this web site is a great resource, thanks to everyone!

 

[mattinky]

To answer most of your questions....Most of these run WOT and cuts in the wood in under 5 seconds. Most are half of that time....

I did'nt realize that they were run for such a short time, and after looking closely at the pics it looks like the power valve govener and the water pump are removed anyway....very interesting. Matt

 

[troutfisher]

I know on some V8 drag motors they fill the water jackets with something, it hardens and stiffens the block. Maybe here too.

 

[romeo]

I have been using the water jackets in the head as the oil tank. I have seen other folks (D. Cahoon) just trim the jackets off. You don't run them long enough to get hot.

 

[blsnelling]

I'm glad someone finally asked about the cooling. I've wondered myself. I'd think these saws would have trouble with cold seizures since they don't run that long before going WOT.

 

[romeo]

Getting them warmed up is a bigger problem than getting them too hot. My old saw was an air cooled snowmobile engine, I ended up breaking all of the cooling fins off.

 

[goof008]

on a '93 bike engine the power valve should be mechanically operated, so you should be able to leave it in place and let it work like it was designed. If you are set on disabling it, I'd leave it open for max hp's! That 125 should have plenty of torque anyway.

 

[romeo]

on a '93 bike engine the power valve should be mechanically operated, so you should be able to leave it in place and let it work like it was designed. If you are set on disabling it, I'd leave it open for max hp's! That 125 should have plenty of torque anyway.

They are driven off the primary or water pump gears but they are right where the bar mount needs to be. It is far more important to have your bar mount pulled in tight than to run the power valve. Having them closed only helps with acceleration under load. If you get her tuned out and jetted right, she will beat you to the wood even with the valves fixed open.

 

[troutfisher]

romeo, thank you for sharing the info, you are saving me a lot of trial and error. Is that sprocket custom? I've purchased race sprockets from madsens, but that looks bigger.

 

[NEK VT]

Saw

I have worked on bikes for years as well and know withe the valve open you have no lowend... But I guess they are reving these up so fast anyway they are at top end in a second. I knew about the cooling issue... It is like sleds for grass drags..... they don't have a cooling system because they are only being run for a second...... running the chain off the water bump? do they regear the pump... seems direct drive off the crank would spin her faster......

 

[romeo]

I have worked on bikes for years as well and know withe the valve open you have no lowend... But I guess they are reving these up so fast anyway they are at top end in a second. I knew about the cooling issue... It is like sleds for grass drags..... they don't have a cooling system because they are only being run for a second...... running the chain off the water bump? do they regear the pump... seems direct drive off the crank would spin her faster......

No, they are direct drive off the crank shaft, but the sprocket runs where the primary gear used to. The water pump and power valve section is where the bar mount needs to be. I start by Pulling the primary cover, all the gears, water pump, power valve section, and sawing off the transmission. Then you have a good surface to mount the side plate.

 

[NEK VT]

I see

I see.... thse things gotta be crasy.......

 

[romeo]

romeo, thank you for sharing the info, you are saving me a lot of trial and error. Is that sprocket custom? I've purchased race sprockets from madsens, but that looks bigger.

That is a .404 x 16 harvester sprocket. You can get them from Baileys for about $50. If you want them bigger you can get them up to something like 22 or 24 pin from custom shops and usually around $100. I know a guy with a KX125 running a 12 pin 3/8, that saw is freakin fast. On the 250's I would stay with the large .404 stuff, your working off lower rpm and more torque.