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2005 rm250-finally got the bottom end power I was looking for

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Had my rm250 for awhile now. Owning ktm's prior, I wanted that bottom end pull back. I added an fmf gnarly, reed spacer, jetted, adjusted powervalve and added 10oz fww. Good but not there yet. So..

I lowered the cylinder with a thinner base gasket! Squish came in at 1.2mm. Base gasket came in at .44mm. Went to a .1 base gasket. I had to make it, but athena does make different thickness base gaskets such as .1 .4 etc. Cometic aslo make a .2mm. Squish came in at .66 to .7mm. I was a bit cojlncernednbecause that is pretty tight. Crank and everything is new and dial indicator showed. Less than .005 play. Could have been less or more but it wasn't enough for concern. Buttoned it up and went for a ride. Long story short it runs awesome. Throttle response is crisp. Motor sounds cleaner. I can now open the throtle and pullna nice wheelie. The powerband trwnsitions smoothly from top to bottom. Midrange hitch is gone. At idle I can get on gas and bike responds. No burble or spool up. Best mod I have done to the bike. I didn't want to cut the base of cylinder yet. I wanted to make small change that can be reversed. As hours rack up I may have to cut head or keep crank fresh. This is a worthile mod for anyone looking for more linear and torquey power. Simple mod that really changes the personality of the rm250. I based the gasket and build of this based on the stock limitations. SomeSomeay have to resort to milling cylinder or head. A gasket can also be made for the cylinder head, but there is mixed opinions on this. Fun mod and rewlly got what I was looking for. Hope it helps. I see it uas heen done before but wanted to share my experience for new members and to share another opinion on port timing and squish mod review.

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Awesome someone that goes a step further. All the bolt on's (and money) won't get you any closer until you take the time to learn and apply.

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Very interesting indeed. Have you tried it on big throttle openings- and if so were there any detonation issues?

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Very interesting indeed. Have you tried it on big throttle openings- and if so were there any detonation issues?

People need to study the effects of what tighter squish creating higher msv does. 

 

Where does detonation usually occur?

What happens when you close up that gap?

What happens if there's a higher velocity of charged air coming back out from that gap?

"It creates a cooling ring effect along the perimeter of the cylinder head to piston"

 

Taken from one of many studies;

 

That still is the secret of the squish-type cylinder head: It concentrates the main charge in a tight pocket under the spark plug, and spreads the mixture at the cylinder-bore's edges too thinly to be heated to the point of ignition. These “end gases” do not burn with the main charge, and are only partly consumed as the piston moves away from top center and releases them from their cooling contact with the surrounding metal. And right there is the disadvantage that comes with the squish-band cylinder head, for mixture that does not burn is mixture that contributes nothing to power output. 

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So you haven't tried it on big throttle openings yet then? 

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Sorry for delay in response. I lost a little bit of over-rev and top end. Its really not that much of a sacrifice because the bottom end pulls into mid better and just makes for a realy smooth usable powerband. Peak hp feels same-just shifting power down the rpm. I made the gaskets. Athena and cometic sell thin gaskets but are really expensive. I did take it apart tonight to check everything over. No signs of detonation. The powervwlve flap and side ports need to be kept tight on this bike. Once it wears and there is play performance will suffer. The pv relise on spring tension, so being clean and moving easy is very important. My squish is on the tight side. If your crank is in good shape-the tighter you can go will make the bike run better. If my squish and bowl were more open-I would expect less gains. I feel moe comfortwble going from my rmz to the rm now. Powerbane is more predictable and I don't have to clutch as much. I was riding smoother and it was more fun to ride. Definitely a good powerband for woods or even tight mx. Faster track where you can open it up wil favor a smaller port timing change and head mod.

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So what exact base gasket did you buy . I have a 01 with a gnarly and want a smoother transition to the the mid . Thanks

Made gasket from heavy envelope paper and gasket shellac. Would love to buy one from athena but gotta spend like $120 to get shipped in.I made 4 gaskets of different thicknesses and just kept mic'ing and seeing how far I could go. Cleaning the gasket off sucks, but it holds up really well. I have done this for years. After owning an exotic bike..u learn to improvise.

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So you haven't tried it on big throttle openings yet then?

Big throttle openings no issue. We just bumping up compression and lowering port timing a hair. I do run a high octane gas mix with pump. If you got low quality gas maybe you will need better fuel. You will lose some over-rev, but it makes it up by heing able to pull another gear. Actually as it flattened off it wa s helpful coming in and out of turns. I could rev it out and not have to worry about shifting and clutching so much. It was much more able to just pull.

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I just ordered a .012 gasket for my bike .it was 47 bucks shipped . I'm hoping to get similar results as you . I'll update when i install it and ride it . My bike is dialed in perfect jetting wise and i run 93 octane i hope I don't have to mix with race fuel . Did you have to. Change your jetting at all ?

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I just ordered a .012 gasket for my bike .it was 47 bucks shipped . I'm hoping to get similar results as you . I'll update when i install it and ride it . My bike is dialed in perfect jetting wise and i run 93 octane i hope I don't have to mix with race fuel . Did you have to. Change your jetting at all ?

It leaned my jetting slightly but I was rich beforehand on closed to half throttle.

.012 is like .4 mm. Not a whole lot thinner than stock base gasket. Stock is .4mm to .5mm. The one that was on my bike was thick. It was .5mm. Pretty sure it was a tusk gasket set. See what squish is at and play in crank and just fo from there.

My plan this winter is to tear the powervalve apart and get it matched prefectly. The main flap is too loose and unmatched from the factory. When the powervalve is matched and operating efficiently, I know the throttle reponse and powerband will be better. Going to spring for a lectron or an atp carb also. I feel if I can get the powervalve functioning better and metering carb-I can go higher on the xhaust port timing and still have the a strong and smooth powerband. Hard to justify the money on that stuff when what I got is working good...but there is always that want for more

Edited by ac717

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So I could t wait for my new gasket to show up . So I made my own gasket out of a cover on a folder . Than coated in gasket shellac as you did . Let everything dry and reassembled . First impresarios when I went to start it was "holy crap" this thing is harder to turn over . You can def feel the increase in compression . I didnt rering the motor just so I can see exactly how the bike would respond to lowering the base . The bike is a 2001 rm250 hot rods crank and wiesco piston have 36 hours on it . After I warmed the bike up i will say i was surprised at my throttle response ! Wow its snappy and crisp . Initial ride will come when I get my new front sprocket . Somehow the old counter sprocket broke into pieces when. I was 3rd gear pinned in the whoops .

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ac, thanks for the extra info- really interesting. I wonder if the improvement is down to higher compression or the changed port timing, but I guess that's going to be hard to work out.  

 

Keep us posted of any more improvements you find.

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Improvements are from from both. There are some fancy calculations we can do to figure out how lowering cylinder and head effects overall power. By lowering port timing that is where we are getting the bottom end grunt. Because head is now lower and the squish is tighter we have raised compression and "squeeze" the fuel harder. This makes a more efiicient burn and creates more overall power. By adjusting the ignition timing---moving stator plate at flywheel-we can change to more over-rev slightly or even gain more bottom. When I make chang es like this, pipes and silencer effetcs are more noticeable. I have a gnarly on now, but a fatty or pc works could fill in and compliment the powerband in the mid top and provide more over-rev. I went reall thin with the base gssket. Dont have to necessarily go that thin. Just experiment and see what you like. Making different thickness base gaskets is cheap and will make a pronounced change-more than any pipe or other bolt on.

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So I could t wait for my new gasket to show up . So I made my own gasket out of a cover on a folder . Than coated in gasket shellac as you did . Let everything dry and reassembled . First impresarios when I went to start it was "holy crap" this thing is harder to turn over . You can def feel the increase in compression . I didnt rering the motor just so I can see exactly how the bike would respond to lowering the base . The bike is a 2001 rm250 hot rods crank and wiesco piston have 36 hours on it . After I warmed the bike up i will say i was surprised at my throttle response ! Wow its snappy and crisp . Initial ride will come when I get my new front sprocket . Somehow the old counter sprocket broke into pieces when. I was 3rd gear pinned in the whoops .

Awesome! Hope you like it. Really brought the power where I wanted it.

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Its cool that you guys are tuning your bikes like the factory mechanics did years ago. Good reading to say the least!;)

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Ride report : so I took the bike out today With the homemade gasket . The power is smooth ! This will be great i the woods . Less wheel spin and should keep me from getting fatigued as quick. The bike lost a little topend power but its nothing bad . The power is close to what my ktm 300 xcw felt like minus the heavier flywheel . For some Reason the bike is running a lot richer . My plug is black and I'm getting a lot of spooge! That really surprised me since my bike before this mod was jetting perfect and the plug was tan . Also my base gasket is getting damp on the right side . So I may have to pull it off . Overall I'm happy with the performance of the bike and i know once its leaned out a bit it will have the bark it should .

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Sounds like the gasket is leaking a bit?

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The cylinder bolts were a bit lose . All is well now . It felt good to pull the wheel up in third and short shift in the air and carry 4th with no effort ! I think the reason its a bit rich is when. I put the gasket on the block , the gasket wasn't 100% matched with the ports were the intake ports are so that may be causing a slight restriction . But the bike runs good so I'm going to drop my main from a 155 to a 150 and do a plug chop and see how the bike responds. Thanks for the idea now I'm going to hold out on sending the motor to eric gore until I need to . Take a look at the piston tho ! ImageUploadedByThumper Talk1383867572.537505.jpg

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