125 rm many MANY mods, how much increase in hp?

[125rmcrazy]

okay i've got a 91 rm 125. i bought it from a buddy of mine last year for next to nothing because it had a seized crank. i work at a machine shop, so this was right up my alley.

i've been working on the bike off and on for about 8 months now, i cant seem to find the stock hp rating for this bike, but i suspect its in the mid to high 20's? anyway here goes..

i bored the cyl to 2mm over, making the displacement 137cc. rebuilt the crank with a wiseco hot rods kit. its been ported and polished, no change in port height just smoothened. the head has been resurfaced and the squish angle tightened to max tolerance with supreme pump fuel, compression at 150psi. Ive got a boyesen rad valve with the best reeds they make to fit it installed, a fmf fatty pipe w/ silencer. timing i left stock. jetting is to fmf's spec with one size up from that on the mainjet due to the rest of the mods.

my question is, how much horsepower increase over stock can i expect from this? my brother has a 88 rm 250 competely stock half worn out and i want to keep up with his riding next season.

thanks!

 

[Chili]

Not that it's important but it always my understanding from the equation I've always used that a 2mm overbore was a 134 not a 137.

As far as the other stuff if you managed to get more than 3 hp extra out of it I'd be surprised unless you truly understand what needs to be done to these engines to make them breath fire. Each of the three RM125's we've owned made over 150 psi of compression bone stock.

 

[Pete Payne]

Yep ,+ 2mm equals 134.
Did the cylinder have a cast iron sleeve in it or did you get it replated ? Don't bore and hone the aluminum with out replating , It will last only a few minutes .

 

[2-Strokes 4-ever]

I'm only patient for so long.........where's Pete Payne's avatar?

 

[MOTO ON RM250]

the only things that added hp were the big bore and porting. everything else just moved power around. im guessing around 30 or 31 hp

 

[MOTO ON RM250]

and btw you should probaly go with race gas

 

[Rich Rohrich]

its been ported and polished, no change in port height just smoothened.

That's not porting, that's just metal masturbation. :)

If you don't change the port time/area values it doesn't add anything to the performance. Smoothing and polishing ports is just eye-candy for the uninformed.

To make a big bore 125 work you'll need to change the porting time/area to suit the additional piston area available. Failing to do that makes for little difference in performance.

 

[whenfoxforks-ruled]

I am wondering about the cylinder material myself? Rich, outstanding comparison on the port fiddling! If the cylinder is iron or you sent it out to be plated, I will bet it has no bottom or mid.

 

[125rmcrazy]

its been resleeved with a cast liner, why do you think that would make a difference to power compared to a chromed cyl? so what changes would be proper with the exhaust port for the extra displacement? and also... doesnt compression over 150psi require race fuel?

 

[125rmcrazy]

i did all this work on the bike myself i am a machinst by trade and been working on 2 stroke engines for years mostly on snowmobiles however. i know that for large displacement snowmobile engines raising the port height advances the timing and allows for higher compression tolerances at the head. but i am unsure for the smaller single cyl eng's thats why i left the port dimensions alone.

 

[125rmcrazy]

how much compression did your bikes run chilli?

 

[Chili]

Unfortunately I don't have any notes on the older bikes, my son switched to a 250F this past season. I do recall his 2005 being over 170 and I know for a fact his KX80 was around 155 on a fresh top end because that bike I took the time to keep track of the compression at different stages of wear.

 

[125rmcrazy]

did you run race fuel for 170psi? im curious because usually on a snowmobile eng, anything above 150psi on pump fuel can detonate when hot.

 

[Pete Payne]

That's not porting, that's just metal masturbation.

Rich ,
That was great !!! I almost pee'd my pants. :)

As far as my advatar , who know's !

125RMcrazy,
Did you set the ports to the same height and width's after you installed the sleeve as they were originaly ? They (the guys that make the sleeves) typicaly leave them smaller than original and you have to come back with a right hand grinder and fix em up .

Br,
Pete

 

[Chili]

We did not run race fuel on any of them, on his 03 we had some slight detonation issues with an FMF SST pipe that required a little boost in the octane to clean it up but it was fine with the stock pipe or if I wanted to leave the main 1 size too fat.

 

[Chili]

Pete's avatar along with many others were lost in a board upgrade a few years back, unfortunately no one seems to have a back up copy of Pete's.

 

[125rmcrazy]

i didnt re-sleeve it myself but yes i noticed the port dimensions in the sleeve were slightly smaller, and yes i took the time to grind and smoothen out all those edges to match the ports of the cylinder.. i figured that that could restrict flow a bit. as for the compression, so i could probably bump it up another 20 psi or so? i dont want to run race fuel in the bike because its pricey and im probably going to be using the bike quite a bit.

another question.. is a timing advance worth it? or would i just be asking for detonation?

 

[Pete Payne]

Chili ,
Let see if we can get me a new advatar ? How do I do this ?

125RMcrazy,
Good deal on matching the ports from the cylinder to the new liner. Most 125's from this era made around 34hp at the rear wheel with a few mods . But could be pushed up to around a good 37 with a =2mmover bore and some good jetting . I will search my dyno files and see if I have something .

Br,
Pete

 

[125rmcrazy]

cool man let me know if you've got anything that could help me out!

 

[Chili]

Chili ,
Let see if we can get me a new advatar ? How do I do this ?

Pete, If you have a copy of the original then you can add it in your Control Panel, if not that would be the same place you would add the new one you want to use.

 

[whenfoxforks-ruled]

Be very careful what rings you put in that bore. Iron liners take different rings.

 

[125rmcrazy]

well i used the only ring that came with the wiesco kit.. i never heard of diff rings for a nicksil/iron lining?

 

[whenfoxforks-ruled]

Nikasil= type of composite or chrome bore. Iron= iron bore. Two completely different bores. US chrome does claim to be able to coat iron bores, did they? Then you can use the thin sissy ring. The thin sissy ring in an iron bore would not last very long, would be my guess. Never knew anyone who tried it!

 

[125rmcrazy]

compression for a 125 rm

hey guys i did a the 2mm overbore rebuild on my bike and i had to recut the head.. surface and combustion chamber due to damage. i kind of eyballed the measurements when i did it.. (i am a machinist, done lots) and i tested the compression and its at 135 psi. all i want to know is how high comp can you run on a 125 on pump fuel? i've got all the dimensions now so i can recut my head again but im just curious how high i can go without detonation? a couple people on here told me between 155-and 170 but im just looking for more opinions?

thanks!

 

[125rmcrazy]

125 rm compression

hey guys i did a the 2mm overbore rebuild on my bike and i had to recut the head.. surface and combustion chamber due to damage. i kind of eyballed the measurements when i did it.. (i am a machinist, done lots) and i tested the compression and its at 135 psi. all i want to know is how high comp can you run on a 125 on pump fuel? i've got all the dimensions now so i can recut my head again but im just curious how high i can go without detonation? a couple people on here told me between 155-and 170 but im just looking for more opinions?

thanks!