125-200cc CR150 squish band

Been reading and searching past threads on this and there isn't much. I've contacted one member who's done things a little differently, and will probably go down that road, but just thought I'd ask if anybody here has experimented with the squish band, clearances, or any other head or cylinder work on the 125/150, and what the results were. Also if anyone has any porting info, that wouldn't hurt either.

Edited to add, Current mods to bike are:

RB modded carb
FMF pipe
.3 base gasket to make squish 1 mm from 1.2
51 tooth sprocket

Each one of those made a noticeable difference, and I'm very happy with the bike - best bike I've ever owned, just looking for a little more.

Arent we all looking for a little more .... I had my head shaved around .2mm and it did make a difference but there was some detonation issues . It needs to be done properly and I went the cheapos route - RB designs in the US are reasonable
Porting work would be good but again its an expensive exercise - well locally here anyway
The 144s do seem a bit flat up top and this may be able to be improved with work on raising the ports . Eric Gorr - Pro circuit or maxrpms are other possibilities. Walt would also be the one to ask .
The 144s are also quite aggressive in power delivery and they may be able to smoothed out

When I overhauled the engine I changed all the engine bearings for micro blue coated bearings

www.microblueracing.com

did make a difference

I was surprised how much the lectron added although that was not the reason why I bought one.

yet enlarging the the piston with 32% coming from a 125cc was the biggest power gain

Robert-Jan

I was hoping for a few more responses and a few ideas.

What I'm planning on is to run without the base gasket, lowering the ports (.5 mm over stock), and reducing the squish down to about .8mm, and/or re-machining the chamber for 1 mm squish and see how that works with the lowered ports. I may also try running the original .5 base gasket and machining the head for reduced squish of .8 - 1 mm and see how that works.
I'm also cleaning the ports up (although they're quite good) and putting a radius on the divider in the transfers.

I contacted Eric Gorr a while back. He took some time to get back to me, but did, and gave pretty detailed instructions about what he has done in the past on these motors. They seemed to be focused on really increasing low end power - at the expense of top end, and that's not really what I'm looking for. I also contacted RB designs, and well, that guy's really a piece of work. Not much chance I'll be dealing with him anytime soon. No need to anyway. After a bit of research, I'm pretty confident I know what to do to the chamber.

Any thoughts?

Its quiet on here

No gasket will be risky ? All in all though I would nt expect too much from just doing head mods especially wince you have already used a .3mm gasket.

My understanding is you really want to raise the ports as the bottom of the 144 ports are lowered more than 125 by the over bore - this affects timing

I went back to the 165 kit as I find it nicer than the 144 . I also like the 125 better if its dialled in jetting wise. I think a well set up 125 can be the most fun of all the sizes .

You may get results playing with jetting - what main jet you running ?

I am not sure if the 39mm RB design carb works all that well for low to mid - Ive got one and put original back on at the moment

Not sure about you guys but we just had christmass lol

Squish thats a toughie.
I shaved 1mm off my 360 head now running at 1.2mm but i wanted it safe so thats why i have it a little larger but the bike is alot happier not having the huge band so its all good.

Taking it to only .8 may be a little tight but im no professional i would personally take asmuch off before disturbing the actual domed part of the cylinder.
Then if it is say .8mm chuck a thicker base gasket in wich will raise your ports to help open them for longer.
More time open more in and more out.. In theory

Wouldnt run it without a base gasket thats for sure ! Way to much chance of pulling in air / puking coolant to my mind.
Not sure what your trying to acchive as the more you reduce the base gasket thickness the more the port timing comes down so does the top end. And as for the tighter squish the only thing that would achive here in the UK is a motor that would last about a hour before it detonated itself to bits as even a stock 144 is a little prone to having a bit of deto without good fuel.
My 144 ran its best on a DEP pipe although not to different from the FMF just a bit more off the bottom and mid and the topend around the same.

Thank you for the responses.

I don't consider the lack of base gasket to be risky at all. I use a Loctite flange sealant that will do the job fine.

The goal is to make more power through the RPM range. Lowering the ports should help on the bottom, but it's only .5 mm. I can then experiment with squish, from .8 - 1mm by machining the chamber.
Other options are to put the .5 gasket back in (move the ports back up), and reduce the squish on the head. It was 1.3mm stock, which is a little much. 1 mm is fine and .8 should be doable on this motor. I'm not concerned with detonation, as I don't see it at 1mm and have better fuel easily and cheaply available if i need it.

So the question is, do I lower the ports, and experiment with squish from .8 - 1mm, or raise them and reduce squish in the head from .8 - 1mm?
I have a spare cylinder, head, and power valves, and I'm a machinist, so doing the work is the easy part


To answer the question about the RB carb, it made a significant improvement in the low to midrange power, which is exactly what I was looking for.
I also have an older Lectron PJ carb that I would like to try, but only after I sort this stuff out. I race woods/fast woods, and natural terrain motocross.

Here is Eric Gorr's response. Because I know he's a busy guy, I didn't ask any tech questions, only about sending my parts in for modification. Oddly, he sent a detailed response, but didn't answer a single question I asked him.

OK, so I took a lot of measurements, read a lot and talked to a few people.

This is what I did:

Smoothed and cleaned up the ports, and radius-ed the transfer port dividers
Installed power valves ground to fit better in the closed position, and blended the ports for a smooth transition
.5 base gasket
Machined the head for a squish of .9

Power is stronger everywhere, and it's especially strong on the bottom. Racing last Sunday (MX), I was doing things I've never been able to do before. There's a small table right after a turn that, if you don't clear it, it can get ugly, so I've always landed on top, hit the gas, and jumped off. It's a spot where the big 4 stroke bikes always pulled me, so running 3rd, I thought I'd try it, and cleared it every time. Never been able to clear it before. I also let another 125 rider try it out yesterday, and she raved about the bottom end power, and how easy it was to to ride, and exit the turns on the gas.
No sign of detonation (on the plug), but I'm pulling the head tonight to have a look, and if all looks OK, I'll machine another .1 - .15 mm for a .75 - .8 mm squish.

It was that much better on the track, so I can't wait to get into the woods with it.
I would strongly recommend these mods.

I don't think you will get closer without detonation and may get it now . May need to run avgas or race fuel
Did you get the top of the head - the bowl machined ? The squish is only part of the story - need to know volume and compression ratio

You may want to try a different pipe for MX - depending on the track a pro circuit or scalvini may give you more hp
the stock pipe may give more top end too

I pulled the head, and everything looks perfect. Not a trace of detonation, and perfectly clean.

I left the combustion chamber stock because I wanted to increase the compression. On the (factory) 144's compression is too low. Even now, at .9 squish, I'm only at 165 psi. Going to go to .75 and see how that is. I think 175 psi would be close to ideal, and 94 octane should look after that fine.

I'm quite happy with the Fatty, but did take the stocker to the track with me last night to try with the new setup. If I didn't forget my sockets, I would have tried it No rush, though. I have a month until my first points race.
The motocross I do because I love it, but my work schedule keeps me from attending all the events, so I run an off road endurance (hare scrambles) series, and tune the bike best for that. I have been trying to trade my spare Fatty for something different, but so far haven't found anything.

You could try my pro circuit pipe if you want to pay the shipping there and back. I went back to my stock pipe for now because it seems to give a a better spread of power whereas the PC seemed to be more top end. If you're hitting motopark sometime you could come by and grab it for the day since I'm not far from there. I know that on sleds a little less compression seems to help top end/over rev but higher compression seems crisp with better bottom to mid. Thicker base gasket with a cut head always works well

Thanks for the offer. If you like the stock pipe for low end, you'll love the FMF Fatty. It gave it a ton more low end, and more power everywhere. Thanks for the offer. Don't usually ride MP, but maybe I'll set some time aside to do that. I'll bring my spare Fatty, you can try that

Head is machined for .75 mm squish. Working all weekend, but probably get it back on the bike Monday, take some measurements and a compression check, and hopefully go riding.

Dont get to hung up on psi values ive found most guages are reasonably accurate but really just use it for base setting so you know for sure when your rings are letting by.

Do the old head volume measurment by using a syringe and a flat piece of glass or plastic with a small amount of grease to stop it leaking.
And calculate trapped volume of combustion chamber piston at bdc an measure from crown to deck height them pie of cylinder to get cc of that then you can work out compression ratio n other unimportant figures there.
If shes more powerful and not grenaded then keep her as is, ive done it myself before and chased more power n lower squish higher cr's an its just one adjustment from holing a piston. An that can happen before you realise your detonating.

Keep a little margin in there for reliability.

I use a good gauge, and it is accurate, but it's only for comparison. I do a lot of (4 stroke) cylinder head work, so this is simple stuff, and even at .75, I think there's still a fair bit of margin.
If you ask around, everybody will tell you that 1mm is the correct squish for a 125, but nobody can tell you why. The kart guys, who run serious 2 strokes are running tighter even than I am, with reliability. We'll test and see. Pistons are cheap, I have a spare head, and cylinder. What could possibly go wrong?

Lol im running 55 thou on 360, could go more but tbh 360 don't need it.
But the factory squish doesn't even work it's so slack.
The kart boys are a good bunch unless they have a broken engine and a race on the weekend...

P.s. pistons for big bore well medium are £156 so in dollar 200 ish not for taking too much risk on.

£156? I think I paid less than that in Canadian $ for a top end kit with O rings and gaskets.

Putting it back together tomorrow, and hopefully riding Tuesday.

One thought, when I put the 51T sprocket on the rear, it made a big difference (way more than I expected) being able to keep the bike right in the meat of the powerband. The way it feels now, I think I can go back to the 50, and it should pull harder.

Yes one hundred and fifty six English pounds sterling for a piston from wosner.
Not cheap enough to not learn from mistakes the first time.
My gearing has been 13-⁴8 to 16-⁴8 not really any big difference apart from using 1st and top speed it's on 13-⁴8.

16:48!!! Man did u have to machine the cases down?! :-0

She would bn pullin 200kph eh?!!!