TE/TC TC 250 2018 squish band clearance

Hi

I´m going to modify my TC 250 2018 motor to get more lowend and midrange power to smooth that violent hit it has originally. I still want it to rev more than std so all the mods must be done so it will still have some over rev, but smoother more "filled up" power curve. No FMF Gnarly for me as it cuts over rev too early.

So far airbox has some extra holes drilled to right hand side near the silencer as these Husky´s are known to have some asthmatic symptoms vs. KTM´s.

I have also done mods to original Mikuni TMX 38 carburetors slides intake side (notch) and fuel feeding improved for the mainjet (drilled four 1.5 mm holes to drain plug groove and opened up those plastic covers original small holes also to 1.5 mm size). It is already much better.

It got even better when I changed red power valve spring to green. Now that hit is not as violent as before. I have that power valve adjustment screw backed up level with the cover so it starts to open power valve early, but green heavier spring prevents power valve from opening fully too early. This way there is wider rev range when power valve adjusts power curve.

I have my ignition map at more agressive setting (connector plugs connected together) as I don´t like that soft map with less ignition advance.

I bought FMF Fatty exhaust pipe and Pro Circuit R304 shorty silencer. Also Boyesen Rad Valve intake reed valve. I have not installed these yet, but I assume these will have a good effect to smoothen out the power delivery and give good over rev.

I bought another used perfect condition TC 250 cylinder head and now I want to modify it to improve lowend and midrange power. These 2017 => cylinder heads don´t have same kind of squish band in their combustion chambers as earlier models for example 2014-2016 has. I measured with caliber what is the height of combustion chamber edge. It is 1.60 mm unmodified (0.063"). It´s a little too much IF piston top is level with the cylinder top at TDC (distance X= 0.00 mm). I was planning to cut that head -0.40 mm to 1.20 mm (0.047").

Is this a good squish band clearance 1.20 mm to this TC 250 2018 motor? I have understood that 1.60 mm is way too much and much less than 1.20 mm is not very practical.

I have in my other bike TE 250 2015 more mods done and it has RK-TEK head with only around 1.00 mm piston to head clearance, but I don´t want to go that far with this TC 250 2018 now. I want to try this time with modified std head instead to save some money and get maybe 70-80% of those benefits of RK-TEK head with these mods.

Next step is to add flywheel weight if it is still too agressive motor after these mods. Maybe 9 oz weight is ok. My TE 250 2015 has heavier flywheel from the factory as it is enduro bike so it has that starter gear in it which is heavier. These mx models have very light flywheels and I think with flywheel weight it is possible to easier get that power to the ground without getting tired so easy. Originally my TC 250 did wheelies everywhere unwanted. It´s nice to have power, but how that power comes out is more important.

My 18 te300 came with almost 2mm squish..... took it to just under 1.6mm and now I'm at 1.25mm. I haven't checked compression yet but I believe this is the ticket for these bikes. I also have similar carb mods as you, aka Graham Jarvis. Jetting is easier but more important seems to be more stable. I'll have to wait and see what the weather brings. It pulls 3rd in single track very nicely.

1.2mm on stock head .....go for it

Thanks for the fast reply. Yes those TE models have even larger squish band clearance from the factory. I had also in my other bike TE250 2015 very large around 2 mm squish. Then I bought also to that one TC 250 head and modified it for this same around 1.20 mm squish. It made a huge difference. I use also SX/TC models CDI box in that bike. There is not same kind of jetting issues in that model as it uses Keihin PWK 36 carburetor instead of Mikuni TMX. I changed that Keihin 36 to Keihin 38 and use Suzuki RM 250 needle NECJ. Jetting is so perfect as can be nowadays. It has V-Force 4 reed valve, full Bill´s Pipes exhaust and RK-TEK head, but with this TC250 I want to try something different so I bought FMF Fatty exhaust pipe, Pro Circuit R304 shorty silencer and Boyesen Rad Valve reed valve. That Mikuni TMX 38 with these Graham Jarvis mods (slide and drain plug and plastic cover mods) is a very good carburetor so I don´t even consider to put Keihin PWK 38 in this bike. Both are great carburetors, but Mikuni just needs these small free mods to work as it should. When I got this TC250 2018 and went for a ride for the very first time after riding my modified TE250 2015 I was surprised how badly it was working. Jetting was way off and it went so much better with those GJ mods. I hope with tighter squish band clearance and these intake and exhaust mods it will get even better.

No SmartCarbs or Lectrons for my bikes as these Keihin and Mikuni are just perfect and I ride only at sea level anyway.

Now FMF Fatty exhaust pipe, Pro Circuit R304 shorty silencer and Boyesen Rad valve are installed. I also put back that red power valve spring and took off green spring as I want to see if it works ok with it. Cylinder head is from a year older TC 250 model 2017. I polished it’s combustion chamber and my friend milled -0.4 mm off from it. Now that very outer edge of combustion chamber has around 1.10 mm to 1.15 mm ”squish band” thickness. In my opinion it doesn’t have practically any squishband at all in these 2017 => newer motor versions because of completely different combustion chamber shape vs. <= 2016. Piston top is level at TDC or just slightly above (maybe a few hundreds 0.01-0.03 mm).

So now this motor has more static compression ratio and should have also more dynamic compression ratio because of better cylinder filling (Boyesen Rad valve) and more efficient exhaust. I kept my Mikuni TMX 38 jetting as it was. 430 MJ, 30 PJ, AS open 1.5, same needle (I don’t remember which right now, but I have it in my garages notes), needle clip in 2nd position from top (second leanest position). Boyesen Rad valve requires leaning out the jetting, but I have already done it before. When I got this bike it had 470 MJ and 42 or 45 PJ if I remember correctly. Way too rich jetting... Because those GJ mods to Mikuni TMX 38 carb helps feeding MJ better it is best to use a leaner MJ (430). Increased static/dynamic compression ratio also works so that leaner jetting is needed vs. std jetting. Same thing with more efficient exhaust system. Motor works more efficiently, it ”sucks” fuel better with higher vacuum so jetting can stay as it is now. Good starting point at least. One thing which I have to say about my jetting is that I run 32:1 premix so more oil than factory spec = leaner, less fuel. I ride only at sea level and temperature is around 20-25 degree Celsius.

I haven`t bought flywheel weight yet and I can’t test my bike properly as I injuried my right knee. I have just tested it a bit and it sounds great, feels strong and starts at first kick. It idles perfectly too.

Today I tested more this modified TC250 and now it’s perfect. Very strong and wide powerband. Low-end and midrange torque is much better and now it ”sings” at much higher rpm = more top-end power too. Even with Pro Circuit R304 shorty silencer. With longer silencer it should rev even higher if I have understood correctly.

I feel like that red powervalve spring is the best option. I like how predictable motor is with that spring adjustment screw adjusted at level to the case. Yes, motor is very responsive vs. with green spring, but it isn’t anymore completely on/off all or nothing type. I’m very glad I did these mods to it. First I was going to just leave it as stock, only GJ carb mods and red PW spring. But this head mod, Boyesen Rad valve and FMF Fatty + Pro Circuit R304 shorty were all quite inexpensive but still well working mods... and what a sound that exhaust has now!!

At the moment I feel like it doesn’t need FWW anymore. I haven’t bought one yet. I run 14/49 sprockets instead of stock 14/50. Motor feels so strong now that gearing could be even longer. Maybe 14/48, 14/47 or 14/46.

I cut my '19 250xc to 1.2mm and it was way better than stock.

Reach out to https://twostrokeperformance.com.au/