Post By storminnorman07
A question for the smokers (YZ85 porting mods)
I've got some questions for the 2 stroke gurus around... I ride a YZ85 mini tard, and the power mods are:
barrel has had 0.4mm decked off the base
ported and polished exhaust port
carb bored out to 28.5mm
I have a good package, but naturally... I want more power! I'm racing against a well sorted CRF150R (23hp ATW I'm told), and a few Anima 150s.
Watching the CRF from the side of the track, I believe the power claims. It hammers. I should have more top end power than the 4 strokes, but I don't.
And with the disadvantage off the bottom it makes for some hard work indeed! I can work with losing off the tight turns if I can make it up end of the straights.
My cylinder is off being honed and re-plated at the moment thanks to a recent seize due to a failed water pump seal, and I have the option to do some more work to the cylinder.
With the base decked, I have lowered the port timing and tightened up the squish (OEM is quite lose). Pinging is not an issue. Seat of the pants tells me I've gained midrange (it's quite aggressive) but I've lost top end - but my well trusted engine machinist assures me this mod won't have made a noticeable difference to the top end end, based on his experience in building race bikes (has worked with factory teams in dirt racing). We've discussed opening up the top of the intake and exhaust port which will give me some top end power, but at the sacrifice of bottom end. And I've been warned to expect a doughy bottom end, and need to use clutch to keep it/get it up to the power band off the turn. My machinist recommends against doing this, though I have read reports from other reputable sources saying to do just this. And I know my application is unique to my machinists experience.
Could anyone offer any insight, recommendations, or past experience? I'm just throwing it out there to gather opinions!
Not a whole lot to go off of but.... I'll throw my 2 cents in.
Originally Posted by wrtimmy
I use to do a bunch of 2 stroke engine builds til about 04 when there was no doubt the old dinosaurs weren't going to cut it against the modern thumpers. That being said I sold off all my 2 stroke notes, port mappings, jigs etc... plus it's been so long those specs may not even be applicable for the newer 2 smokers. I can say that what you have been told is correct. Raising port heights will yield you more mid-town to top end power but your bottom end is going to suffer. How much?... depends on how hot of a port you put to it. Without knowing more my suggestion would be to go ahead and put the hotter port to it and if your builder/machinist is good he/she can deck and rework the combustion chamber and alter quench ring and angle to re-establish some bottom and mid. Or http://www.vhm.nl/?navi=cylinderheads an aftermarket head if rules allow it. The aftermarket heads make it a little easier cause ultimately to get what you want you need to be in race fuel and really do an aggressive combustion quench rework. But most guys want to avoid running expensive race fuels. With the aftermarket heads they generally offer a few configurations for the combustion inserts. This will allot allow you to play around and see which one works best for your application but would also give you the ability to swap out for something less aggressive to run on pump if this is also a bike you play ride too.
Hope that helps some,
Originally Posted by storminnorman07
That's some excellent help, I really appreciate the response! You hit the nail on the head re: fuel - I don't want to run race gas (though we do get 98 unleaded at the pump here in Aus., and a 100 octane 5% ethanol blend which I'm not game to use).
Time spent on the very bottom of the rev range is very minimal on our tracks. I was hoping that with the cylinder base already having 0.4mm taken out, that opening up the top of the ports slightly may bring my bottom/mid back to OEM (which is OK - I don't mind a softer hit right in the middle of the turn, it makes it a bit easier to ride), but give me the added advantage of the additional top. If I lose a little bit of ground on the exit of the turns, or more to point - not lose anymore than I already do, but can make it up at the end of the straight I'll be happy. At the moment, I'm losing ground but not being able to make it back up.
I think I'll bring up with the machinist this "...put the hotter port to it and if your builder/machinist is good he/she can deck and rework the combustion chamber and alter quench ring and angle to re-establish some bottom and mid" and see what he has to say knowing in more detail what's already been done.
Last edited by wrtimmy; 06-20-2016 at 06:16 PM.
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