A question for the smokers (YZ85 porting mods)

[wrtimmy]

G'day everyone,

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
standard head
ported and polished exhaust port
ported intakes
standard cases
carb bored out to 28.5mm
boyesen reeds
HGS pipe

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!

Thanks!!!

 

[china911]

Unfortunately with smaller capacity 2 strokes the meat of power is not equally shared that well... l did a bit of work years back on a super kart with a yz 80 mounted in ... same deal overall took power from one department ,increased midrange thou lost top end .. the 4 banger will always push forward with its torque towards mid to top end .. consider also rider and machine weight ..

 

[wrtimmy]

Thanks for the reply mate. I did consider weights, the CRF150 vs YZ85 isn't much in it. If anything the YZ is a bit lighter, and I've got 5-10kg on the rider also.

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 ground on the exit of the turns, 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.

 

[my67xr]

Ringo race's Motard's so he may chip in a bit of info for you

Maybe try something like a Torque plate/reed spacer to help it mid/top end, they fit in between the reed cage and barrel.
The V Force 3 Reed Valve is slightly deeper on the engine side to help with torque the same way the one's above do.
Adding a flywheel weight (eg Steahly etc) will help it develop more torque too.
Bike may need rejetting to suit any of those mod's, but you won't know till they're on

 

[wrtimmy]

Thanks mate,

I race against Ringo :fencing: - he actually raced my bike round before last while I was away and picked up 2x 1st, and 2x 2nds (just after I rebuilt it from a seize/water pump failure, then I go and seize it first round I come back again!). So I know in the right hands it's competitive, but this wasn't against the beast of a CRF, and the guy is practically a midget. I think he's got close to 15kg on me at least?!

Hadn't considered a reed spacer - my understanding is they effect the mid-range more than the top.

I had this recommendation over on another forum which I'll bring up with the machinist and see what he thinks.
"...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..."

The motor as is, has more mid-range than standard, but a bit less top. What I'm hoping to achieve is more top than OEM, without the mid-range going less than OEM. I'm hoping the combination of the decked cylinder/lower ports + the opened up top of the ports for top = what I'm after?