Posts
73
Joined
1/23/2019
Location
Tuckerton, NJ
US
Hey guys. Recently we made the transition to the new yz65. The boy has been on cobras for about 3 years now and is at the top of his class racing ECEA hare scrambles. We have one season left on the 50 but he will also be racing the 65 this year as a secondary event and seat time. The yz65 runs great, except on the bottom. I would venture to say that it runs better than his cobra 65 except it doesn’t have the bottom end pull that the cobra does. As most know, in a hare scramble you can find yourself in some situations that really scrub the speed off and that’s when we see a problem, the bike bogs. Also, dead engine starts are inconsistent because of this off idle bog. I realize that he may be coming off the clutch to quick, before the engine has a chance to build enough rpm to bypass the bog in my experience this seems to be a little more of an issue than a normalcy. I’m currently running the carb with stock needle, needle position, and main jet. The bog seems like a lean bog to me so I bumped the pilot jet one size and adjusted air screw accordingly but I haven’t seen much improvement. What jetting specs are you guys using? Any one else experience this issue? A 28mm carb on a 65 really sticks out to me and comes off as a bit much but what do I know compared to Yamaha engineers? When it starts good it starts great but it’s inconsistent and coming out of slow corners is usually a bog fest
Again, probably not a lot of info on such a new bike of small bore. Like what has already been said, probably not going to accomplish what you're trying to do with jetting alone.
The Shop
Any and all help is welcome. Also looking for fork oil capacity specs if anyone has them.
A lean bog will indicate pretty well on the plug. My suspicion is that the bike is rich, not lean, however. OEMs tend to err on the side of caution and deliver their 2-strokes with rich jetting.
If you have black or oily residue on the flat part of the plug adjacent to the threads, that's rich pilot circuit.
Here's how to tune your pilot circuit:
Turn the air screw all the way in, then one full turn out. Turn the idle up enough that the bike will not stall. Now, while idling, turn the screw in, back to the original spot, and back out. you should hear it "miss" or stumble in some range of those turns; put the airscrew in the middle of those two misses and you're good.
If the bike dies or misses with the airscrew turned in, but you can turn it all the way out with no miss, your pilot jet is probably too big/rich. If it dies or misses with the airscrew turned out but does not miss when turned in, the jet is too small/lean.
Your final airscrew setting should lie between one and 1-1/2 turns out. One turn = 360 degrees.
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