Looking for YZ65 jetting insight

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1/23/2019 4:53 AM

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

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1/23/2019 6:03 AM
Edited Date/Time: 1/23/2019 6:06 AM

Highly unlikely you will get much more low end power by jetting alone. If he is never going max speed in top gear you could try installing numerically higher gearing. He will likely need to learn to hold the throttle open fairly far while slipping the bejesus out of the clutch exiting turns.you should also consider buying a spare clutch and gasket in case it fails at a race.

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1/23/2019 11:50 AM

I would try going down on the pilot rather then up. Unless you are 100% positive that it’s lean. I have had bikes bog off the bottom yz 125, and a beater ttr 125 and one was to rich and one was to lean but acted the same. A jet is like $5. I would try it. Just cause it’s a small engine doesn’t mean it should bog. I’ve had plenty of 65’s that didn’t bog at all. Especially not a new one. My little brother has an 04 kx 65 (he doesn’t care to race just rides) and has no bog.

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1/23/2019 4:16 PM

Not sure how much of this applies to the new YZ-65 and how much aftermarket there is, but if I was looking at making a two stroke a better woods/harescramble bike I would: 1. Look at flywheel weights. 2. Look at a different pipe. 3. Look to see if anyone was modifying the power valve system.

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.

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2018 KX450F
2005 KX250
2003 KDX 220
1984 KX 80
1982 RM 80
1980 RM 80
1977 XR 75
1969 Honda Mini 50

1/23/2019 4:19 PM

I’m not looking for more power, I’m looking to clean up the bog off the bottom. I’m aware of pipes and flywheel weights, etc but this bike has a distinctive bog that should not be there

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1/24/2019 8:01 AM
Edited Date/Time: 1/26/2019 6:48 AM

EAmato88 wrote:

I’m not looking for more power, I’m looking to clean up the bog off the bottom. I’m aware of pipes and flywheel weights, etc but this bike has a distinctive bog that should not be there

Does the bike bog when you open the throttle quickly on the stand, or only when it's under a load ?

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1/26/2019 5:35 AM

I agree with Blackjack31. I dropped both the pilot and main one size and my son’s 65 runs really well. Still has a slight bog at 1/4 throttle till motor is warmed up then it’s good.

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