Yamaha YZ85 with 105 big bore kit

[CR85RACER2]

I am thinkin about getting a 04' Yamaha YZ85 with a 105 big bore kit my friend said with a YZ 105 kit you have to run the bike hard or you will foul plugs a lot and he said I would have to rebuild it a good bit. I am also wondering how much it is for a 105 big bore kit piston and rings.

 

[Patman]

Uh, your friend might want to learn how to properly jet a carb, that will pretty much resolve most plug fouling issues. My son's CR85 is a 102 and I can honestly say he can run it easier than when it was an 85 and it is fine but he doesn't. As far as more frequent rebuilds... well I guess if you happen ti run the 105 harder than you run it when it's an 85 then sure anything run harder is going to wear faster than something that's run easier.

 

[CR85RACER2]

Thanx Patman

 

[buffsabs11]

is a 105 cc big bore kit .080"over????

 

[Patman]

is a 105 cc big bore kit .080"over????

Nope. For a 105cc YZ the stroke would also be increased so you'd have a bore of 51.0mm and the stroke would be 51.4mm. Using the formula I provided in your other thread that would give the 105cc. :) (quit being lazy! if you'll just try I know you'll figure it out)

 

[buffsabs11]

patman, im sorry but i cant figure it out. the when you multiply 47.5 by 47.8 you get like 2000 something! and what is the difference between bore and stroke?

 

[robwbright]

It's a bit complicated, but . . .

Formula for Volume of Cylinder:

http://www.1728.com/diamform.htm

http://www.math.com/school/subject3/lessons/S3U4L4DP.html

 

[Patman]

patman, im sorry but i cant figure it out. the when you multiply 47.5 by 47.8 you get like 2000 something! and what is the difference between bore and stroke?

OK we're making progress check your original thread. :cool:

 

[david01]

hey guys i got a question, my son has an 06 yz85 other then the normal up grades exhaust,rad valve,and a rear sprocket with two more teeth. we both want more out of the engine it has plenty of power but can use more what is the best route to take. iam in IRAQ now so i want to give him all the support i can since i cant be there with him. we have a spare engine that i got on **** and is in good working order so we want to use it for this project.

 

[Patman]

I would have suggested sending the engine to Eric Gorr but his shop is closed for a while :( So he did give some good suggestions for companies to try http://www.dirtrider.net/forums3/showthread.php?t=132493

 

[david01]

ok thanks i will look into it.

 

[husabutt]

hey guys i got a question, my son has an 06 yz85 other then the normal up grades exhaust,rad valve,and a rear sprocket with two more teeth. we both want more out of the engine it has plenty of power but can use more what is the best route to take. iam in IRAQ now so i want to give him all the support i can since i cant be there with him. we have a spare engine that i got on **** and is in good working order so we want to use it for this project.

TH Racing builds a great stuffer motor. This will give you the added boost without having to go the big bore route.

From their website:

KTM 85, YZ 85 and RM 85 Options
Package 2 (SX85, RM85, KX85) - T H Racing 85cc modified motor - $950 If you would like a cooler running motor than a big bore and AMA legal mod class motor. The Stuffed Motor may be your best option. our engine gets completely rebuilt and cases get machined, stuffed and reshaped to reduce crankcase volume and improved flow. Your cylinder is ported, exhaust ports are polished, and matched to the newly shaped cases. This along with a several other TH Racing trade secret internal modifications produce the modified motor of choice for many national level 85cc racers
YZ 103.5cc big bore Super Mini - Add $300 to package 2 or 3 price.