Freestyle SJ 650 advice /help

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BlastinGaston

If its broke I Rode it
Location
Chesapeake Beach Maryland/Lake Gaston N.C.
Hello good people of the X.

I don't post alot because I normally find the answers to my questions throughout the form.
But I need some help with this one. I have a square SJ with a stock SBT 650 that I was thinking about boring out and making a little bigger, but I was as I was talking with some friends that do this kind of machine work I was told point blank "if you do work to an sbt motor IT WILL BLOW UP" so before I do anything to the motor I wanna get advice from the experts. Any ideas, comments, or advice is greatly appreciated.
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
I have upgraded a 650 in the past and it was kind of a fun project.
We took a 650 cylinder, sport-ported it, and bored it up to a 717cc size. We put it on 62T cases with stock dual 38 carbs. Shaved stock head for 180psi compression, and a Riva red pipe.
It pulled just as well as a lightly ported 701. Nice little motor.

BUT: we did that only because we wanted to see what we could do with a 650. It's not a cost effective way to build a motor at all.
In my opinion, you would be much better off selling the 650 short block complete and buying a good used 701 shortblock. Most everything from your 650 will transfer over.
You would need a 701 intake manifold (which one depends on whether you get a single or dual carb 701). If you get the single carb 701, your old carb will even work.
All your electrics will transfer right over. You might want to get a 701 CDI because it has a higher rev limit, but the 650 CDI will work in a pinch.
It would also be good to get the 701 exhaust manifold (if you go with the stock exhaust), but the 650 one will work.
 
yurkanip

yurkanip

Location
Great Pacific Northwest
I agree with droste14.....but in your words exactly...boring it out and making it a little bigger....Basically yo uwant it to run like a 701. ?
Keep your crank, and use it in a 61x case, and get a 61x cylinder and pistons. (all 650, 701 and 760 use the same crank) Everything else will swap over. (exhaust pipe is a tiny bit smaller than a true 701 exhaust but it works for the time being, but if you are upgrading a pipe will complete your motor).
electronics wise, everything else is the same EXCEPT, you will need an 61x cdi. That is it. Use your original single carb, and put it on a 61x intake/manifold.
I got a set of cases for a good deal. Buy new crank seals, a cylinder and you have a 701....simple as that.
 
snowxr

snowxr

V watch your daughters V
Location
Waterford, MI
Bore the 650 out to a 701. Use good pistons, and it'll be fine. There's nothing wrong with SBT's engine parts that will make it blow up, because most of the parts are OEM Yamaha that they start with. The crank may not be built to be used in a World Finals quality race ski, but it's fine for most recreational riders with slightly modified skis. Their pistons are fine, too. They're not top of the line forged Wisecos, but they're every bit as good as most on tha market.

To build the best motor, you need to have parts that are machined and assembled correctly. If you don't know the ins and outs of these engines, work with someone who does.
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
IMHO, just boring it out to a 701 will not give you a power increase that's worth the money you're putting into it. The exhaust ports are TINY. The transfers are so low that mid and topend will never be good (without porting).
 
B

BlastinGaston

If its broke I Rode it
Location
Chesapeake Beach Maryland/Lake Gaston N.C.
thanks for the advice mat, it looks like i have some things to think about but i think im leaning on the just buy a 701 and be done with it side. ill prolly keep the 650 as a spare just in case, BTW i forgot to mention i do have the red riva exhaust in the ski now, and i am looking to go to dual carbs. looks like i have some shopping to do. thanks again
 
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