750 "big bore" kits

oberling

XS650 Member
Messages
20
Reaction score
45
Points
13
Location
Montana
I see multiple different options available for 750cc big bore kits. They claim to be able to produce about 65 bhp if set up correctly, but I hear others saying that they don't do much without a port and polish as well as a camshaft swap.

I'm not really looking for major horsepower gains, but it would be nice to increase torque lower down in the rev range and I'm planning on pulling my motor at some point this spring to replace the cam chain tensioner and a few other things. Anyone done just a 750cc big bore kit and Mikuni carbs who can report back?
 
Another option that could be cheaper would be 680 cc with bored stock liners and oversized pistons. It depends on what 750cc kit you select and the cost of boring and pistons. I have heard of a 700cc using stock liners but not sure if that is factual.
https://www.xs650.com/threads/ccing-a-new-bore.45134/

If you go with the 750cc kit remember the bike is 40 years old and an old design back then, you will notice a gain just don't expect miracles.
 
I have a big bore kit, vm34s and an almost completely open exhaust(running some cheap baffles) with torque inserts and I can tell you compared to a stock bike it’s a completely different beast.

Much more responsive, more down low grunt and a better top end performer that really helps reduce the vibes at cruising speed.

Pulls very nicely through the rev range.

I had some issues with a slipping clutch but got it sorted with new cork and stiffer springs. Hasn’t slipped since.

Mine is installed on an 81.

It may also help that I have ditched a fair bit of weight on the overall bike compared to stock aswell.
 
I have a big bore kit, vm34s and an almost completely open exhaust(running some cheap baffles) with torque inserts and I can tell you compared to a stock bike it’s a completely different beast.

Much more responsive, more down low grunt and a better top end performer that really helps reduce the vibes at cruising speed.

Pulls very nicely through the rev range.

I had some issues with a slipping clutch but got it sorted with new cork and stiffer springs. Hasn’t slipped since.

Mine is installed on an 81.

It may also help that I have ditched a fair bit of weight on the overall bike compared to stock aswell.
700cc or 750cc ? What cam are you running? Stock head?
 
XS650 owners who have ridden my 750 say it is much quicker than a stock 650. I have pods, BS38s, reverse cone shorties. Significant gains were realized when I spent the time to get the jetting right.
Further, the 750BBKs seem to range from 9:1 to 10.5:1 CRs which is a big step up from the stock (?) 8.4:1.
At worst , correctly jetted you should see 15% increase in hp. 750 being 15% bigger than 650. If that follows, you go up from stock ~45 hp to ~53hp. Add a Compression Ratio modifier and 55-57 hp is maybe realistic.
To achieve 65hp will take other work, cams and porting. But, then you will lose the beautiful flexibility of the 3k to 5.5k rpm that make the XS so treasured.
(The hp numbers above are for reference and some may question the accuracy of the stock 45 ish number I posted. If it is wildly inaccurate I apologize)
 
Last edited:
Another option that could be cheaper would be 680 cc with bored stock liners and oversized pistons. It depends on what 750cc kit you select and the cost of boring and pistons. I have heard of a 700cc using stock liners but not sure if that is factual.
https://www.xs650.com/threads/ccing-a-new-bore.45134/

If you go with the 750cc kit remember the bike is 40 years old and an old design back then, you will notice a gain just don't expect miracles.
I have been running a 700 (77.5 mm bore) since 2008. Now it's worn and I'm going to 78 mm for 706 cc. That's the max.
 
If your kids are hungry do not do a 750 kit. If the kids are out of college, on their own earning the big bucks then it is time to do something for yourself. 750BBK is the perfect solution...
 
You guys finally made me do it before they're gone and I have to live with regret (sounds like an infomercial): Ordered the $450 thing:shootme:

Still in Honda land for a month or two, then an original XS2, then (maybe).....

Edit: 3rd one just graduated 2 weeks ago, Cum Laude, two more to go (Junior & Freshman) - thank goodness for 529's;)
 
If your kids are hungry do not do a 750 kit. If the kids are out of college, on their own earning the big bucks then it is time to do something for yourself. 750BBK is the perfect solution...

Kids are gone so...
Had to peek.......it'll stay in the box for a while....
20230111_120620 (2).jpg
 
Back
Top