Mike's Big Bore 750 kit opinions

jonasblack

jonasblack

XS650 Enthusiast
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I am thinking about doing the upgrade to 750. I was wondering what everyone's opinions on this were and what the procedure would entail? new carbs? new jets? is it hard to do?

I am running the stock carbs on my 81 xs650 special, with a jet kit.


any input, thoughts, and ideas exchanged here would be great.
 
I think it's much more economical to just go to the biggest bore on the stock sleeves. You won't get to 750, but you probably would be happy for the dough you spent.
 
The 750 upgrade is rock-solid, but only gets you a 14% increase in displacement (744cc vs 653cc). It's somewhat noticeable, and can be dropped-in with little retuning, but the true benefits happen if it's part of an overall engine performance upgrade plan.
 
TwoManyXS1Bs said:
The 750 upgrade is rock-solid, but only gets you a 14% increase in displacement (744cc vs 653cc). It's somewhat noticeable, and can be dropped-in with little retuning, but the true benefits happen if it's part of an overall engine performance upgrade plan.
Click to expand...

that would include new carbs and cams etc?
 
Yep. And headwork, exhaust, valves, valvesprings, rockers, ignition (maybe), suspension rework, rephasing (good reports there), and, and, and, $$$, $$$$, $$$$$...!!!
 
I have done both and have bought and installed the Mikes 750 bore kit on 5 motors I built for people and like TWOMANY said unless you do the other upgrades or you have deep pockets and just want the look I would just freshen up the motor you have. You can do 1mm over bore with your stock jugs for less than $400.00 and still use the stock stuff you have and have a happy motor. Big bore kit plus labor you are close to $1000.00 and you can freshen up a complete motor for around $1500.00 BUT you can save even more if you can do it yourself. Labor is what kills you in most shops.
 
jonasblack said:
alright! well, im out!

would a nigger rear sprocket give me a little more pep?
Click to expand...

What do you mean your out? You want more power start in small stages,first on the list would be getting the head ported,that's were you see the most for your dollar,not some big bore because in the end the engine will still be handicapped by the restrictive exhaust ports and intake ports. It's your money but spend it wisely.
 
I like Mike's kit. I had done a lot of the performance stuff on a 650, except for a cam. I did the head, intake, exhaust, several sets of carbs.
I would recommend doing all this before the 750 kit. Just adding the 750 kit to your engine won't require carb tuning if all you do is the 750 kit. If you do the 750 kit and something along with it carb tuning may be needed.
I like the extra HP of adding 14% more cc's, but one thing that surprised me was the reduction in vibes. On the stock 650 as the rpms reach 4 to 5 thousand rpms the vibes increase drastically. The 750 the vibes don't change much at the higher rpms.
I think this is from the forged pistons. The weight difference smooth's out the vibes.
I think on any rebuilds I do from here out will get the 750 kit or at least forged pistons.
Leo
 
The lighter you can get the reciprocating weight of the small end of a 360 firing twin,you'll most likely see a vibe reduction. JS MotorSports has been suppling light weight pistons and long rods for Nortons with one goal in mind to reduce vibrations and all his customer testimonials have been positive. I've managed to deduce my KZ long rod pistons down to 245 grams,not the super light piston I was hoping for but it's dome is ideally suited for my welded up chamber.
 
If you go with the 750 kit I would put a better cam than the stock cam. I haven't ridden a 750 with the stock cam basing my evaluation 650 that it lacks power low end Top end it comes alive 4000 to 7500 it starts building horsepower it might have 20 something HP at 4000 rpm and building. Better low end with the #1 shell cam is why I changed and it. Better low end and builds HP to 8000 RPM if you dare to go there. Depends what kind of driving you do and what gears your running. Lots of choices as far as cams go a good set if new R/D valve springs to replace the old stock is a good idea. The stock cam works well and better open road and in it's power band. Not knocking it good cam not my choice. Leo runs a stock and it works good. My carb choice is the Keihin 34mm PVK EX 500 ninja the best choice my opinion and others who have tried them. Best carbs on the bike so far light years away from the old BS carbs that came on the bike.
 

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