Power

Just so everyone knows, 95 octane in Europe and 91 octane in North America are the same. Europe and Australia use "Research Octane Number" to get the number and North America uses "Anti-Knock Index" to get theirs. My question is I heard these engines recommend a 95 octane rating, is that ROM or AKI octane rating. I assume Japan uses ROM and 91 is the lowest you'd use here in North America.
 
Most people don't understand that lowering the compression on these a bit smooths them right out.........major improvement in reducing vibrations... maybe a bit advanced for most to understand......took me a while to figure that out and I am sure glad I did.......xsjohn
 
I bored my xs out to 770 cc and the viberation at high revs is really noticable but it pulls quite well, its a fucker to kick over. My friend has bored his out to 900cc, they will go to that on the stock liner, fuck knows what that feels like at 70mph....I have biult several xs650 bobbers, i wont ever take them out to 750cc again, its a bit quicker but much better as a 650, top speed is not much different and its a smoother ride..after all its only pulling one person, if you change your sprockets to keep the revs down you can pull 90, fuck me thats gotta be enough on a hardtail......................
 
81-650chopper said:
I agree with one member on here with the porting and polishing. That watch what you take off cuz yes these ports are definetly large enough compaired to the size of the valve. I've found in my experiences that wether it be a car or snowmobile or bike any motor that the simplest thing you can do is first get in there and smooth out the bends. Don't take to much off just smooth it out. Then the most important part most people overlook is "port matching". You want to have the exact same size hole on the engine side and manifold. Easiest way to do this is place just the gasket or mounting clamp on without the exaust pipes or carbs use a marker to color in what part of the port is smaller or larger than the gasket and grind that down. The objective is too have a smooth flow from engine to exhaust. You can gain good power just from this by eliminating flow resistance.
Click to expand...

Listen to this guys!!

I've quad race alot when I was young with a very low budget and the first thing that I always do on motors is a light p&p with the most important ''port matching'' for sure! You guys spend alot of $$ on big bore, cam and rephased but you'll never get full potential of all thoses mods w/o a good flowing head. No needs for race cuts, 5 angles seats... bla bla... until u race, just smooth the casting irregularity, don't polish the intake side and port match... bamm! :bike:
 
i am a big guy, i run 6'4 and 265lbs

I love my xs650 but i struggle keeping up to my friends on bigger jap bikes.

Do you guys think the 750 kit would give me that extra nudge i need to help me keep up and such?
 
no i dont think it will. It will pull better and get away quicker but thats all....you must remember that the gearing is the same and your rev limit is the same wich means your top speed will be the same...My xs has been bored out to 770cc it has awsome power but out on the highway top speed is about the same..I have changed my rear sprocket and fitted one with less teeth on it..I think i went from 34tooth to a 31 tooth.. The final drive sprocket has 17 teeth as standard, and you can change this for a 18tooth, thats the biggest that will fit....One more tooth onthe final drive is equall to 2teeth on the wheel..But all this does is keeps the revs down when your criusing..I reckon i guy your size needs 1000cc to get you going........
 
I'll take my Road Star if keeping up is important!

An XS is a different ride. It's not a UJM, and you can't expect it to perform like one. A great deal of my riding time is spent on 2-laners, between 50-60 MPH, and that don't take a whole bunch of bike.
 
A big bore will help you can run a lower taller gear 18 front 30 rear a bigger eng.will pull that gearing set up.and raise the top speed,may not keep up with big cc 4 cyl.bikes but hell if that was what we wanted thats what we would have wright!
 
If all you do is pull of your cylindes and slip Mike's 750 kit on your bike you will gain about 7 hp or so.
You will get almost the same increase with just a head clean up. As others said don't enlarge the ports just clean up the rough spots. Matching the ports is also good. On my 75 I did all this when I redid it as a 2nd over. Carb tuning, intake and exhaust helps too.
Do these things first. Then If that's not enough then add the 750 kit.
With all the work my 750 is getting about 10-12 hp over a bone stock bike. That's seat of the pants dyno runs.
As far as vibes go when I put on the 750 kit the low rev vibes increased almost enough to notice but the vibes stay about the same at all rpms. No great jump in vibes at around 4000-4500 rpms like stock.
I think this come from the different weight pistons. Not sure if the forged are heavier or lighter than the cast but the vibes are definitly lessened.
Leo
 
I agree with Xsleo. The mikes 750 kit and a good cam with some port work will give you more power than you need. :D I also like the big bike sound.
 
I have been thinking of a rephase on this engine, or doing another 750 with a rephase.
The rephase reduces vibes, the 750 kit reduces vibes, together it shouldn't vibe much at all.
It might be just the thing to make a long distance tourer.
Leo
 
For anyone out there who has completed the 750cc conversion I am wondering what you did for carburetors? Can you keep your original carbs or do you need to get bigger carbs to keep up with the increased air flow. I know I will need to upgrade the amount of fuel given to the engine to avoid a melted piston or detonation, I learned that the hard way two weeks after I bought the bike. Just hoping that someone can share their expertise with me.
 
XS659 said:
For anyone out there who has completed the 750cc conversion I am wondering what you did for carburetors? Can you keep your original carbs or do you need to get bigger carbs to keep up with the increased air flow. I know I will need to upgrade the amount of fuel given to the engine to avoid a melted piston or detonation, I learned that the hard way two weeks after I bought the bike. Just hoping that someone can share their expertise with me.
Click to expand...

Go with your stock carbs to start with. You might find they work just fine as they can probably flow more air than you'll need. Then later, if you're not happy you can try something else. I know it seems counter intuitive but you might find you need to switch to smaller jetting when going to a bigger bore. Not always, just sometimes.
 
XS659 said:
For anyone out there who has completed the 750cc conversion I am wondering what you did for carburetors? Can you keep your original carbs or do you need to get bigger carbs to keep up with the increased air flow. I know I will need to upgrade the amount of fuel given to the engine to avoid a melted piston or detonation, I learned that the hard way two weeks after I bought the bike. Just hoping that someone can share their expertise with me.
Click to expand...
The stock carbs will work minor jet change. If you holed a piston the cause most likely over advanced ignition timing. If you don't fix that first you will hole the 750 pistons as well. :thumbsup:
 
As I said earlier I did most of the head, carb, intake and exhaust work when I did the engine as a second over. When I added the 750 kit I assumed I would need to rejet the carbs, so I spent about two weeks playing with the carbs. After all the adjusting I was right where I was as a 650.
My guess is that if all you do is the 750 kit your carbs won't need anything, if they do they needed it before the 750.
Leo
 
Me, I just used Mike's 750 big fin kit. 80 mm pistons.
.80 mm isn't an option. The over bores for the stock cylinders are in .25 mm increments. as in first over is .25, second over is .50, third over is .75 over
They list one over size for the 750 kit of 81 mm.
Leo
 

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