I am rebuilding my 79 XS650 which had frozen motor. I got it all apart and am considering boring to 80mm but am wondering what else is needed to do a quality job? Are the standard cylinder studs able to handle the increase separation force? A friend mentioned I may have to change timing advance and repilot the carbs. Where can I find out details of how to do this sucessfully?
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What is needed to mod 75mm to big bore 80mm
- Thread starter 79Dano
- Start date
750 bigbore kits need to have the sleeves replaced from the original 650 sleeves. They are good to 706cc on the 650 sleeves.
MikesXS is the go to for big-bore kits for value as they also incorporate larger fins for cooling and it is a bolt on kit. All that needs to be done is to make sure your con-rods, and crank bearings are good and the pistons will go straight on the con-rods.
Jetting is basically the same as for the 650 engine unless you change the headers and mufflers, then jetting changes are needed. To get the most from your 750 engine the headers do need to be changed for it to breath properly and the head also needs to have work done to it also. Just a basic clean up of the casting of the ports will help but a good rework of the inlet ports, (needs to be done by someone who knows the XS650 engine), will get the optimum from the flow.
Caution, The XS heads ports are already on the point of being to large so cleaning the casting or any reworking of the ports needs to be done by an XS650 expert.
Can buy sleeves to install into your existing barrels. That requires the barrels to be bored to accept the larger 750 sleeves then the 750 sleeves to be bored and honed to accept the 750 pistons. to buy 750 pistons from Wisco or some other manufacture can be expensive. By the time you add up the cost of sleeves the boring of the barrels and sleeves and the pistons. ....................
No comparison compared to MikesXS 750 kit
MikesXS is the go to for big-bore kits for value as they also incorporate larger fins for cooling and it is a bolt on kit. All that needs to be done is to make sure your con-rods, and crank bearings are good and the pistons will go straight on the con-rods.
Jetting is basically the same as for the 650 engine unless you change the headers and mufflers, then jetting changes are needed. To get the most from your 750 engine the headers do need to be changed for it to breath properly and the head also needs to have work done to it also. Just a basic clean up of the casting of the ports will help but a good rework of the inlet ports, (needs to be done by someone who knows the XS650 engine), will get the optimum from the flow.
Caution, The XS heads ports are already on the point of being to large so cleaning the casting or any reworking of the ports needs to be done by an XS650 expert.
Can buy sleeves to install into your existing barrels. That requires the barrels to be bored to accept the larger 750 sleeves then the 750 sleeves to be bored and honed to accept the 750 pistons. to buy 750 pistons from Wisco or some other manufacture can be expensive. By the time you add up the cost of sleeves the boring of the barrels and sleeves and the pistons. ....................
No comparison compared to MikesXS 750 kit
