700 to 750

Hello everyone I’m rebuilding my Yamaha maxim 700 the motor has low compression I’m only getting 50 to 60 I know it’s the rings because I just redone the valves and I did the oil test and it shot up to the 150 170 range anyway I’m thinking of converting it to a 750 i would like to know what all the differences of the two motors are

 

is this an air-cooled or liquid?

 

Air cooled

 

You have the bore of the 750 and the stroke of the 650.

However, your pistons are different p/n from the 750.
Your crankshaft believe it or not is the same p/n as the 82 & 83 XJ650 Turbo.

I suspect to convert to the 81-84 XJ750 dimensions you will need the crank, rods, pistons, cylinder block, maybe even the head.
Rings and quite possibly crank and big end bearings after you go the plasti-gauge test.

When you say you did the valves, do you mean you adjusted them or you actually re-lapped them?
If it's just a valve adjustment, I suggest taking the head and cylinder off and see what's going on.
Measure the rings for wear in the cylinder without the piston, measure the bores, measure the pistons.
Could be just stuck rings, others have had success just soaking the cylinders in strong solvent without tearing it apart.

If the motor is licked then find a good 750.

 

I actually lapped them and everything but what you’re saying is I need different cylinders and head to?

 

The 700 makes more power than the 750. Leave it be in that regard.

 

Oh I’d love to but I can’t not find any rings for the air cooled 700s man

 

You can't find rings for the 750 either. Not oversize ones anyway.

email @chacal at info@xj4ever.com to see if he has any oversize rings for the 700.
Even if he does go ahead and see if the rings are just stuck (very likely). Use a 50/50 mix of ATF and Acetone. Put a teaspoon down each sparkplug hole and let it do its thing for a few days. Run the engine and rechek compression.

IF no rings area avaialble, and the compression does not improve from the soak (try several times, and don't check while wet), then look for a used engine to swap in. If you change displaement be sure to get the TCI.

 

that would have been the time to look over the cylinders, they were ready to slide off.
You could build a 750 with your pistons cylinders and head but you would need to measure your compression ratio once you swap crank. There’s a 4 mm difference in stroke which means you need to raise the head 2 mm plus you’re cramming more into the combustion chamber requiring raising it even higher.
I have done this procedure with my XJ-X. You assemble #1 piston in the cylinder and smother the rings with grease so it holds liquid. Position it TDC then bolt the head on. Fill the combustion chamber with a measured amount of fluid then you can determine how much shimming is necessary. You would add this under the cylinder block so you’re still using a stock head gasket.
You can compare 750 and 700 pistons and make sure to use the one with the shortest deck height.

 

You also need to compare rod lengths.
There are several sets of 891 jugs and pistons on eBay from Germany at the moment, pretty reasonable price. You could build a 772 with that stuff using your existing crank.