My 1975 650 B project.

[northtexas_firefighter]

I got my first bike from a family. A 1975 XS 650 B. It has been sitting in a storage unit for at least 5 years and wasn't running due to a stuck piston. I tried to free the pistons but ended up having to dismantle the upper part of the motor. After disassembling the upper and freeing the stuck left piston, I determined that the lower was needing to be examined, cleaned and rebuilt (debris and crusty oil residue and gasket material noticed inside crank shaft area).

So here is where I am in the process. I'm waiting for a good day to separate the lower housing. Only thing I have left it to remove the 18 bolts that hold the casing together and separate the case.

My question is this. Since I will have the motor apart, what modifications would be a good thing to do at this stage. I already plan on swapping out the 5th gear for the overdrive gear but not sure what other modifications I should do to improve performance of the motor.

Any other suggestions would be appreciated.

 

[gggGary]

I am beginning to believe that the crank should be welded when the bottom end is open.

 

[XSLeo]

The 5th overdrive may not work in your 75. It was designed to work in the late 78 up engines. There may be enough wear on your shafts to let the new gear work.
On the lower end if everything checks in spec, not really anything to change. Just clean, inspect per the book, reassemble. With one stuck piston I might just measure things up and go to the next oversize piston, rings too of course.
Lap the valves, check valve stem to guide wear. If excessive replace guides and maybe valves.
New front chain guide and timing chain.
Should be good to go.
Leo

 

[650Skull]

the 5th gear overdrive gear is not designed to go into engines earlier than 77. I believe some have done it...BUT.......................... Can get a later model gearbox and it will swap straight in.

I have a friend who has built the top end twice on the same engine and has never welded the crank.

Depends on how you intend to ride the bike, (red-line through the gears=welding crank or wheel stands=welding the crank), or how much it takes to push the crank back together. Talking to people who have been working with these engines for 20 years and more they will tell you the press has to push 20Tons to rebuild the crank. If it takes any less, then the pins need to be welded.

Because we can never know how the PO, and the PO's before that, have ridden the bike there will always be the possibility the crank has or is walking or could in the future.

Some on here have used a 12ton press to rebuild the crank.............now that needs welding.

The rule of thumb for a Yamaha mechanic was, the crank should be able to be taken apart and put back together 2 times, and on the third time it would more than likely have to be welded.

Again, it also depended on the tonnage it took to press it back together

 

[northtexas_firefighter]

Thanks for the advise and assistance. I know the motor has been disassembled before but I don't know how far they took it. I wasn't planning to disassemble the crank shaft unless I had to and after hearing about needing a 20 ton press, I think I will just take it to a professional if that is the case. lol. I was just planning to remove and clean the parts to try and prevent further issues afterwards. Plus, with the oil on the outside of the motor, I want to replace all the oil seals. I am sure they have some dry rot going on from sitting in a storage unit for 5+ years.

As for the 5th gear overdrive, I have heard that it will fit but, before I do, I plan to measure the shaft before to make sure it will work. I am also running into some conflicting info for the bike. By the date on the sticker this should be a 76 since it was build in the later part of 75 but when I called Yamaha they said it was a 75. I don't think there was any difference in parts between the years though so that shouldn't be an issue. Another issue I am running into is that the previous guy that modified the bike has replaced some parts here and there already. I just found out that I had a later version of a fuel tank (not exactly sure what year) and not sure what else he replaced. I was informed that he was going to make it a custom chopper but stopped mid build and reassembled the bike. However he did not get all the wiring 100% correct. I will have to check that out after rebuilding the motor and get things back together.

Any suggestions with the electrical components? I hear that there is potentially issue with charging and ignition system that I should replace. I know I may be getting the cart before the horse since the motor in pieces right now but planning for the future and adding things to the parts list to acquire.

 

[650Skull]

Mounting of the front brake calipers,Exhausts and gauges all say its a 74TXA/75B. They were identical. Tank looks like a 76/77.

On the charging system check out gggGary's thread

These others may help with your electrics.
1
2
3

To help with identification look here

 

[TwoManyXS1Bs]

Bottom end: Examine all bearings & races, listen for grittiness when dry. Clutch pushrod bushing replacement is less messy when shaft is out. Examine shift forks, looking for lopsided wear marks/scratches on sides of engagement ears, same with slots on gears. Look for excessive rounding of dogs/cogs corners. Check neutral detent plunger & spring. Check fitment of shiftshaft to shifter arm, some swaged fittings loosen up, weld helps.

 

[CDNTX650]

If you want the 5th od, just buy the countershaft gear set from a 78+ and swap that in so you can use the 5th, should only need the one set. The gears are same externally, the only change was to the shaft splines and inside part of the gears.

Dont jump on pma bandwagon. Have 3 rotors and all are good, solid state voltage regulator is a great upgrade.

Posted via Mobile

 

[CDNTX650]

My crank will be back tomorrow from being rebuilt. Left the main bearings alone as they all looked okay. cant decide if i want to weld or not. Im pretty gentle on my machines.

Posted via Mobile

 

[XSLeo]

On some of the VIN charts they tell you the model year is from August to July. So in this case a 75 was built from august 74 to July 75.
A 76 starts August 75 and goes to July 76.
The main difference between the 74 and 75 was the 74 was painted brown, the 75 black.
Leo

 

[650Skull]

I am also running into some conflicting info for the bike. By the date on the sticker this should be a 76 since it was build in the later part of 75 but when I called Yamaha they said it was a 75. I don't think there was any difference in parts between the years though so that shouldn't be an issue.

as XSleo says the August to July was the production run for the year, i have found, and confirmed, there can be a difference either side of those months and the bike still was numbered and produced as the year in between the Aug/July.

Big difference between the 74/75 and 76 was the 34mm forks on the 74/75 had the brake caliper mounter in front of the fork, the 76 34mm forks had the brake caliper mounted behind the forks

 

[northtexas_firefighter]

Well, if I go by the date on the sticker, it should be a 76 since it says 9/75. BUT, the VIN shows to be a 75. Plus, another confirmation is that the break caliper is in front of the forks. I'm I think that the thought about the fabrication date on the sticker method for determining the year of the bike is not 100% accurate.

 

[650Skull]

July /August is a rule of thumb. ID no's are the true confirmation. yours is only the next month on.

 

[northtexas_firefighter]

Motor almost completely apart. Only thing not disassembled is the valves (no valve spring compressor) and rocker arms (damn rocker arm covers are stuck and don't want to turn to come off). Found a guy locally that will redo the head and check/cylinders for a descent fee.

Afterward, I'm thinking of having the motor painted. Not completely sure but considering it since the case is separated. Will make a more through decision after I get the outside cleaned and inspected. If only I had access to bead blast everything.

 

[5twins]

The rocker arms don't really need to come apart, as long as there's no excess wear evident on any of them. The caps over the ends of their shafts are best broken free with a hand impact before you tear the motor all apart.

While it's torn down, a good upgrade is the elephant's foot valve adjuster screws. The original screws are a poor design and the tips wear out. Stock replacements will do the same. They can also dimple and wear the valve stem tops. These elephants foot screws eliminate all those problems. The rockers need some grinding done to them to install the screws so it can only be done during a tear-down.