TX750 Timing - Stumped

Pardee

New Member
To start from the beginning, and to keep a long story short, I inherited a '73 TX750.
As most know, a TX750 is definitely not a desirable bike.

This bike sat for approximately 20 years in a barn and needed some TLC.
After getting the bike home, it needed more than a little cleaning.

My first goal was to get the bike running, a dead bike is a dead bike.
After rebuilding the carbs, installing a new battery, plugs, and a few minor parts, it fired and ran pretty good.

When my budget build was about 80-90% done, I noticed the one-way starter bearing / mechanism started acting up.
I was familiar with the concept as I had to replace one on a new ATV that I previously owned.
I knew the starter was good, and was able to count that out.

I pulled the flywheel and noticed the springs were stuck and the mechanism was not working properly.
I purchased a rebuild kit for the one way mechanism, and cleaned / installed the new springs / dowels.

Unfortunately, things started going down-hill from here.

I installed the flywheel / chain / gears without any problems.
I referred to the Service Manual when setting points and timing.

I was able to get the bike to fire again, but something was not right.
After shutting the bike down and attempting to fire it back up, I checked the timing and noticed the points and flywheel markings were not correct.

I could reset the marks and get the bike to fire, but after every running attempt, I would have to manually set the points. The only way I was able to get the points back in line was to pull the clutch basket and manually turn the gear / shaft for the points to ensure they were opening at the correct time. They seem to be jumping.
I am not sure if the woodruf key is worn, as it was not when installed.

This is where I have let the bike sit.
I have not touched any other internals of the engine itself.
The bike has approx 5K original miles and has not been modified.

Any suggestions would be great.
I'm not afraid to dig into the internals, but would rather learn from those who know from experience.

Thanks in advance.
 
Re: TX750 Jumping Timing

So, yesterday and today I pulled the case covers off again and double-checked alignment of the points / gear, primary drive gear, and position of the right piston.

I even checked the cam chain tensioner to ensure it was still within spec.

Once everything is lined up and put back together, I attempted to start the bike. Using the electric start, on the first push of the button, the right cylinder will sputter, as if it wants to fire (note: this bike is a stubborn starter). When attempting to push the electric start button again, the timing is off and the right cylinder will backfire.

Checked the points and it is off once again.

I am currently double-checking the Service Manual to ensure I have not missed anything.
 
Sounds like something is slipping. How is the points cam located on the shaft? Some bikes use a small pin that can drop out when you aren't looking.
 
The cam installation is fool-proof.
The shaft itself has a notch-style end and can only be installed one way.

Check out the image attached.

yamaha-tx750-19731974-contact-breaker-governor_mediumyau0841b-6_f770.jpg


I am going to double-check item #11, as I see there is a dowel further in on the shaft.

I did just pull the flywheel to ensure the woodruff key was not sheared.
 
Found the culprit.

Refer to the diagram on my last post.

The small philps screw on the left side timing plate had no threads left.

I was tightening the large screws with my screwdriver and as I passed by the small screw, it moved.

Pulled it out, replaced it and the bike fires every time.

Such a small part has caused me so many headaches.
 
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