Carbs or Ignition?

[XSLeo]

I think if your crank and rods are in spec, the repair manual tells how to do the measurements, I would just use it. To break down the crank to replace just the rod bearings is a lot of work. I did two awhile back, not hard to do with proper tools just time consuming.
If you do break the crank down replace all the bearings and use new rods. You will have them all off anyway to press out the crank pins the rods are mounted on. The parts will run you $500-$600+ the rebuild of the crank.
The new 5th gear is popular. The cam chain, new front guide is necessary.
The 750 kit a new set of clutch plates helps, the extra power can make the stock clutch slip. I might try roughing up the old fibers and cleaning off the discoloration and burrs off the steels a bit with sand paper. A new bearing with more rollers like Mike's sell is a good thing too. If it still slips, go with stronger springs, if still slips then a set of Forodo clutch plates will help.
Or just clean up the steels and put in the Forodo's and heavier spring to start.
Leo

 

[JR Hammer]

I did the math---- it's right around $500 minus tranny bearings and crank work.,,, ugh! Damn! Ack! Fuuuu!

 

[pamcopete]

JR...,

I did the math when my trusty '78/E developed a bad case of low compression. Instead, I bought a new to me '81/H with just 6,000 miles on it for $900. Everything was new!

 

[TXA Phoenix]

JR...,

Your problems start at about the point where the ignition advance is supposed to be at max. If the advance rod is binding, then the engine will run out of enthusiasm at about 3,000 + RPM. Check your advance for binding like in this video:

Check your timing to see if it achieves maximum advance at about 3,000 RPM

I see in the video that your springs spring back very quickly. Are those springs original Yamaha springs or are they an aftermarket pair. I'm using ones from Mike's XS and there is some free play in the system as if they are too weak.

 

[pamcopete]

That video was made with my '81/H so the advance parts are all MikesXS, including the springs. If anything, the springs were too tight so I had to stretch them a bit to get the correct advance timing. Contrary to what people think, loose springs will actually result in not enough advance at full advance because some of the available advance movement is used up at idle leaving less for full advance. The idea that loose springs will result in too much advance is because loose springs are common as the advance mechanism gets older so there is more total movement available due to worn holes in the weights and worn pins that the weights pivot on rather than the loose springs themselves.

 

[JR Hammer]

oh yeah-- that's good advice.... I did eventually find that the mixture was a tad bit lean and leaning out at higher revs

but yes, changing out the springs and clips really did the trick--- that would be my #1 suggestion for anyone still running the stock yamaha springs is to get fresh springs/clips in there for sure

so it was actually both, the carbs and the ignition

thanks