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The hydraulic clutch is most likely air locked. Bleed it. When it gets hot, it's losing it's pressure because the fluid is most likely boiling.

Totally agree!  I went to a couple parts stores and couldn't find a bleed nipple.  Only thing stopping me from ordering one is general frustration, but cooler heads prevail I suppose.  Was gonna just bin the whole system for the simplicity of cable, but if I can get the hydro work right I'd rather not throw more money at it.

you can get a bleed nipple in a banjo bolt fitting. had to do that on mine, been good for over a decade. (13 yrs maybe?)

also, i made an adjustable rod for mine so i would get full throw. is it a necessary mod? absolutely not. but its fun when it works well.

this is an OLD picture, when i first did it.

Super clean install!  The guy who installed the hydro on mine must have been an amateur Machinist.  There is a secondary pushrod that has a cup to house a ball bearing where it contacts the primary pushrod, and there is a third ball bearing where the secondary pushrod contacts the piston.  The secondary pushrod is stepped so that the piston end fits through the OEM cover and is cupped to hold the second ball at the secondary/primary interface.  It's really a pretty smart design that someone put some effort into.

It does have a bleed fitting, but it just has a pinhole in the top, almost like the nipple was cut off).  I tried bleeding it by pumping it up with the valve closed, holding the lever down, and cracking the system to allow the pressure to release, then closing the valve before releasing the lever (all done with the master open and level, keeping a close eye on the fluid level.  In a perfect world that should work, but it the technique offers no visual confirmation (bubbles in the line) to verify a complete bleed.  I could "bench bleed" the system by just dropping the whole slave into a tub of hydro fluid and bleeding it down like that.  Maybe I'll give it a go.

This afternoon I pulled the plates out and did find some pretty distinct grooves in the leading edge of the clutch basket lugs.  So I'll pull the basket out and true it up.  Maybe that and the bench bleed will get me in shape.

Sooooooo....  Here we are three years later haha wow.

I had to step away from this project for several reasons.  Two new babies!  One being a little boy who is now almost 3, and the other being a HD Shovelhead that's about 45 😜 Anyways, I think in my intro I probably explained the origin of this bike having belonged to the father of a close friend who inherited it when his dad passed.  I was charged with it's repair and care.  Recently, my very good friend (who lives out of state) decided that the best option for this bike was to release it to my ownership.  Naturally that inspired me to dig back in, so here are some updates

If you care to review this thread please do, but I'll give you the readers digest condensed version:

74 rd350 won't shift when hot, approximately 20 minutes of ride time.  Lucas 10-40 semi.  Shifts cold.  Been through the clutch case one million times and everything checks out.

So yeah.  Threw a new battery in, cleaned the tank.  I had run the carbs dry when I parked it.  Started right up, and ended up, as expected, right where I left off, but with a new sense of hope and some new ideas.

I wanted to verify that the issue was driven my heat, and sure enough, after about 20 minutes I lost my shifting, parked it in the shade, and about 20 minutes later was able top back on and enjoy my renewed ability to shift smoothly through all gears.  That got me thinking, why haven't I looked into a potential clutch release issue?  Well the short answer is because it has been working before it stopped, but that's not really adequate method. 

This bike has been converted to a hydraulic clutch.

Red Flag!

Cool, but had already been a PITA anytime I worked on the bike.  The install is pretty cool.  The cover was left fully intact, with an unknown slave cylinder modified to take screws through the factory holes for the release cover.  At some point in my mania I cracked the system, and the bleed valve does not have a nipple on it, so I have not been able to confirm an honest bleed.  I am highly suspicious that the hydraulic clutch is the culprit.  Even when cold I found that with the engine off, the bike in gear, and the clutch released, that there was noticable drag.  In neutral in rolled smooth and free, but in gear I could feel the drag and also hear the clutch load up the flywheels before encountering piston resistance, at which point it would turn through the drag but with quite a bit of friction.  Not enough to move this piston in the bore, but just shy.  So  I pulled the clutch out and noticed that it was short a disc!  The stack finished in a friction rather than a steel.  I'm guessing this was done to give some extra release room for what is most likely a hydraulic system that doesn't have adequate throw.  I also found that the outermost friction that make direct contact with the pressure plate was cracked, and all the friction discs measured fat on one side (probably from the many years of sitting in oil before I ever saw it).  So I replaced the clutch with one of the HD sets from Economy (fzr plates, new steels, new springs), installed it annnddd... Still won't shift hot 🤣. It did improve the bikes power delivery noticeably.  The FZR frictions have metal core which makes them much heavier than the all cork discs that came out, and I think I had been slipping under power based on the condition of the discs and the missing one.  But yeah still won't shift while hot.

So my last hurrah before pulling and splitting the engine is going to be to convert to a traditional cable pull clutch release, and also to look close at clutch basket for burrs on the lugs.

Wish me luck.  I really need some wind therapy right now.

We shall see.  I noticed that the shift shaft had what seemed an inordinate amount of thrust play, to the tune of about 2mm.  That's when I decided to review the assembly procedure, and already knew I had installed the washer on the inside.  When I re-assembled it in the correct order, the thrust play was reduced to around .5mm.

It's funny, these bikes are dead-ass simple, but as such, very unforgiving of any mistakes.

Your symptoms mimmick what my RD400 was doing. Had to send the transmission out to be rebuilt by R&D Motor Sports in Clearwater, FL (next door to Chuck).  While it was there I had them modify it for racing, as my RD is for road racing.

Good luck.

Rob

Fingers crossed the issue is resolved!

look for a chip on your dipstick end. glad ya found it, i think.

did you have the thin washer behind the shift arm and the case on the shaft? that one can cause fits if its missing ( i think thats rd and not triples)

Back to this, I may have one last glimmer of hope.  I took the cover back off and ran back through the assembly procedure and realized that while I did install the shift shaft washer, I installed in the wrong place!  I had it installed behind the shift arm on the clutch cover side, but it looks like it's supposed to be installed on the primary/generator side, behind the e-clip.  So I switched it around and have yet to complete the re-assembly - just need to hook up the oil lines, install the pipes, and fill the case.  Should be able to test ride it tonight about an hour after I get home from work.

Wish me luck.

I think you need to split the motor and pull the trans out, remove the gears from the shafts and examine the gears and shafts. Sounds like you have a gear or two binding or galling on a shaft when things heat up. Now that it has done it a few times I'm afraid no oil in the world is going to fix the problem.

Definitely not what I want to hear =(

I think I know where the black plastic came from. I think it is a piece of the cushions on the back of the basket. I dont think it will effect the shifting but it is a good sign that your rubber cushions are worn and deteriorating. I had to replace mine, it's a little bit involved but you can do it.

That's an interesting theory, I've never seen the results of failed clutch basket insulators.  I think the piece I found was too large to have worked it's way out from behind the clutch basket anywhere I can see, but I guess it's possible.  My clutch basket does have some play.  The little piece I found, though, had five sides of a perfect cube with the sixth surface appearing to have been snapped off of something.  Doesn't really add up to any engine component I can think of.

Also, the issue came back. 

So after I took it all apart, not finding anything definitive, I put it back together, filled the case back up, and took it up and down the street.  Did this a couple days in a row back to back, and everything was working beautifully.  Today I decided to ride to work since my test rides had been smooth.  Well, it didn't go great.  About 15 or 20 minutes in, the shifting started getting sticky again.  First to second about 40-50% miss on first attempt, 30-40% on second attempt, and second to third, 100% miss on first attempt, probably 90% on third attempt, to the point where I'd have to down shift all the way back into first and try again.  So now I'm at work, hoping that maybe its a heat issue and the bike just doesn't like the gear oil I'm using.  If that's the case I guess I can expect it to shift well after sitting all day, and start giving me problems when I'm most the way home.  The first time it happened, it was also after having been riding for 20 or so minutes straight.

I'm always a proponent of a high end specific 2 stroke oil as RD's do have a problem if ever run low on oil.
They will get seizure on the gears to the shaft if borderline on lubrication
Chuck

I'm open to suggestions!  I get the Lucas stuff because its readily available in my area... I use the 10-40 semi synth gear oil, and also the synth 2 stroke oil.

look for a chip on your dipstick end. glad ya found it, i think.

did you have the thin washer behind the shift arm and the case on the shaft? that one can cause fits if its missing ( i think thats rd and not triples)

I looked for a chip and didn't find one, I'm pretty sure the plastic was foreign.  Just dont know how it got there.  I did have the washer in place, though.

Got the bike back together and took it for a quick ride up the street and shifted through all six gears up and down buttery smooth, so the problem is fixed, just hope it doesnt come back!

Well I seemed to have fixed it, but I dont know how, which worries me.  Pulled the cover, and clutch.  A small piece of black plastic fell out when I pull the clutch, but no idea where it came from and I was pretty attentive while putting this thing together.  I'm thinking maybe there was some debris trapped inside my funnel when I filled the case.  Odd thing is, it wasnt trapped in the shifter assembly as far as i can tell, but it was large enough to interfere, had it been.

Anyways i pulled the assembly apart and messed with it, put it back together and now it works.  I also found that one of the springs for the claw wasnt in place, but not sure if that happened during disassembly. 

At this point it shifts into all the gears, although it is "sticky" at times - this is while spinning the rear wheel by hand until the dogs time, so I guess that's to be expected. 

Are there any elements of the gear change assembly that will impact shifting quality with wear?

Haha thanks!  And dont sell yourself short, if you are able to pull the cover and clutch assembly to diagnose something like this I certainly wouldn't describe you "far from a mechanic"! 

Anyways, got as far as getting the cover off before "duty called".  Maybe later tonight when duty goes to bed I'll run out to the garage and pull the clutch off to see what's going on.  There was a small amount of metal on the magnet and some flakes in the oil, but I dont think it's anything really bad.  Especially since the shift drum and kicker aren't supported by bearings.  We shall see.

Hey guys, looking for some ideas on this.  Just starting to ride this thing after a prolonged case swap and lots of little repairs.  The bike has been running great (still need to work out the jetting) around the neighborhood, has made the trip to the store and the gas station under it's own power, so earlier this week I took it to work and had a flawless ride home.  The bike seems to be running better and better, and today I took it to work again, then made a couple extra stops on the way home.

So everything's great, then I seem to miss a shift, finding a "false neutral" between 3 and 4.  No biggy, hit it again and ride on.  Then it progressively gets harder and harder to successfully shift from 2-3 and 3-4, giving me a false neutral maybe 1 out of 3 shifts.  Then it just wont shift up from 3-4, the shift lever physically wont budge beyond 3rd.  Then 3rd starts fighting me until it wont shift physically make that change, and so on, until I ride home about 7 miles in the bike lane in 1st gear.  I stopped at o'Reilly's on the way and checked my gearbox level, and it seemed a tad low but not much.  There was fluid on the tip of the dip stick after dipping but not threading.  I added maybe 3 oz and continued on my way.  I did get 2nd back after that, but still locked above that.  I've been using the Lucas semi-synth 10-40 wet clutch motorcycle oil.

If you search my name 92stroker you'll find a recent thread asking for help with pawl plate orientation.  I had tested this thing multiple times and it went through all the gears on the bench, stopping below 1st and above 6th as it should.  I've put about 45 miles on it and everything worked perfectly until today.

I'm hoping that it works after a good cool off but I'm not betting on it.  If so I'll have to try a different oil.

Any thoughts?  I assume I'm just gonna have to drain it and pulled the cover to look at the shift claw and drum assembly.

I didn't race them, but I did fix the problem!

As it turns out, that outer gear that slips over those little spring loaded dowels is directional!  So I pulled it back apart and flipped the gear.  Viola!  Working like a charm now.  Kinda embarrassed that I flipped it in the first place, I'm usually pretty good about laying things out so they go back together correctly.  Learning experience, and some knowledge to share with the group if it ever comes up.

The bike started instantly on the first kick after being down for over a year with every system disassembled and cleaned or rebuilt.

Hey guys, just finished rebuilding my engine, oil pump, petcock, bike is ready to ride except for a strange oil pump issue;  the priming wheel on the pump will turn counter clockwise no problem, but it wont turn clockwise.  I know that clockwise is the correct direction, so what gives?  It doesnt seem like I could have installed anything "backwards" but something is wrong. 

Any ideas?  Do I have to pull this pump back apart?  Shouldn't it free spin in both directions?

Thanks in advance, hoping to ride tomorrow!

That's what I thought it was for, thanks for the confirmation! 

Still not able to find any info on this subject.  I have a digital copy of an rd350 shop manual that for some reason doesn't include any transmission info, searched the tech section on this forum, even did a google image search (along with a half dozen different keyword searches and I'm striking out.  This is the only thing keeping me from putting the engine back in the chassis and finishing the rebuild.  Does anybody have an old engine laying around that would be will to investigate?

Really appreciate any help.