74 RD350 - Anything more than 2 strokes is a waste

Guys,

I'm back in action with another bike. Some of you may remember the Scrambler I restored over the winter of 2013 - http://www.dotheton.com/forum/index.php?topic=44487.0

I made a trip from NC to NY last December and ended up picking up a 74 RD250 with a clean title on the drive up. This beast hasn't been run in 20+ years, but is all together. I finally cleared my plate enough to start on this bike. I don't have a concrete plan just yet, but I'm leaning towards cafe bike, hence it location in the cafe racers page. My roots are moto and supermoto, so I'm considering a more upright riding position with some sticky tires.
I really like the look of a modern fork, expansion chambers, and a clean looking tail that lines up with the tank and ends at the center of the rear wheel.

I'm on the fence as to rebuild this as a 250, or locate some 350 cyls/heads/primary gear to build it as a 350. I've always liked the feeling of riding a slow bike fast (not that the 250's are that slow). Any input you guys have is appreciated in trying to solve this dilemma.

That's all to be decided in the next few weeks. In the meantime, it's teardown and degrease.

Re: 74 RD250 - Anything more than 2 strokes is a waste

I started the evaluation last week and did some prep work getting it ready to be torn down. Prep work you say? - With the engine in the frame, I cracked the front sprocket nut loose and used my impact driver (not to be confused with impact gun) to break free all of the phillips head machine screws on both cover sides of the engine. This worked much easier than trying to do it on the bench with the engine sliding around. I also pulled the cyl heads and have been spraying penetrating oil on the cylinders/rings for the past 2 weeks so that they'll be free when it comes time to teardown. The only issues I'm seeing initially are that the trans rolls in neutral with no issues, but the kickstart mechanism is locked up. I'll address this on teardown.

There was an oil tank leak that went unaddressed for 30 years, so there is a quarter inch thick layer of funk on the engine and frame. I'm going to expansion plug the intake/exhaust on the cylinders and use the pressure washer, purple power, and elbow grease to get the engine clean. I have a feeling that the frame is going to require the same.

Re: 74 RD250 - Anything more than 2 strokes is a waste

If you need anything let me know i got 11 motors and crates of engine parts and a shelf of jugs and heads

Re: 74 RD250 - Anything more than 2 strokes is a waste

Looking forward to the progress.

Re: 74 RD250 - Anything more than 2 strokes is a waste

This has good potential, i'm signing on

Re: 74 RD250 - Anything more than 2 strokes is a waste

Good looking bike! Looking forward to seeing your vision for it come to life.

Re: 74 RD250 - Anything more than 2 strokes is a waste

just finishing my RD 250
next season i´ll do the 350 conversion.cyls are already sitting in my shelf.

if you stick to using the oil pump...i know its a religious question dont forget to swap your oil pump gear wheel that grasps the primary.the 350 wheel has less teeth...

Re: 74 RD250 - Anything more than 2 strokes is a waste

Do any of you guys have a pdf copy of the service manual for the 250/350?

Re: 74 RD250 - Anything more than 2 strokes is a waste

I don't have anything major to report. I got the bike down to a roller for cleaning. There was what seemed like 20 years of oil on the top of the crankcase, frame, and swingarm. It took some engine degreaser and scrubbing, but I got it 80% of the way clean. My plans are to soda blast the engine cases one they are apart and I'll finish cleaning the frame one the kickstand/wheels/swinger are off.

The top end was seized in the cylinders. For weeks I've been hitting the top of the pistons with penetrating oil. I was finally able to get the clutch side cylinder free, but the mag size won't budge. It's soaking in penetrating oil for the next 4 days as I'm traveling. If that doesn't work, I'm going to try some of the heat/cool methods I've researched on the interwebs. Then I'll be able to knock the engine/trans apart for cleaning/inspection.

The carb slides are also seized and one of them is scored pretty good. I had them soaking in PineSol overnight - cleaned up the varnish and removed the paint, but didn't touch the rust in the bottom of one float boal. I'll have to see if I can find replacement slides or another set of carbs.

Still on the fence as to whether or not I'll rebuild it as a 350 or stick with the 250. I'm guessing it'll cost an additional $500-$600 to go 350 considering cylinders/heads/bolts/primary + OP gear/350 carbs.

I didn't have a whole lot of time to play yesterday, but I did make some progress on pulling this dent out of the tank. I've had good luck using a hot glue dent puller from HF. I was slowling working the dent out but ran out of glue. I ended up having to buy another kit just to get the glue sticks, which will work out well. Having a second drawbar should make things go easy.

I also scored a swingarm and shock from a 250 Ninja from ebay for $25 each. That's cheap enough for me to play around and decide if I'm going mono or not.

Re: 74 RD250 - Anything more than 2 strokes is a waste

I have a little RD125 that is in about the same shape as this bike is. I havent had a heart to do much (modifiying) to it as its survived this long as stock.

Re: 74 RD250 - Anything more than 2 strokes is a waste

Interesting project, I will be following to see where you end up with it! I too am a supermoto guy and my Suzuki 500 two stroke ended up like this:

Re: 74 RD250 - Anything more than 2 strokes is a waste

I looked im down to 1 complete 250 top end and 1R5 top end i can sell. the rd350 jugs are running low enough that im gonna keep what i got

Re: 74 RD250 - Anything more than 2 strokes is a waste

johnu said:

Interesting project, I will be following to see where you end up with it! I too am a supermoto guy and my Suzuki 500 two stroke ended up like this:

Click to expand...

This bike is disturbingly nice. I'm striving for the openness between the bottom of the seat and top of the rear wheel.
Can you share a few details on your build? Forks - Shock - Swingarm: did you fab that swinger shock mount?

Re: 74 RD250 - Anything more than 2 strokes is a waste

bradj said:

I looked im down to 1 complete 250 top end and 1R5 top end i can sell. the rd350 jugs are running low enough that im gonna keep what i got

Click to expand...

No sweat Brad. Thanks for taking a look and letting me know. Unless I find a good deal on a 350 setup, I'm probably going to stick with a bore/hone/FTE with the 250 goodies.

Re: 74 RD250 - Anything more than 2 strokes is a waste

jchek779 said:

This bike is disturbingly nice. I'm striving for the openness between the bottom of the seat and top of the rear wheel.
Can you share a few details on your build? Forks - Shock - Swingarm: did you fab that swinger shock mount?

Click to expand...

Thanks. Here is a link to my build thread http://www.dotheton.com/forum/index.php?topic=20290.0 and here is the blurb from the botm contest which gives most of the details about the bike, I did fab my own mounting point on the narrowed YZF swing arm:

My T500 started off as a 1971 frame with a 1975 GT500 engine. I love air cooled two strokes so I decided I would like one with modern handling. I thought about building my own frame initially but lack of time and certain equipment dashed that plan. However I did cut off a good chunk off of the frame including the rear engine mounting point and the swing arm pivot area, I fabricated new frame tubes and the swing arm pivot mount to get it closer to the counter shaft centerline. I also used a swing arm from a YZF600 but I cut it and made it 30mm narrower also modified to be a linkless mono shock. The biggest challenge on the bike was to get the chainline correct, for this I had to make my own offset counter sprocket (probably the part that gave me the most satisfaction knowing I made it myself) and machine the rear sprockect carrier from the EX650 wheel that I used.

The forks are from a ZX636 Kawi with a modified steering stem and top clamp. I made a custom adapter to suit the KTM bars and clamps. Rear shock is EX650, I made an adjustable mounting so the ride height can be changed. Rear brake is a modified KTM Brembo unit. Rearsets are modified Triumph Daytona units.

The tank and seat I made from scratch from 3003 aluminum, all done using hand tools and oxy/acetylene welding (I don't have tig). I also made my own expansion chambers which was a lot of work but I got them exactly how I wanted them to look.

The engine has been ported (eric kalamaja, sundial motorsports) and I am running 34mm mikunis with it.

The bike came out exactly as I imagined it would. Now I look forward to giving it a good thrashing on the street and track.

Ask away with any other questions, I don't know much about RD's that would be Brad but I do know about fabbing and modifying frames.

Good luck with the project,
John.

Re: 74 RD250 - Anything more than 2 strokes is a waste

Not to derail your thread - but have any of you RD guys ever messed with an RD125?

I am having trouble finding parts. Where do you guys get parts from for the most part? I use z1 enterprises for a lot of my KZ stuff and they dont even list the RD125.

The bike it pretty low on compression on one side but I haven't messed with it since early college about a decade ago.

Bad picture but its all there

Re: 74 RD250 - Anything more than 2 strokes is a waste

johnu said:

Thanks. Here is a link to my build thread http://www.dotheton.com/forum/index.php?topic=20290.0 and here is the blurb from the botm contest which gives most of the details about the bike, I did fab my own mounting point on the narrowed YZF swing arm:

My T500 started off as a 1971 frame with a 1975 GT500 engine. I love air cooled two strokes so I decided I would like one with modern handling. I thought about building my own frame initially but lack of time and certain equipment dashed that plan. However I did cut off a good chunk off of the frame including the rear engine mounting point and the swing arm pivot area, I fabricated new frame tubes and the swing arm pivot mount to get it closer to the counter shaft centerline. I also used a swing arm from a YZF600 but I cut it and made it 30mm narrower also modified to be a linkless mono shock. The biggest challenge on the bike was to get the chainline correct, for this I had to make my own offset counter sprocket (probably the part that gave me the most satisfaction knowing I made it myself) and machine the rear sprockect carrier from the EX650 wheel that I used.

The forks are from a ZX636 Kawi with a modified steering stem and top clamp. I made a custom adapter to suit the KTM bars and clamps. Rear shock is EX650, I made an adjustable mounting so the ride height can be changed. Rear brake is a modified KTM Brembo unit. Rearsets are modified Triumph Daytona units.

The tank and seat I made from scratch from 3003 aluminum, all done using hand tools and oxy/acetylene welding (I don't have tig). I also made my own expansion chambers which was a lot of work but I got them exactly how I wanted them to look.

The engine has been ported (eric kalamaja, sundial motorsports) and I am running 34mm mikunis with it.

The bike came out exactly as I imagined it would. Now I look forward to giving it a good thrashing on the street and track.

Ask away with any other questions, I don't know much about RD's that would be Brad but I do know about fabbing and modifying frames.

Good luck with the project,
John.

Click to expand...

Incredible build John. I spent so much time reading through your build that I felt guilty for not working on my own bike.

Re: 74 RD250 - Anything more than 2 strokes is a waste

I was finally able to get the mag side cylinder off the engine. It had been soaking in penetrating oil for days but that didn't seem to do the trick...so I resorted to fire. The top ring was above the ex port, so I was able to puddle a small amount of brake clean and light it. It was a small amount that burned for about 2 minutes. The idea was to allow the heat to expand the piston/rings, the the cooling contraction would unfreeze whatever was rusted together. Whatever I did worked, but the cylinder still needed to be knocked off with a plastic mallet.

Re: 74 RD250 - Anything more than 2 strokes is a waste

I went back to the garage last night to play with some new toys that came in the mail.

The bike didn't have a key, so I was able to find the proper match on eBay. I was finally able to look inside the tank and see what was in there - the good news is the rust isn't too bad. I think a soak in vinegar may be able to save it.

Then the fun stuff - my swingarm and shock from a later model Ninja 250 showed up. I have less than $50 into these so i'm fine with them working or not. I laid the shock in there to get a feel for how it would look if I used the stock swinger. For whatever reason, I'm not a huge fan of that look. I'm worried that the additional swingarm bracing will take away from the minimalist look that a cafe bike has. I took some measurements and checked the travel to see where the rear wheel stopped with the RD shocks bottomed.

So I started modifying the Ninja swingarm. It was about 11mm too wide to fit the RD frame. I was able to cut/grind both the pivot axle and swingarm down to fit. The good news is that the swingarm bearing bores are still deep enough to reinstall the bearings. The swingarm adds about 4-3/4" from S/A pivot to rear axle centerline. I'm not a huge fan of stretching it out that far and may modifiy the rear of the S/A to get back close to 2". If I did this I'd lose the ability to bolt on a rear disc. The Ninja S/A is symmetric and has a lower linkage mount that I was thinking of using to mount the shock directly to by running it upside down...but that's going to require more fab work than just building my own mount. I could also use a linkage shock...So many options here.

What are you guys experiences with ride quality/handling when you add 2-4 inches of S/A length from the pivot? I'm worried that the bike will turn slower than I want.

Disclaimer - The shock is just hanging in there. Please don't get too caught up in suspension geometry just yet. I'm going to do the homework. This was the first mock up.

Re: 74 RD250 - Anything more than 2 strokes is a waste

Here are a few more shots