Yahama rx50 rebuild

Rosario Miller /

Alright I'm the proud owner of a yahama rx50 it has the same PowerPlant as the ysr50 but looks like a "cruiser". The bike had been sitting in the back of my garage for to long so I decided to fix the throttle cable and take it for a spin.

Long story short it's running like shit it bogs and bogs until you get into the PowerBand then runs like a runaway train.

The bike has always had stupidly low compression. You could push the kick starter down with your pinky finger ( I'm dead serious). It also has a oil injection system that seems like a ticking time bomb. Tops out at 35 but should go 55

The bike was sitting in a "shed" most of life's and still has the original tires from the Mesozoic age, I have had to put near 500 miles on it and have somehow not busted my ass.

It's sad to pull a bike apart at the start of summer but It really needs it. I am going to try to keep a consistent build thread going on it but no promises.

Thanks for reading, please send a prayer out for parts availability.

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Re: Yahama rx50 rebuild

Dirty30 Dillon /

The injection system on these is very robust, so it should be serviced and maintained, rather than deleting.

I have like the same bike, and this and most other factory 50shifties have a light compression feel at the kicker, but typically put out pretty decent compression if you test them.

Re: Yahama rx50 rebuild

Rosario Miller /

So I haven't yet found a recommend compression level for these but this is what I'm getting with the throttle held wide open.

As for the oil injection system I haven't had any problems with it. I have just heard countless horror story's of it turning engines into paperweights.

Have any pics of your rx50? Would love to see it.

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Re: Yahama rx50 rebuild

I have the dt50 and it was about 90psi when it had good compression.

On one hand these motors come apart super easy, most stuff in them isn't even a press fit and they are super easy to work on. There are some tricky gaskets in them so you'll want a good gasket set before you start.

BUT

Your compression looks good, your motor is probably fine, my guess is the air filter is disentegrating or you have a cracked intake boot. Worst case the shaft seal on the crank is bad. All things that can be fixed without major surgery.

I would start with the easy stuff, tires, tubes, chain, cables, clean the carb, replace fuel and injector lines, run it on premix for a few tanks. Check the wheel bearings and brakes. These old Yamaha are incredibly tough. My dt50 has like 40 psi of compression and 8000 miles on the stock top end and still runs, they are just really picky about air leaks anywhere.

Re: Yahama rx50 rebuild

I had a TY50M and it had the same engine in it. It wouldn't do more than 35mph ever. Apparently the porting on them differed depending on what model it was in. My friend had the RD50M and it was a little bit faster, both bikes were stock. I read recently that there wasn't any real form of restriction in them other than the porting. He put an Autisa 60cc kit on his with a Micron exhaust and it hit 50mph no bother.

Re: Yahama rx50 rebuild

Dirty30 Dillon /

> I M wrote:

> -------------------------------------------------------

> I had a TY50M and it had the same engine in it. It wouldn't do more

> than 35mph ever. Apparently the porting on them differed depending on

> what model it was in. My friend had the RD50M and it was a little bit

> faster, both bikes were stock. I read recently that there wasn't any

> real form of restriction in them other than the porting. He put an

> Autisa 60cc kit on his with a Micron exhaust and it hit 50mph no bother.

That's some UK restrictions for you. RX50 us spec has no such. My very much not dialed in RX does 47mph with a failing tank liner, full airbox, oil injection and factory pipe

Re: Yahama rx50 rebuild

Dirty30 Dillon /

> Rosario Miller wrote:

> -------------------------------------------------------

> Have any pics of your rx50? Would love to see it.

I just took it out to run it a bit, planning to do some clean up while I'm on paternity leave.

IMG-20220506-WA0001.jpeg

The plan is to retrim the front fender, delete the battery box and tuck that wiring, pull the OI tank and replace with something less egregious, build a back rest for easy ridin, and plop some tires on it.

I have a DT100 intake I was thinking about putting on there to run a PWK, but honestly may just leave the engine bone stock and enjoy it.

Re: Yahama rx50 rebuild

Rosario Miller /

Thanks for the picture! The bike would definitely look better with the oil tank replaced. I believe from factory there wad a side cover but that's impossible to find now. Do you know why the engines seem to oxide and rust like that?

I worked on tuning the carb a bit it runs alot better now but still does go all that fast. A 60cc kit doesn't seem like a bad idea 🤔. Also my carb is missing the idle screw with I'm sure is effecting proformance. It's been raining the last week so I haven't been able to test it any more. I do have some parts in the mail I'm exited to share when they get here

Re: Yahama rx50 rebuild

I used to have trouble with the piston rings sticking in 2-stroke bikes that sat unused for a long while.

Re: Yahama rx50 rebuild

Dirty30 Dillon /

> Rosario Miller wrote:

> -------------------------------------------------------

> Thanks for the picture! The bike would definitely look better with the

> oil tank replaced. I believe from factory there wad a side cover but

> that's impossible to find now. Do you know why the engines seem to oxide

> and rust like that?

>

> I worked on tuning the carb a bit it runs alot better now but still does

> go all that fast. A 60cc kit doesn't seem like a bad idea 🤔. Also my

> carb is missing the idle screw with I'm sure is effecting proformance.

> It's been raining the last week so I haven't been able to test it any

> more. I do have some parts in the mail I'm exited to share when they get

> here

Yes, the factory covers arent really an option any longer, so replacement of the tank is my thoughts rn.

I don't know if a 60cc kit is even available any longer, but that's be cool.

If you're missing an idle screw, that is 100% why your bike is running bad. That's a large airleak leaning our your mix.

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