RD 250

[forexer]

a co-worker of mine was telling me he bought a 1975 RD250 brand new and its been sitting for years. He also said that he has tried taking it to several local cycle shops and some have tried to get it running but failed and the others won't touch it. It will run but not well enough to power the bike down the road.

I volunteered to take a look at it for him to see if I could help him out. I've never worked on a RD250 however I'm assuming there very similar to the RD400. Since I don't own a service manual for the RD250 motorcycle I was wondering if some one could please let me know what size the stock main and pilot jets are for the carbs and also what is the correct timing number BTDC for today's fuel.

Thank you.....

[m in sc]

similar yes. here's some general specs, for a 75RD250 but pretty much the same:

reply  #6 courtesy of socal:

https://www.2strokeworld.net/forum/index.php?topic=40.0

however, 1972 should be a ds7, rd250 wasn't introduced until 1973.  so, make sure its a reed valve motor to be an rd, and the frame/engine prefix should be '352-"

IF not, the engine prefix will be DS7 if its a ds7... then specs are different etc. 

[Striker1423]

Stock timing was 2.0mm btdc.
Typically we run 1.8mm for today's cornsoaked fuel.

Pilot jet depended on the year. But, assuming it's actually a 1973 RD as a 72 would have been a DS7.

You need to look at the air filter. If it's a baffles style the main jet is a 90 and a 25 pilot. If it's not baffled or open it's a 120 with a 30 pilot.

1 3/4 turns out on air screw was stock.

[forexer]

Thanks for the replies and sorry for the confusion I got the dates mixed up - the bike's definitely a 1975 RD250 - and not a 1972 as previously posted before editing....

I'll also check the air filter to determine which jets to install - Thanks again!

[m in sc]

75 -should- be baffled stock.

[Striker1423]

75 -should- be baffled stock.

True. I think that's if it got the latest service bulletins back then. Mine was actually unbaffled FWIW. Weird cause everything about it looked original. But, neither here nor there. Air filter will be the tell.

[m in sc]

look int eh tech library, i just scanned my chiltons manual carb spec sheet for yamaha 2 strokes. ironically, doesnt have the leaner jetting for baffled air filters in it.  but the rest of the info is really good.

[forexer]

Stock timing was 2.0mm btdc.
Typically we run 1.8mm for today's cornsoaked fuel.

Pilot jet depended on the year. But, assuming it's actually a 1973 RD as a 72 would have been a DS7.

You need to look at the air filter. If it's a baffles style the main jet is a 90 and a 25 pilot. If it's not baffled or open it's a 120 with a 30 pilot.

1 3/4 turns out on air screw was stock.

I will definitely check the air filter next week when the bike arrives
What size jets does the Chilton manual call for?
Striker suggested using a 90 main and a 20 pilot
and the attached link provided by Russ for the 1975 shows a 120 main and a 30 pilot

[forexer]

EXCELLENT!

Thanks Mark - I must of had a brain fart because I just saw your post where you loaded the Chilton's manual info into the tech library 

FYI: the main jet and pilot jet sizes for the RD250 matches the link SoCal posted   

https://www.2strokeworld.net/forum/index.php?topic=40.0

( reply  #6 )   Mains = 120  Pilot = 30  Question has been answered - Thank you!

[Striker1423]

The jet change was a technical bulletin from Yamaha to reduce the intake reverberation that typically hit the riders ear drums the entire time.  So, Yamaha came up with a baffled air filter to redirect and dampen some of the sound. This air filter restricted flow and required a down-jetting of the carburetor or the bike would run too rich.

[m in sc]

yup. he's right. my take on this when I've run across it a few times was to dremel the baffles out of the air filter then jet to the older settings. but thats just me. .02

[forexer]

Compliments of Mr. Striker - for those who may not know what the difference is

The baffled filter was a paper filter that had internal baffles like this.
https://images.app.goo.gl/MZQDWJYZYLpv6JAc6

If the center portion of the filter is open all the way down then it is NOT baffled. Like this.
https://images.app.goo.gl/r5zCnQmFeZWirGXz5

[SoCal250]

Just as an FYI...
The jetting & filter change was a mid-year update for the 1975 model year. All RD250Bs built from engine number 352-304783 got the baffled filter and corresponding jetting.