75' RD350 chain swap to 520..

[RDnuTZ]

If it was me, I'd get the exhaust + intake sorted and a basic state of tuning to use as baseline before messing with anything else. Always solve for 1 variable at a time and build off that. On the clutch grabby thing, an old dirt bike trick we use on bikes sitting for years is oil change using detergent ATF to break loose clutches. Fill, fire it up and just sit there in neutral exercising clutch in and out until you feel clutch effort change. Then ride it around a bit still exercising the clutch. Get it good and hot, then drain ATF and see what it looks like. If dirty, repeat process. If relatively clean fill back up with your regular oil and try it out. About 50% of the time the clutch works normal with no slipping or grabbing. YMMV 

[grcamna3]

I see mentioned here about other things to go from 250-350 top-end swap,which leads me to two questions: will the original RD250B 'tall slide' carbs(one guy local to me said 250 carbs are 26mm,but the RD350 carbs are larger at 28mm)work ok with the RD350 top-end, & are changing all the cylinder studs out from the 250 up to the OEM 350 studs a good upgrade ?

[m in sc]

rd250 carbs are 28mm. ds7 carbs are 26mm. (wrong 250). the jetting is obviously different but the carbs themselves will work just fine.

[SoCal250]

I see mentioned here about other things to go from 250-350 top-end swap,which leads me to two questions: will the original RD250B 'tall slide' carbs(one guy local to me said 250 carbs are 26mm,but the RD350 carbs are larger at 28mm)work ok with the RD350 top-end, & are changing all the cylinder studs out from the 250 up to the OEM 350 studs a good upgrade ?

The 73-75 RD250 and RD350 carbs are the same, both are Mikuni VM28. Yamaha changed the design in the middle of the 1974 model year, going from a low-top design to the high-top design. This was done "to improve the waterproof design of the carbs and to prevent slide sticking". (See Yamaha Tech Bulletin M4-061)
The switch to the high-top version affected all bikes built from the following engine numbers
RD250A 352-206520 and RD350A 351-213364.
No reason to change out your carbs.

Cylinder studs have been discussed on here recently. You can swap out the shorter 250 studs for the longer 350 version 90116-08064-00. Technically it's the proper thing to do but it's not required.

[RDryan]

If it was me, I'd get the exhaust + intake sorted and a basic state of tuning to use as baseline before messing with anything else. Always solve for 1 variable at a time and build off that. On the clutch grabby thing, an old dirt bike trick we use on bikes sitting for years is oil change using detergent ATF to break loose clutches. Fill, fire it up and just sit there in neutral exercising clutch in and out until you feel clutch effort change. Then ride it around a bit still exercising the clutch. Get it good and hot, then drain ATF and see what it looks like. If dirty, repeat process. If relatively clean fill back up with your regular oil and try it out. About 50% of the time the clutch works normal with no slipping or grabbing. YMMV 

Wow,that is very interesting. I actually have almost a gallon of ATF on a shelf,tempted to do this

[grcamna3]

I see mentioned here about other things to go from 250-350 top-end swap,which leads me to two questions: will the original RD250B 'tall slide' carbs(one guy local to me said 250 carbs are 26mm,but the RD350 carbs are larger at 28mm)work ok with the RD350 top-end, & are changing all the cylinder studs out from the 250 up to the OEM 350 studs a good upgrade ?

The 73-75 RD250 and RD350 carbs are the same, both are Mikuni VM28. Yamaha changed the design in the middle of the 1974 model year, going from a low-top design to the high-top design. This was done "to improve the waterproof design of the carbs and to prevent slide sticking". (See Yamaha Tech Bulletin M4-061)
The switch to the high-top version affected all bikes built from the following engine numbers
RD250A 352-206520 and RD350A 351-213364.
No reason to change out your carbs.

Cylinder studs have been discussed on here recently. You can swap out the shorter 250 studs for the longer 350 version 90116-08064-00. Technically it's the proper thing to do but it's not required.

I prefer to change the cylinder studs to the longer 350 part# you posted.
Will all 8) be the same part # ?

[SoCal250]

I prefer to change the cylinder studs to the longer 350 part# you posted.
Will all 8) be the same part # ?

Yes, 8 pcs. A stud extractor tool will make removal easier. There are some discussions on here about it.
Some examples:
https://www.summitracing.com/parts/wmr-w83203
https://www.summitracing.com/parts/lil-71200

[grcamna3]

I prefer to change the cylinder studs to the longer 350 part# you posted.
Will all 8) be the same part # ?

Yes, 8 pcs. A stud extractor tool will make removal easier. There are some discussions on here about it.
Some examples:
https://www.summitracing.com/parts/wmr-w83203
https://www.summitracing.com/parts/lil-71200

Thank you very much for this information.
I'm going to start collecting parts(as I locate them)to do the swap,and then go for it.
I'll run it as a stock 250(including stock gearing and chain,for now)after I go all through the list of things I would normally do(including the full 50 yr. old electrics cleaning and servicing)when buying a new-to-me bike:by the book.

I plan to completely do the front-end next,including tapered steering bearings and .60Kg Race-Tech straight rate springs.

[grcamna3]

you can run like 100 hp thru a 520, rd isn't going to do anything to it. fwiw john at economy cycle sells the whole kit specifically for an rd

Ok Mark. 

[Yamanatic]

Just an aside, 1970's TZ250/TZ350 sprockets are 520 and some TZ rears are drilled for 4 or 5 stud mounting - TZ countershafts use the same spline as RDs. The alloy rears are very pretty!


 
WW

[grcamna3]

Just an aside, 1970's TZ250/TZ350 sprockets are 520 and some TZ rears are drilled for 4 or 5 stud mounting - TZ countershafts use the same spline as RDs. The alloy rears are very pretty!


 
WW

70's TZ sprockets compatible? Ok 

[Vincenthdfan]

Does anyone make steel rear sprockets for 520 chains on the RD's?  All I could find when researching this was aluminum, which I dont care to run based on past experience.

[grcamna3]

Does anyone make steel rear sprockets for 520 chains on the RD's?  All I could find when researching this was aluminum, which I dont care to run based on past experience.

I prefer steel on my RD road bike also.

[Vincenthdfan]

Does anyone make steel rear sprockets for 520 chains on the RD's?  All I could find when researching this was aluminum, which I dont care to run based on past experience.

I prefer steel on my RD road bike also.

Ever found any for RD's?

[grcamna3]

Does anyone make steel rear sprockets for 520 chains on the RD's?  All I could find when researching this was aluminum, which I dont care to run based on past experience.

I prefer steel on my RD road bike also.

Ever found any for RD's?

I haven't found any yet.