Beginner RD250 project questions

[mlakritz]

Hi, brand new to the forums here...So I am starting an RD250 restomod project on a '73 RD250 blending it with some superbike style and vibe. My challenges right now are trying to spec the engine upgrades that give me the best balance of reliability, cost and power, kind of in that order. My 2 stroke experience is many years with a CR250 but never cracking the cases or anything like that so i'm starting pretty blind here. I've got a good set of expansion chambers already, custom for the '73 but beyond that, do I go for a overbore kit with pistons/rods/bearings (if so, how big, 300, 350?) If i go that route, do i need to completely upgrade the transmission and camshaft as well? Can I get decent power increases without an overbore kit (with the chambers alone) and by doing some mild porting, upgrading reeds, maybe the head etc? For philosophical reasons I want to keep the costs down as much as is possible/reasonable. I want to produce something original and unique without breaking the bank. I know i'll spend more money than I intend, and i'm well aware of the mantra "speed don't come cheap" but I can start with good intentions at least. Any thoughts or tips would be awesome. Attached a pic of the bike right before I stripped it. I've had it for over 35 years and struggled with doing a stock restore or something hooligan-ish...hooligan won out easily in the end. :-)

[m in sc]

an aftermarket 350 topend is the easiest/cheapest way but you also need to find 350 heads & 350 wristpin bearings.

chambers, jetting and a really solid ignition will keep you very happy w it. TBH, fitting aftermarket replacement 350 baffles to the stock 250 pipes and the 350 jugs will make a difference as well.

there really isnt any benefit to over boring stock cyls to get power on these. just, isn't. you'll break thru the liner before it makes any difference

also, if you don't know when the crank seals were last done, plan it it.

https://www.2strokeworld.net/forum/index.php?topic=5650.0

and welcome to the forum.

[SoCal250]

Welcome!   your bike appears to be a 1974 RD250A based on the Amber Brown paint. It should have a frame and engine number starting with 352-2xxxxx. A '73 would be Butterfly Blue.

Looks like it's all stock. Just needs a new taillight lens. Do you have the carbs? I would recommend not going overboard on the mods, and keeping all the take-off stock parts. That way it can always be put back to stock if you or the next owner wants to. An original stock bike will tend to will have a higher resale value than a modded one that's been personalized.

If you want to increase the displacement you can't really do that with an overbore since you're only gaining a few millimeters of bore at a relatively big cost. Better to just install a RD350 top end on it. The 350 top end from '73-75 will both right on. There are a few threads on this site covering it. If you want to keep costs down, some members here have had good luck with the Chinese cylinders/piston kits available on eBay. A few threads on that topic here too.

EDIT: looks like Mark was faster on the keyboard and posted while I was still typing. 

[Jspooner]

What they ^^^^ said. Personally, FWIW, i would not worry about reliability. I had a very modded 350 and had no issues. If you take your time and build it right and listen to the experience on the forum you will be fine.

[mlakritz]

wow, thanks for all the quick replies. seems then like a 350 top end is the way to go, had no idea you could just bolt one on, i'll search the forums for info as suggested. to answer a couple of questions above, the head tube tag/sticker says '73 on it but I guess that must just the production date, not the model year since the numbers match what was mentioned above for a '74. I do have the carbs and the tail light lens but the slides seized from sitting for decades so i took them off to free them up. This bike will be with me til i'm dead so no need to worry about resale value or anything, the goal here is to build a fun little ripper with some fresh style that makes babies cry as it flies by. thanks again for the advice, keep it coming, i'm sure i'll have tons of questions! really appreciate all the historical/institutional knowledge here.

[RDnuTZ]

re: bolting on 350 top end on 250- Isn't there a difference in cylinder studs and/or bolts that needs to be addressed? Could be a Senior memory fart 

[Jspooner]

re: bolting on 350 top end on 250- Isn't there a difference in cylinder studs and/or bolts that needs to be addressed? Could be a Senior memory fart 

There is a difference in length, the 250 studs are shorter. I have done it without changing the studs and have not had any problems.

[m in sc]

same. 

if you want to get the 'correct' ones yambits sells them new, but it def isn't necessary

[SoCal250]

Economy Cycle sells the studs: Yamaha 90116-08064-00
It's not that costly to replace them all

[m in sc]

didn't know john had them. sweet. 

[RDnuTZ]

when swapping 350 top end onto 250 lower end, does oil pump need to be swapped as well to maintain proper mixture ratio?

[m in sc]

absolutely not. been there done it. as long as the primary gearing and clutch stay '250' it's fine. 

[Brad-Man]

absolutely not. been there done it. as long as the primary gearing and clutch stay '250' it's fine. 

Just go one tooth larger on the front sprocket or you'll be WAY wheelie-prone!

[RDryan]

re: bolting on 350 top end on 250- Isn't there a difference in cylinder studs and/or bolts that needs to be addressed? Could be a Senior memory fart 

There is a difference in length, the 250 studs are shorter. I have done it without changing the studs and have not had any problems.

same. 

if you want to get the 'correct' ones yambits sells them new, but it def isn't necessary

If I may jump into this thread and hopefully anybody else may find my experience useful. So as some of you may recall I am into this 250/350 conversion myself and to late to turn back. I have the parts required. The cheap Chinese 350 top end from Ebay along with a pair of mismatched 350 cylinder heads that I did my best to clean up.Also have the 350 wrist pin bearings exhaust manifold studs,and a gasket kit. I even have the correct 350 cylinder studs...and the old 250 top end is already off the bike.

This is where the road to hell is paved with good intentions.  With three of the 250 cylinder studs off the engine I only have five more to go. I can say for sure having measured the shorter 250 studs there's a difference of a quarter inch in length compared to the 350 studs. I hope you fellas are correct that it isn't necessary to use the 350 studs because I am having a hell of time trying to free up at least two of the studs on this engine. I've been using the double nut method with a third nut being the special nuts that bolts down the cylinders and heads, I like that better than clamping down on the stud with vise grips.   Also had to heat em up with a Map gas torch and I tried PB Blaster and I just can't get a couple of these studs free doing all that.  The one closest to the oil pump I managed to partially sing/burn the oil tank sending line, the three holed grommet it goes thru and a bit of the fancy rubber protective Vape boot routing the alternator lines.   Yeah I did make a heat shield with some metal scrap to protect most of that stuff after the fact but too late to cry over that. 

I'm gonna try to get as many of these studs out as I can but I think I may have to settle on not getting them all.   Just seems like it would be nice though as the 250 studs only seem to allow a wee bit more of three full turns with the special nuts, unless I omit using the washers?...that's probably not a good idea but it would give some of the nuts on whatever remaining old 250 studs more purchase.

Well in theory I think more threads(350 studs) to bolt down everything is better but it seems like a rabbit hole to try to get out the old 250 studs. I don't want to ruin the threads. if they will work just fine may not have a choice unless  you want a bigger project extacting studs or even ruining the upper case half?  

[RDnuTZ]

you need a stud removal tool like the 1 Mark recommended some time ago. I used to struggle with removing using Primitive Pete methods as well until I saw Mark's, so I went to NAPA and got 1 of these and they come out easy (using Blaster does help too)

https://www.jegs.com/i/JEGS/555/W83203/10002/-1?msclkid=1e0b40afb9fa13619be7275c8384e7d7&utm_source=bing&utm_medium=cpc&utm_campaign=WP_US_Bing_Shopping_555_Hybrid_Category_Catchall&utm_term=4581664973605614&utm_content=555%20Products

I used it multiple times on various sizes and it's great.