The good thing about center plug heads are you can easily spin it in the lathe to machine & reduce the squish. I made a tool back in the 70's using an old plug & straight steel rod. Still use it once in a while. Re break in oil etc I'd be the last to ask haha. I just rebore one day & race the next I'm embarrassed to say. MarkT has it sussed with oil & jetting i see so that's good.

I agree with RT325 on "break in".   I ran the engine hard as soon as I could after just a normal warm up.  (You do want to make sure it has oil if pump condition is unknown as you've mentioned.)

I discovered this "break in" method after a late night rebuild and no time to do anything but race it the next morning.  I was running in a stock class at the time...  they had about 150 yard drag race to the first corner and normally everyone was pretty much evenly matched and got there the same time.  I had done a couple practice starts and then it was time to race...  I easily won the race to the first corner for the first time ever!  I won that race.  That engine ran strong the rest of the season and I went from mid-pack finishes to top 3. Only "mod" from before I siezed the engine from running out of gas at full throttle was the lack of a slow break in.  I've used that "brutal" break in process ever since and it's never let me down.  But this is a touchy topic and I'll never recommend you or anyone do what I do. 

Here's a site that might explain what I stumbled across by accident decades ago.  www.mototuneusa.com/break_in_secrets.htm

 

Thanks this is fantastic information. 

I think I did have it too rich in the midrange on my Yamaha and no amount of jet needle height changes made a difference main jet was great with plug chop and going one size up and down I noticed power drop and then egtd getting a bit high I think the solution would have been to change the atomiser but I never did. 

Mark do you reckon synthetic for break in matters ? 

Thanks this is fantastic information. 

I think I did have it too rich in the midrange on my Yamaha and no amount of jet needle height changes made a difference main jet was great with plug chop and going one size up and down I noticed power drop and then egtd getting a bit high I think the solution would have been to change the atomiser but I never did. 

Mark do you reckon synthetic for break in matters ? I am going to warm it up and do hard break in as that’s all I’ve ever done and I’ve never had blowby on the rings but I never thought about using synthetic oil. I’ve always just used synthetic I only now question myself reading about it. 

ar any rate if the rings don’t bed then I’ll just buy more lol 

Well I’ve started the bike over for the first time. Kicked over on 3rd attempt and idles beautifully. 

cold starts first kick with choke and have a perfectly steady idle so it seems like no air leaks. 

I noticed the autolube pump wasn’t stroking as I suspected. This pump doesn’t have a bleed wheel but it pumped the oil through the feed tube once I pulled the cable full open. checked the pump stroke and it was basically nothing. Could this be because I changed the oil pump gasket on the side cover? Or as they wear do they stroke less ?

anyway didn’t have any correct thickness shims so I now have 0.35mm pump stroke which is more than the 0.2-0.25 with throttle closed but should be ok for now will just waste oil. 

I have ordered a pack of 100 m6x10x0.1mm shims lol that should allow me to get it spot on. Will run it closer to the 0.15mm (minimum stroke) I think. 

the cam washer is not worn which is good. I borrowed some shims off my Yamaha 175 oil pump I removed some time ago and noticed the cam washer is heavily worn. 

all fun doing final adjustments. Got a few small parts to paint and then I can take it for a test ride

Wonder if I can fit the reed cage from rd350lc lol

would only leave 3mm wall thickness on the cylinder housing on each side. Would probably have to reinforce with aluminium weld but 

This image is hidden for guests.
Please log in or register to see it.

This image is hidden for guests.
Please log in or register to see it.

Probably not worth the effort so far as performance goes.
Could run a boost bottle on that inlet manifold where the ballance pipe fits for the twin cylinder.
Not sure if that's any advantage in reality.
It's a crying shame they make the manifold mount hole spacing the same on all these models but restrict the inner size so "WE" cant go play.
They realy should know haha.
If you did a bit of research on amazon or aliexpress they have quite cheap double reed setups, two up two down for various motors so if measurements are up there you could be in luck.
Ok i went looking. Here's a link to get you in the correct area i hope.
www.aliexpress.com/w/wholesale-reed-valv...productlist.search.0

I went from ct175 to dt175 reeds and boyesen two stage trees on my 175 and noticed good bottom end improvement. 

The larger reed cage does decrease inlet void and the rd350 setup has stuffers on the carb side of the reed block to encourage velocity at lower throttle openings so possibly helps a bit there. 

I am gonna put on a 28 or 34mm flat slide once the chamber is done so I will probably increase the size of the reed cage. A rd350Lc reed cage is 50x50 whereas rd350 standard is 43x40 from memory (similar to what is in the rx125 stock so increasing reed cage size might help. 


will be interesting to see will do one mod at a time. I have a spare rx125 cylinder so will keep one stock and do some porting in the other one. 

for now going to run stock. Will break engine in and do a power run on a local dyno then I’ll do some mods. Then swap fibre reed into stock reed cage first and see if difference. I like doing one thing at a time by seat of the pants then do chamber cylinder head and carb and do an after power run. If I can pick up 4-5hp I should be happy. (Aiming for 18hp which should be readily achievable. 

I don’t know if boost bottles ever really did anything. I wonder if it was just a problem due to large carburettors and low intake velocities causing excessively boggy throttle response when on off throttle. I presume as they improved expansion chambers and carb atomisers (needle jet orifice) I guess they eliminated the problem as they disappeared pretty quickly. Maybe it was a scammy thing they did in the 80s dunno I know many people still swear they work. 

some boost port work will probably help too. 

 

Those dual stage reeds are crap for power.  They do give you a neat boost in throttle response but kill top end power compared to stock reeds.

I’ve got single stage phenolic reeds to put in my rx125 from ebc. 

Reckon they’re going to be better or are stock reeds the way to go? 

I don’t love the metal reeds in theory  but I know Yamaha made them very good quality.