November 20, 2011

Small tribute to my Tzr250 engine

Well, I'm parting out my 1kt engine on evilbay and I've been comparing some old pics with recent ones. Many memories keep coming through...

This is how about it all started.

And some of the mods I've done: 

Rough marking at the very beginning while trying to flow the crankcases and open up new passages from the reed valves directly to the transfer ports. 
Ported cylinders, from memory the timings were about 198 degrees for exhaust duration and about 130 degrees for the transfer ports. All passages were flowed and port angles were altered. Two auxiliary exhaust ports have been opened up that had a timing of about 202 degrees and the exhaust valves were modified to work in conjunction with the new ports. 

Lightened flywheel that eventually decided to crack after a few years.
Reprofiled cylinder heads. Higher compression and I've converted the stock hemi-spherical design to the TZ250 taper profile. 
In the mean while, I have tried quite a few carbs on this bike and every pair worked great. 
I believe that the worst pair I ever had on this bike, was the stock Mikuni 28mm carbs. 
Then fitted a pair of Rgv250 vj21 32mm carbs, very simple and straight forward conversion, way better than stock but still not that great. 
Next pair was again from a Rgv250, this time 34mm Mikuni carbs. Very good results but still kept looking for more... and eventually ended up with a pair of Lectron 36mm. 
This was one of the biggest improvements I've made to this bike. Excellent results from the very beginning and unbelievable throttle response. 
A Lectron carb, due to it's unique design allows for up to 45 degrees positioning to the manifolds that allows for a completely straight inlet to the reed valves. 
This way, a common carb would have flooded badly due to the existence of a pilot jet (Lectron carbs use only the needle as a fuel metering device). So, I made a couple billet aluminum manifolds to accommodate the bigger carbs.

Lectron carbs were modified as well. I've grind the stock power jets and fitted handmade ones that are externally adjustable. 
And the last serious modification and also my favorite one, was the fabrication of expansion chambers.
Even though most of the things were surrounded by a low budget theme,  tons of rewarding experience has been gained through this project which would have been impossible otherwise. Lots of mistakes as well, but as mr. Phelps once said "The man who makes no mistakes does not usually make anything". 

Have fun, keep the two strokes alive and kicking. 


  1. Rip TZR...keep up the good work mate !!!

  2. is that a torque spacer behind the billet aluminium manifold? if so what is the gain was?

  3. Nope, it is just a stock spacer that is polished out.

  4. My TZR250 story:-
    All true, that's why it's a bit boring, so I'll write it in brief note form...
    1/ Working as foreman mechanic on Citroens in 1987. Bought a nearly new Citroen BX with duff engine and used company resources to make it saleable. Sold it and quit my job.

    2/ Went to Collin Appleyard Yorkshire the Yamaha dealers and bought brand new last 3MA? model TZR 250 from proceeds of selling the fixed BX car.

    3/ Put an advert in UK Motorcycle News early 1990: "Breaking Brand New TZR 250 all parts"

    4/ Was an overnight success. Really. Almost. Prob. was I was broke so I was sending out all of the parts via a courier with a prepaid envelope from courier so buyer could put money in for parts when they arrived. 100% honesty. All money from 1st advert comes back two days later.

    5/ One guy building a 'special' wanted front-end, tank, wheels etc. Posted them all in a massive box and waited. And waited. Part were 'lost' (I'd sent these parts via Royal Mail but couldn't afford postal insurance).

    6/ Six weeks later, I tracked parts down to a British Rail Depot in Crewe, England:
    'We've 'ad your box for ages, mate. We're using it as A TABLE to eat our dinner from!!!!

    7/ After more stress, I eventually got my TZR bits back.
    Chickened out. Bought the parts I had sold, as used parts. Put the bike back together.
    Sold the bike to a dealer. Entire enterprise, I just about broke even.
    BUT before selling this cursed machine - or so it seemed - I had a 380 mile 'road race' via M6 and other motorways on the very best bike I have ever owned/ridden.
    In conclusion, I was one of the very first people who started buying brand new motorcycles and breaking them for spares. Even a TZR 250 has an endless amount of parts. The new price was about £3,000 in 1999 for a new-old-stock bike. The potential profit for selling those parts is about eight-fold. At very cheap prices. There's just so many bits.
    Only trouble nowadays is that everyone is doing it.
    Thanks. Sorry if it was boring, but it's true. I have never been as scared in my life when that box went 'Missing' with no job, no money :-)