nr's blog

A sheep in wolfs clothing 1 April, 2002

Filed under: The Racing years — nr @ 6:17 pm

Monday, 8th April

Well, things are now moving fairly rapidly towards the start of my season, and my return to GP250 racing. Firstly, I’ve repainted the bike, as by the end of last season it looked like an entry for Scrapheap Challenge:
front
left
right
back
The tank is borrowed from my friend Sol, ‘cos I’ve not quite finished spraying mine yet. (It must also be stressed that that’s not my car in the rear view shot. My car is currently festering somewhere with a blown head gasket. I hate four strokes).

I’ve also spent a day on the dyno just to check the motor is up to race fitness ready for the new season. I’ve get the dyno curve scanned in at some point, but let’s just say that 64bhp is about 15bhp down on where it should be and is certainly not going to win me any races. Unless I enter the TZ as a TZR. I hate two strokes…

Tuesday, 9th April

…well, I decided to do things the methodical way, and strip each barrel seperately to see what the problem could be – so the top barrel got it last night. Here’s what I found:

  • The barrel has been timeserted in the past, and one of the timeserts has worked loose. This meant the head wasn’t sealing to the barrel properly, leading to failure of the O-ring, and ingress of water into the combustion chamber. This is obviously a Bad Thing.
  • The upper reed block is a bit of a mess. The rubber portion of it has, in the past, become unbonded from the metal part of it. (Kind of hard to describe). So, last time I put it together I tidied it all up, and used Yamabond to seal the two parts together. Only I overdid it on the gooey stuff, and some of it ran down into the inside of the reed block, where it would disturb the airflow.
  • One of the reeds is ever so slightly chipped.
  • The powervalve has been clouted at some point by a ring, so was a bit ragged on one corner. Not much I can do about this, so I just dressed it with an oilstone to remove the sharp bit.

So, would this account for being 15bhp down? Probably not, but it’s certainly not going to help matters. I’ll get the thread(s) reclaimed, re-assemble the top cylinder, then start work on the bottom one.

 

Later that same day…

Well, looks like the barrel damage is a little more severe than just needing a re-thread. Firstly, there’s not enough metal left in the thread to be able to just add another timesert. So, we’ll probably need to drill out the remains of the old one, fill the hole with weld, and start again. Something I don’t really want to do, as it will weaken the barrel.
Secondly, there’s another crack appeared between the exhaust and transfer port. This is irritating, as I only had it repaired at the end of last season… Still, the chaps at Cambridge Motorcycles have agreed to see if they can get it repaired again free of charge seeing as it really shouldn’t have cracked again

All this leaves me in a bit of a dilemma. Do I carry on with the same barrels or get my spare set tuned and use them instead? In theory it would be a Good Thing to have the spare set tuned anyway, so I can use them in an emergency without having to worry about it at the time. Then again, the only way that would be of benefit is if I get my main pair repaired as well.

I’m developing a phobia of internal combustion engines now…

Thursday, 11th April

Well, I’ve had a good look at the options, and decided the first thing to try is a run with my spare barrels to see what kind of effect they have. On first glance, it would appear they have already been tuned anyway, and I have to say, they went on with a minimum of fuss and the squish came out at .65mm on the bottom cylinder and .68mm on the top. Which was suspiciously good. The pistons are of unknown vintage, so if it survives the dyno run (which is by no means a racing certainty…) and makes more power, I’ll put some new ones in there before I race it.

The top reed block has been cleaned up, and now I just need to find some yamabond to put it back together again. I guess this is a long shot, but if anyone out there knows where my tube of yamabond is, can they let me know please? Ta.

The powervalves have been aligned as perfectly as I can manage in my garage. I suspect this will always be a weak point with these motors, as the whole arrangement of pulleys, cables, cranks, fulcrums (fulcra?), levers and motors is just designed to go out of alignment as soon as you start it. There are times when I wish I rode a Honda. Then again, their system is prone to going out of alignment too, so it’s not solely a Yamahaha thing.

Perhaps I should take up pigeon racing. I’m sure you don’t have to worry about these things then.

Thursday, 25th April

My bike is for sale

 

 
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