nr's blog

Pump It Up 27 March, 2016

Filed under: Motorcycling — nr @ 3:35 pm

I’m always a bit nervous taking a recently rebuilt bike for it’s first MOT. Firstly, is it going to break down and leave me stranded somewhere in the middle of The Fens, within earshot of banjos playing? But, more importantly, will the MOT tester take one look at it, and laugh? I normally try to mitigate the effects of the second of these problems by taking it to the splendid chaps at Cambridge Motorcycles. I know for a fact that Spike will laugh at the fact that I’m still riding a 30yr old two-stroke that’s cobbled together from bits that that I found on eBay, and all held together by cable ties, and ancient Whitworth fasteners and plumbing fixtures. Whereas Phil will just laugh at my hair. So, that takes a lot of the uncertainty out of the process.

So, Project TZR was tentatively fired up, and ridden to Cambridge a few days back for the inaugural MOT. And it passed. It’s not perfect – there’s some play in the rear suspension (my bush is slack and floppy. Something that I guess comes with age), the tyres are down to the legal limit (already knew about this, so I have a pair of Bridgestone S20 Evos in the shed waiting to be fitted). And there was something else, which I’ve completely forgotten about now. So, I’ll get some bushes machined out of bronze to rejuvenate my bouncy bits. The standard Yamaha bits are a) no longer made, and b) made out of plastic anyway, which is why they wear out so quickly. And the tyres will be thrown onto the rims when I get the chance.

But, more importantly, how did it ride? Well, let’s get the good bits out of the way. The brakes are easily good enough to overcome any last vestiges of friction afforded by the ancient front tyre, but also have plenty of feel. Lovely. A lot of people look at TZRs with the single front disc and assume them to be underbraked. Not at all – I reckon they’ve got a near perfect balance of feel and power. The handling, despite the aging and saggy bush, is actually OK. I reckon there’s still some damping left in the shock. Which is a shock, ho ho. I didn’t want to push too hard though, as it was damp, and remember what I said about the less than grippy front tyre? The rear is worse. But still. All felt OK, and to be honest, I didn’t even notice the clonky rear.

The engine, on full throttle, is glorious. Pure two-stroke pleasure. On part throttle though… oh dear. Not quite unrideable, but not far off. Hunting and surging on a part throttle, the only way to get it through town was to constantly accelerate up to 30mph, then coast down to 20mph and start the process again. If you were caught behind me, I’m sorry. It must have been infuriating. So, where to look? first thought was needles, so I’ve ordered a new pair of needles and emulsion tubes (and a few other jets and whatnot) to refresh the carbs. Basically, every jet and gasket for both carbs, for £25 including postage. Bargain, if they’re any good. But also I invested in a leakdown tester, as I had a feeling that an air leak would also cause these kind of symptoms. Leakdown testing is a simple theory really. Whip the pipes and carbs off, plug up the resulting holes, pump some air in there and see where it leaks out from. The leakdown test kit I bought is a brilliant example of 3D printing. Super, and it just fits perfectly. So, I plugged everything up, and started pumping with an old bicycle pump (you don’t want any more than about 6PSI). Normally, you want the cases to hold 6PSI for 6 minutes. Mine were leaking air faster than I could get it in there. This was easy to find:

P1050861_arrow Those two red arrows point to splits in the carb rubbers. I think I know what’s happened – someone has used that balance pipe to pick the engine up before. I’ve done it myself on one of my old racebike engines, and had exactly the same splits appear. I don’t really want to buy new carb rubbers, as they’re expensive, and very hard to find. So, I ran some cyanoacrylate glue into the split firstly, as it’s good at that kind of thing. I then used some Loctite sealant which is more flexible to cover the split. A couple of days later I retested, and it’s now holding pressure.

So, still a few things to do, like the tyres and rear bushes. But more importantly, give it another whizz up the road to see if the part throttle fuelling has improved. I’ll wait until the carb jet kits turn up, and do that while I’ve got the carbs off. Then I’ll put it all back together, and go and make some smoke around The Fens. And those banjo players will have their work cut out to keep up.

In other news, I’ve been helping Sol put together his project Katana. This is really the antithesis of the TZR, being a big bruising fourstroke. Good fun though – pop over to for a look. It’s going to be brilliant when it’s done.


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