Looking for performance

The main problems with bike were:
– the main mounting for the exhaust was missing and the combination of cable ties and lockwire wasn’t really doing a good job.
– the engine was running, but there was no low-down power at all and getting off the line was becoming a bit of a joke. Some playing with timing and carb didn’t seem to be doing the job.

So, prior to a trip to Belgium I spent a couple of hours stripping the bike down and prepping another frame, with a complete exhaust mount. Sprayed the new frame as before with aerosol black Hammerite Satin which seems to do a good job. Didn’t strip the engine, as I already had a spare engine following on from a rebuild during last season, so had one on the bench so to speak.

After a week in Belgium (good trip over on the KTM), back in the UK for the royal wedding and more importantly a whole day of bike rebuilding. A few issues needing resolving as a rebuilt the bike:

– steering head bearings, needed a new set
– clutch arm, during the strip I’d seem to have misplaced the connection from cable to clutch arm. Faffing with this and nicking one of the one of the TY250’s took about 1.5 hours, which is a pain.
– exhaust, the mounting of the silencer etc

Checked the compression of the old and the replacement engines, with old showing 75 psi, whilst the new 125 psi.

Existing engine, reason for poor performance

Removing the cylinder head and the barrel from the used engine revealed why there was loss of compression and power. The piston had cracked from the skirt up towards the rings (there are two rings on this piston). Might account for the problem getting from the line, but amazed that it was a least a little competitive and that it hadn’t completely broken up in the engine.

Starting up the new engine, it had slightly more than just a nice top end rattle with a metallic knock also. Though the bike ran okay and seems to have some power, decided not to continue the running in until I’d had a chance to resolve the noise.

Wire insert kit

Going through the piston and barrel selection (this new piston and barrel are 0.25 oversize), gave me a couple more options, but decided to stick with the existing piston as this had a new conrod, little end etc. Tried a different barrel (which seemed to a closer fit), but which needed more faff time to get ready as someone had butchered the exhaust bolt threads. Out with the new 6mm wire thread inserts, but took a couple of attempts, as the threads needed to go deeper than the tool would allow.

Used the same piston, ring, little end and gudgeon pin, the latter two of which were new…but same top end clatter. Time for a think and for a barbeque.

The thinkended up carrying on for another week and a trip to Madrid and Antwerp and with just the afternoon to go before the meeting at Abbeycwmhir decided to have another look. Some inspection of the top end components by neighbour Gavin identified the problem as being a worn gudgeon pin and a quick 15 minutes later, then engine was running again, though still with a slight knock. Also changed to another 0.25 piston and a fresh ring.

End can

Some further fettling and modified the exhaust rear can as it seemed to be restricting the engine. A quick run round the wet field seemed to indicate that there was some power and response. The carbueration was a little off, and I retarted the timing slightly and all seemed okay.

Will see what happens at Abbeycwmhir tomorrow….

Leave a Reply

Your email address will not be published. Required fields are marked *