Tweaking the Jumbo

Following the first couple of trials, the Wye Valley classic trial and the 1st Sammy Miller round, the Vic Ashford Trial , it was time to look at what was working and what isn’t working with the Jumbo and make some alterations if needed.

Following the Wye Valley trial, there were a couple of problems with the bike;
– it was clear that the kickstart shaft was knackered and I must have put the bottom kickstart knuckle back on the shaft about 10 times; so much so, I gave up turning the bike off and left it runnng for most of the three laps.

With Gavin rebuilding the 320TLNW I had sold him a couple of weeks before, he noticed that his kickstart spline was also screwed, so suggested a modification. The problem is that the kickstart shaft is hollow, with the gear selector shaft running inside. Therefore, you cannot overtighten the kickstart bottom knuckle as it pinches the selector shaft and it tends to slip on worn shafts.

Therefore the solution was to put a line of weld on the outer half of the splines on the shaft, so that they lined up with the gap in the bottom knuckle, giving it something to tighten against. Drilling or trying to attach the knuckle to the shaft in someway is a little tricky. The only downside is that having a 2-3mm line of weld on the shaft makes it a little awkward to remove to the clutch cover…

Other encountered problems on the Vic Ashford trial
– decompressor cable broke, so luckily the modified kickstart shaft held up for 28 of the 30 sections.
– chain tensioner spring also broke
– leaking fuel cap
– lack of low down power, especially in first gear.

My soldering skills (or lack of them) was to blame for the cable and the tensioner was a little tired, so simply replaced the spring. Need to make another fuel cap seal, so will carve another one out of an old inner tube.

The power issue was a little puzzling, as the Jumbo has a fearsome reputation, so some further investigation required. One thing to try first, was to repack the exhaust mid box, as when it was looked at originally I took out the packing. In hindsight, this has changed the back pressue characteristics and probably contributed to the missing spurt of power in bottom. More on this after the next trial to see if it has improved the situation.

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