I like rock’n’roll (the genuine stuff, eg Buddy Holly, Eddie Cochran) as opposed to what many call rock’n’roll but isn’t (usually rhythm’n’blues, rockabilly). I generally dislike classical and blues, but the latter is beginning to describe how I feel……
Why? The project is going backwards and getting me down. I’ve had a real problem starting it recently, and as that usually requires a visit from my son, it means it’s not been started for a few weeks even though he’s tried a couple of times. (1,000cc and relatively high compression doesn’t make for easy starting). I’d presumed the mag was to blame, given sparks are very weak (if at all) at the sort of revs encountered during kick-starting - and so I built a 60* distributor to replace the mag. And today we tried it.
A few pops and bangs but no start. And so we put it on the rollers. Started, but only at exactly ¼ throttle, with lots of backfiring and overheating. Clearly it was running way too lean.
Confused and getting rather pizzed off, I looked up some internet comments I’d saved from others trying to tune Mikuni carbs. Seems these carbs are eccentric (as opposed to concentric). Previously she’d started fine on choke, but would not run without and so I’d upped the pilot jet from 15 to 35 (more normal for 500cc). Seems others had had the same issue – do that and it’ll never start on choke as it’s now too rich. Then there’s the quirkiness of Mikunis to consider – the pilot jet affects the entire throttle opening, including the main jet.
I also had another issue – previous runs had produced an oil pressure around 25psi (half of what I’d expected) and so I’d changed the pump from a plunger to a gear type, the latter advertised as giving twice the flow. But now I had less pressure – a measly 15 – 20psi. What was going on? I decided to remove the gauze in the tank in case that was causing a restriction, as it did appear to be rather fine.
And so back to the rollers to check oil pressure. No change, but I could set a tickover and it now definitely wasn’t lean. How, why? Do you ever feel like giving up?
I always feared Mikunis would be a nightmare to tune, but chose them because my favourite (Amal) were too large in MK2 configuration. Unlike Amals, Mikunis have a nightmarish range of jets to choose from (as far as I can see, 26 different needle jets and 15 needles, giving 390 different combinations). Add that to the fact jets overlap, the chance of getting fuelling remotely right is nigh impossible. Seems the best one can hope for is that it actually runs! Excellent, no doubt, when set up on a factory engine, but an absolute nightmare on one-offs. Tomorrow it’s another look at the oil pump.
