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Here's another trap that catches some, especially those that tune on an inertia dyno. Here's how it goes: you've been tuning your bike on the inertia dyno and it's making good power, more than it has ever made. All the signs show the mixture and timing are both on the safe side so the next weekend you head out to the track full of confidence. It leaves the line hard and pulls like a train - or at least it does until you click it into top and feel that heartbreaking sensation of a piston sticking. What the hell happened, it was on the safe side on the dyno? Why did it grab?
What happened is this: methanol likes a rich mixture to start from cold, requiring more enrichment than petrol. No problem, you've learned to give it plenty of fuel and it fires up instantly. You sit on it and back the mixture off as you warm it up on the dyno for half a minute or so, the engine sounding clean and crisp as you gently blip the throttle. After the brief warmup you click it into gear and commence the dyno run, which being on an inertia dyno is over after maybe 4 or 5 seconds. Power is good, there's no sign of leanness and everyone is happy.
Why then did it seize at the track? While you were starting and warming up the bike only a very small percentage of the fuel was vaporising and being burnt. The rest was collecting in the crankcase and forming an ever larger puddle at the bottom. Because you like to be a bit gentle on a cold engine you weren't revving it hard and the puddle remained in place. Once the dyno run commenced though and revs increased the puddle was stirred up into a mist that passed into the cylinder and was burnt. That mainjet that you carefully selected after multiple dyno pulls wasn't the only source of fuel - a significant proportion was being drawn from the crankcase pool. And because the dyno run was so brief it was all over by the time the puddle had been completely consumed.
Out on the track though it wasn't so good. It started out well but after a short time the puddle was all gone and then the only fuel supply available was from the mainjet - which the engine clearly showed wasn't enough on its own. How do you manage this? I think the main thing is to be aware of the possibility of this happening and being the cause of unexpected lean-outs. Try to make your tests long enough to eliminate the supplementary fuel effect. Don't let the engine idle for too long and lean the idle mixture out as much as you can and as soon as you can after startup. Don't be surprised to find that the idle fuel can be shut off completely for quite a while before the engine starts to lean out - the crankcase fuel builds up quickly and takes a while to be used up. And be wary of an engine wanting a suspiciously small mainjet - it might suggest it's getting fuel from somewhere else.
Another factor to consider is temperature. Methanol's cooling effect is well known and that makes it a very good choice for engines like the big air-cooled Buls that are normally badly undercooled when the horsepower level is increased significantly. But there's more to it - methanol is also very sensitive to temperature and needs large adjustments to the A/F as temps change. It's much more sensitive than petrol in this respect. It also requires a different way of thinking - with petrol we focus on keeping the engine as cool as possible to maximise VE and power. Methanol on the other hand likes some heat and a heavy load to really burn well and of course we have to think of the pipe too. It's not unusual to be in the situation where the challenge is not in cooling the engine but in getting enough heat into it. We need to get some heat into the motor (especially the head) quickly.
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