Depends on the specific engine, atmospheric conditions on the day and how well your driver works the throttle. Others might have a diff view, but IMO Rotaxes are very tolerant to lean jetting as far as seizures go, but very intolerant if you want the most zasp out of the engine. If I was only using one "safe" jet for my youngest sons Mini engine I would go 142, but 145 would give you an extra safety margin. If you only have a 158, I reckon it would be so rich you might as well leave it in the tool box and let the carb work through the emulsion tube! It will be cheaper in the end to buy some jets - A very rich jet will kill a £10 spark plug in 20 minutes running - buy 3 or 4 jets at £3 a go and get the jetting something like and you will already be saving by the time you need one new plug!
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