Start the flight in a wet 2, and before you know what happened it's leaned out and scorching through the pattern like the police are after it. 11x4 or 11x5 prop seems to keep it happy, but I suspect it's in a runaway condition.
This works for LA 46's, and (in my miniscule experience) with Tower 40's and the like. It leaves stuff out, and I've never had an FP 40 to try it on.
Tach it before you launch. If it runs away (yes, going to lean and "scorching through the pattern" is running away), then try launching it slower (by richening it up). My biggest problem with just doing taching and tweaking is finding a needle setting where it'll stay in a good 2-stroke, without either staying slobbery rich for the whole flight, or going into runaway. If you can't find a needle setting where it'll reliably stay at a good RPM, then try the following:
- More castor in the fuel (you should be running about 22 percent total oil, with at least 10% castor).
- A smaller venturi (or pile filters made of pantyhose over the venturi that you've got -- that works well, is adjustable, and filters are always good)
- Make sure the glow plug is hot, and in good condition
If it were my engine and that didn't work, my next step would be to ask here.
If it
does work, then once you've established a baseline combination of fuel/prop/plug/venturi that works, play around with different props until you're really happy.
Do a search. I can't remember for sure, but if I recall correctly the FP 40 is an OK stunt engine if run right (in a wet 2-stroke) but the 40 LA can be a real bear -- and even then, there are folks who are happy with the LA 40. Someone will correct me if I'm wrong about the FP 40 being better than the LA 40.
So, having said that, how would one go about "retiming" this engine? Are we simply removing metal here and there, or does it require degree wheels, micrometers and machine shops and such?
Machine shops simply remove metal, precisely.
I've never retimed an engine. The closest I've come is to build a new cylinder for a Cox 09 TD, which wore out about 15 seconds after it was broken in (next time I'll be more diligent about getting all the lapping compound off the parts!). Some claim it can be "simply" done with a Dremel tool, and they may be right. But whether you're filing on it by hand or using a milling machine to do it super-precisely, you have to know how to not damage the plating on the liner, and how to restore the piston-liner fit when you're done, or all you'll end up with is a horribly scored-up P&L that doesn't work worth a damn.