quote:

ORIGINAL: steve111

Getting back to your question Oscar... . Youll probably find a few different opinions on diesel tuning, but there are some good general principles. You should be looking to run the minimum compression necessary to get an adequately smooth run. As far as the needle goes, I find a peaked setting (any leaner and it will misfire), and then back it off a few hundred rpm to give it a bit of fat for unwinding in the air, manoeuvring,etc.

Something worth mentioning is that a traditional contrapiston shouldn't be treated as something to be constantly wound up and down and messed with unnecessarily. If you do this it will wear and become loose. Obviously it's meant to be adjusted or it wouldn't be there, but try to keep it to a minimum once you've found a good running setting. (Steve Rothwell, who knows a bit about model diesels, has engines on which he hasn't adjusted the compression setting for years).

When starting, I usually open the needle a turn or so, but try to leave the compression where it was. With a choke or two and maybe a judicious prime to the side of the piston, the engine should start. Some engines may need to have the comp adjusted to get them going (particularly if a muffler stops you priming the exhaust), but even then it shouldn't be more than a quarter of a turn or so.

Once running, the engine may be misfiring, but as long as it keeps running, I leave it alone for a bit rather than making a lunge for the tommy bar. Remember it hasn't warmed up yet, and the misfire will likely reduce as the engine heats up. If you gradually wind the needle in, you'll hear the engine speed up even though it might be misfiring. If it stops speeding up and just misfires worse, then you're a bit too lean. With a bit of running and a leaner mixture, the engine will have warmed up and may well no longer be misfiring. If so, great. Otherwise, you can increase the comp just enough to get rid of the misfire, and maybe readjust the needle if necessary. Remember that the engine will need a minute or so to warm up, so there's no need to rush.

So in summary, I'd recommend: just enough compression, mixture a bit rich of peak, and don't mess with the comp too much. I'm sure there are those who are much more knowledgeable than I am who will have more to add, but hopefully this will be of some help.

Steve

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ORIGINAL: SBOT

Steve,
Thank you, That is just the info I was looking for. One question though. How do I tell the difference between an under compressed run and a overly rich run.
Oscar