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RE: choices
If that's an original Elfin .15 I'd grab it for old time FF or as a collectible.
The PAW and MVVS are both quality engines and I like both. |
RE: choices
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 |
RE: choices
I always blow air through the NVA with a piece of fuel line to get an initial setting on a new engine. I close the needle then open slowly while blowing in the line. Once the air starts to get though, give it another half turn or so and you should be pretty close.
Blow through one that you're already set and you should get an idea of how much air. |
RE: choices
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 I went and moved the comp screw on my R250 and still haven't got it back to where Steve had it. <br type="_moz" /> |
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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 |
RE: choices
Undercompressed will misfire. Too rich will put out a lot of oil and, if you turn the needle in the engine will speed up. If you are using a commercial fuel, your exhaust will be between clear and honey colored when all is well. Some say clear indicates a little rich.
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RE: choices
SBOT There is an other factor here also, its harder to get a handle on this on the smaller engines .10 and down the larger ones say 40 are easier to set up Lets assume you have over primed and rich needle the engine more fuel than needed this takes up volume in the cylinder and raises the compression now its over compressed as soon as this burns off you could be in an under compressed state engine missing, lower rpms, a quick turn in on compression ( not much) maybe 1/4 -to half turn will correct this then you adjust then needle you then can do the balance for a prefect run condition yiu may need a tweek on compression and needle the needle is only going to be a half turn or so, then you get that nice hum as pointed out harsh heavy sound over compressed , missing under compressed or lean you have to find the balance between the two. and if you change props bigger less compression smaller more a tach is nice but you can hear it when set right or wrong martin Forgot if you change fuel formulas the Ether/Kero % ratios effect the above rule of the thumb more ether less compression needed less ether more compression if you lose ether from your fuel the engine will be very hard start or not at all require excess compression which an be harmful to engine, tough on crank and rod and run hot |
RE: choices
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 What Jim said. Unlike glows, diesels won't generally 4-stroke when rich, if that's what you're getting at. A better question might be, "How do I tell the the difference between an under compressed run and an overly lean run?" Both will cause misfiring. I guess I'd say that being too lean will cause a harsher misfire, whereas undercompressed is more of a crackle. If in doubt, open the needle a bit and see what happens. Don't worry, you'll work it out pretty quickly with a bit of experience. Something to watch for with your CS is that the compression is prone to unwinding when running, due to the fit of the contra and the design of the comp screw. You might need to make a locking lever out of some aluminium. The CS NVAs are also a bit suspect, and some of them don't meter properly. |
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I bought a couple of MVVS 1.5 diesels on fleabay. I like what I got BUT, both had casting flaws in the cases. one had a pin hole in the bottom of the case and the other near the intake. I cleaned them up with laquer thinner and brake cleaner, then pressure fit some JB Weld through the pinhole(s). result, 2 perfectly good engines. run like a top. nice plain bearing sport engine, with mufflers, no ear piercing scream.
The PAW engines I have, all have excellent cases-perfect. conclusion: check the mvvs engine cases very carefully before buying. I could not see any of this in the pictures on fleabay. |
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