stunthanger.com
Engine basics => Engine set up tips => Topic started by: Tim Wescott on September 17, 2010, 07:42:56 PM
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Well, that was a fun hour.
It's a 7/32" bore, on an OS Max 25 (not FP or FS or any nonsense -- just a cross-flow OS Max).
Things calculated out to needing something a hair bigger than 7/32 -- I'm going to finish this one, then make one with a 15/32" bore, and see what I can see.
Spraybar will be 1/8", or a hair under. I was kinda surprised -- the carburetor hold-down screws are obviously M3, I expected smaller holes. Note that I'm not complaining. I just got a 5-44 tap & die set from McMaster; the thread's probably a bit coarse but I'll find out! If the needle's too hard to set I'll try scheming on a finer thread (there are M3 x 0.35 tap & die sets out there, but they're spendy) or I'll experiment with less slope on the needle itself.
I fantasize about making a #5-64 tap -- but that's probably best left a fantasy.
We'll see how it works.
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Uh---------Tim-------uh---- Do you REALLY mean 15/32s??? Or perhaps 15/64ths? I would think that 15/32nds would be more than just a "tad" larger.
Bigiron
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Uh---------Tim-------uh---- Do you REALLY mean 15/32s??? Or perhaps 15/64ths? I would think that 15/32nds would be more than just a "tad" larger.
Well I'll just open up the crank to match this venturi and -- oops!
Yup -- 15/64ths. I want a drill index full of number and letter drills for Christmas.
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Back in the 60s, in the combat circle I saw a Johnson with the venturi bored out to paper-thin walls, and crank port opened up to match. It looked like about 15/32"! On bladder pressure of course. I didn't get to see it run, just bite the dickens out of anyone who tried to start it.
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It's done! It runs! (well, at least it got through an ounce of fuel -- more to come). Now to run some more fuel through, just to make sure. This is an engine that I bought at a rummage sale in the early 1980's, along with the all-styrofoam RC Cub to which it attached, and ran just once or twice since (mostly long enough to find out that I couldn't readily transition to a transmitter that used one stick for elevator and another for rudder -- the plane had a short life).
The engine just got the whole crock-pot/antifreeze/scrub off the residue treatment, and a brand new venturi, of course. I'm not all that experienced at engine teardown, but it looks to me like it's probably broken in but not at all worn -- which should be perfect.
And I saved what -- $15? $20? For only four hours of work!
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Back in the 60s, in the combat circle I saw a Johnson with the venturi bored out to paper-thin walls, and crank port opened up to match. It looked like about 15/32"! On bladder pressure of course. I didn't get to see it run, just bite the dickens out of anyone who tried to start it.
Hot rodding engines is great fun. It's even more fun if they actually work when you're done.