Large Model Association Forum

hi all,

today i mounted and ran up the first of 2 z38's, one for me and one for a friend. my friends one happened to be in the mount first and i was very impressed with how easily it fired up and ran nicely.

i havent touched the needles yet but i have noticed something that seems odd to me. on a few occations when i havent flipped it hard enough, it has bounced of compression and happily run backwards until i stopped it. is this a common trait of the 38?

cheers

nick

Yes.......they can kick back and run backwards if the starting flick isn't hard enough, but they won't throttle, you'll just get a pretty rough idle.

Phil

Phil Clark wrote:Yes.......they can kick back and run backwards if the starting flick isn't hard enough, but they won't throttle, you'll just get a pretty rough idle.

Phil

now you tell me. Just gone through a whole thread on this on RCMF... I have one which is a pussycat, the other one runs backwards 2 or 3 times when starting, and I have been through everything from woodruff key to mixture to plug....

Well, just got back from flying. Despite trying every bit of advice given to me, leaning out the slow running, setting the prop at 12 o'clock, whacking it as fast as possible, the bloody 38 starts backwards. Not just once, it does it a dozen times, then, for no apparent reason, starts the right way and runs like a sewing machine. Running out of options, the other one I have starts easily. (as did this one for four years!)

could it possibly be that the engine is actually getting too 'wet' or over primed, and then running backwards on the prime like glows do?

No, I have let it run backwards for 30 seconds, stopped it, and it still starts backwards. Going to swap the magneto for the one which works fine to see if it is something there. If I had any hair, I would be pulling it out...

I've got a z45 which doesn't run backwards but is the most spitefull engine I have ever had and I've had a few!. It kicks back and tries to get your fingers every time you try to start it, I've tried various prop positions and the fuel mixture is ok because when it starts it is fine. The prop position has always been in the same place for all of the Zenhoa engines I've had for optimum start. I've given in to it now and use a well padded glove!!

These problems sound like ignition timing faults. I don't know how you would check this on Zenoahs. Petrol engines will kick back badly if too far advanced - running backwards could be retarded. ( Early BSA Bantam motorbikes would set off in reverse at traffic lights until they fitted a long delay cam to keep the points open after TDC ).

this thing of mine does both! It bites, and runs backwards. The timing cant be altered if the woodruff key is intact, and it is. The engine runs perfectly once it starts the right way. Its just very odd....

Have you tried a heavier prop to give more flywheel effect when starting? ..........just a thought, not that I know anything about petrol!

Have you checked the flywheel - magneto gap?

Phil

Actually, no, Phil. As I said, the engine runs sweet as a nut, so not thought of that one. I shall give it a try.

John- I would only ever use wood props on petrol, this is a Menz 19x10, but changing props makes no difference.

Even if the gap is a little on the wid side, it'll run OK once going, so it's worth checking. Loosen mag bolts, fold a regular piece if A4 printer paper in half, clap this between flywheel and mag (use magnets to do the clamping rather than finger pressure) and tighten the bolts........this sets the gap spot on.

Phil

Cheers Phil. Might be a while, as I'm off on holiday, but I will report back.

I line my props up with the prop hub mounting bolts (38's and 45) then give it a hard flick, both types start no problem.


Sean.