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OS .91-P Tricky Tune

Old 10-26-2014, 11:48 AM
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kram
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Default OS .91-P Tricky Tune

I bought two of them 6 years ago for my Bf-110

Just started them 2 months ago.

One of them runs great, the other is VERY sensitive on the high-end needle

Running 12-8 MAS 3B props

Peak at 9,700 rpm, but one click richer turns only 9,100.

Then it quit in mid-flight. Fortunately, I was able to bring it back.

Set the low end a little richer, but still very touchy at high end

The other one is running OK

They're inverted, of course, and about 3/4" below the tank center

Both have new F plugs, Cool Power 4-stroke fuel

Any ideas?


Thanks,

Mark
Old 10-27-2014, 07:20 AM
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Bax
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You need to check for trash in the fuel system. It's a leading cause of sensitive needles. When you get trash in the system, the engine doesn't get enough fuel, so fuel pressure becomes lower than is needed and a little adjustment of the needle affects fuel flow a lot. You could have trash anywhere in the system, but usually it's either at the needle seat or in the regulator.
Old 10-27-2014, 02:16 PM
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Thanks

Makes sense.

I'll be able to put it all back together if I take the cover off the pump?


mt
Old 10-28-2014, 06:24 AM
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The pump should not be disassembled. If you take the regulator apart, make sure you keep the small internal parts together and in the correct order. The diaphragm may be glued to one of the covers.
Old 10-28-2014, 08:18 AM
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Thanks for sticking with me.

My review of the owner's manual convinced me not to try taking the pump apart

I did take the needle valve and the regulator apart, paid close attention to the small bits and blew them out with air and fuel

Didn't see any dirt or particles.

Now reassembled and headed for re-start

OF NOTE: while taking the engine off the plane, I'm pretty sure the needle valve (where I said it was touchy) was only a click or two open from completely closed!

Does that make sense?

With inverted engine and tank an inch above the carb, I would expect the needle valve to be tight: the other one was 24 clicks, but not ONE click!

Thanks for help!


Mark
Old 10-28-2014, 01:58 PM
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Spent two hours starting and tuning both engines this afternoon.

Other engine works fine: high needle set by rpm and smoke to about 25 clicks (3/4 turn) open

Problem engine seemed better: got the low end and the high functioning well and transitioning well, although the high end did seem a little touchy, I was thinking the clean-out was a success.

UNTIL I shut it down and counted clicks to close: 4!

That just ain't right

And, essentially, it's the same as it was before the clean-out.

Do I need to send it in?

Remember, I'm running it inverted and one inch below center tank. I am not a mechanical whiz, but I am not a novice, in fact I have 5 of these in exactly the same type set-up and this is the only one I'm having trouble with.

Maybe I should switch out one from my other 110 and see if it works OK.

Seems like a lot of trouble if I'm just gonna have to send it in anyway.

Thanks for your advice,

Mark
Old 10-28-2014, 02:02 PM
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That's FOUR clicks.

Peak woulda been 2 or 3
Old 10-29-2014, 06:42 AM
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It sounds like the needle seat has been damaged at some time. That's why the needle turns so far in when trying to lean the engine. You need a new carburetor body to fix that. The needle seat is integral to the carb body, so it's not a part you can easily replace all by itself.
Old 10-29-2014, 07:58 AM
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OK, I'm sending it to you

Not like you have time to watch this, but here's the video of my maiden only 3 days before, where both engines seem well-tuned

The left is the one acting up now

BF-110 MaidenFlight-H.264 for Video Podcasting.m4v

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