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Author Topic: LA .46 rear needle valve  (Read 1127 times)
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montmil
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« on: February 24, 2012, 08:16:11 AM »

I'm slowly returning to my C/L youth with a Sig Chipmunk. I'd like to use the LA .46 as a powerplant. My wonder is about the rear needle valve / plastic rear plate; it gets the occasional bad rap. Is this warranted or just someone's $0.02?

Monte
old c/l vet - born again newbie Roll Eyes
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Monte Miller
Denton, Texas
greggles47
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« Reply #1 on: February 25, 2012, 12:06:01 AM »

Hi Monte,

I've only ever used a remote needle on a Magnum 15. I couldn't tell the difference except my finger were much safer.
People say there is some lag between making tune adjustments and the engine responding. I never noticed any difference.

I've never used a motor with a plastic pb, so won't comment.

The real test is whether you are happy using them. They should not affect how the model flies.

Regards

G
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piper098
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« Reply #2 on: March 20, 2012, 10:56:14 AM »

Monte,
I have ten LA .40 engines and six LA .46 engines. Over the last 10 years I've used these O.S. LA .40 and .46 engines in profile stunt planes ( i.e. SIG Twisters, SIG Banshee, Brodak Pathfinder, Brodak Cardinal) and in a full fuselage stunt plane (i.e. Buccaneer 46).  These LA engines have powered the planes that I used to win numerous trophies in AMA sponsored control line stunt contests in the midwest. Therefore, although I may not be an expert, I do have years of successful experience using these engines.
Based on my personal experience:
PLASTIC BACK PLATE. I have never had a problem with the stock plastic back plate. HOWEVER, I always remove the back plate attachment bolts and install washers under the bolts to increase the contact "foot print" with the plastic back plate. I am also very careful to not over tighten the back plate bolts. A sure way to get a back plate air leak, and the resulting crummy engine run,  is to over tighten the back plate bolts and crack the plastic.
REMOTE NEEDLE VALVE. I have replaced all the remote needle valves with needle valves through the venturi. I never experienced the lag in response to needle valve changes that some complain about. However, the remote needle valve arrangement did cause me problems.
(a) When starting the engine I put my finger over the venturi and turn the prop over to draw a fuel prime into the engine. You can see the fuel moving through the transparent fuel tubing into the engine. However, as soon as I would remove my finger from the venturi you could see the fuel flow back out of the engine. So when you would try to start the engine by hand it wouldn't start because it didn't have a sufficient fuel prime. Guys using electric starters could supposedly just grind away for 30 seconds to get fuel drawn back to the engine. But at contests you want a quick one or two flip start. With the needle valve installed through the venturi the fuel prime is drawn into the engine and it stays there.
(b) When I first started using the LA engines with the stock remote needle valve arrangement I could not get a consistent engine cut out at the end of a flight. The engine would sound like it was quitting and then kick back in and go another lap and then repeat this stumbling routine for 6 to 10 laps before finally quitting. This was not very impressive when flying before stunt judges at a contest. Simply installing the needle valve through the venturi eliminated this problem and gave me a positive engine shut off at the end of the flight.
Best regards,
Dennis
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Re: LA .46 rear needle valve
Re: LA .46 rear needle valve
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montmil
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« Reply #3 on: March 20, 2012, 01:12:42 PM »

Thank you, Dennis and "G". This is terrific info I can apply to my project.
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Monte Miller
Denton, Texas
tonsberg
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« Reply #4 on: December 25, 2012, 08:52:36 AM »

Dennis has it nailed as to the use of the rear venturi ........ don't! As to the plastic backplates. Well I have run with plastic backplates and using the mod of a washer, you can keep them leak free. However, at some time they will give up and leak. Better to sort the problem before that happens. Metal backplates are hard to fins thesedays. But Curtis Shipp over on Stunthanger, makes them and they do the job.

Andrew.
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David Murrell
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« Reply #5 on: April 12, 2013, 03:20:25 AM »

I fully agree, except I also use some Loctite gasket sealant on the flange face - belts & braces perhaps

Cheers
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takEon
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« Reply #6 on: March 19, 2016, 02:04:16 PM »

The plastic one on my LA .46 broke off when dropped ( my fault), so I went ahead and changed to a metal backplate from an OS FP .40 that I had laying around. 
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TimWescott
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« Reply #7 on: March 19, 2016, 04:39:01 PM »

Dennis nailed it, but never the less I can't keep my mouth shut:

  • Any backplate will warp and leak if you over tighten -- it's just that the plastic ones are more flexible, and it takes a whole lot less to overtighten them.  Reef on an aluminum backplate and it'll warp.
  • My flying buddy is a long-time motorcyclist.  He has some Loktite gasket goo that he really likes; when he gets a 46LA with a plastic backplate he takes it off, applies the goo, and reassembles (tightening the screws very lightly).  He swears by this procedure.
  • You can fix the on-again, off-again end-of-flight dance with a clunk tank.  You get a clean run to the end and then the engine cuts off.  The downside is that you get no warning.
  • If you fear that you may still crash, and you're flying profiles, it's a lot easier to mount a remote needle valve where it won't hit the ground.  I rotate mine in place so they're running up behind the cylinder.  If your backplate has a built-in needle valve mount you may have to make an adapter.
  • You can buy needle valves and venturis from Tower Hobby.  You want the 25FP needle valve assembly, and either the 25FP or the 40FP venturi (the 25FP venturi is about the right size.  The 40FP venturi is too big for stunt, but you can tune it by piling layers of pantyhose material over it as an adjustable filter).
  • Some people like to use the ST-style needle valve.  These work well with the 40FP venturi, because they're bigger.  But -- because they're bigger -- they require drilling out the crankcase.  I like keeping my engines stock, so I don't do that.
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