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Laser Engines - Technical questions

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trebor15/01/2018 09:17:07
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1424 forum posts
173 photos

I've got a saito 82b that has this problem mounted in a Piper Pawnee, three times its caught me out doing low passes and then cut out at the edge of the field. It runs ok rich but try to clean the fuselage afterwards. I've got an old saito 82a that's no problem, the only thing different is it has slightly better compression.

Don Fry15/01/2018 11:32:55
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2074 forum posts
26 photos

Padding a tank is always good. And I would have thought an 8 oz tank is good for that engine/airframe, giving you some wriggle room.

Martin Harris15/01/2018 15:30:56
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7322 forum posts
183 photos

Vertical position does seem to be slightly more critical with an unpressurised tank. I know that Jon disagrees with it fundamentally, but a nose up test will sometimes reveal if this is the problem - although your installation looks pretty much ideal.

Edited By Martin Harris on 15/01/2018 15:35:42

Jon - Laser Engines15/01/2018 16:39:09
3448 forum posts
142 photos

This has been previously discussed and i dont want to get drawn into the argument again.

As the manufacturer i can say wholeheartedly that the nose up test will not reveal any tuning issues as it is unrepresentative of any condition seen by the model in flight. It will only give misleading information regarding the state of tune of the engine and we do not recommend it.

This is the official position of Laser engines and it is not open to further debate on this thread.

 

Please note that the above is not a personal attack on Martin. I just dont want another week long debate like last time as it wont change our recommendations. 

 

Edited By Jon Harper - Laser Engines on 15/01/2018 16:52:03

Martin Harris15/01/2018 18:25:41
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7322 forum posts
183 photos

I fully agree that the debate has been aired fully - I appreciate Jon's position and arguments and we have agreed to differ on this subject.

Geoff Sleath15/01/2018 19:49:31
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2262 forum posts
165 photos

I flew the Laser 62 I rebuilt over Christmas and New Year last week and I'm afraid it didn't perform as well as I'd hoped in that it exhibited the often experienced glow engine faults I'd forgotten about viz:

1: Choking up if left ticking over too long before taking off and losing power on the take off run, resulting in an aborted take off.

2. Unreliable slow tick over making landing difficult and certainly not giving me the confidence to do a touch and go

3. After cruising round on about half throttle for a while when I tried to open the throttle to climb vertically and perform a stall turn there was enough of a hesitation to discourage me.

It starts easily and sounds great. I have a brand new OSF glow plug and I'm using Modeltechics Laser Mix as fuel with an APC 12x6 prop. The angled glow plug position makes it difficult to apply power but that's easily overcome.

Now I guess the main problem is carb tuning. I set the main needle first to max rpm (approx 10k rpm) but it doesn't seem to have a sharp peak. Perhaps I should monitor the engine speed with my tacho as I trim the needle? I've tried adjusting the slow running needle in small increments but I haven't been able to effect a significant improvement. I'm sure the tank plumbing is OK. I have new tubing to both the clunk and the pressure/overflow (which goes to a forward facing open vent) but I haven't checked the pipe to the clunk itself internally. It's a SLEC square tank and its centre is just slightly lower (2/3mm perhaps, it's hard to judge) than the Super Tigre carb's spray bar.

I'm sure there is somewhere on here a thread about tuning a Laser carb. I just can't find it. If someone could point me to it here (or elsewhere) I'd be grateful. I really would like to get this sorted. I know I've become used to the reliability of electric motors but I did also manage to get the odd glow engine running reasonably well in the past and I know I can again.

Geoff

Denis Watkins15/01/2018 20:24:21
2356 forum posts
121 photos

" Rebuilt " Geoff suggests that this motor is run in, but many new parts may need running in, preventing smooth low end yet

Crankcase filling and slowing the motor, especially in cold weather can be overcome by fast tickover held on the throttle stick to keep the motor hot

And going to full chat now and again in the air to clear the fuel

Apologies, but all my motors run on pressure, no matter what make, and they run faultlessly until the summer when we pick up dust and pollen off the runway, and the needle needs wipe

An odd feeling of non positive needle is usually a dirty needle as you have a new plug, the plug is the next culprit

Geoff Sleath15/01/2018 20:42:48
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2262 forum posts
165 photos

All I did to the engine was strip it to clean out the castor crud which was blocking the drain from the crankcase to the cam box and unstick the piston rings so there was some compression. I replaced all three ball bearings which don't need any running in. So really nothing to run in. I removed both needles and ensured the carb is clean.

Here's the thread about my rebuild.

Geoff

extra slim15/01/2018 21:14:54
379 forum posts
47 photos

Sounds like rich low runner. Worth trying the old blow down a piece of fuel tube, wind in until no hiss, then open up until you hear a hiss, carb 99% closed.. usually not a bad starting point. Can't remember if the carb has an O ring on slow runner, occasionally these aren't seated and distorted or just perished.. you'll get there

Frank Skilbeck15/01/2018 22:13:02
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3890 forum posts
91 photos

Geoff, I was out at the weekend with my 30+ year old Laser 61, problem I had was getting it to stop!

The needle valve on the Super Tigre carb is not very sensitive and I do find a tach makes setting up the top end easier.

My procedure is as follows, tune for top end revs and the richen slightly to drop say 200 rpm, the back to idle and adjust the low end mixture, if it bogs down and splutters on pick up then lean it a bit, once set then check the top end. I must admit I don't really touch my needles, just do a quick rpm check and then fly.

At the weekend mine wouldn't start, plug was showing amps but pulling the plug and it wasn't glowing, new plug and it started first touch of the starter.

Jon - Laser Engines15/01/2018 22:28:14
3448 forum posts
142 photos

Geoff your symptoms are of an engine with unhappy fuel mix. it could be tuning or it could be something else. Now the engine has run double check the valve clearance has not moved, also how is the compression? if its somewhat soggy then that will not be helping the idle.

The fact that the idle runs down slowly suggests the engine is cooling off and this could be down to a rich mix.

Edited By Jon Harper - Laser Engines on 15/01/2018 22:29:03

bert baker16/01/2018 00:55:34
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1025 forum posts
216 photos

I going to do my own conversion to petrol img_1961.jpg

Jon - Laser Engines16/01/2018 08:40:50
3448 forum posts
142 photos
Posted by bert baker on 16/01/2018 00:55:34:

I going to do my own conversion to petrol img_1961.jpg

Have fun. dont bother using a butterfly carb unless its got a very small bore and an acceleration pump. If you can find a rotary barrel carb you might have some success but i gave up as it was too unreliable and fussy.

I dont wish to be doom and gloom, but its not likely to work too well. Thats why its taken me so long to sort out our production engines. Its a bit more complicated than some would have you believe

bert baker16/01/2018 11:01:24
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1025 forum posts
216 photos

It just for fun,I have had two Runtronic units that were due to go on a pair of Saito 120 Golden Knights, for a Roy Lever Mega Models Mosquito. But that's another story

I have a small Walbro pumped carb, with internal acceleration pump.

I did measure it and it is about 8-9mm bore,

I am in proses off making a insulator, and now thinking of best way to use if possible crank oscillation pressure.

One concern is heat build up in bottom end of crank case,

As I see it I will still need to be vented,

Jon - Laser Engines16/01/2018 12:28:53
3448 forum posts
142 photos

Dont be too concerned by the crankcase, it will be quite happy if left alone. What you can do is fit a second nipple to the backplate and use that for pulse pressure. An alternative is to use intake suction for the pump. Depending on which carb you have it might be set up for both

Gordon Whitehead 116/01/2018 13:44:42
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174 forum posts
108 photos

I used this setup with a Perry pulse pump and Cline regulator when experimenting with petrol on a 4str. If there's a nipple on your carb pump to take crankcase pressure pulses it might work for you, to save adding a second backplate nipple.

Here's where I found the idea: http://saito-engines.info/pumps.html

The last part of the article describes an interesting "pumped tank pressure" system that was apparently sold with the Saito 220.

Gordon

Edited By Gordon Whitehead 1 on 16/01/2018 13:52:30

Geoff Sleath16/01/2018 17:20:37
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2262 forum posts
165 photos
Posted by Frank Skilbeck on 15/01/2018 22:13:02:

Geoff, I was out at the weekend with my 30+ year old Laser 61, problem I had was getting it to stop!

The needle valve on the Super Tigre carb is not very sensitive and I do find a tach makes setting up the top end easier.

My procedure is as follows, tune for top end revs and the richen slightly to drop say 200 rpm, the back to idle and adjust the low end mixture, if it bogs down and splutters on pick up then lean it a bit, once set then check the top end. I must admit I don't really touch my needles, just do a quick rpm check and then fly.

At the weekend mine wouldn't start, plug was showing amps but pulling the plug and it wasn't glowing, new plug and it started first touch of the starter.

This engine hadn't run for years until I 'found' it in the club hut just before Christmas and decided to refurbish it by stripping, cleaning and fitting new bearings. Once I get it set up then I hope not to touch the needles like you.

One of the best glow engines I had is a G34 Super Tigre which just ran and ran, first in the Precedent Fun Fly I used to pass my 'A' and eventually in a Limbo Dancer I built from the plan. So I have some experience with ST carbs. I'm going to set the slow needle by the blow down the fuel pipe with a tiny throttle opening method and then set the main needle using my tacho. I'll then follow your advice and see where it leads - hopefully to a well running engine.

Thanks

Geoff

Gordon Whitehead 116/01/2018 18:46:03
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174 forum posts
108 photos
Posted by Geoff Sleath on 16/01/2018 17:20:37:
I'm going to set the slow needle by the blow down the fuel pipe with a tiny throttle opening method

If I remember correctly that's the method Neil Tidey used to recommend for achieving the initial slow needle setting. I think the throttle opening would have been set using 18swg wire. With cheeks puffed out and ear drums popped out too, I always found it a bit difficult to tell when the hiss of escaping air had arrived. I still use the method, though slightly modified. Instead of blowing down the fuel pipe, I squeeze air down it using a disposable syringe. Much easier!

Gordon

Martin Harris16/01/2018 19:19:23
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7322 forum posts
183 photos

It was certainly advocated by the legendary Mick Wilshere for those carburettors on their originally intended locations.

Jon - Laser Engines16/01/2018 20:01:30
3448 forum posts
142 photos

Once I get it set up then I hope not to touch the needles like you.

This is something I hear often and I'm pretty sure its the cause of a great many engine related issues. It is vital that the engine be tuned correctly for the day on which it is being used as factors such as temperature, humidity, air density etc all make a difference to the tune of the engine. In general slow run needles need no adjustment once set but main needles must be checked on the first run of the day. Often little to no adjustment is required but its always worth making sure the engine is spot on before attempting to fly it.

The blowing through pipe trick for the slow run works fine but it is only a base setting to get the engine running. As your engine is already running it wont really help you as it must now be tuned up for its specific installation.

Tuning with a tacho is also not recommended as it leads to excessively lean mixture and 'rpm chasing' in most cases. RPM chasing comes about due to the natural drop in rpm an engine sees once it warms up. If rpm is checked on the first full throttle blast you may see 8500 for example. If you then adjust the needle, are satisfied with the tune, then check again you may see 8400 due to the engine warming up and naturally loosing a few revs. This is usually viewed as the tuning adjustment causing the rpm loss and in our experience there then follows a prolonged period of full throttle operation with leaner and leaner mixes being pushed to try and reclaim 8500rpm or more. During this time the engine gets hotter and hotter, and slower and slower with the user still trying to screw the life out of the needle in search of more rpm.

This video shows me setting up a Laser 150 on the test bench. Both needles are set well rich in the first instance and the engine was given a short warm up. I start with the main needle and tune it as easily as you can see. The slow run is then leaned off slowly, with accelerations in between, until it bogs down. This is a lean cut so the slow run is teased open until I can slap it about as much as I like. as you can see from the video symptoms of a rich idle are a slightly fast idle rpm that is lumpy and wont sit still. Once leaned off you can see the rpm drops and is very stable. Throttle response when rich on the low end is also sluggish and lumpy. Note that setting up the engine takes a matter of minutes only and is not a drawn out process. I hope the video is of use.

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