OS GT55

[BalsaBasher]

Hi there

 

I was "lucky" enough to witness an accident out our field yesterday and was given an OS gt55 to repair.

 

Engine went in deep and I spent the evening taking it apart and flushing out as must of the dirt as possible.

 

I could then inspect the damage:

 

(1) Carb mounting broken

(2) Venturi trumpet damaged

(3) Rear cover plate mounting foot broken

(4) Wires from Ignition unit frayed

 

Pics to follow

 

 

[Steve Hargreaves - Moderator]

Ouch......sounds very expensive....

[BalsaBasher]

This is how it came home with me - a sorry bag of bits

This is the damage to the cover plate - I am either going to replace - can order for about $40 us from tower hobbies - or will try a local engineering firm to weld up..........will see how this goes.

This I will repair using liquid metal ,I have managed to find the bit on the right that broke out , and the left can be moved back into place and then built up , machined and re-tapped . I am a little concerned bout the hairline crack that is visible. Any opinions ?

Here we ave some braiding missing and the sensor/battery connections have been damaged.The latter is easy enough to fix . I do need some assistance regarding the damaged screen - would what is left be sufficient. Do I need to extend new screen over the damaged portion and clamp on both sides? A replacement unit is more than half the price of a new engine.

 

Any advice from engine guru's ?

 

Cheers

 

Hilton

 

[Steve Hargreaves - Moderator]

For 40 bucks I would say a new back plate is the way to go.....you are relying on this to keep the motor attached to the model after all!!!

 

Liquid metal should be fine for the carb mounting......keep it neat won't you....

 

Not a model petrol engine guru but I do know you will need to "replace" that braid such that the gap is covered....can you take the plug cap off?? If so might it be possible to use the outer braid from a length of co-ax (TV Aerial) cable? If you cut a length & strip off the outer jacket (carefully so you don't nick the copper braid) you should be able to slip the braid off the plastic inner & maybe use that....you might need to use a few layers to make sure the inner cable is fully covered. You will also need to make sure that the new braid is electrically bonded to the existing jacket......soldering would be the best option. Test the braid for continuity (& zero Ohms!!) across the new section....cover in heatshrink & Roberts yer mothers brother......!!!

 

It probably won't look very neat bit will save a few quid!!!

 

Failing that would a commercial replacement CDI from, say, Just Engines be an option....much cheaper than the OS version!!!

[BalsaBasher]

Thanx for the reply Steve - Agree on the cover - replacement already ordered. I think the plug cap removal is the way to go - even though it has been crimped on I think cutting it off and then using the braid to slide back over and then refitting .Plug cable will be 5cm's or so shorter so not really an issue.If this fails a new cdi unit.

 

What do you think of the hairline crack @ bottom right at carb inlet - I presume the cavity is there to provide suction to diaphragm fuel pump ? Not to sure how that can be repaired.......... or if I should even try? Just fill the crack and build up around it with liquid weld.

 

This is the area I am referring to:

 

Edited By BalsaBasher on 24/01/2011 11:51:16

Edited By BalsaBasher on 24/01/2011 12:01:49

[Steve Hargreaves - Moderator]

H'mmmm....don't know is the short answer. That groove is obviously there for a good reason.....what is on the mounting face of the carb? Is it for the fuel pump diaphram as you suspect

 

Can you see the crack from the outside or thought the carb mounting hole? I might be tempted to ignore it initially & see if the carb still works when the engine is back together....if not then the crack must be leaking air in somewhere. Anaerobic threadlock might be a solution....you can get stuff that wicks into threads (ie when they are mated) & then sets. Might be an answer....??

 

Failing that I might be tempted to fill it with liquid metal & really press it into the crack then grind it back with a dremel when set.....I can't see the shape of the groove being critical as long as it allows passage of air.....

 

Might this stuff be an alternative to liquid metal......you would need to be pretty skilled with a good blowtorch methinks.......that said though the only alternative is a new crankcase which I'm guessing is megabucks!!!

[Steve Hargreaves - Moderator]

Another option for your braid might be these guys. doubt they would sell you some direct but ask for a sample or a distributor.

 

Such braiding is often used to shield cables to prevent EMI.....

 

I can't stress enough though that you should make sure the braid is electrically continuous....the CDI unit often uses the braid as the "earth" for the high tension & if this is not in good condition then the HT will fry the output side of your CDI unit!!!!

[BalsaBasher]

Have taken note of comments regarding braid - there is however a separate earthing strap for the cdi - so less of a risk. The crack does not seem to have gone right through - reason for me thinking to just leave it as is.

[Kozmyk]

I don't know what the dia of that CDI cable is but you get that kind of braiding on audio cables; balanced microphone cable and even guitar leads (up to aboout 6mm dia). Bigger than that and I'd be looking at the screen from multicore cable. Another source might be the screen from computer cables, RS232 and the like.

I had a reel of RS 25-way knocking about somewhere, I'll see if I can dig it out.

[BalsaBasher]

Posted by Kozmyk on 24/01/2011 13:57:26:

I don't know what the dia of that CDI cable is but you get that kind of braiding on audio cables; balanced microphone cable and even guitar leads (up to aboout 6mm dia). Bigger than that and I'd be looking at the screen from multicore cable. Another source might be the screen from computer cables, RS232 and the like.

I had a reel of RS 25-way knocking about somewhere, I'll see if I can dig it out.

Thanx for the offer - but I live in South Africa

[Biggles' Elder Brother - Moderator]

That crack is a problem. If you don't do something I would think its only going to spread due to engine vibration. The problem is - cracks spread from the tip and you are very unlikely to be able to get any sort of filler into the crack right to the tip. I'm really not sure what to suggest - but doing nothing is not an option! Having said the filler wont get to the tip there might not be an alternative. You could try the liquid metal solution then bench run the engine for at least a couple of hours and see if there is any suggestion that the tip is "travelling"? Tricky one.

 

BEB

[Kozmyk]

Posted by BalsaBasher on 24/01/2011 14:30:39:Thanx for the offer - but I live in South Africa

A few inches of braiding wouldn't cost much to post but I haven't found it yet anyway.

 

On the matter of leaving it uncovered.

The remaining braid on your lead might maintain the earth path but it's not going to help with the HT leakage at all.

Best get it covered one way or the other rather than leave it unshielded.

 

Edited By Kozmyk on 24/01/2011 15:37:36

[Martin Harris - Moderator]

Can you see the ends of the crack? If so, drilling a small hole at each end centred just beyond the end of the crack should stop it propagating. Fill crack and holes with liquid metal or high temperature epoxy - if possible, V out the crack a little as well.

 

As you suspect, the groove is the air duct to allow crankcase pressure pulses to operate the pump diaphragm.

[BalsaBasher]

Posted by Martin Harris on 24/01/2011 23:04:27:

Can you see the ends of the crack? If so, drilling a small hole at each end centred just beyond the end of the crack should stop it propagating. Fill crack and holes with liquid metal or high temperature epoxy - if possible, V out the crack a little as well.

 

I would be able to get to the ends , I think ................. and drill small holes . I bit of judicious work with a dremel and the groove could be v'd out a bit.

 

Fixed the screening last night - so will put up pics of that later and have re-connected sensor and battery connections.

 

 

 

[BalsaBasher]

I have been in contact with Brian Winch and he suggests drilling holes in from the side and then adding 2 or 3 small screws to prevent the crack from spreading. i.e. side of carb manifold. This is now my plan of action.

 

The rear crankcase cover left the states on wednesday so should have it next week if all goes well.

 

A bit of luck and the engine could be given a test run again next weekend.

 

Will add some pics for those interested.

 

Have a 2metre span stik that might just work with this on it

 

Cheers

 

Hilton

 

 

[Former Member]

[This posting has been removed]

[BalsaBasher]

Hi all

 

I have finally received the replacement cover from the states - was the better of a 6 week wait . Our postal service has a lot to answer for here as it took a week from America and then the balance to get to me.

I have re-assembled everything and plan to do a test run this weekend. Just need to find a decent block of wood to mount the engine on

 

Will let everyone know how this pan out.

[BalsaBasher]

Hi there all.

 

I have managed to mount the engine and get some fuel running through it.

 

I am however struggling to get it to run consistently.

 

I have had to reset needles to the factory recommended 2 turns out and prime with choke closed until petrol reaches carb . Then a further 6 flips , open choke and flip a further 4-5 to draw mixture into engine.

 

I then hand flip and get a 1/2 to 1 second run after 3 or 4th try. If I increase priming engine does not start at all , trying an electric starter just floods engine.

 

Any suggestions on what I can do to get it to run.

 

P.S have checked there is a nice strong spark at plug end and the ignition battery is fully charged.

[BalsaBasher]

Any suggestions ?

[Myron Beaumont]

BalsaBasher

Have read your thread several times to get the picture so to speak .I don't have any experience of model petrol engines ,only full sized. You refer to resetting to factory needle settings ie 2 turns .What was the setting when you last flew ? It would appear to me that there's something wrong with the carburation once the engine has started .I gather that you need a rich mixture to get it to fire up -but then it suffers from too much petrol like using your starter motor as well.Can't be blamed on an air leak (or can it) or a jambed shut choke ? Stating the obvious I know but I think you have a carburretor malfunction brought on by something that maybe you havn't spotted . Another crack maybe ?

I wish I knew more about model petrol powered engines as I intend to go that way myself.I've never even seen one except in my lawnmower and strimmer but their carbs are not quite the same I gather.

Not much use am I but let us know that previous needle setting

[Craig Spence]

Hi BalsaBasher,

I have read your thread and it is very interesting. It's a shame as the engine costs so much so to replace the larger parts would probably end up more than the cost of the engine.

I can't say I have any real experience with these engines or machining parts etc.. But what I can say is that I can more than always make the best of a bad situation, even if I don't have the tools I'm quite inotive with what I've got, family trait apparently lol!.

I personally would not drill holes into the crack, if it fails again you just have more work or even end up scraping it.
I would like suggested above use some high heat resistant epoxy and wick it right in there, then I would wipe off the excess and join the carb.
This would in theory pull the crack in a little, once the parts are hand tight I'd let them set or support the area where the crack is in a vice or g-clamp.

Once it has all set I would remove the part again, and lightly smear the mating surfaces with heat resistant epoxy and then rejoin, this will ensure the whole set up is air/ fuel tight where it needs to be.

Once this has all been done I'd concentrate on ensureing the damage doesn't spread, I would do something a little simpler for this. A strip of thin steel or similar strength metal, silver soldered across the two parts and along the hair line crack.
This would act as support across the whole structure.

As I said though, I just make the best of what I've got. Hope it all gets sorted, they look excellent engines. Expensive though and looking at this thread I can see why I don't go too up Market lol, if I did that to one my engines and asked the wife to replac it I think shed pickle my jewels lol!.

Good luck.