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Carburettor Set Up

Do you know how to set up a Thunder Tiger GP 61?

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Rob Cope08/10/2007 23:30:00
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Hi Flanker,

I wrote a reply, but lost it somehow...

The TT GP61 starts and runs up to half throttle at almost any needle setting between about 0.8 turns and 1.8 turns. However, if I want it to reach above half throttle, it really needs to be up at the 1.8 end. Otherwise it'll either die as soon as you throttle up above half way. Sometimes (perhaps at 1.2 turns) it'll throttle up nicely and then slow and suddenly cut out.

To me, this means lack of tank pressure. It's a rather non-standard looking silencer, so I'm wondering if I should pick up factory TT GP61 silencer and try that out. The one on there is a sort of barrel shape (horizontal cylinder) fed through the side wall of the barrel from the engine exhaust port. The outlet is a fairly narrow (10mm?) tube in the opposite side wall. The pressure nipple is near to the top (when inverted of course.)

I find it hard to get to the model shop these days as it's a way away and I hate to lose a weekend day I could be flying. Still, the winter season is on its way, so there will be more opportunities... he he. Thanks for the advice. I shall get a new plug or two. I've been tempted to cut my losses and just replace the engine several times, but I feel it must be possible to get this running. It feels so smooth and there's not much play anywhereas far as I can tell without dismantling.

Regards, Rob

Flanker .09/10/2007 08:30:00
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Dude DO try a new plug! As to mixture if the motor starts ok and only cuts at around half throttle it means that the two needles are out of balance. Start it and get it to run at full throt by adjusting the MAIN needle 'till it peaks. (it will sag then stop if you screw in the main needle too much) after it peaks richen (un screw) until you hear a slight rpm drop. Close the throttle slowly untill the engine sounds like it is going to stop, the move the throttle to the lowest keepable idle, wait 30 seconds the wack it on to full and listen to what it does. If it just stops dead, then your mix is too lean, screw in your air bleed screw. (unscrew if twin needle) and try again. keep doing this untill it will throttle properly.

If it does transit to full but in a "bubbly" and smokey , coughy sort of way the your idle mix is too rich and you need to screw out your air bleed needle.

DO NOT TRY THIS until you have fitted a new plug !

 Keep your adjustments small. Jolly good luck sir! A final point, when you master this, and it is well worth practicing, your engines will always start fast, and the club pit flickers become jealous.  F

Tim Mackey11/10/2007 19:13:00
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ROB. I do not advise keeping the cardboard ring /sleeve as a way of centering the prop on the shaft, it will NOT last. If you send me a the cardboard ring, I will happily make you up a replacement of the same dimensions in alu or tufnol which will last. Email me via the button in my banner.

(freebie of course)

Rob Cope16/10/2007 00:20:00
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94 forum posts

Hi Timbo,

You are very kind.  I have other avenues I should explore before putting you to the trouble of turning a custom component for me.  Thanks very much for the offer though.

Regards, Rob

Tim Mackey16/10/2007 08:56:00
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'tis is absolutely no trouble - honestly - 20 minute job
Rob Cope17/10/2007 01:16:00
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94 forum posts

Well Timbo, see what you can make of this little lot!!! I think it's a bit of a challenge.

The Thunder Tiger GP 61 seems to have a shaft diameter a little in excess of its thread clearance diameter. It may be a metric shaft with a 5/16" thread cut on the end!

This has complicated things somewhat.  I've measured the (two) bits of cardboard up... and, well, I'll have a go at describing the stepped diameters as best I can.  You might prefer a drawing.  Please let me know and I'll have a go. 

The measurements are as accurate as I can measure them.  However, I haven't a vernier caliper or a micrometer, so they're read from an ordinary outside caliper and a 1/64th inch rule.  If this is pitifully inaccurate, please let me know.     I have deliberately avoided halves of 1/64" in the measurements and biased outside diameters to the smaller side and inside diameters to the larger side, but it's close.

Anyway, ... 

Inboard we have a sleeve 11/32" long. The inside diameter is 21/64".  The outside is 3/8".  This carries the spinner back plate and extends into a clearance hole on the back of the propeller.

Outboard, we have an 11/16" length cardboard sleve, but for a one-piece construction this needs reducing to a 5/16" length.  The outboard sleeve inside diameter is 5/16" for the shaft thread clearance and outside diameter is 23/64" to carry the Graupner prop.

The cardboard sleeves actually just touch. There's a small overlap possible on the properly engineered bush. It will be "inside" the inboard sleve, so the inside of the inboard 12/64" hole can be shorter than 11/32".  This I measure as 1/4".

So, you see it's a bit of a pain really.  I can't see why the Thunder Tiger GP 61 should have this odd arrangement.  But for that, I'd be quite happy to drill/ream the prop inside diameter out to make the outside diameters of the bush a consistent 3/8".  Please let me know if this would help.  Perhaps the inside taper is not necessary as the plain shaft on the engine might centre everything up with the prop drilled/reamed out to 3/8".  There's plenty of diameter on the Graupner to spare.  However, the bush will be thin and might not carry the potential side-load of a prop strike, not that there will be any of those of course...

Assuming the inside diameter could be made to fit only the shaft, enlarging the prop centre hole would simplify things to a 5/8" bush with outside dia 3/8" and inside dia 21/64".  It's still a bit thin for comfort and would require drilling/boring from a slightly thicker diameter before slowly turning down the outside diameter... possibly in stages to avoid over-stressing the walls... as they're only 3/128" thick!!! Cardboard cereal packet size...

Perhaps I should bore everything (prop and spinner) out to 7/16"!!!! ?

Regards, Rob

Flanker .17/10/2007 08:21:00
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All this sounds a little odd. I'll pop out/down in a minute and measure the shat (or better shaft) on my TT 61.

I normally have to open up props to fit my engines and I do it on a drill press. I go for a friction fit on the thread, ie rotating the prop will screw it on with ease. This seems to work fine for me. I balance the prop after this operation. If your shaft is loose in most new props then you do have an odd motor.  That being the case measure the prop hole diameter & the shaft diameter and send em to Timbo. If he can tear himself away from breaking post records that is ! Lol Hope you get all this sorted. 'Course you could just go an buy a new motor, but that would spoil the fun wouldn't it ? Good luck F

00117/10/2007 09:05:00
2212 forum posts
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re 2T and 4T, in Spanish a 2T is a two stroke motor and a 4T is a four stroke, T is for 'tiempos' which translated directly means times, so, Timbo, you may have picked it up from there?

Rob Cope17/10/2007 10:27:00
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94 forum posts

Hi Flanker,

This is not a new Graupner, it's the one I got with the 'plane second hand. It is however a new spinner and that has a hole that's too big too! You're saying if I buy a new prop, that would sort out the concentricity? OK, there is the tiniest hint of a chip on a tip anyway looking more carefully at it, so perhaps that would be best.

As for the spinner, it's just too large a hole, but... it came with two bushes - one for 1/4" shaft and the other for 5/16" shaft. However, they're both too small because of the excess size of the crankshaft on the GP 61. I can see if you put a prop only on the GP 61, the prop has a recess on the back to cater for this sort of design, but the spinner has no such facility. It also came with bushes "having a flange at one end" that I also cannot see how to hide/use/miss. This flange has no use - and can only jack the spinner back plate off the engine thrust washer, causing more eccentricity/wobbling. I could just grind the 5/8" bush down until the flange is gone...

What I can do is go buy a new prop and see where I stand then. Perhaps I'll need only the bush for the spinner which would be a single inside+outside diameter. This would simplify things. I'll go do that this week and get back to you all. Many thanks once again.

Regards, Rob

P.S. Thanks Richard for the T info. So, duo-tiempos and quatro-tiempos, yes?

Tim Mackey17/10/2007 17:28:00
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Ok Rob...if you need the bush for the spinner doing, just yell - I have loads of aluminium rod here.

Incidentally would the flanged bush just not fit in the other way around, so the flange bit was inside and the prop fitted up against that ? 

Flanker .17/10/2007 19:07:00
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And drilling out the bush should be easy any way. Yeah splash out on a few props, coz you always break 'em when you really want another fly !

Y pues si, motores en españa se llaman quatro tiempo y dos tiempo, o con mas facilidad 

2T y 4T. ¿Claro? Muy bien.  Hasta lluego todos y mucho suerte,  F

Rob Cope17/10/2007 20:56:00
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94 forum posts

Hi Timbo,

With the flange inside (foremost) the propeller would not fit flat against the full width rough inside of the spinner back. This would lead to both potential slipping and eccentricity / vibration / weakness. I feel it is better to make another bush/sleve with no flange and the correct inside diameter. It will be very hard to drill out the existing one, even with my pillar drill as the piece is so small to hold.

Yes, Flanker, thanks for the encouragement, I know. They're only a few pounds each. I'll go buy some!

Regards, Rob

Tim Mackey17/10/2007 23:55:00
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Gotya. Anyway, send me the dimensions of the bushing you require, and I'll knock one up for you on the lathe
Rob Cope18/10/2007 00:22:00
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94 forum posts

Oh Timbo,

You're so keen... I'm now going to ask, just to passify you! Wow! How many societies have this sort of help from members? I'll send the best info I have privately once the new prop is in my hands - and possibly the vernier inside/outside caliper!!! I know I need it.  This is just a stimulus...

Regards, Rob

Flanker .18/10/2007 09:30:00
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622 forum posts
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I don't know if you have "lidle" supermarkets over there but they are a great sorce of good cheap tools, I bought a fine digital vernier there the other day and it is great. A "must have " tool imho. Go well and yeah I know of very few sports where we all take care of our own so well, I have very fond memories of my first RC club, Wrexham model aeroclub, they were ALL so kind and unstinting in their help, and taught me to fly so well. I remember Archie & co working out how to teach two DEAF lads that turned up. They invented a touch n wave sign language just for these two, and got 'em through their "A" s in double quick time! Kind actions being their own reward does seem to be the motto of us flyers. Hooray. Alas I know not how they fair now, but if any Wrexham folk see this I wish you all well. Ahh hiridd !
Tim Mackey18/10/2007 09:39:00
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LIDL actually

Wrexham club is still going strong, and their friendly relaxed club funday is a must on my calendar every year, as is the scale day. Pay a quid or so towards the running costs of the day, and get raffle entry, free flying all day, AND free burgers /hotdogs / and drinks too!

As you know too probably......the site is one of the most relaxing and scenic flying sites you will ever find, with a good bunch of lads and lasses.

Only seriously bad problem which absolutely MUST get sorted and quickly....

NO STICKY BUNS FOR AFTERS!!

Rob Cope30/10/2007 00:11:00
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94 forum posts

Well guys,

I know it's late!!! Short and sweet. The Graupner arrived and I drilled it out - 8mm was required. 7.5mm was not enough. The prop fits perfectly now and my cardboard sleve is looking less worrisome. However, I do promise to buy a vernier (quite possibly digital 0.005mm and all that!) Timbo and supply details of the piece and I thank you for your help in this. Joyfully it will now be rather simpler being a single inside/outside diameter bush for the spinner.

Thanks ever so much everyone for your encouragement and help. The spit has a broken leg at the moment, but as soon as that's sorted out and I've got the bush sorted, I'll let you know how I got on.

Regards, Rob

Rob Cope09/01/2008 21:12:00
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94 forum posts

So,

Just to round up the thread and put it to bed...

For christmas I got the digital caliper - nice - in a tool case - half a thou and hundredth of a mil absolute accuracy it says. Seems to work pretty well and repeatably - even after being switched off for days.

It turns out the shaft is around 8mm, but 8.02-8.04 depending on the angle. The bush DOES actually fit in the back of the spinner!!! if you push really hard. It's getting looser now. Perhaps its the colder weather?

The bush ID was too small (as discussed) - 0.302" (or 3/10") so smaller than 8mm (actually that's 7.78mm) , but I measured the ID of the prop nut and it was 8.10mm. This encouraged me to go for the "boring it out" approach (thanks Flanker) on the pillar drill with an 8mm HSS bit.

Don't listen Mr "Health & Safety fascist" but I held it in my fingers (on lowest drill speed of course) and offered it up ever so slowly by hand and on the 3rd attempt managed to get it to cut gently all the way though (my hand - no the bush.) It did-na fit! Hole dia 7.79mm - so my 8mm HSS drill bit is actually 7.8mm. Doh!

Anyway I rolled a piece of glass paper up and threaded the bush on it and with a slow twist ran it up and down - and eventually arrived at 8.08mm which was a nice just-loose fit on the propshaft! So now It all works.

Thanks for your offer/help/encouragement everyone. With the exact measurements, I could proceed along the sensible lines you'd all described. I now have a new prop (without the tiny chip) that fits perfectly and is balanced - and a spinner that fits and is concentric. And I didn't have to slow Timbo's posting rate at all! Happy New Year everyone!

Regards, Rob

P.S. All we need now is the weather to be able to try a flight or two...

Tim Mackey09/01/2008 23:30:00
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Nice one Rob,

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