spitting 2-speeders

[richard norris]

I know this subject has been much discussed but I wonder whether anyone has tried opening out the pilot jet little by little to richen the pilot and just-off-pilot mixture? Certainly my carb needs the pilot screw nearly home but the cutaway seems just about right. No air leaks evident. The bike starts easily, runs well and idles pretty well when warm, just tends to spit just as you pull away which can be interesting, also when on very tiny throttle openings e.g. in slow traffic. Any coments/suggestions? I love this bike!
Happy Christmas to everyone,
Richard Norris

[BRIAN MARSHALL]

Hi Richard,
You don’t mention what carburettor you have fitted…..? This problem can also be caused by over-retarded ignition, but I suppose you have already checked that. Certainly the factory were well aware of it as being a particularly intractable problem with some machines, that no amount of fiddling about with carb., plugs, and timing would cure; and I think that if a bike gave this trouble at the factory it would be fitted with gauzes incorporated into the transfer port gaskets. Even some short-stroke Flyers were so fitted from new. When the late Tom Wess’s 1928 Flyer was stripped for perhaps the first time EVER by Tim Sharp a few years ago, it was found to have gauzes fitted, complete with about 75 year’s build up of dead flies (pickled in petrol !), and assorted bits of leaf, straw, gunge, and small feathers…
Good luck !
Brian

[dave bushell]

Could also be running weak on one cylinder only. Make sure that you have the mixture guide/splitter plate fitted in the crankcase manifold which is fitted to prevent mixture bias to one cylinder.
Dave

[TedParkin]

Brian is right. Check your gauzes Mate!!!!!!!! I had the same problem on mine!!!!!! New gauzes, no problem!!!!! Ted 😆 😆 😆 😆

[richard norris]

Thanks to you all, your advice will save time and money on micro drill bits. Nice not to try to reinvent the wheel. It’s an Amal 276 carb in good order, ignition is well advanced, I have managed to start backwards on one occasion (embarrassing). My motor is gauze-free, who supplies them?
Oh, and it’s the LH (nearside) cylinder which spits,
Richard

[BRIAN MARSHALL]

This might seem like a silly question…How do you know that it is the L/H cylinder that is spitting back ? !
Sorry I can’t supply new gauzes. I have other gaskets/packings in stock but not those. You might have to make them using quite fine mesh gauze in a sandwich of two plain gaskets with plenty of silicone sealant to seal the edges of the sandwich, and then use a double thickness gasket on the other side so that the transfer port goes down squarely.Let us know how you get on.
Brian

[richard norris]

Not a silly question, I’m the one who asks silly questions. When I got it you could see a puff of blue smoke where the LH transfer port didn’t bed completely. That is ok now after carefully trimming the gasket to fit and the spitting is certainly improved. Richard Tann advanced the ignition as well before handing it over and that helped a lot. He has also tried a baffle in the past without success. Luckily I can live with the spitting and work round it by avoiding small throttle openings (!). I’ll have a go at making some gauzes, hours of innocent fun in prospect.

[BRIAN MARSHALL]

Hi Richard,
I don’t know if you have knuckled down and made yourself a pair of gauzes yet….I have found an old but unused one in a box of gaskets that came from the late Bruce Selous. I could send it to you to try on the side of the engine that is spitting back. If that is an instant cure you will be sure what the remedy is ! If not you could put it on the other side to make absolutely sure. I would like it back to keep as a pattern, but at least it will give you a confirmation of the problem, and also you will see just what gauge of gauze ( !! ) you should use when making them.
Brian

[richard norris]

Thanks Brian, that would be good, will send it back by return of post. I’ve made some up with anti-spatter gauze (non-stick!) but have yet to give them a good try.
Cheers, Richard

[BRIAN MARSHALL]

Hi Richard,
I have posted it to you first class so you should in theory get it tomorrow. When I examined it wearing my reading glasses I realised that it has TWO layers of gauze, one each side of the gasket material, which is interesting. I presume that they work in the same way as the gauze screen in a Humphrey Davy Miner’s Lamp, preventing the flame from going all the way through the transfer port. If the Spares Scheme is interested in having my lonely old original gauze as a pattern/sample, please let me know.
I would think that the gauge of the mesh is fairly critical, as it was in the miner’s lamps, and that they will probably take the edge off the performance of the bike, but hope that is acceptable with the reward of sweeter low speed running, and less chance of a carburettor fire !
The only Scott I’ve owned which had this problem was my 1927 long wheelbase Flyer, which had a low-compression engine, ( piston crowns lower, and stamped “T” instead of the usual “R” ) . On two occasions it managed to blow the R/H crankcase door off ! I didn’t fit gauzes, but managed to cure it, after various experiments. I will keep my remedy to myself for now, but would mention that the bike was by far the most tractable and sweet Scott I have ever owned. It would pull top gear down to a walking pace, and then accelerate away without any four-stroking or spitting back. Provided that the bike was rolling there was no need to change gear, and I rarely used bottom gear at all. When the three-speed box was introduced it was advertised as being an emergency bottom gear, and so my riding technique was entirely as intended by the factory , something that is not normally possible with later higher compression and/or long-stroke engines.
Brian

[richard norris]

Got it this morning Brian and many thanks – it’s quite a work of art isn’t it? And the mesh is much finer than the ones I have made. I would be interested to get some made if anyone knows how to go about it. I’ll take some photos and send it back to you, will post photos here assuming it isn’t beyond my competence,
Thanks again, Richard

[richard norris]

The spitting seems to be cured by fitting fine gauzes (40 strands per inch one, each side of the upper transfer port gasket) and using no.3 slide cutaway, needle position 4 (second from weakest) and 190 main jet to take account of the ethanol. Any comments appreciated.
In passing, is anyone know an expert on Amal 276 carbs, the various types – I suppose different models had different pilot jet sizes – which is correct for the 2-speed Scotts?

[BRIAN MARSHALL]

Hi.
I’m very pleased that my suggestion to fit gauzes did the trick !

As regards the Amal 276 carb, this is basically the same as the 6/151R fitted to Flyers, except of course for the horizontal fitting and clip instead of flange mounting, and a Super engine will not be too different from a shortstroke Flyer in its fuel needs.
I would agree with 6/3 slide and 190 main jet, and would fit a new needle as wear on them is easily missed. The worst thing for a Scott or any other two-stroke is to run weak, so I usually fit the needle clip in the middle notch, and this is rarely far from being correct. As the late Jim Best was always saying “I like ’em to run a bit rich”. Petrol is cheaper than pistons….
The pilot air screw should be gently screwed all the way in and then screwed out one and a half turns, and if the carb is in good order this again is usually about right. I always have the throttle-stop screw backed right off so that it doesn’t touch the slide, otherwise you tend to get snatching on the over-run.
Brian

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