Before I start taking various bits off the bike to fix over the winter I could do with some idea if the following are normal 1948 ‘Scott’ behaviour or indicate faults to add to the list;
1. I checked my timing and its spot on with full advance. However, on the open road, the advance-retard lever appears to make little or no difference to how the bike runs. Full advance does speed up what passes for a ‘tickover’ but thats about it. I have confirmed that the magneto slip-ring turns when the lever is rotated 😆 The magneto is claimed to have had a recent rebuild.
2. At present the bike is very lean at zero and light throttle openings. Thus, when hot and coming to a stop I still need to drop the air slide to stop the revs fading away. As soon as I open up I need to raise the air slide again to keep it running clean. The (standard as far as I can tell) carb has been stripped and cleaned and appears in excellent condition with un-blocked pilot drillings. At present in town I’m constantly playing with the air lever to keep it running clean. Am I correct that raising the needle and/or reducing air bleed through the pilot adjuster should help? At least I don’t have to fiddle with the advance lever as well!
3. The bike will not go into first gear with the engine running without pushing it forward a little whilst maintaining pressue on the gear lever. Thats a real nuisance at lights.
4. The bike cruises in a relaxed manner up to about 55 mph. Above that it starts to rapidly feel less relaxed. Thus, cruising at 60 felt a bit too much even though there is plenty of power in reserve. Is that me being over-sensitive?
1 Longstroke engines are generally unresponsive to the advance and retard lever, the opposite of shortstroke engines. I believe that your engine has been altered by Roger Moss, and may be different to the norm…..
2 See what happens if you screw in the primary airscrew (gently !) onto its seating, and then undo it one and a half turns. This should sort out your slow speed running, BUT there are some very fine internal drillings in the carb, and you might have a speck of gunge in one of them. Gently pushing some appropriate diameter fuse wire thru’ the drillings may help. I can if necessary lend you a spare carburettor to try, or can have a look at your carb for you. It is best not to adjust the throttle stop to achieve a reliable tickover as this tends to make the engine snatch and jerk on a closed throttle/over-run situation.
This can be a bit tricky to get used to, as you have to keep the throttle slightly open when at a standstill so that the engine doesn’t stall. A little paint line on the twistgrip may help you until it becomes ‘second nature’. There shouldn’t be any need to use the air lever except when starting from cold.
3 Your gearbox has thixotropic grease in it, instead of oil, and this may be the root cause of your problem. Fill up the gearbox with paraffin, and then spin it over for a few minutes by turning the back wheel with the bike in gear. Then remove the oil level plug and carefully lay the bike over on its RH side. WATCH THE RADIATOR DOESN’T TAKE ANY WEIGHT ! Allow the gearbox to drain out into Management’s best turkey roasting tin, then repeat the paraffin rinse so as to get rid of the grease. Then fill up to the level plug hole with 140 grade gear oil. With the bike in second gear, NOT neutral, adjust the short external linkage between positive-stop mechanism and bellcrank arm, so that the second gear pair faces are level with one another. This should sort out your problems, assuming that there isn’t excessive play in the positive stop mechanism, and that there is no clutch drag.
4 I don’t know what your gearing is, but assuming that you have a 21T final drive sprocket, your bike should be perfectly happy cruising at 65 to 70 mph, but by experiment you will find that there is a speed at which you have to open the throttle a lot more to gain more speed. This causes a lot more induction roar to be audible, which can be a bit worrying to a new Scott owner, until you get to recognise it !
I hope that this rambling sorts things out for you, and would say “Don’t start pulling it to pieces unless convinced that it can’t be fixed by normal tweaking”.
I fully support Brians comments, but in addition to those, since he has not mentioned the carburettor needle, I would suggest that you take a look at the needle position. It ought to be 3 notches up from the bottom position in a normal Scott Amal carburettor.
1. Its good to know that Scott engines can sometimes be unresponsive to a/r. It has been breathed on by Roger so, as you say, may not behave as standard in any case.
2. I stripped the carb as it felt to me that the pilot was blocked but it made no difference. I removed the jet block and made doubly sure that all the fine drillings were clear. The main jet is a 190 and the needle was 3 notches up so as they should be. As it sounds like this behaviour is not normal I guess it must be the airscrew adjustment (I left it set as it was) or a worn needle and/or jet block. I’ll have a play with the pilot adjustment next year (the bike is parked up downstairs now) and see if I can get it better. I’m actually used to running with no tickover as my old GT750 triple used to buck like a bronco on the over run with expansion chambers fitted so (as many do) I wound the tickover right down. The problem with the Scott was that small throttle openings were just not enough to keep the engine running and its rather all or nothing. Brian -thanks for the offer of borrowing a carb. I will take you up on that next year if jet adjustment doesn’t sort it.
3. I did suspect the black gunk in my gearbox could be causing the difficulty in gear engagement in addition to the very loud ‘clonk’ on down-changes. Again, its great to know its not normal behaviour. The clutch is not a joy to use either at the moment so I was planning on stripping the clutch/transmission down to check everything and get to know the bike better. I suspect that the gunk was added for the reason and that oil ends up draining out through the seals 😛 I have lots of receipts going back to the 70’s for lots of engine & chassis stuff but nothing relating to clutch and gearbox so I suspect they are due a freshen up!
4. It could well have been induction roar that was making me feel a bit nervous. I’m not used to ‘open’ intake systems. I look forward to being braver with the throttle next year 😀