First run but not all is well.

[steph1960]

Well three years have passed (viewtopic.php?f=1&t=2344) and I did my first long run (80 miles) on my Scott yesterday which it completed successfully. Overall I was pleased but something isn’t right with the gearing or power delivery.

This hasn’t been a restoration, just doing what was needed to make it work and road worthy but with other commitments and projects time flies, as you know.

It starts third or fourth kick from cold and first time from hot and revs up freely and ticks over nicely. On the move it pulls reasonably well but not as well as I would expect, as the revs increase it sounds choked and progression from that point is slow, 45mph was where it was happiest. It bogs down from a standing start and this gets worse at the journey progresses, slipping the clutch is the only way to avoid stalling it. I spent a lot of time in second gear as this kept the revs up but it was sluggish up hills and felt choked when trying to pull beyond 50mph. I am suspecting worn bores but being inexperienced with two strokes there may be another problem, it felt as though the gearing was too high but on inspection after the run the sprockets were correct 40T rear and 19T front

I guess I am going to have to pull it apart and have a look but some suggestions as to a possible problem would be helpful.

Thanks
Steve

[BigKev]

Is the Exhaust restricted by anything up the pipe? KP

[steph1960]

Thanks KP.

I will remove it and have a look, its been standing a long while so you never know.

[dave bushell]

A 19 tooth sprocket is sidecar gearing. Solo should be 21 teeth. What effect are the choke and levers having ? The position of these can have quite a lot of effect on performance. For instance, if you progressively close the air lever, does the engine pull better? If so, it could indicate a weak mixture caused by air leaks, poor fuel flow, blocked jets etc. Operating the advance/retard lever could indicate a timing problem.
Dave

[steph1960]

Thanks Dave
I didn’t know that but 21 teeth would make things worse wouldn’t it?

The mag is reconditioned and I set the timing at 7/16 BTC on full advance, this was the sweet spot at which both pistons were firing at the same BTC setting, I didn’t think to move the advance/retard lever so I will try this before I set about removing the exhaust. I found that closing the air lever made the situation worse giving very little acceleration.

Steve

[dave bushell]

7/16″ may be too much. I usually go for 5/16″ btc, fully advanced. Anyway, you can reduce the advance on the mag lever and see if it makes any improvement before retiming if necessary.
Dave

[BRIAN MARSHALL]

7/16″ is definitely too much, and with modern petrol 5/16″ should be the absolute maximum or you run the risk of crank-breaking ‘detonation’. Do make sure that your petrol is really ‘fresh’.
Brian

[steph1960]

Thanks Dave and Brian
I will check again but I found that 7/16 was equal to both cylinders if I was to set one at 5/16 the other one will be firing too soon. Could the cam ring on the mag be wrong?

Steve

[BRIAN MARSHALL]

Yes, it sounds as if the ramps on the cam ring are not at 180 degrees from one another, a fairly common problem with Lucas mags, but rare with BTH and Bosch mags. Probably a job for a magneto specialist… There is a remote possibility that your cranks are not quite at 180 degrees to one another if the keyways were mis-machined, but I would check the magneto first. I once came across an engine with a longstroke conrod on one side and a shortstroke rod on the other, which would also cause a similar problem, ditto a mix of longstroke and shortstroke cranks, and ditto pistons…. Lots to check !
Brian

[BRIAN MARSHALL]

I should have asked if you know the history of the engine ? Did it run well prior to its hibernation, or is it an AJS ? (Autojumble Special !).
Brian

[dave bushell]

It’s also not unknown for engines to be reassembled with the pistons reversed. They will still run but be gutless!
Dave

[steph1960]

Thanks Brian and Dave
I bought the bike as a complete non runner and the engine had been rebuilt in the late 70’s and had only covered 400 miles but it hadn’t been run since the early eighties. All I have done is get it running and make roadworthy to use. I had it running very soon after getting it home but it was a difficult starter and wouldn’t tick over, this was traced to a poorly sparking Lucas MN2 mag with a slipping cam ring. I had the mag reconditioned by Dave Lindsley which included a new cam ring so I am assuming this is correct, he does come highly recommended. That’s all I know about its history and none of that is proven, for all I know the engine could be worn, it does sound very nice ticking over and revs freely when not under a load though.

I will check the cam ring and timing tonight if I get a chance. Apart from this and checking for a block exhaust it looks like it will need an exploratory strip.

Steve

[Ian Parsons]

If it starts easily then the bores and crankcase seals should be OK and I would not be looking for air leaks. The timing should be set to 5/16th ” BTDC on full advance on the leading cylinder and measured for the other cylinder . It may not be as far out as you think.

I would also çheck the carburettor jet and slide to make sure that they are correct.

[steph1960]

Thanks Ian
Its comforting to know the engine should be in good health. During the run on Sunday I noticed I had a comforting light haze from the exhaust but it did smoke more under load, on removing the plugs last night the electrode colour was a browny/grey, is this normal as I was expecting some oily deposits?

I did some checking last night and found the cam ring to be accurate at 180 degrees. On full advance and equal to both cylinders the optimum setting is 7/16 BTC and on full retard this is 1/8 BTC so if I run at half lever setting it should be around 5/16 as Dave suggested. I will try this at the weekend and see if I get an improvement.

I will carry out the setting you suggest and see what measurement I get on the other cylinder, do you think it is best to run at the setting you suggest or as above?

Do you know what the jet and slide should be for a 1947 Shipley Scott please?

Steve

[Barrie]

170 main, slide 6/4 .
If you have any grey on the plug this indicates that you could be running with too much ignition advance.

[Author]

Posts