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South Croxton
Leics. LE7 3RE

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   Check list for engine that will not start

Do you have an engine that does not run correctly?

Try the following steps to discover the cause.


1)  Remove door retainers

2)  Kick over to blow out doors

3)  Inspect face seals to see if any problems.  At the TDC position there is a cut out each side to park the rod while the crankpin is rotated past during assembly.  This leaves a thin face for the door seal and it is sometimes broken causing an air leak.

4)  Mop all oil out of crank chambers.  Only squirt a little oil into big end with oilcan through holes in outer roller plate if obviously dry

5)  While you are there, check for assembly accuracy using the following method

If you wish to test the integrity of an engine when you are building it, assemble without rings and big ends without roller plates.  Measure the distance of the rod to the crank cheek each side and record it.

Rotate the engine 20 revolutions forward, then stop and record the distance of the respective rods to the crank cheeks.

Rotate the engine 20 revolutions backwards and repeat the measurement.

The rod big end should not have travelled sideways by any noticeable amount.

If your engine is assembled, Take off doors and remove crankpin screw and outer roller plate.

Remove spark plugs, then check that rods are against the inner roller plate and rotate forwards as before.

Are the rods against the inner roller plate and if so, is the roller plate entirely free to rotate, or has the rod “wound over” and trapped the roller plate making it stiff to turn?

Have the rods “wound out” so that there is now a gap between the inner roller plate and the rod?

Rotate the opposite direction and recheck.

The rod should not track across the bearing to any great degree.

Problems associated with inaccuracies in these areas will cause the following:-

Outer roller plate blue and dished outwards

Blue rollers

Inner roller plate with wear on side faces

Very rough engine if little end out of alignment causes rapid rod wag sideways.

Rapid big end wear

Rapid little end wear

Rapid wear of bores for gudgeon pin in pistons

Loss of power.

Because of the accuracy required to get an acceptable result from a design that was of necessity a compromise between mechanical integrity and gas flow efficiency, even in the early lower powered slower revving engines which are more true to Alfred’s original design, the company took as its logo, the “Made to limit gauge”

Personally I prefer to control the stance and position of the little end by fitting control washers either side of the rod little end within the piston.  This may not have been economical for the Scott Company to consider, as, after all, they were a commercial concern trying to survive in a difficult commercial world. Anything that increased production costs and thus retail costs would have been difficult to justify.


6)  Check all round crankcase.  The Scott "Long Stroke cranks are weak and break.  About 30% of cases have had holes punched in from broken rods and an air leak can come from a badly welded up repair

7)  Seating faces for "Transfer port covers on crankcase and barrel are not always on a common plane and    may not hold the gasket firmly in all places.  Sometimes a piece where you can not see breaks off and an air leak results. 

I do not use gaskets, but assemble with Silicone RTV (as you use round baths) This works fine as it takes up gaps of varying size.  If you wish to use gaskets, I suggest a little RTV as extra security.

8)  Check fuel flow is ok (min 400 ml / min) at the bottom carb feed and tap filter is not blocked.

9)   Ignition about 33 deg BTDC at full advance (For a standard engine without gas flow enhancement)

10)  Main jet for a standard 17 / 19 bhp 600cc engine with Amal 276 15 / 16" choke carb is 190 (ie 190ml /

        min at 500 mm head through a hole of 4D length)  (Standard Engine)

11)  Best plugs are NGK Iridium BR6EIX long reach or BR6HIX short reach. Set gap 0.015 / 0.018”  Mag runs twice the speed of a four stroke, a smaller gap needs less voltage to fire and  gives a mag an easier time with no perceptible difference in output.

12)  Check mag spark  Note.  A large percentage of starting problems are due to a weakened magneto performance.

The fact that it will produce a spark in air, does not mean it will will perform under compression.

Take out brushes and clean slip ring and brushes. If the engine still will not fire, take plug gap down to 0.007" as a test and try again.  If you have had a spark in air at the original gap but it will not fire in the engine at 0.007" gap. then it is likely (but not absolutely sure) that the mag is not the cause of your problem.

Regap plug to 0.015" / 0.018" and continue with other tests.

13)  Assemble

14)  Try to kick start.  If it will not start, spray 1.5 secs ether or WD40 type damp start  down opened carb and try again.  If it starts briefly but then stops, the spark timing is OK but the fuel system is not performing,  OR you have a bad air leak.

15)  If it did not fire, swap plug leads over if it is a new rebuild, as it may be reversed here.

16  If you get it running but badly, keep it going and spray with WD 40 or a petrol spray  (WD 40 is safer!) all round doors, transfer port covers and anywhere that the cases have been repaired by welding.  If you spray on a point where the engine runs cleaner, you will most likely have found an air leak.  Do not stop here, but spray all relevant areas, as there might be more than one leak.

17)  If you do not find any faulty feature and the engine still does not behave, then the problem is more deep seated and could be from--

18)  Damaged crankcase glands.   This is usually caused by poor crank assembly and the cranks shaking loose and damaging crank and flywheel tapers.

   The crank then runs out of true and the gland which is controlled in axis by its fit on the one inch diameter crank stem, instead of running flat against the  gland face in the cup, runs like a swashplate and admits air.

19)  If the condition of the bore and pistons is not reasonable then running problems can obviously result.  Check if the problems have been becoming steadily worse or if they are a sudden and recent problem.  It is obvious that wear related problems do nor manifest themselves suddenly. © 2003

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