J.A.P. 4B and 84S A guide to maintenance and overhaul


The modified engine type 4B, has a much improved performance especially at higher RPM than previous types 4 and 4A. This is due to improved breathing capabilities brought about by an increase in the inlet tract diameter. This ' Big Port' engine has a 32 mm inlet port as against 28,5 mm on the previous engines and is fitted with the new Amal 932 Carburettor which is adapted for methanol fuel. The need for a large bore inlet tract has been realised for some time and it is a logical conclusion to the larger inlet valve modification done when the engine was converted to 4 stud in 1949 but no large enough carburettor suitable for methanol fuel existed before the introduction of the new Amal 932 which is now available and has proved entirely satisfactory under test. In fact, this new concentric type carburettor is much more efficient than the previous type 27/013 twin float chamber track racing carburettor. The new 932 Carburettor is mounted on a 1,5" inlet pipe by a mounting clamp and is removable by slacking off 1 Allen screw in the clamp, this also allows the carb to be turned to facilitate removal of throttle valve and float chamber. The better breathing facility of the new engine makes the use of ' hairy' high lift camwheels used by some riders, particularly on the Continent, unnecessary as these are only used to try to help the cylinder fill properly which it can now do more efficiently without, and any camwheel with greater lift than the engine was designed for causes unnecessary mechanical troubles. The basic design of this engine, developed over forty years of continuous racing embodies Electron crankcases, Dural con rod, big end and main bearings in caged rollers and needle roller rocker levers. The "Marelli" Polar-Inductor type magneto is driven by 3/8" pitch roller chain and lubrication is of the total loss type supplied by Pilgrim Duplex pump driven from camwheel. The engine is extremely simple to overhaul, requiring no special tools, is completely reliable and efficient in service.


Bore: 80mm.

Stroke: 99mm.

Capacity: 497 cc

Compression Ratio: 15 : 1

Weight: Net 63 Ib.; Magneto 2 ¾ Ib., Carburettor 2 Ib.

Magneto: Marcelli Rotating Magnet type. L.H. rotation.

Carburettor : AMAL type 932. 12½° downdraft.

Oil Pump: Pilgrim Duplex B.I.

Sparking Plug: N.G.K. B-9HN or B-10HN (½" reach).

Fuel: Methanol (Methyl Alcohol CHgOH).

Oil: Castor Base (Castrol R.40 or Newton R.).

Engine Sprocket: 19T (for ½x 5/16" Renold Chain). 16T to 22T available if required.

Fits and Clearances

Standard Valve Timing: (Camwheel Part No. 18989)

Exhaust: Opens 65° before BDC. Closes 34° after TDC.

Inlet: Opens 44° before TDC. Closes 62° after BDC.

Magneto Ignition Advance: 34° before TDC (15.1 Comp. ratio).

Piston Clearance: .006" at bottom of skirt on thrust faces (at right-angles to gudgeon pin).

Piston Ring Gap: .008" (minimum).

Tappet Clearance: .002" (engine warm).

Magneto Contact Breaker Points : .015".

End Float of Con Rod between Flywheels: .010" to .012".

End Float of Flywheels in Crankcase: .012" to .015".

End Float of Camwheel: .003".

End Float of Cam Levers: .003".

End Float of Rocker Levers: .002".

Compressed Length of Valve Springs: 1 5/16" (valve closed).


General Overhaul and Maintenance

More than forty years of development work has been put into the production of the J.A.P. speedway engine, therefore you will not be able to effect any radical alteration which will greatly improve the performance of this engine. The emphasis on tuning and overhaul rests on absolute cleanliness and accuracy of fitting when carrying out any overhaul operations. The following paragraphs give an outline of the points where care is required.



Due to the high power output of this engine a careful check must be kept on big-end wear and any signs of up and down movement must be rectified immediately by fitting a new crankpin and/or big-end liner and rollers. This is a straightforward operation, being particularly careful to examine the con rod for cracks, any scratches should be carefully polished out longwise (along the length of the con rod) remembering that cracks start on the surface, usually from scratches across the rod. When reassembling, the con rod should have a side float between the flywheels of .010" minimum. When the flywheels have had several crankpins fitted the tapers ' sink' causing the wheels to ' nip' the con rod and this is remedied by fitting an oversize crankpin, this oversize is in length between tapers (not diameter). There are two oversizes. Part No. 11453/1 (1/32" longer between tapers) and 11453/2 (1/16" longer between tapers). The flywheel assembly must be accurately trued between centres after assembly and the crankpin nuts properly tightened, this requires a jig to hold the assembly and proper tools and should be carried out by an experienced mechanic. Under normal speedway conditions the life of a con rod is one season's racing and it should not be relied upon for longer than this as a broken rod can be a very expensive business if it wrecks the crankcases.



Before assembling the flywheels into the crankcases there are several points to be checked. The camwheel should be checked and tried in its bushes with the magneto drive bolted on, the camwheel should rotate freely with adequate end float (.003") and there should be no slackness in the bushes as this will affect valve timing. The cam levers should be checked for wear on the rollers, pins and bushes and then assembled on the cam lever pivot pin with the distance piece and the magneto drive again bolted on to ascertain that they are free to move and have correct end float Check wear of main bearing liners, if it is necessary to remove one of these the crankcase must be heated to remove the old liner and the new liner inserted while crankcase is still hot, insert in correct position to engage the liner retaining screw, remember do not overheat as Electron is highly inflammable although not dangerous except in the form of filings or swarf. Oversize liners +.003" and +.005" on outside diameter can be supplied where crankcases require these. When fitting the flywheel assembly into the crankcases it is most important that the con rod should be exactly in the centre of the cylinder barrel and that the assembly should have approximately .012" end float, this is not critical provided the actual end float can be felt by pushing and pulling the driving spindle when crank-cases are bolted together tightly. To obtain this, thrust washers are supplied in three thicknesses, two of each thickness will, by trial and error, enable the correct position and end float to be obtained. Check that the top faces of the crankcase are flat and level as otherwise the cylinder barrel will not be vertical, all crankcases are machined in pairs and cannot be separated for this reason.


Piston, Barrel and Cylinder Head

Valves, valve guides and springs require careful attention and should be renewed as necessary. The valve seating in the cylinder head requires to be kept narrow and about 1/16" is the amount to be aimed at. In attempting this do not take large amounts of metal out of the cylinder head as this cannot be replaced and by doing so you are reducing the compression ratio and hence the efficiency of the engine. It is advisable when valve seatings become very wide to have hardened valve seat inserts fitted which will last longer than the original cast-iron. Mica washers are fitted underneath the valve spring cups to adjust the valve spring length to 1 5/16" when valve is closed, at this length the valve springs should exert a pressure of 145 Ib, if this falls below 100 Ib. the valves will bounce, causing severe damage.

Check piston ring gap in cylinder barrel. This should be .008" minimum. The piston clearance is determined by the piston manufacturer and it is only necessary to be sure that your cylinder barrel is correct in size (80mm. or 80 mm. + .010" etc.) and that it is not oval or taper. Modern methods of manufacture of piston rings ensure extreme accuracy and it is usually safe to assume that if the piston ring, as supplied, has the correct gap when put into the barrel then the barrel is correct in size, ovality can also be checked by holding up the barrel to the light with the ring positioned in various stages up the barrel. It is advisable to have a honed finish to the cylinder as the ' cross-hatched' pattern enables the rings to bed quickly.

Fit piston to con rod; it may be necessary to warm piston to remove and insert gudgeon pin. (Do not use force.)
Check that gudgeon pin circlips are tight fitting and secure in piston and assemble cylinder barrel.
Before fitting cylinder head, anneal the copper joint washer by heating cherry red and plunging into water, check that the cylinder head sits on the copper ring and this can be ascertained by inserting a 1½ thou. feeler gauge between the head and barrel after cylinder bolts are tightened.


Compression Ratio

This can be checked with a 50 cc burette, using water as this will not penetrate past the piston rings if the barrel and piston are well smeared with oil during assembly and the surplus wiped off before fitting the head.

Lean the engine over until the plug hole is level and allow water to fill combustion space up to bottom of plug hole. This amount (say 35 cubic centimetres) is then added to the swept volume of the piston (497 cc + 35 cc = 532 cc) which is divided by the measured amount (35 cc) and would give a compression ratio of 15.2:1. Compression plates are available in three thicknesses 1/64", 1/32" and 1/16", the 1/64" compression plate will increase the combustion space by 2 cc. This knowledge will save re-measuring should the original assembly need to be modified. Turn engine upside down to empty water out and if the head is not to be removed again a little methanol fuel can be introduced into the head, this will dissolve the water, preventing rust, empty methanol out and a few turns of the engine will allow the remains to evaporate.


Ignition Timing

Higher compression ratios require less ignition advance than the lower ratios and for 500 cc type 4, and 4A engines a general rule can be given which is 1 ° of advance for every 1 cc of combustion space, therefore 15:1 compression ratio (i.e. 35 cc) requires 35° advance before TDC.

For 500 cc Big port engines type 4B subtract 1°, therefore, 35 cc requires 34° and so on.

The above rule does not apply to 350 cc engines but 15:1 compression ratio requires 34° on 350 cc engine.


Valve Timing

Before commencing to ' time' the camwheel and magneto the timing disc and pointer must be accurately set up to give exact Top Dead Centre. Set the pointer to read TDC by visual means, looking through the plug hole, this, however, is not accurate enough for our purposes and a small ruler or other object should be inserted in the plug hole to prevent the piston coming to the top by about 30°, turn the piston down the cylinder and up the other side until it again touches the obstruction and if the two readings differ move the pointer until they agree. Before commencing to time the camwheel the backlash between the teeth of the camwheel and pinion must be determined and pinions are supplied with different thicknesses of teeth for this purpose, new engines generally have standard or +6 sizes and these, together with —6 and +10 will usually cater for all variations. Remember that the pinion nut is a left-hand thread as is also the flywheel nut on the other end of the timing spindle. These are the only left-hand nuts on the engine. Assemble the camwheel on to the pinion and bolt the magneto drive on. The backlash can be heard rather than felt when rotating the camwheel spindle if assembled without any oil. Any tendency to ' bind' will also be felt by absence of end float in the camwheel and cannot be permitted. Too much backlash will cause the teeth to chatter and fail in service. Assemble the cam levers and distance piece and push rods, without outer covers, fit valve end caps and adjust tappet screws to give a clearance of .003" on inlet and .004" on exhaust. Spin push rods to ascertain that these are not bent. Push down the lower halves of the valve spring covers so that the valve collars (cone end) may be rotated between finger and thumb. As the valve itself is integral with this collar, it will be impossible to move when the valve is resting on its seat but as soon as the valve opens it is possible to rock the valve collar back and forward between the fingers and the actual time when the valve opens can be read off on the timing disc.

If unable to obtain exact timing try to balance the errors across the overlap period when both valves are open. When both valves are open it should be possible to feel the amount of backlash between the camwheel and pinion teeth and should not measure more than 1° on timing disc. The timing pinion has five evenly spaced keyways and by trial and error the pinion can be positioned within 3°. After timing valves re-adjust tappets to .002".


Valve Clearance

It is now necessary to ascertain if there is sufficient mechanical clearance between the crown of the piston and the valve head. This clearance should be 3 mm. The standard tappet adjuster spanner is approximately 3 mm. thick and can be used for this purpose by inserting it between the valve end cap and the rocker lever with valve closed. This will open the valve 3 mm. more than normal. Then turn the engine slowly over the overlap Top Dead Centre and ascertain if the valve touches the piston. Do each valve separately otherwise they will hit each other. If there is insufficient clearance a compression plate must be fitted irrespective of the effect of reducing the ratio which can be compensated for by an increase in the magneto advance.


Special Camwheel (Part No. 23154)

Our special camwheel with increased overlap period gives a valve timing:

Exhaust: Opens 90° before BDC. Closes 53° after TDC.

Inlet: Opens 60° before TDC. Closes 80° after BDC.

and in addition to the above checks care must be taken with the ' special' camwheel to ensure that the valves do not ' clip' each other during the overlap period and it may be necessary to slightly reduce the valve head diameters to ensure this.

Do not forget to fit the oil tube, spring and washer and ensure this is in place correctly by inserting a straight piece of wire through the magneto drive and into the ball end of the oil tube so that this will lead the ball into its cup, when the magneto drive is finally placed in position remove the wire.


Magneto Timing

This is very critical, see paragraph on compression ratio. Check sprockets for alignment and fix magneto to give correct tension of chain, check contact breaker point gap (.015") and take up slack in chain when timing, remembering these sprockets run in opposite direction to crankshaft. Should it be necessary to re-time, loosen camwheel nut and tap camwheel sprocket round with brass drift, this will lift sprocket off its taper, after final checking that ignition timing is absolutely correct fit oil pump driving thimble before replacing cover.

Should engine performance be down or if you have any doubts about the magneto, have this tested by a competent magneto repairer and in any case have it periodically overhauled and tested.


Oil Pump

This is actually two pumps, one supplies the big-end via the oil tube and this is the one furthest away from the engine, controlled by the front adjuster and it should always be set to give the maximum flow. The inner pump, controlled by the rearward facing adjuster supplies the rocker box only and this should be adjusted to supply a very small quantity (one drop every 2 pulses of the pump). Any excess oil will be drawn through inlet valve guide causing oiling-up of sparking plug. Check function of oil pump regularly (a few drops of castor-base oil smeared on the window will keep it from clouding over).



The Special Methanol type AMAL 932 carburettor is designed for use with alcohol fuels under gravity feed. This carburettor passes large quantities of fuel and it is necessary to ensure that the fuel taps and pipes will supply enough fuel and the vent hole in the tank cap is sufficiently large to prevent starvation.


84S Type Engine

This engine nicknamed the ' Short Stroke' engine is not really a short stroke engine in the proper sense of the term which infers a shorter stroke than bore ratio. Its 90 mm stroke is shorter than the 4B engine which is 99 mm stroke. The 84S type engine is not intended to be a high revving engine and it's low speed torque and power is similar to the 4B engine but it will achieve higher RPM up the straight. Do not under-gear this engine, the best results will be obtained using the same high gear ratio as the 4B engine and certainly not more than 1 tooth extra on rear wheel sprocket from what would normally be considered JAP gearing. The main differences apart from the bore and stroke are a shorter and strengthened alloy con rod, an alloy barrel with steel liner and ' squish' top piston, and a larger inlet port and carburettor (34mm).


Maintenance procedures are similar to the 4B engine but the 84S engine requires more ignition advance and we have found 40° before TDC to produce the best results. When assembling the engine particular attention should be paid to the ' squish ' clearance between the piston and cylinder head, remembering that when engine gets hot the con rod will expand slightly in length so an allowance of about ½ the thickness of a cylinder head washer should be allowed when the engine is cold to prevent the piston touching the cylinder head when the engine is running hot.


Bore: 84 mm.

Stroke: 90 mm.

Capacity: 499 cc

Compression Ratio: 14 : 1

Weight: Net 59 Ibs.; Magneto 2 ¾ lbs.; Carburettor 2½ lbs.

Carburettor: AMAL type 1034. 12½° down draft.

Ignition advance: 40° before T.D.C. (14 : 1 Comp. ratio).

Valve Timing: As for special cam wheel pt. No. 23154.