Maintaining, tuning and riding your four-cylinder FN

I had a phone call the other day from an enthusiast who has just purchased a four-cylinder FN and wants to get it running. Some of the people he had talked to told him he was wasting his time because it is impossible to get them to run properly.

Just this morning I've been out on a very pleasant 40 km run on my FN, and I'd love to share my grin with anyone who has enough patience to get their FN running nicely. Yes, it is possible but it is rather tricky. As time allows I will add my "tips" here in the hope that someone may find them helpful.

If you need advice, or wish to straighten out my mistakes, please email me. Keep in mind that I have no (useful) qualifications relating to motorcycle engineering, and so I take no responsibility in relation to any of the tips listed here.

Valve and ignition timing
Fitting the magneto to a lightweight four
Remove the rear wheel
Dismantle the rear hub and bearings
Remove the rear sprocket
Understanding FN threads

Valve and Ignition Timing

  Inlet opens Inlet closes Exhaust opens Exhaust closes Ignition
1914 Four 8o after TDC 26o after BDC 50o before BDC TDC 32o before TDC
1914 Single 19o after TDC 22.5o after BDC 40o before BDC TDC 27.5o before TDC

The settings for the 1914 single come from an article Overhauling the 2 1/2 h.p. F.N. in The Motor Cycle in 1919. Those for the 1914 four (mechanically-operated side valves) come from my interpretation of the marks on the flywheel of a 1914 engine - where I have assumed that the exhaust closes at TDC. Pretty conservative timing: I hope the 1914 T.T racers had a bit of overlap on the valves! Of course in reality it will be impossible to get all four cylinders within say +/- 10 degrees of these settings, but it shouldn't matter very much.

Fitting the magneto

With the linkages connected, there is about 30 degrees of advance/retard available on the magneto. Remember that on a four-cylinder FN the magneto is directly geared to the crank shaft and runs at engine speed, the two cams giving two sparks per revolution. I have not seen the official recommendation for FN ignition timing, but my bike runs well set up to fire at top-dead-centre on full retard, or up to about 30 degrees before top-dead-centre fully advanced.

(In his book Le Mythe des 4 Cylindres en Ligne 1904-1954, Claude Reynaud gives, for the 1913 model, a figure of 5 mm before TDC for the ignition. With stroke of 57 mm and 110 mm connecting rods, this equates to 31 degrees before TDC. Sounds about right.)

Because the magneto is driven by gears, it can be removed (by undoing the three bolts) and replaced with care without disturbing the timing. Just note carefully the orientation of the contact breaker before you start. If the magneto drive gear has to be removed, however, you will need to retime the ignition as there is no key on the taper.

If the timing has been disturbed, it can be reset as follows. (Perhaps there is a better way, but this works OK.) Fit the drive gear loosely to the magneto. DO NOT TIGHTEN YET. Fit the magneto in position on the bike - you will have to remove it again in a minute. Remove a spark plug (pre-1910) or a cylinder primer from your favourite cylinder, and rotate the engine until the piston is at TDC on the compression stroke. Check that it is the compression stroke! If everything is OK, the brush in the distributor should be in contact with the terminal appropriate for this cylinder. If you think there may be something seriously amiss (e.g. if the engine can't be made to run) check this for all the cylinders.

Now, holding the engine at TDC by holding the flywheel, rotate the magneto armature gently until the points are just opening with the magneto at full retard. Gently remove the magneto without twisting anything. Push the drive gear firmly home on the taper and tighten. Refit in same orientation - if the motor is still at TDC, the points should just be opening on that cylinder.

To remove the rear wheel

  • Notice that the rear stand pivots on the back axle, and so is useless for performing this task! I have made a stand which uses a thin (say 6mm) metal rod which passes under the rear chain stays just in front of the rear wheel and is supported a few inches outside the frame. It is worth making something like this before you start. FNs are too expensive to drop.
  • Disconnect the foot-brake rod from the brake arm at the rear wheel.
  • If you have a handbrake fitted to a contracting band around the outside of the drum, you will need to disconnect this too.
  • Remove the bolt clamping the brake torque say to the frame. This bolt doubles as the band brake pivot.
  • Undo the three screws on the rear bevel cover, and remove the cover.
  • Undo the rear axle nut and withdraw the rear axle. The stand will come free and the wheel can now be pulled (gently) back and down to remove it. (This is where you find out if your home-made stand keep the back of the bike high enough off the ground to get the wheel out easily.)

To dismantle the hub and get at the bearings

Have a look at the wheel. You have pulled out the main axle, but there is a hollow "inner axle" still in the hub. There are actually three sets of bearings in the hub, one of which doesn't rotate as the bike is ridden (except when you're pedalling - but if you've got it running properly you don't need to do that!). The problem is that the bearing track pits badly where the bearings sit, and the pedalling action becomes very jerky. To remove the bearings, you will need a spanner for the large nut on one end (1" AF) and a piece of flat steel to act as a screwdriver for the other (sprocket) end.

  • In the vice, clamp a piece of 3mm flat steel so that it sticks up about 5mm.
  • Lay the wheel, sprocket side down, so that the piece of steel engages in the slot in the end of the axle.
  • Undo the nut at the bevel end with the 1" AF ring spanner. RH thread.
  • Remove the tab washer.
  • Using a peg spanner (two 3-mm pegs separated by 32 mm) or otherwise, unscrew the bearing cone. RH thread.
  • You can now remove the inner axle, and the pedal sprocket with its quick thread.
  • Notice the spacer that lives in the centre of the hub. The larger diameter end is closest to the bevels - note the indexing peg when you are putting it all back together.

Removing the pedal sprocket

  • Hold the quickthread in soft jaws in the vice, with the sprocket facing upwards.
  • Undo the retaining ring. RH thread.
  • Unscrew the sprocket. LEFT HAND THREAD.

Understanding FN threads

Why can't I find nuts and bolts to fit my early FN? If you don't own an FN, this might be just a bit boring, but  for the first time owner here's some vital advice. Threads on early FN motorcycles are very strange, and are for the most part unique to FN. There are some rules of thumb that may help.

  • So far as I can see, there is not one metric thread in an early FN four. Email me if you find one!
  • The FN drawing office seems to have worked in inches, so the diameter of all bolts, the diameter of all screw heads and all of the "across flats" measurements of hexagons on nuts and bolts are in fractional inches. You will need mostly AF (American) tools to work on your FN. Don't put away the metric spanners (OK, wrenches if you must) entirely - you'll find a couple of uses for them if you look hard enough.
  • So, yes, all those front fork nuts really are 5/8" AF, the cylinder bolts 7/16" AF, etc.
  • Many of the threads related to the front forks and wheel are British Standard Cycle (BSCy), that is 26 threads per inch (tpi). The back axle too.
  • Almost every other thread on the bike is strange. Most are 2 tpi more than the equivalent diameter Whitworth (BSW) thread, so you'll mostly find 1/4-22, 5/16-20, 3/8-18 and a few instances of 7/16-16.
  • So when you get your FN basketcase home, and you want to fit the tank to the frame, keep those 1/4-20 (BSW or UNC) screws away! All the tank mounting hardware is 1/4-22. Don't be tempted to screw in anything else - it won't fit and all you will do is strip the 100 year old threads.
  • Note that 5/16-20 is very close to 8 mm 1.25 mm pitch. It's close, but it isn't the same.
  • Of course there are other threads on the bike. You'll find quite a number of fine 5/16" threads - these are 5/16-28. My favourite is the turnbuckle that supports the magneto, which uses 5/16-28 left and right threads. Just for consistency, the screw that clamps the top of the turnbuckle to the frame is 5/16-28 too.
  • The small screws in the big end caps, oil windows, etc. are 3/16-30.
  • The really small screws around the place are 1/8-40 (conveniently the same as 1/8" BSW, but I'm sure this is just a coincidence).
  • Left hand threads occur in the strangest places - be vigilant. For example, the ring that holds the glass in the 1910 fuel filter (on the seat post) is a LH thread. Explain that if you can.
  • Real enthusiasts will find the one 7/16-16 Left Hand screw on the bike. Enjoy it when you do. (OK, it's the lock screw for the spur gear on the drive shaft.)
  • Parts that screw on and lock rings to prevent them coming of are often found in LH/RH combinations.
  • Most of the large diameter fine threads are 24 tpi, except the flywheel which is 28 tpi (LH) and its locknut which is 28 tpi (RH).
  • My 1923 FN four uses a few of the strange "FN threads", but there a quite a number of "ordinary" metric threads.

You've just got to love it! Working on an old FN is fun, but it's not for the faint-hearted. Just remember - if you are missing a nut or a screw, don't bother to look for one. You'll have to make it.

Copyrightę Leon Mitchell 1998-2007

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