feetupfun

It is fairly common for premix fuel to form a gum on the surfaces it is in contact with, if the bike is left with fuel in the float bowl between rides. 4 weeks is long enough for this to happen. If you are going to leave the bike for more than a few days between rides, it is prudent to run the float bowl to empty at the end of your riding session or to drain it after your riding session. The main two places where this gum tends to cause problems are at the float needle valve and inside the pilot jet orifice. Your problem may be from this, or it could be one of a few other causes for a float needle valve not sealing off properly. To diagnose the cause of the problem, the carby will need to be taken off the bike and the float bowl taken off to allow internal inspection. It's good that you have learned that owners manuals are to be treated with suspicion.

The difference you are feeling is not an illusion. It is real and yes it is due to differences in geometry and mass distribution. The small differences in total mass between bikes are not a good guide to how a bike will feel to ride. People's bodies also have different geometry and mass distribution. Which bike feels best to each different person is why people should try before they buy. There is no single "best handling" or "lightest handling" modern trials bike for all riders.

The main reason why the 250 handling feels heavier is the increased gyroscopic effect of the heavier crankshaft/flywheel assembly.

Model 27/49/80 brake hubs are cast aluminium with a cast iron friction surface. The front 27/49/80 hub also has equal diameter spoke flanges. There are many things you can do to make the brakes work very well. As you already have mentioned that some of them have been done, here are other things that are important. Wheel bearings have no play. Drum friction surface is smooth (not grooved), parallel (not tapered) and round. Bikes that old usually are woeful in these areas unless the drum has been re-machined or resleeved. The friction material is of a modern high friction type, available from specialist shoe relining people. A thinker lining than off-the-shelf shoes have is usually required to achieve a big enough diameter for the lining OD on the backing plate to match a worn or remachined drum. I'd be surprised if the radius of off-the-shelf new brake shoes matches the drum curvature of such an old bike, unless the drum has been releeved back to original diameter. If you can pull the lever all the way in, then a likely cause of this is that the radius of the linings does not match the curvature of the drum. The springiness that allows you to pull the lever in is probably the shoes being flexed away from their relaxed shape. This flexing may well provide a contact witness on the shoes but makes the brakes useless. So my suggestion is to have the drum machined smooth and parallel and the shoes relined thicker and machined back to match them to the new drum diameter.

Impressive. Are you saying that motorbike fork tubes are manufactured to that degree of accuracy on diameter?

What stage are you at in the rebuild? I'm trying to work out why you need to compress the springs

There is variability in tube diameter when tubes are made new and when they are re-chromed. The process of finishing them (centreless grinding) is not a precision process, nor does it need to be.

Welcome Martin

What do you need help with?

If you want to do it the original way, from memory there are wiring diagrams in the owners manual and in the generic 5 speed Bultaco service manuals.

If you want to run lights, a total loss (12V battery) system running LEDs is a popular (cheap) option. The standard lighting supply is only nominally 6 volts and the voltage is highly RPM dependent. At idle and low RPM the standard lighting is very poor. However for the ignition electrics, there's nothing wrong with the standard points/coil system design/equipment.

Modern fuel + premix oil + float bowl not drained often means that you need to physically clean the pilot jet and its passages (rather than blowing air through)

You can test this yourself by moving the axle forwards then undoing the spilt link and overlapping the chain ends on the rear sprocket. If one link difference is too much, the normal fix is to fit a "half-link". These are readily available

Welcome AJ. There is an OSSA forum on here where you might get more info. Go to Forums, then Bike Specific Forums, then OSSA

I don't have a motor apart to measure but there are two pushrods to minimise clutch drag (the ball between the two rods means that the actuator end push rod doesn't spin while the clutch is disengaged)

If you can get the studs out you can use countersunk head screws instead and sandwich the spacer

Even heavily corroded crankshaft halves can usually be returned to serviceable condition by an engine re-conditioner and rod kits are readily available

Yes that's true. Many shocks do have a floating piston to separate the oil and gas. Standard Falcon shocks don't have a floating piston or a bladder or a piece of closed-cell foam. The oil and gas mix.

They look like early Falcons. The valves are there to allow them to be pressurised with gas. They need to have gas pressure inside to work properly.

https://www.frankiemountain.com/en/compra/bultaco-alpina-exhaust-new-bultaco-alpina-muffler-model-138-bultaco-1591

Having a stepped spacer like that makes it super easy to push the old bearings out compared with a spacer that has the same ID as the bearings

If you want to be certain of having enough oil, set the oil quantity by level rather than volume. A base-line starting point is 125mm dip to the oil surface with the forks fully compressed metal-to-metal. At that point, extend the forks fully and look down and see if the top end of the damper rod is still covered by oil. If it is exposed, add enough oil to just cover the top end of the damper rod. Then compress the forks again and see what the dip to oil is. More than 110mm dip is enough air space. I suspect that 48mm preload may be too much preload. I have the same forks, damper rods, oil weight and same springs and weigh 220lb dressed to ride and run about 32mm preload. My preload spacer is not the adjustable one from Magical Springs.

Warm the aluminium with a hot air gun and the bearings will emerge with much less persuasion

I reckon I could work out pretty much all that you wrote except what does the F stand for in FNG?