Thumper

Petrol Tank and Fuel line

Where the outlet screws into the tank isn’t great quality, unscrew the outlet and apply either hot glue or araldite around the threads, screw back in and apply more glue around the outer section to reinforce it.

There is only a coarse strainer on these until the fuel hits the carb (small strainer in the side of the carb body) so flush out the tank well before starting the bike. On Mk1 reps there is no spring around the fuel line to protect it from the exhaust (its very close) but on Mk2’s the required spring is there, although the fuel line may be a bit short and make tank removal awkward, best to put a new length of pipe on. The spring clip that comes supplied is hopeless and will need replacing with a better one.

Check to see if there are rubber blocks between your frame and tank, Mk2’s have said rubber blocks. If you don’t have them buy some foam tape from B&Q and stick it to the top side of the frame, in fact stick it anywhere the tank touches as the vibrations will soon wear through the tank. A rubber tap washer (again from B&Q) underneath the front tank mounting bolt will help and prevent it from coming loose.

The filler neck of the tank can wear and make the cap loose fitting, if this happens smooth some chemical metal around the inner lip of the tank (only a thin film) to fix it.

After more long term use the petrol tanks on the reps aren’t very good quality at all and will throw the filler cap far too easily, plus, like the genuine tanks they aren’t keen on being painted, the rep paint peeled after a couple of weeks and looked rough. I bought a genuine tank and cap for £30, you don’t need the rubber blocks under these tanks as they much thicker plastic and a much tighter fit, plus you get the convenience of a fuel tap on the tank itself

Radiator and Cooling System

The bracket that the radiator mounts to isn’t the best, and touches the rad when tightened up. Vibration will eventually make the bracket wear through the radiator and leak. To prevent this remove the rad and glue a piece of old inner tube or foam tape to the bracket itself. If yours is already leaking don’t worry, the radiator is copper and can be soldered up.

To fill the system with water remove the radiator bleed screw (top left on a mk1) or remove the breather hose (top left on a mk2) and fill the bottle, blow on the neck of the header tank to force the liquid into the rest of the system, keep filling the tank up and blowing into it until liquid comes out of the bleed screw (that you’ve already taken out) rotating the back wheel so the pump circulates water will help to remove any unwanted air.

The header tank cap is pretty poor quality but is a different thread to the original polini part, its best to swap both tank and cap (part no 143.330.003 Coolant Bottle/Cap 911 & 911GP2) which will cost you £6-£7 but well worth it.

Bodywork

The fairing is held together with nuts and bolts which are a pain to remove, especially when you’re rushing around in the pits trying to make the next session. Replace these with original polini quick fastners (part no 143.305.023 Nutserts 5 mm qty-8 £8.95)

A good idea is to remove the black front cowling which the forks popke through as it lessens the chore of removing the bodywork.

The stock bodywork is very brittle and will snap easily if you’re unlucky, standard polini parts are quite cheap and a lot stronger.

Brakes

The twin front brake on these bikes is very powerful, but if badly set up can have no feel to it whatsoever. Replacing the plastic brake levers with metal ones may help.

First, slacken your front brake cables so that both operating arms on the calipers drop back to their rest fully, then adjust the play in the cable out so that they are equal, put the bike on its stand and get someone to push down on the rear to lift the front wheel, loosen the locknut and allen bolt to wind the pad away from the disc, then adjust the knurled knob to bring the inner pad as close to the disc as possible WITHOUT binding. I adjust it up till it binds then back off just enough to clear, do this on both calipers first, all the while keep spinning the wheel to ensure its free. once your happy that the inner pad is set right adjust up the allen bolt until the outer pad touches the disc and locks the wheel...then back off enough so its clear - DO THIS ONE CALIPER AT A TIME! When your happy its right lock up the locknut and repeat on the other calliper, you should find the brakes will be very sharp and enough to put you over the handlebars.

Alterntatively you can buy some decent quality mountain bike levers from Halfords for less than £20 and remove one of the front discs, I personally found this to give the best ‘feel’ with plenty of power

The rear brake is floating and is adjusted via the allen screw and locknut. Check the mounting bracket as mine was bent and pushed the calliper out of alignment.

Clutch and Chain

The standard clutch is very similar to a 3 shoe cag clutch and is non adjustable so no use whatsoever, bin it. Replace it with your choice of Polini or Bizeta 2 shoe adjustable clutch. You’ll also need to swap the drum and pinion, make sure you get the one for rear wheel driven pump models. These will work well with the polini yellow springs set to bite at 6500-7000rpm, or 7500rpm if you want a bigger hit when it engages. Getting your shoes relined with Kevlar (ask Vaughan)

Either Bizeta yellow springs or bizeta blue springs with weights both work well.

The genuine polini or bizeta clutch housings have a slightly larger lip on the inside which interferes with a good fit. I’ve ground this lip off, relying on the four mounting bolts to centre it up, with no ill effects.

The standard chain isn’t very strong, replace it with Bizeta, its £15.00 a metre, one metre will be ample.

Replace the rear egg shaped sprocket with a decent one and all your chain tension worries will go away, the original sprockets are egg shaped and make setting even tension very difficult. To change the sprocket you will either need to drill holes to fit your water pump pulley too, or mod the pulley by one of the following two methods

1) Use chemical metal or Quik Steel to glue the pulley to your back wheel.

2) Drill 3 3.5mm holes and tap them out to 4mm from the outside of your pulley in towards the middle, so that when a grub screw is fitted it will clamp the pulley to the wheel itself.

Mk1’s come with a 6 tooth pinion installed but Mk2’s come with an 8 tooth, for better acceleration swap it for a 6.

Buy the polini chain guard that slips onto the front part of your swingarm section, it only costs four quid and saves the chain battering the swingarm to death.

Exhaust

The exhaust silencer, which looks nasty with the downturned pipe, is held on with 3 screw, these do work loose and should be loctited in place, the mounting bracket isn’t too clever, remove and refit it to be sure. You can swap the silencer for the polini part but this will need to be welded onto the existing system (Part no 143.285.004 Exhaust Muffler – 911 Racing Carbon Fibre - £22.90 or 143.285.005 Exhaust Muffler 6.2hp 910 Steel - £14.50)

Replacing the mounting bolts with proper polini rubber mounts is a good idea to cut vibrations (part no 214.0104 Exhaust Rubber Support Mounting Bolt – M6 - £4.00)

For a good increase in power fit either the genuine 911 race exhaust or preferably the 911 GP2 Evo exhaust, both fit straight on with no mods needed.

Frame etc

If you have a Mk1 the subframe is held together with rivets which aren’t that strong, especially if you’re fond of pies, replace these with 5mm bolts and nyloc nuts. The Mk2 has this done already.

The stock rubber footpegs aren’t very strong and may vibrate loose, loctite them on when they they do break replace them with polini ones (part no 143.763.001 FOOTPEGS STANDARD - £4.10)

The chunkier Polini pegs also fit and are much more comfortable than the skinny type, plus they withstand crashing better.

Be very careful when tightening up the fork clamps, the bolts screw straight into aluminium.

Engine

Some units can be very tight from the factory and can stall when combined with the awful standard clutch, careful running in is recommended if you have this problem.

On Mk1 bikes the woodruff keys may be made from chocolate, swap these for good quality ones from Polini

Tighten your cylinder head bolts – mine were loose

There has been a problem on one of the bikes with a very sloppy fitting flywheel, the problem was that when the but is all the way tight its still not up against the flywheel, put a washer on after the flywheel, before the but to cure this. Mk2’s may already have this done.

The main bearings in these motors are awful. Better to rebuild before they let go completely.

The bearings you need are 6202 C3, and the seals are 15mm bore, 24mm outer and 5mm deep. These are available from your local bearing supplier and the total cost of a rebuild if you do it yourself will be less than 20 quid.


Thanks go to Rick-GSX, Beanie-Polini and chute for writing up this amazing guide!

Tom