The Philips headed screw between the two pipes is the bleed screw

Two Stroke on a budget – Aprilia RS125 (Part 8)

Aprilia RS125 air filter housingTwo Stroke on a  budget Episode 8 – The fun continues.

The electronic ink had barely had time to dry on the last episode when the postman threw another couple of parcels through the back door. Fortunately he had opened it first. The first small package just happened to be the thin plastic tube I had ordered to go from the oil pump to the carb. Only it wasn’t – after waiting 11 days for the bloody stuff they sent me 5mm instead of 3mm. Happy I was not. The second parcel was a bit better though – this was the air filter housing I only ordered 4 days ago. And it was the right one.

Excitedly I went to the workshop and put the filter box in place and then scurried off to find a suitable Jubilee clip. Did I find one? Did I buggery but what I did find was a length of 3mm piping I didn’t know I had! Ho hum.

So next job was to check the oil pump set up and fit a new length of pipe. The pump has a mark on it’s body that matches up with a mark on the quadrant when the bike is running at tickover. It puts more oil in as the throttle angle increases. Mine had been set to deliver a bit too much oil so I wound it back a touch, still on the safe side but much closer to the manufacturer’s marks. It’s easy enough to adjust there is a 10mm adjuster on the pump end of the cable. Getting the new pipe fitted was a bit of a chore as it’s quite awkward to manipulate in the small space available. I got it on there soon enough though, cut it to length and clipped it in to the clip moulded in to the engine casting. I left the top end open so that I could bleed it through.

Aprilia RS125 top endThat turned out to be a bit harder than expected. You can bleed as far as the pump because there is a screw on the pump body that undoes to allow any air out. Easy enough, if you see bubbles there is still air in the system, leave it open until all you get out is oil. There is no way of bleeding the pipe from the pump to the carb though so I disconnected both ends and used a surgical syringe full of oil to bleed it through. It just makes sure the engine doesn’t get starved of oil. I guess I could have run it on some 2 stroke mix until the pump pushed the air out but I felt happier with my method – especially as I didn’t want to put the knackered exhaust back on so I could run the motor only to have to take it back off again.

The Philips headed screw between the two pipes is the bleed screw, take it out and any air will come out followed by oil. Note the mark on the quadrant is not quite aligned with the mark on the pump body, I corrected that just after this photo was taken.

Next job was as simple as they come but essential – change the gearbox oil. The Rotax engine has a 6mm Allen head bolt, remove it and let the oil drain. Check the bolt for any bits of metal – it has a magnet sticking out the end of it. There were a few tiny bits, not unusual in a bike gearbox but nothing to worry about. I cleaned it, put it back and put 600cc of quality 10W40 back in there. It’s a bit weird on the Rotax, instead of putting the oil filler on top of the engine it is in the side, which makes filling a bit tricky. I cut a length of tube that went on the end of a funnel so I could bend the tube through 90 degrees and make the job as easy as it should have been in the first place. You also have to be a bit careful undoing and doing up the filler plug – it’s plastic and softer than a very soft thing that’s been left soaking in soggy juice. Bet they cost north of a tenner if you break one too.

Next job was to replace the choke cable, which was quite uneventful and not really worthy of too many words. The left hand switch pack comes off, you remove the old cable, fit the new one and then fit the other end to the choke plunger. I had to go hunting for a new return spring as the old one was rusty and mangled. Its important to adjust the cable so there is a tiny amount of slack with the choke lever in the off position – you don’t want the choke slightly on during normal running. A couple or three cable ties help tidy the job up and give it a finished look.

bespoke Aprilia RS125 fairing standNext job was to make a fairing stand off as one had been broken before I got the bike. They are not available any more so I had to make one. It should be aluminium ideally but as we are still on lockdown I had to use whatever I could find – a length of brass as it turned out. I bough this to make a fitting for a steam engine but need as must and all that.

Anyways, first job cut it to length – 41mm should do. The bandsaw is the best tool for that job.

Next transfer it to my trusty old Henry Milnes DF41 lathe, it’s about 65 years old but still works perfectly. Face off both ends then drill a hole down the middle of it.

Now turn down one end to 8mm ready to take a thread, the hole in the frame has already been tapped ready at this stage.

bespoke Aprilia RS125 fairing standNow put an 8mm thread on it to match the one I tapped in the frame. Screw it in to the frame and the jobs a good un. I marked the top of the piece when it was done up so I could put through the hole for the R pin that fixes on the panel.

bespoke Aprilia RS125 fairing stand installedIt was an expensive thing to do time wise but given the lack of availability it seemed like the only way to go. Material cost was zero as it was stuff I had lying around and probably bought about 10 years ago from a steam rally or something. I knew it would come in handy one day.

Only other job I could do with the stuff I have on hand is to fill the cooling system with appropriate coolant. There are as many discussions on the interweb about which coolant is best as there are about which oil is best. I had some Motul stuff I had bought for a previous job so that’s what I used – they all have to come up to the same minimum spec. and I have never had any reason to believe one is any better than any other. Some people like to test with water first, which is not a bad idea as coolant is expensive and if you do have a leak you can identify it and sort it without pissing expensive coolant all over the floor. As these little bikes don’t take much – the manual suggests 0.8 litres of 50/50 water / coolant – I wasn’t too worried if I lost 400ml of coolant so just went for it. It was fine.

So that’s it for now, I am back to waiting for parts to arrive – I still need the lock set that I ordered , I’m still waiting for my seat to be recovered and I still have to sort out the bodywork. I alsi need new tyres but can test with the ones I have.

Seeya next time, Dave.

Article provided by David Powell of Boston Bike Bits.

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