1934 Ultima BMA 100cc Motorcycle Clean up part 1

Published on 16 April 2024 at 16:36

Having had a proper look and discovering a missing carb part I scoured the auto jumble looking for a suitable carb that would at least allow me to run the bike, in doing so I learnt an important lesson in how no to buy things at auto jumbles.

I knew more or less what I was looking for - a carb with a 19mm spigot mount for a 2 stroke engine, I was quite pleased when I spotted one. The guy wanted 30 quid for it, which seemed ok to me, except I had no idea if it was going to be any good so I asked him if 25 would buy it. He was quite upset by that and suggested it would be a lot more on ebay and insisted on looking up the part number. Sure enough he found one and, yes, it was a lot more. So with that he snatched it out of my hand, put it in the back of his van and pretty much told me to fuck off. I was thinking maybe I should have paid him his asking price but then about 10 stalls down was another carb exactly the same. This buyer seemed to be quite happy with his 15 quid asking price, which I paid him without haggling, I felt quite happy that I had saved myself some money and got what I needed, I was very happy that the rude seller hadn't got my money, I hope he and his carb will be very happy together.

Anyway, I got it back to the workshop with a number of other purchases that I had made and tried it on the bike. It fitted and I got the engine started but it is a long way from running properly. I need to raise the needle a bit to richen the mixture up but at least I know the engine runs. The needle is a bit weird in this carb as most needles have a series of grooves in to which goes a circlip so you can vary the height of the needle and therefore the fuel mixture at any given throttle setting. In this carb the needle has a screw on top so it is infinitely variable over quite a wide range, I should be able to get it just right although it will be a faf because you need to stop the bike and open up the cab each time you adjust it. I wasn't impressed with the compression though, it felt a bit weedy compared to my other little 2 strokes so I thought a quick barrel off inspection was in order. The engine has clearly been worked on very recently, it was spotlessly clean, sadly it was obvious it had the wrong piston rings in it. It has a weird sized bore - 48.5mm and at 2.4mm the rings are quite thick, I could not find any in the UK of the correct size. Fortunately I happened across a company called Barnwell that can make rings of pretty much any size. I gave them a ring and was delighted to get proper, good old fashioned service from somebody that clearly knew what he was talking about. I gave him the critical dimensions of the bore, the piston and the rings that I had and he confirmed that they were indeed the wrong ones. I think they are modified Villiers ones, no idea but they are very sloppy and have 40 thou gaps in the ends where 10 thou would be much better. They will be making them for me, the cost is an unbelievably low tenner for the two, I shall be delighted if this is all as it should be. It will take a week for them to get here but I can live with that, I have plenty of other things to do in the mean time.

I still hope to find the missing carb part but I think I can get the Villiers one working, the engine did run on it even with the dodgy piston rings. I don't see the point in trying to tune the carb until the compression is somewhat better, I will give the barrel a good hone before the new rings are fitted. 

It's quite a strange little bike with some rather peculiar features like the kickstart that can be operated from either side of the bike, when running the kickstart becomes the foot rests. It's also highly unusual for a 100cc bike to have twin exhausts, the ones on this bike have no real silencing capability so it is unusually loud. It is fitted with a headlamp but it has no electrical system to power it. That wasn't unusual at the time, it was fairly common to have battery powered lights or small dynamos like the ones commonly fitted to bicycles. This headlamp is clearly not the right one as it has an aperture for a speedo and no speedo drive. I have a battery one that would be more period correct that I may put on.

Once more I found myself trapped between the restore it to original and keep it as it is to preserve it's identity sort of dilemma. I am pretty sure it was used in a circus, the colours would certainly suggest that, it's not normal to have white wheel rims and red spokes, for example. The red, blue and gold used on the tank suggest circus, as do the patches of wear that suggest somebody spent the best years of their life standing on the tank while some maniac drive it around the big top. I think it would be wrong to lose that but whether I just retain that in photographs or whether I keep it because it needs to be kept remains to be seen. I shall decide soon but first I want to sort out the mechanical side of things and get it running properly.


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