Ultima BMA 100 Part 2

Published on 16 May 2024 at 16:33

It's funny how the same words in a sentence can have totally different meanings to different people. I had found a supplier on line that said they could make piston rings in any size. I phoned them and had a chat with a guy that seemed to know exactly what I was talking about and said that his company could make new rings to my exact sizes for just 4 pounds fifty each. Naturally I was delighted, so I placed my order and waited a week for them to arrive. Imagine my dismay when the postman brought my two rubber o rings that are about as much use as a chocolate teapot. I think the guy I spoke to had assumed they were for a compressor or something although I'm pretty sure most of them use steel or iron rings. So, they now have to go back at my expense and I have to go elsewhere for the parts I need.

Eventually I found another supplier, the rings are a weird size, they are unusually thick at 2.4mm - most rings tend to be in the 1-1.75mm range. The ones I have now ordered are much more expensive - 28 quid plus post plus the sodding VAT. Add all that up and the bill is well over 40 quid, which hurts a fair bit given that common sized rings are literally a couple of quid each. I am sure that there will be other engines that use the same size ring as wither standard or as an over bore size but most people only list by diameter. In the old days you could go to an engineering supplier but they are few and far between now. It was taking far too much time so I had to bite the bullet, I am still waiting to receive them. In the mean time, I received the cylinder hone I ordered so the bore is now deglazed and ready for the new rings to be fitted

I had to pull off the flywheel to get at the engine sprocket, which had been worn so badly it was almost smooth, I have no idea haw somebody was riding it like that for the last couple of thousand miles, I have never seen one this bad before. The chain was missing, I am pretty sure it would have been worn well beyond use. O course, it was a special one that I couldn't buy off the shelf so I had to make one. I did that by getting a standard 15 tooth sprocket from a Chinese bike  no idea which one - and grinding out the center part so that I had  a hole about 25mm diameter. I then cut out the middle of the old sprocket, turned it down to 25mm and pressed it in to the center of my new sprocket and welded the two together. It's not ideal, the heat may well have impaired the hardness of he sprocket but I figured the bike won't be used for doing many long distance journeys, it will most likely outlast me with the use it will get. It all worked out rather splendidly, I had a chain of the correct size so now that's back together. I noticed on the flywheel there were remnants of blue and red paint so I thought I should reinstate that, I am glad that I did, I think it suits the rest of the bike rather well now.

With those bits attended to and the correct rings received and installed it was time to see how well the bike would run., I wasn't expecting miracles as it was obvious the carb would need a tweak and I had no idea what to set the ignition timing to. The Villiers carb has a fully adjustable needle which is great but you have to take the slide out of the carb every time you want to tweak it. I took a guess and put some fuel in. Amazingly it ran - not brilliantly well but for the first time on many a long year it fired up. It soon died though and despite all my best efforts refused to start again. Numerous times I took the plug out and checked for spark - that was fine. I checked the timing, that looked good at about 34 degrees of advance. The plug kept coming out wet so obviously fuel was getting through.

I was still convinced that the timing may have slipped so I dug out my test strobe and a 12 volt battery. I checked it first with the plug out to make sure the strobe worked ok, which it did. I then put the plug in and lo and behold no spark detected by the strobe. I retried this a couple of times and it was consistent, with the plug out I got a spark, as soon as I put it in the engine nothing. I had tried 4 plugs in all, none would work reliably. As an experiment and because I had no idea what else to try I closed the gap on the plug right up from 25 thou to about 8 thou. It started first kick. It didn't run brilliantly but it was good enough to enable me to progress. I have not backed off the advance to about 25 degrees, it seems to tick over better than at 34, I have tweaked the carb as best as I can but I think it's still a bit rich at wide open throttle.

I took it for a test ride, it actually goes remarkably well for a 90 year old 100cc but I don't think much of the handling. It was off road to be fair, I have been waiting for a dry day to try it out on tarmac but we haven't had one yet.

there is a short video here - https://www.youtube.com/watch?v=99uBZV9V6cU&t=30s


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