Chapter 9. Gentlemen Start your engines – an exhausting time in more ways than one

Chapter 9. Gentlemen Start your engines – an exhausting time in more ways than one

This past week has been a little sad in some ways. I quickly went from having what was a functioning motorcycle to an empty space on my workshop floor. The next stage is to strip the machine for painting, and I managed to get the whole thing apart in an afternoon, the various components are now stored away in boxes, on shelves, or under benches. I am trying to tag all the small parts because I know so well that a little detail, I spent hours working out, can easily be forgotten.

I consoled myself however with a couple of bike rides. Based on what I considered to be safe practice and published advice from the VMCC and Trail Riders fellowship I have been out on my BMW café racer and my 1974 custom Harley 1000 XLCH. Both bikes went incredibly well considering the layover – as enthusiastic as I was to get out. I took essential tools and tyre inflation kit just in case and have filled up a large jerry can with some of the hard stuff, so I don’t have to stop too often. That said I could sense I was approaching a great viewing point on Monday and couldn’t resist the temptation for a photo-opp. Can anyone tell me where this is?

Exhaust headers.

Anyone who knows me eventually realises that I try to do any job myself – rather than employ a skilled professional. Partly it’s because I’m tight-fisted, but also, I guess that’s the way my father was. I’m always encouraged by one of our neighbours who’s often to be seen teetering at the top of a ladder doing some painting or fixing something in his garage. He’s a few years older than me but still rides a pretty quick looking sports bike, so there’s a common thread. Anyway, when I built my BMW I fabricated the exhaust headers and it went swimmingly well. I ended up with some neat looking pipes, well tucked in, and tuned to optimum length. It was therefore a no-brainer that I would build the pipes for Marge. The BMW came with stainless pipes so I had purchased pre-formed stainless steel mandrel bends, you can get them in a variety of curvatures, and the equivalent straight pipe. Its then relatively straightforward to cut the bend sections to the correct angle, join them together with the straight pieces, tack weld in-situ on the bike and then complete the welding on the bench.

For the BSA I could have bought standard pipes from Armours, but as I’ve said that’s not really my thing, plus they looked too curvy, very much for a touring motorbike; so I went the same way as before, to get more of  a racer look. I used stainless again. The pluses are that it doesn’t corrode and most importantly if I need to repair or modify it after use then I don’t have the issue of weld-contamination – one of the disadvantages of TIG welding where cleanliness is next to godliness. Disadvantages are that the initial stainless finish doesn’t look very vintage and strangely enough the sound. I now have stainless pipes on my Commando, having spent my younger years replacing chromed mild steel pipes every few years. (As an aside, my wife’s wedding present to me was a set of silencers for the Norton, needless to say that didn’t go down too well with her Mum as they took pride of place at the reception alongside casserole sets and crockery.) Straight away with the stainless pipes I noticed the sound was not as good – it must be the resonant quality of the material.

The first stage of the process is very straightforward. Most exhaust header systems are based on standard imperial (inch) pipe sizes. The BSA B31-2 is a twin-port design, popular at the time, and has two massively oversized headers of 1-3/4” diameter. Nearly all pipe stockists will have the equivalent 38 mm material available. If you are designing an exhaust system from scratch it would be sensible to optimise the pipe diameter, but with existing port diameters there’s no point in changing this figure. Irrespective of pipe diameter the optimum header pipe calculation mainly depends on exhaust valve timing and engine r.p.m. You also need to account for the displacement. There are plenty of calculators on-line and this is the one I used. Given that the valve timing is fixed the r.p.m. has the biggest effect. I tried using 250 cc but settled with 125 cc the BSA being a twin-port. The calculated length would have been 48.75 inches with 250 cc, so not a lot of difference. 4500 rpm by comparison would have resulted in a shorter length of 43.6 inches. The confirmation I needed was when I then set out the pipe layout on my outline drawing, measuring from the valve head, and the total exhaust header came to exactly the same point as the stock system. This is not strictly correct as the length is modified by the change from a straight pipe to a silencer but is good enough at this stage. We all know that in practice exhausts are developed by “exhaustive” testing on the dyno and then on the track.

Well that was the good part. The internet is great for on-line shopping but when I came to buy the material I couldn’t nail down the exact supplier I had bought the BMW stainless from. They’re all called something like “Exhausts-r-us” so its not straightforward and I expect that most of them are stockists and get the parts and material from the same source. I went ahead and ordered the stainless, but when it arrived, the mandrel bends were not that good, and I wasn’t even sure the grade of stainless was the same. I know the bends are 304 stainless and bought 308 filler rods – which can even be used for welding stainless to mild steel. This is probably not the best selection but I could only go with what was available.

I mocked up the system using cardboard templates for the bends between the straight pipes and cut the metal bends to lengths. Unfortunately, at this point I discovered the tube diameter mid-way through the bends was someway off and was difficult to form into a round 38 mm diameter. Tack-welding in situ was a messy affair, so not many photos, but even then you can see a lightly different finish on the pipe. Making a clamp set-up for the bench is critical for welding with someone with my level of skill and dexterity (both pretty minimal) and I used an Argon purge to inert the inside of the tube. Even then stainless has an alarming tendency to burn through so it’s a juggling act with the welder amperage to get a strong butt weld and not leave holes. In practice the only way, for me anyway, is to do it in two passes, grinding out the first weld then patching up with the second pass. I was happier with the port arrangement as I made up two short lengths of half round pipe, clamped the outside diameter and completed a lap-joint, leaving a split pipe to go over the exhaust port connection. This should work well with vintage style clamps, not the Jubilee clips you can see in the photos. There’s a lot of work to do on these but I have managed to achieve a reasonable surface finish on a test piece, and they didn’t blow on the first run – see later!

 

 

Start-up

You’ll know from my blog a couple of weeks ago that I got the BSA started OK, and it was definitely one of the “ups” in the project. When I set out on this venture life was very different and I had in mind bringing the bike into Brooklands on my trailer, starting it on the rollers and a couple of illicit runs in the car park, before some shake down rides at Dunsfold. Alas it has not come about, so I needed to think again. From way back you will recall that my gearbox was damaged, mainly in respect of the kick-start mechanism – there isn’t one – so that was an issue. The road outside my house is quiet, slightly down hill, and not strictly a through road, so it would have been possible to bump it down there. In these times however I thought it would be a little insensitive to my neighbours who put up with a fair bit of noise in the best of times. When I had the primary drive and clutch fitted I tried a couple of bump starts in my back garden, which consists of a windy path across grass, bit all that happened is the rear wheel locked up and skidded.

I came up with a revised plan, to bench test the magneto on my lathe and use the Kehin carburettor look-alike fitted to my 250 BSA C15, which together with the known compression and correctly set timing should eliminate many of the unknowns. The engine also had a decompressor, and I recalled an article where a someone had bump-started a bike by dropping the decompressor lever.

I went ahead and ordered the handlebar control, together with all the other controls from Feked, just before the shut the on-line side of the business, so I got lucky there. It was relatively straightforward fitting the cables and levers, although I do have a problem with the clutch lift which I will come to another time. I had partly commissioned the front brake, realising that if I got the bike started I would have to be able to stop.

I timed in the magneto, using marks that I had made during the bench test and my timing disc. Again I have no set-up data but have adjusted it to provide 5 degrees to 38 degrees full advance. Unfortunately my planned half-hour job to transfer the Kehin carb over turned into a whole day, when it wouldn’t quite fit. It looked a lot smaller than my Amal 276 but bosses on the casting meant it just wouldn’t fit between the inlet flange studs and seat-tube. I could have taken the head off and fitted it but given I would have to anneal the head gasket and adjust the valves, etc. decided to try my Amal that has been sitting in a box for 9 months or so. It looks in really condition, although the main jet I know is too big. I checked the float level per the attached pic although there isn’t much data available it looks about right, connected the cables and it fitted in really nicely. I adjusted the screws 1-1/2 turns in / out for the throttle stop and pilot air bleed (although I now know the latter is probably too much) and was set to go.

Except – all those last-minute jobs, I had to make up some footrest brackets and fix the seat properly as I didn’t want to jump on and fall straight off. There’s no photos here as I was playing catch-up, but I did make a temporary gear change lever. I had a minor scare when it wouldn’t select. However I took the gearbox top cover off to have a poke around, and lo and behold it selected OK. Its hard to believe that the if the screws holding the cover in place (the one I dropped in a while ago) protrude even a couple of millimetres under the plate, then they jam the selector mechanism. Easily fixed and all set to go – again.

I rigged up an old battery to provide power to the ignition and fitted an-line fuse and cut-out switch.

The first time I come to start a bike or car engine I have that gut-gnawing sensation that otherwise occurs when I take an old bike in for its MOT. Thank goodness for the Historic vehicle cut-off date of 40 years, I now only have one bike that needs an MOT. In the past I must admit I have mainly been successful. The first major rebuild where I stripped my Mini engine – literally every nut and bolt and ran out of money in the process – springs to mind. After several hours (days) fruitlessly pushing it up and down the road outside my parent’s flat, my mate Pete banged the horn in frustration and it coughed into life, then cut out. I realised I had connected the horn wire to the distributor and vice versa – lesson learned. My best experience was when I had completely rebuilt my Harley from the crankshaft flywheels up, nudged the starter solenoid to make sure it was wired correctly and it fired up in my workshop – boy is it loud.

Getting back to Marge, one miscued attempt when I ran down the garden, dropped the decompressor lever and nothing; when I realised I hadn’t turned the ignition on. Second attempt, and off it went, having to grab the front brake and pull in the clutch quick. I had to hold it on the throttle but otherwise it was all good apart from the oil pump tell-tale plunger. After a few seconds I cut the engine, stripped the timing side and realised I had got a small lump of gasket sealant in the feed line from the pump. I cleaned everything up and used ordinary grease between the timing cover and mating flange, just as my Dad used to. It may not be 100% oil tight but I would rather it that way. A day later second start – again forgetting the cut-out switch, probably because my next door neighbour was watching, but try again and all good. There’s a neat little adjustor on the oil feed and I could get the plunger lifting easily. I’ll start with over-oiling and work from there. The carburetion was better with the Amal pilot screw ¾ turn out – and I think that may be correct? And a little more on the throttle stop. The magneto advance worked well and it revved cleanly, although I’ll have to wait for a road-test before making further adjustments. There’s a problem with lack of clutch lift, which is only marginal, but I could select gears and did a few figure of eights around the garden, then left it idling for the short video I posted two weeks ago – Chapter 7.

Whether I would have made Mallory 1000 bikes – I’m still not sure, but at least I feel I’m over the hump, and getting on with rubbing down and painting now.

All the best and stay safe.

Gareth