It’s been a quiet summer on the Scott racing front. All kinds of responsibilities and activities fill a Summer when you’ve got two very small children and I must admit to at least attempting to lead a balanced life, especially when the sun is out. Also, doing a bit of Scott work for other people does mean that I’m spending time on their engines rather than my own! It’s all good though and I was looking forward to getting to the September meeting at Cadwell park as this is my favourite meeting of all.
The Flying Squirrel (remember, it’s a Flying Squirrel and not a Super Squirrel as I discovered this year..) was still on the bench looking a little forlorn at the beginning of September. I had removed the top end after the seizures at Anglesey because I wanted to check the pistons and bores. The seizures had occurred on both pistons at the rear corners and the left hand piston was a little distorted because of it. I spent quite a bit of time filing the damage out carefully. The bores were fine, although I flex-honed them to freshen them up a bit.
The last time I had assembled the engine, I used a gasket compound called ‘three bond’ which has good gap filling properties and remains flexible. I used a tiny smear on various surfaces that I had scraped flat, just to ensure a good seal. I had a hell of a job getting things apart and in fact the bond was so good that it pulled the devcon epoxy out of some corrosion damage on one of the carburettor flange faces. There’s such a thing as too good! I think I’ll use it where a face isn’t good, but back to a silicone sealant where it’s needed.
So, with the engine back together and the repaired radiator back in (thank you again, Graham Moag), the bike just needed a few bits sorting out to get it ready. The front mudguard needed to be replaced and the L/H footrest needed straightening. A few other little bits as well.
Inevitably, some of these things ended up being left to the last minute, which was OK since I’d arranged to have the day free before I went to Cadwell just to make sure everything was finished. Unfortunately, my wife had a work deadline which meant that I had to look after our children on this day. This was an error in my calculations! Instead of an unhurried day and early afternoon departure to Cadwell, I frenetically worked on the bike from around 5pm till 9.00pm on the Friday night, then packed the van and went to bed at 10.30pm. At 1.00am I arose, drank coffee and set off to Cadwell… over 6 hours away.
The drive wasn’t so bad and the morning was crisp, with a low mist that hugged the warm ground. It was obvious that it would clear and clear it did. It was indeed a beautiful morning welcome from Cadwell Park.
I arrived pretty much as people were starting to get up and got the bike down to Scrutineering early. No problems here and I signed on and started to check the bike over myself.
Practice for solos was called and the bike started immediately. The twin carbs breathing without bellmouths because I knew I needed to make some that actually flowed properly and I hadn’t done testing to base a length on. I figured it would do at this point.
The Scott seemed willing at low revs although I was concentrating on the clutch, given the seizures it had experienced at Anglesey. Sure enough, at the bottom of the park straight it came. I was quick to get the clutch in and coasted to the marshall’s post to wait for the end of the session and the recovery truck.
Back in the paddock I started to go through the fuel system. A gentleman called Peter, who was interested in the Scott, was kind enough to help as I went through the fuel system checking the flows through the taps, lines and banjos.
It’s dangerous to assume but sometimes you forget when you have assumed. I had the bike on the dyno at the beginning of the year and had thought that this would show up any problems with fuel supply. It wasn’t that I was expecting it to give problems.. and it didn’t. I’d made sure that the fuel lines were all of a descent size and had put the lot together being as careful as possible to avoid flow related issues. The dyno runs were completed without the suspicion of a seizure, so I hadn’t thought there was likely to be a problem.
I did flow tests all the way to the banjos that fit to the bottom of the carbs and was surprised at the results. They were barely able to flow the potential of the main jet. With my new found assistant’s help, I drilled every fitting out and found that I’d increased flow by a couple of hundred cc’s per minute to each carb. Part of the flow improvement was that I removed a connector pipe between the feeds to the two float chambers from my twin taps. I’d thought that this was was a good idea in case one tap became blocked but in effect the T- piece connectors were just another cause of a pressure drop. Amal main jets are rated by cc’s per minute flow (albeit under certain controlled conditions that I wasn’t trying to emulate) but at least now I had almost double the flow specified by each jet. I thought that should cover that as a possibility of seizure.
I was therefore hopeful when I got out in my first race, though aware that I still may be missing something.
It seized at the end of park straight on about the second lap. Bugger.
I started to think about the history of this problem. Roger had presented my future wife and myself with the exhaust as a wedding present in 2011. She was overjoyed of course :). It had been made by Gibsons in the south east and they’d had the bike to fit it to properly. They hadn’t run any calculations but had copied my dad’s pipe which he’d supplied them with for that purpose.
I went to the Prescott hill climb early in 2012 as the first outing with the bike and .. guess what.. it had seized at the first corner. It sound so hard edged and got so hot that I’d ended up fitting another head gasket at the next outing (BHR meeting at Lydden) simply to get it to stop overheating. It didn’t overheat (though it still got very hot) but it was slow. I thought perhaps that all this was a sign that if I wanted the power, it was a case that I’d need to make the engine dissipate heat better. Maybe my radiator, which is very small, was incapable or that my block (cast iron) was too slow in getting that heat away. It started to look very expensive.
That was why I switched to methanol in the first place…that I thought it might provide me with an easy alternative.
Methanol had worked, but was there another problem that it was hiding?
I had been wondering somewhere of course whether the pipe was in fact unsuitable for some reason. Since none of the problems existed before the pipe, it seems like a clear possibility. I talked to Rex Caunt (BSA Bantam tuner) on the Sunday and told him that I wondered whether the stinger outlet was too small and that it was the exhausts inability to get rid of the pressure quick enough that was causing the heat build up. He gave me a ‘rule of thumb’ to work out whether this was the case.
Apparently, the inside diameter of the ‘stinger’ pipe outlet is normally around 60% to 62% of the inside diameter of the beginning of the header pipe (around 50mm).That would make it around 30 to 31mm ID. I couldn’t get to the stinger, but I could measure the silencer outlet and it was around 26.8mm. That’s under 54% of the ID.
When I start to think about it more, I think there’s another reason why it might need to be bigger. It’s a 2 into 1 pipe and although the operation of the phases are separated, there’s not the time to dissipate pressure that you get with a normal 1 pipe for 1 cylinder operation. In fact it makes sense to me that the outlet should be bigger than a standard pipe for this.
So in terms of racing it was a terrible weekend but in terms of development, I feel happy that I’ve got some new direction for the off season. I’m going back to basics with the fuel system and the exhaust and maybe even make some experimental pipes simply for the dyno. I also have started to realise what the dyno may or may not be useful for. I’ll still use the dyno to show what relative power I’ve achieved but I wont assume that it’s all I need for testing.
I’ve got lots of reasons to be excited about this winter. Roger finally took his bike to Motoliner in Maidstone to have the frame and forks checked and straightened after Steve Plater’s crash at the beginning of the year. It will be good to see that make progress.
The Moss Silk Scott needs to start moving to the next level… a dry build to see what kind of tank and seat unit it’s going to need and where the exhausts can go. I really look forward to moving forward with this.
I’ve got my lathe to finish scraping/ re-building, my milling machine to re-commission and tool up for and various other engines and gearboxes to work on in the meantime.
I still have a dream of making my Flying Squirrel the machine to beat in the vintage class and will be yet again edging toward that over the winter. I can but try!