March 2015 update

After I decided to return to running petrol in the Super Squirrel racer, I thought that I’d re-profile the cylinder head ‘squish’ chamber, given that the compression was probably on the high side and I really wanted to concentrate on trying to create a head shape which made it easier for the flame front to spread.

The original head chamber and the beginnings of work to the other side.
The original head chamber and the beginnings of work to the other side.
The old shape worked well on methanol but I know that I am more likely to suffer from detonation with petrol and so I thought it prudent to make some changes accordingly.
Finished re-profiling and blending. I have not idea whether it'll be better but it seemed like the right thing to do.
Finished re-profiling and blending. I have not idea whether it’ll be better but it seemed like the right thing to do.
I also asked Alan if he’d braze a couple of fixing points onto my exhaust headers, to allow the installation of exhaust temperature sensors as this information may be useful in determining the correct jetting.It will be interesting to note the changes anyway.
The K type thermocouple has platinum probes which will screw into the nuts and poke through a 2.1mm hole.
The K type thermocouple has platinum probes which will screw into the nuts and poke through a 2.1mm hole.
I’m also intending to monitor the temperature underneath the spark plugs, to see what that tells me. I’d like to monitor coolant temperature also but I’ve not yet picked up a sensor for that. Whether any of this stuff gives me absolutely necessary information is questionable, but it will certainly be interesting.
I’ve also been working on the detail of the new improved twin carb set-up. I’ve not actually set them up(!) but I am almost at the point when I can put it all together.
Lots of time spent on these, scraping base flanges flat and careful flow work to match manifold.
Lots of time spent on these, scraping base flanges flat and careful flow work to match manifold.
I’ve spend quite a lot of time matching the carbs to the inlet manifold and also the flow going in. I also have to clean the jet block fuel and air apertures and work out whether I need to increase the size of the pilot jet apertures. I’m not bothered about tickover of course, but the pilot is the only source of fuel when the throttle is shut and I wonder whether, at the end of a long straight, this is necessary for cooling the piston.
A bit of cleaning and fettling for the jet blocks
A bit of cleaning and fettling for the jet blocks
Maybe it’s not, but nip ups quite often happen when you shut the throttle and it strikes me that this might be related. At any rate, the carbs were orignally destined for something like a 350 single running a one hundred and something main jet. I’m going to be running considerably more than that so I’m surmising that the pilot jet should be appropriately modified too.
I’m also planning to try different lengths of intake tract for the carbs as I extended the intake timing duration a little and it will benefit from some length and velocity on the inlet.
Otherwise, I have to do a lot of cleaning of gritty parts, and then get it back together for testing.