Thursday, 10 May 2018

Dealing with Fuel Vaporisation

As I wrote a few days ago, whilst out on a shakedown run I had put about 6 miles under the wheels of the SQ4 when the misfire appeared. Power was really down and I had to drop to third on all the hills. Forgetting what the Four can be like in hot weather I was worrying about the ignition system, but managed to crack on home - I was overtaken by a military M20 at one point, as I could only manage 20 mph.

Back home, I put the bike back in the workshop and drained the oil tank, the sump and removed the filter. Sitting by the oil tank, I could hear the fuel boiling in the float chamber, and my Infra Red thermometer revealed that the float chamber was at 80°C and the carburetter body, near the cylinder hear flange was over 100°C - and remember, I had already spent 15 minutes draining the oil at this point.

I was clearly going to need the carburetter heat insuator that I bought a while back (in 2015!), which I had not fitted since I had the distributor the wrong way around - with the square 'condenser bulge' forward. This reduces the space available for the carburetter/spacer combination.

You'll see from the picture that there is very little room for the spacer before the carburetter hits the distributor, even with the bulge facing to the rear.


Pulling off the carburetter, it is immediately obvious that the existing, original studs barely have enough thread to allow a full nut to be used to retain the carburetter, so there is no chance of a spacer fitting in there as well:


Fortunately I have some 5/16" 303 stainless round bar knocking about, so we can make some longer studs. Here goes:


I've made them 1/4" long than stock. At this point I realised that whilst I have 5/16" BSW taps, I have no die. Now there will be a short pause...

RDG produced a suitable die, two days later:


Using the tailstock die holder made short work of the studs, but one of them has a CEI section which is a little longer than it wants to be I think:

Trying the carburetter in place, an unforeseen foul condition is revealed when the BiStarter hits one of the tank mounting bolts:


As expected, one of the studs is marginally too long in the CEI section:


There is still a few millimetres spare before the float bowl contacts the distributer:


So that's it. Stud adjusted, the carburetter goes on nicely with the 3 mm phenolic spacer, a paper gasket and some Threebond. The rear offside tank bolt has to go in afterwards.


Preliminary results show that it is pretty effective. Warming the bike up after the work, at rest, reveals the following temperature readings with the engine running:
  • 162°C on the fins near No.2 spark plug
  • 107°C on the inlet manifold flange - near the offside carburetter stud
  • 39°C on the carburetter flange, 1/2" from the 107°C measuring point but on the other side of the phenolic spacer. That's a 68°C differential across the spacer.
  • 22°C on the float bowl.
Whilst this looks encouraging, we must remember that this is a result of conducted and radiated energy - any convected heat is going upward towards the tank, not rearward toward the carburetter ; a 10 minute warm up is not a valid representation of 25 miles in a 25°C ambient at 70 mph, and we are not at full operating temperature yet.

But it's encouraging nonetheless.

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