big TT carbs
- Thread starter vibrac
- Start date
The higher hydrostatic head must be small compared to the diff pressure. Any ideas on or measured venturi pressure?
I bored out a couple of TT carbs back in the 1970s and so as to not disturb the slide seat and needle jet holder I chucked it offset onto a modified face plate and bored the choke area higher leaving the bottom alone except for the outer edges. I also did this on Monobloks but not on GPs as large ones were available.
John Mead
John Mead
The main jet is totally immersed so it doesn't matter how deep it is immersed. All that matters is the height of the exit of the choke tube with respect to the height of the fuel in the bowl. If that is the same it doesn't matter where the main jet sits.
I actually have a Superflow flow bench and on my always-increasing, seldom-crossed-off-item, to-do list are various kinds of carburetor measurements to make. Measuring the pressure drop as a function of flow speed would be very easy to do since it only requires a pressure sensor attached to the inlet to the choke tube.
I can't tell from the photograph (and also because I don't have my own TT in front of me for comparison), but did you make the mold for the casting from a standard 10TT9 with additional clay stuck on to increase the thickness at the inlet and outlet, or was the mold made from its own carving with other dimensions beefed up as well? Also, did you make any of the other parts (e.g. jet block), or did everything else come from a sacrificial TT (or a stock of parts)?
Be careful you don't trip and fall down there !!
David has that worked, Or is it ready for tuning, The float level looks very low, I was thinking of putting a Concentric on it's side with a remote float, Like Ken told us about, For use on my Comet with a Rear head, Not sure where the float lev
I did not make the casting, although I wish I did. I do not have an original to compare it to, but I would not be surprised if it were done by a pattern maker and the shrinkage seems to be well accounted for. All the internals are Amal MK2. The cap on top was made so the spring could nest up inside and allow the throttle valve to to clear the bore. I will try to take some more detailed photos.
Glyn Johnson has an original on his Flash and I do have some photos. I will hunt them down.
Bill, this was prior to tuning and when I put it on the dyno I had to raise the bowl immediately. But, the very next run (on the single racer) was 42 bhp. Because it was MK2 I did not have a single tuning item to change out. I was hoping just to see what it did with the 28" long 2" pipe. We were all quite impressed, but had nowhere to go. This is also with a standard size head, not the big port. I put the 32mm 10TT9 on it and got 40, which was quite heartening, but by the time we put the straight through silencer on it was 37. I did many runs with different lengths and 28" open pipe made the most power at 7,000. So, it is very easy to get power from a straight pipe, but the minute you put any absorber silencer on it you loose quite a few hp. So, the racing silencer requirement works heavily against the Vincent.
The port velocity is very low with this set up leading to speculation that a smaller pipe with a megaphone would provide more power and it certainly did on Lindsay Kyle's bike. But, this is the stage of tuning that is wildly expensive. You have to make several custom exhaust systems that will probably not work and spend hours on the dyno. When you are done with the exhaust, you need to tune the intake length for the exhaust you have ended up with it there is a reversion issue, which there usually is, and so it is exactly the sam tuning on the intake as you do on the exhaust. It is a little cheaper to lengthen and shorten manifolds, but there is still hours of dyno time involved. Then you have to make sure there are no drivability issues. Usually, you give up somewhere in this process and just go racing!
To return to the carb, I will say that although testing was limited, I was using jets in the 300 plus range, so whatever issues were present in the original which required huge jets seems to have been dealt with. John Renwick said he tried to use the best of both the TT and MK2 when making it.
David