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big TT carbs

vibrac

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VOC Member
Amal had some 1 7/16" TT carbs made up for fast Vincent twins and singles just before the Amal GP came out in 51-52.
Our Racing Comet really needs one of these now limited as we are to early Gardners or TT carbs in up to 1948 class. I have been using an early 1-3/8 Gardner but its really only clean at speed not out of the corners and as far as I understand the track amals were much the same. Nowadays the biggest TT available is 1-3/16, I suppose a good lathe guy might squeeze a bit more out of the block, I know that was possible with the 1 1/16 TT carb because Ron Kemp did it.but is there another way? I dont need the whole 1-3/8 I would be happy with 1-7/16 (and also to take it with that to the scrutineers.:cool:) I have seen some new big Amals on machines so is someone making new blocks etc for them?. any information gladly received.
 

davidd

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VOC Member
I received a request for one just last week. They are almost impossible to find and I think only a handful were made. Some were said to have shown up on Nortons and Velos, so it would be worth asking anyone associated with those marques. I was never able to find one, but I ended up with one made by John Renwick, but it was not a clip fit. I believe he said he got the casting from someone in the Scott Club. It would be worth asking around.

There were some tuning issues. I am told that you have to use jets in the 1200 plus range for gas. The Factory also used some type of emulsifier tube to counteract the angle of the carb at as a straight port carb. Carleton ran one successfully. The carb was specifically designed for the big port head, which had an 1-7/16" intake tract. I considered casting them up, but there are two issues. One is finding someone who can re-design the carb to work properly and the second is finding a machinist to do it at a reasonable price. The one I have is made with MK2 slides, jets and needles. On the dyno no other carb has produced as much power!

I hope others have some answers!

David
 

Magnetoman

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VOC Member
Nowadays the biggest TT available is 1-3/16, I suppose a good lathe guy might squeeze a bit more out of the block,
I'm just back from the Middle East or I would have posted something about this sooner. I have a vintage 1-3/16" Amal 10TT9 mounted to a BSA C11 head in my office (doesn't everyone?...). I'm fairly certain this is the largest casting Amal made for this carburetor (other than the special one mentioned earlier in this thread, that I had not been aware of before now). Since I'm too tired to do anything productive, I just disassembled it to make some measurements.

The bore for the slide is 1.498" dia. so that represents an upper limit on how much it could be bored out. However, the OD of the casting around the bore for the air itself is only ~1-1/2" so we couldn't get to the 1-1/2" without breaking through that wall. My TT is flange-mounted and there are two flats machined in the OD to allow clearance for the bolt heads. Since the spacing between those flats is 1.375" a brave person could increase the bore to, say, 1.374" without breaking through the casting. That certainly would push the upper limit to the limit. However, 1-5/16" seems somewhat reasonable, in that it would leave 0.031" wall under those two flats and plenty of material everywhere else (i.e. 0.094" walls) to provide the necessary mechanical support to keep vibrations from snapping the carburetor in two.

A conservative racer (if that's not an oxymoron) might find 1-1/4" a safe place to stop with the boring bar on my carburetor. However, filling in those flats with epoxy (which only would be needed to stop possible air leaks, not to provide support) and using Allen-head bolts makes the 1-5/16" perfectly reasonable. Once could even contemplate a bit less than 1-3/8".
 

vibrac

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VOC Member
I was looking for 1-3/8" bore (as was pointed out earlier I remembered about the 1-7/16 special Amal TT's that were made without realising they were even biger than I am contemplating) certainly I would not think any engine I could build would need bigger than 1-3/8 for short circuit Vintage racing in UK indeed thats what the port in my new Godden head has, so an 1-5/16 would be a good compromise it would not be the first engine that went well with a port slightly bigger than the carb, so what MM suggests may be a solution( though I would doubt my lathe work standards could accomplish it:(). Now the concept has rolled round in my head for a few days I seem to remember when Ron opened up his 1-1/16 TT to 1-3/16 that he did something with the main jet position so that it did not stick up to far into the rebored body and remained at the same height as it was with the 1-1/16 at this distance in time I cant remember if he a cut off the brass tube or put a spacer underneath. Does that sound like a good modification?
 

Magnetoman

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VOC Member
I'll look at my TT again tomorrow ...
Even though my TT is NOS, 60+ years of oxidation has made the jet block reluctant to come out of the body and I don't have the time to make a drift to gently force it out. However, like the later Monoblocs and Concentrics there's a brass choke tube in the jet block that protrudes ~1-2 mm into the air stream. If the bore were increased to 1-5/16" (i.e. by 1/16" in radius) the height of that choke tube would have to be cut down by 1/16" to keep the same amount of protrusion.

I can't tell without disassembling it if on the TT the choke tube is pressed directly into the jet block or is in the end of the separate piece that mounts the needle jet and main jet. That separate piece itself screws into place in the jet block. Depending on how it is attached, it might be possible to make a 1/16" spacer to lower the position of the assembly, which in turn would lower the position of the choke tube if it indeed is part of that assembly. However, since in any case the carburetor would be modified on the lathe I can't see any particular advantage to lowering the choke tube that way even if it were possible.
 

Magnetoman

Well Known and Active Forum Website User
VOC Member
That would put the main jet deeper in the float bowl level and maybe be an issue.
No, that's not an issue. The flow rate through the main jet is solely a function of the pressure difference across it (atmospheric in the fload bowl and less than atmospheric at the choke tube due to air velocity). As for the needle jet, as long as it isn't dropped so much that the static fuel level is above it, that won't matter either. Well, it will matter just a little, but should be easily taken care of by moving the needle up or down and/or using a bigger or smaller needle jet than otherwise would have been needed had its position with respect to the fuel level not changed.
 
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