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TT Carb improvements

Forums

Inter MS 1935:

After a full motor rebuild I treated the "new" engine to a carb rebuild with new jets/slide/needle etc to make sure I was on a clean sheet.  It proved somewhat difficult to get started but now that is cured i am not happy with the carb transition phases, slow run to medium speed especially. 

I get quite a bit of "8 stroking" which clears as I go up the speed range and it spits back through the carb quite a lot.  Watching down the intake the fuel looks to be quite "blobby" rather than an  even spray.

Does anyone have any tips or mods to make to the TT carb to improve its drive ability?

 Thanks

Jon       

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Hi Jon,

   You might want to try re-fitting your old carb parts and see how it runs then. If it runs O.K. then fit one new part at a time and re-test to see if you can pin down the problem. Check all the new parts for burrs or machining cutter marks that may be affecting the flow.

   Another reason for uneven running may be because of changes to the valve timing and/or the exhaust system, both of which can affect the settings especially the slide cut-away.

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I thought that TT carbs did not come with idle circuitry.

That is why when you see a Manx worming up the rider is continuing to "blip" the throttle???

Mike

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Though half a century since I last raced with a TT instrument (speedway JAP). Hard to remember, but it never worked well until fully warmed up. Cold it behaved as you described. Could it be that easy?

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Mine Model 30 either starts within two or three kicks, or not at all... but once going it runs fine from cold (with the slide slightly open of course).

What slide etc do you use? Did you change in the hope of improvement?

My 1932 notes say it has 

Flange mark 34/041 10TT34

Needle 10

Set in middle notch (of 7)

Needle jet 109

Main jet 320

Bore 1 5/32

Throttle slide 6 (cut away 3/8")

 

Spot on Richard!  The needle jet was as old as the carb (1935) with the taper at the top end of  the jet.  The replacement was the more modern style with the taper at the bottom and a "well" at the top. This radically affects your needle position.  Still deliberating on best solution; the  original jet may be cleaned and left in place or I experiment with lower needle and cross drill the  "well" to reduce the lift.

 Valve timing was changed but only back to standard overlaps after a radical arrangement was  found on strip down.  Maybe the original builder knew a trick or two.   Will continue with the carb  tinkering before I delve back into valve timing. 

 Michael; the TT has a pilot circuit for starting.  For road use the slide has been modified with a  small hole (~2mm) in the upstream face to maintain a semblance of tick-over. Its a bit of a brute  to kick over if you stall at the lights. I gave up bump starting a few years ago and fitted a kick shaft and motocross style lever. 

 

  Thanks both,

 

Jon  

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The TT carburetor has a pilot circuit.  (Unlike road carburetors, it meters fuel not air).  What the TT does not have is a throttle stop, so the slide closes fully when you close the throttle.  It give stronger engine braking, and means that the engine will stop when the throttle is closed.  It mattered because there was no kill button on the magneto ignition.

I have been looking at a TT, and wondering about fitting a screw into the top of the slide to create an idle stop

Paul

The small hole seems to be the "old school" method Paul.  When I researched the reason for the hole it came up in several models websites (Velo,BSA...).  Were you thinking of jacking the slide up from the top of the slide cover. Similar to adjusting the throttle cable short...  Or something else??     Keep  us  informed :-)

 

J

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The Dellorto SSI carb is like the TT , no stop but has a slide holding rod that can be adjusted from the carb top. I think that a couple of solder blobs on the bottom of the slide would be a simple solution.

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Amal TT came from Amac TT which developed from 1924, and was in production by early 30's, before Dellorto company existed in 1933.  Unless of course Dellorto took theirs from a predecessor Italian firm?

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I see that the first web site hit for Dellorto spare parts says the throttle stop above is 'no longer available'.  But it does give a good picture of one.

My PO put a threaded collar on the throttle cable to allow the slide to hang.  It's useful to set a tickover when on the stand, but not stable enough to be of use on the road (ie,g, at traffic lights).  At least it does not make permanent alterations to the carb.

 

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I have an SSI29  carb on one of my bikes, It has the slide stop feature . Not easy to use as its under the tank. Its not as reliable as one would like and I will be putting a couple of solder blobs on the bottom of the brass slide to hold the slide up a little.

 

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