How to tune a '97 British TA

by Bill Dixon

Disclaimer: Neither Bill nor me can give any warranty. If you do, you will work on your own risk!

A couple of weeks ago, I asked about jetting. There was a good deal of really useful advice, and some unanswered questions. I said I'd let you all know how I got on.

The history is that I put a Micron (race, therefore not actually legal) can on my '97 TA. My OEM exhaust was getting scruffy because the chrome decoration was starting to flake. The Micron is a close fit through the Givi rack. If I ever change it again I'll get an oval can. To stop the can from buzzing against the rack, I put a couple of bits of butyl rubber insulation sliced from a heat resisting electical cable between the rack and the can.

The upshot was a great sound but no more power, perhaps even a slight loss - a certain amount of popping and spluttering on overrun and a slightly gutless feel to the throttle.

I looked at the plugs: completely white, indicating hot running/lean mixture.

I ordered a DynoJet kit from M and P. This was a leap of faith, since I couldn't get anyone there to tell me what is in the kit. After nearly two weeks of waiting for them to get it in stock, I got fed up and cancelled the order.

Instead, I ordered a couple of .130 main jets from TTS Engineering and fitted them myself.

This was only slightly tricky and moderately time-consuming:

  1. Remove seat, side panels, tank.
  2. To change the plugs, I unbolted the left hand radiator, but kept the pipework connected, allowing me to swing it back and out to get at the front left plug. I tied it back with some tape, carefully.
  3. Remove airbox, carb tubing and the carb-to-inlet-port ducts (fiddly) Note the orientation of these ducts - they have a bend in them, so there is a right way round and a right way up. Leave throttle and choke cables in place.

    Now you're ready to take the carbs out....but take some precautions...

  4. SO (the clever bit) remove the diaphragm cover from the top of left-hand carb, taking care of the spring and the four screws. This gives just enough headroom to slide both carbs out through the gap on the left. Now reinstall the cover. DO NOT attempt to separate the carbs.
  5. Tape the carbs in place against the frame, protecting the control cables. Now you can get to work on the float chambers. Clean the outides again and your hands if in any doubt at all. Your mum should be proud of you at ths point.
  6. Remove one float chamber, undoing the four screws evenly and not losing them. Petrol will spill. Let it, as it will carry away any dirt which may have settled in the float bowl. The main jet will have a slot, so you can undo it with a screwdriver. Put in the new one, keeping it as clean as you possibly can and replace the float chamber before doing the other carb.
    By the way, the orginal jets I found were .115 (2)
  7. Put the whole lot back together. You'll have to repeat the trick with the vacuum diaphragm.
  8. When you start the engine, there will be no fuel in the carbs, so you'll have to crank for a while. Full choke will help.

The result is terrific. I have no idea what the actual power is and I've yet to examine a plug to see how it's running. Fuel consumption is certainly up, maybe because I've been enjoying myself. There is a slight amount of soot deposit on the walls of the can outlet, and a tendency to miss slightly when coming off full throttle. Overall the result is great, although I have the suspicion that a .130 jet is just a bit too big. Miles per imperial gallon are about 50 (40 miles per US Gallon, 18km per litre), but then I have been enjoying myself.

Bear in mind that the cross sectional area of a jet is in proportion to the diameter squared and that fuel flows faster through a big hole than through a small one so the mixtre will be rich by at least (130/115) squared = 1.27 i.e 27% richer. Of course I didn't touch the needles or their jets and the fuel has to pass through that restriction too so the overall result may be less than 27%, especially on part throttle.

One other thing I've noticed is that the engine runs cooler. At high speed, the temp gauge used to creep up. Now it doesn't.

I'll see about getting it tested for power and emissions and let you all know how that goes.

Bill Dixon.

Some remarks by Deti:

1) Oops - you really can't get them out of the frame? It works with my oldstyle 32mm carbs in the following manner:

  1. Remove all control cables (throttle and both chokes)
  2. Remove air filter dome (of course ;-) )
  3. Loosen the connections between carbs and cylinderheads (of course ;-) )
  4. Pull the carbs off the heads
  5. Turn them vertically and horizontally by appr. 90 degrees and fumble them out. It's a little bit more easy to tell than to do, but it works.
Anybody being about to work on the carbs should try this. It's much better to have the carbs on the bench than fixed to the frame!

2) Remember - this is a '97 TA!!!! Older TAs may have the following main jets:

128 front
132 rear
That means: mounting a 130 on the rear carb, will make the mixturer go even leaner for rear cylinder!

Josep M. Lopez kindly sent me a table of all European TA carbs jettings. Check it out before starting rejetting ...

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