Carburettor Rebuild – GT750

With the engine in the frame, and the electronic ignition fitted in my GT750 project bike , it was time to dig into my box of bits to see what I could do in the carburettor department.

The local Water Buffalo folks have been urging me to use the Amal style VM 32 carbs as used on the early GT’s, as these do provide better throttle response and also flow better than the Mikuni constant velocity (CV) BS40 carbs used on later models. In the interests of simplicity, I plan to start with the later style CV carbs and then look at replacement options later as they are (I think) easier to set up and will require minimum modification to my current configuration. If I were to use the VM carbs, then I would have to find a three-into-one throttle cable, as well as the handle bar mounted choke lever and its cable assembly, fiddle with the air inlet boots on the  engine as they are a different size,  modify the air outlet from the air box as that is also different (or switch over to carb mounted air filters) and switch to a cable actuated oil injector setup rather than use the CV style pull rod.

Frankly – I just don’t have the energy to start making this number of changes at the moment, and since I have access to additional CV units via the local CVMG club to supplement the box of parts I’m starting with if required, then CV it will have to be. One change I will be making is to rejet the main jet on the centre carb to match the two outside ones as I will be using a three into three exhaust pipe setup, rather than the stock setup in which the centre cylinder exhaust was split into two pipes.  For a main jet, I’m starting with a 115 rather than the stock 110 and we’ll see how that works. The other thing I’d like to do, but will probably pass on for the moment is to install the ‘anti-surge’ fix that was detailed in Service Bulletin GT-36, March 11, 1977 and which can be found here. As I will not be using a stock exhaust system I thought I’d wait and see whether it will actually be required.

I used the Keyster KS-0029 kits to rebuild the carbs – everything they supplied fit just fine, but I was slightly disappointed that the small o-ring (part number 13295-31210) for the needle jet was not included, nor was the fuel filter (13376-65010). Both of these are still available from Suzuki, but it would be nice of Keyster to add them to the rebuild kit just to make it fully complete. These kits are available all over the place and can be picked up on eBay for between $15 USD to $30 plus shipping.  Luckily the kits do include new starter plungers which are not available from Suzuki, as these were all noticeably worn at the shoulder  where the actuator engages it. The photo to the left is slightly out of focus, but gives the general idea.

 The other problem I had (other than the usual corrosion and cleaning items) was that the oil pump arm was missing the small plastic ball used to connect the carbs to the oil injector pump rod.  Each of the available carbs I had all showed the same problem as the metal bush the ball mounted into was so badly worn that it wouldn’t stay. Fortunately, Fred in the local CVMG had a spare carburettor set I could use for parts, and so I swapped out the shaft as can be seen in the photo to the the right. My younger son Derek handled the metal polishing of the carbuerettor caps, and he did a nice job of making them look almost new again !

So – now I finally have a set of carburettors ready to go so the next step will be to see if I can get the engine started !

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