Balancing Twin Carbs

The first step is to get the throttle slides both opening at the same time. This is easy when the carbs are off the bike and I always check this when I have them off. It’s a bit harder on the bike especially with splayed heads. The way I do it is wind the throttle stop screws right out so the slides don’t rest on them. Then place a 3/8” drill bit (metric equivalent will do here if you’re not a purist) underneath each throttle slide. Now adjust the throttle cables so that both drill bits drop at the same time and there is a little bit of play at the twist grip. Now wind the throttle stops to get the bike idling. Make sure you do this with the bike warmed up. The next adjustment is the air screw which the manuals tell you to adjust until the fastest idle is achieved but to be honest I don’t bother with this, I just wind the screw out to the factory setting and leave it at that. The air screw only affects the carb at 1/8th throttle so as long as the bike takes of okay from rest it is okay. (Anyone have a different view on this?)

Crankcase Sludge Trap

The trap in the crankshaft is hard to remove but is a must do whenever the bottom end is apart. I learnt this lesson as a youngster when I rebuilt my 500 Trophy in a corrugated iron lean to at the back of the commune (yeah alright, bloody hippies, but this was in the very early 70’s and I was not the only TOMCC member who lived there, let’s just give him a nick name, say Micky Rat, to protect his identity). I couldn’t undo the screw cap, they’re bloody tight, and thought what harm can it do if it’s that hard to get off it should be okay. Big ends seized when I opened her up on the way back from Whatawhata.

Follow the handbook to remove the cap. When that doesn’t work try heating the crankshaft up and try again. The last resort is to use a hammer and cold chisel on the slot to free it up.

Now for the clever bit – remove the flywheel bolt that retains the trap tube, take your Shadbolt extractor, see photo, and insert it in the trap until you can see the hole in the threaded shaft. Now drop the pin down the bolt hole, Shaddy uses one of those Allen keys with a handle and holds a magnet against the shaft to hold the pin while lowering it down the bolt hole. When the pin locates in the hole in the threaded bar he removes the magnet and withdraws the Allen key. Now wind up the nut drawing the trap tube out of the crankshaft. Simple eh? I had a grader wheel stud from the days when my Dad worked down the Oto County that I could jam up the tube and sweat it out but this tool makes it easy. When you reassemble the crank British Spares can supply an Allen head cap that is apparently easier to remove.

Better Scavenging

If your 650 engine no is between DU44394 and DU887147 then you can reduce running temperature and therefore extend engine life at no cost, next time you have the crankcases apart. From DU44394 an oil pump with better scavenge capability was introduced and after DU887147 the crankcase oil level was increased by shortening the length of the scavenge pipe. All you have to do is shorten your scavenge pipe by cutting 5/8” off the end. If your motor is pre DU44394, and you have fitted the later model pump the same modification obviously applies. I made this modification to my 62 Thunderbird and didn’t notice any difference. I sold the bike (bugger) soon after so don’t know whether it improved wear.

Hinkley Valve Adjustment

Follow the manual to remove the valve covers etc and measure valve clearances. This is where my brain starts to hurt as my feeler guauges are imperial, and I think in Thou, and I have trouble keeping track of what shim I need and where it goes. Try this spreadsheet Valve Shim enter clearance in thou, existing shims in mm (if you can’t read the shim use a vernier Caliper to measure thickness) then the required shims are calculated.

Measure clearances and enter in green cells or if in mm enter in blue cells.

Remove camshaft and record shim thicknesses in pink cells

The new shims you need are in the red cells.