NC30 Carb overhaul part 2

The first post about the carb overhaul ended with “now I just have to synchronize them”… Well, nothing ever seems easy on this bike.

For those of you who don’t know, synchronization is the procedure by which you make sure all cylinders are idling at the same throttle. If you don’t do this, chances are that a few cylinders will want to go faster and will have to pull the others along. This results in a bad idle and the cylinders that are doing all the work will run hotter than the others. The procedure is to hook up a set of vacuum meters to the intakes on the cylinders. Then, with the engine idling, you adjust the screws in the throttle linkages between the carburators until the manifold pressures are the same.

This is simple, in principle. The problem starts when you try to do this in practice. There are 3 adjustments screws that each adjust the #1, #3, and #4 cylinders against the #2 one. These screws are all between the carbs, facing down. To get to them, you either have to jam your hand in between the engine and the carbs, which is not going to fly since this is done with the engine at operating temperature and you’ll burn your hand badly, or you have to come up with some creative tools to allow you to reach the screws. (I’d show a picture but it’s impossible to capture.)

Once I’d worked out some way to reach the screws, I started the adjustment. I had problems with the #1 screw. I wasn’t sure I was turning it the right way, because it seemed to not have the result I expected. Eventually, the screw became really loose and when I tried to screw it back in, it popped out. Sigh… After trying to get it back in, I bit the bullet and took the carb assembly off. It turns out that this screw was not original. The standard screws are M4x0.5mm fine pitch, but someone had jammed a standard M4 screw in there, and thereby messed up the threads. This is a 2mm thick metal piece with not a lot of room.

I pondered my options. I first thought the thread was good enough to hold the standard screw in there, so I ordered a new original screw. Alas, when I tried to screw it in, it popped out under the spring pressure. No go. Next I wondered if there was some way to repair the thread. The material is too thin to use a thread repair insert like a Time-Sert. I thought about trying to repair it with JB Weld and re-thread the standard screw, but threading in JB Weld, in such a thin material, was also unlikely to last. The last thing I wanted was for the thing to pop out after a few days, necessitating yet another carb removal.

In the end, I opted to drill up the hole and tap a new M5 thread. This seems to work, knock on wood. I could only find standard pitch M5 taps and bolts, so it’s a rougher adjustment than it should be, but that’s life.

After putting the carbs back on, I tried to yet again synchronize. I was getting the hang of how to get to the screws, so it was a lot faster than before. However, the results weren’t making sense to me. The Haynes shop manual says that the three screws adjust #4 to #2, #1 to #3, and the front (#2/#4) to back (#1/#3), but it seemed to me that when I turned the screws, something different was happening.
I posted a question on the very useful 400greybike forum, and someone responded “that’s your problem: it doesn’t adjust front to back”! Apparently the Haynes manual is incorrect on the procedure…

This movie is toward the end. I did a few more adjustments, but this is pretty good:

Now we’ll see how it runs on the road! It would be nice to finally get to take this thing for a ride, but we’re not quite there yet — it needs new front brakes first.


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