CA002 CARBURETTOR

My Viper has been running a bit rich and I couldn't tune it out so I recently replaced the main jet, needle jet and needle (Monobloc 376 carb).
The plug is a nice brown colour now, with the needle at its lowest position, but I'm getting popping in the exhaust on the overrun which I didn't have before. I'm sure you covered this in one of the Q&As but I can't find which one.
Unburnt fuel in the exhaust seems the logical answer, i.e. richness but this didn't happen when it was running richer. I think I remember you mentioning air leaks in the exhaust system but mine seems to be sealed OK and it's something I've never had trouble with. And I haven't touched the exhaust since long before I changed the jets.
Timing is correct as well.
So can you let me know where to look in the answers, if it's there, or give some advice. - Frank Dunning 15/4/2003

I don't think this is covered specifically elsewhere, but the Service Manual is quite interesting in what it says about banging in the exhaust - it can be by either too weak or too rich a pilot mixture, and it's the latter that also requires an air leak in the exhaust. In both these cases it occurs with the throttle closing - if it happens with an open throttle then an ignition fault is more likely.
Assuming it's not just a pilot screw setting that's too weak, one cause of a weak pilot mixture can be an air leak between carb and manifold. If you've had the carb off maybe the manifold seal isn't good now?
It's unusual to have "the needle at its lowest position" (ie the clip is in the top notch of the needle), and this too would cause weakness just above the pilot stage.
I would try seal the manifold joint with a gasket compound or silicone, and try raising the needle - if either makes a difference to the popping then you know your cause is a weakness on small throttle openings. - TW 15/4/2003

Yes of course, the service manual, I knew I'd read about it somewhere. If all else fails read the manual!!
I was so convinced the banging in the exhaust was caused by rich mixture I hadn't tried to richen up the pilot screw. Anyway that worked fine.
The O ring on the carb flange, is new so I doubt if that's leaking. There is a spacer (insulator?) and a new paper gasket which I've smeared with silicon.
I tried the needle on the second notch last night and it definitely runs best with it set at the top notch. This was also the case with the old needle and jets, which is why I changed them on the first place. It was far too rich on the recommended settings. I've checked all the jets and they are the correct ones for that engine.
This carb has always run on the rich side. What else can cause that? Could it be the float chamber level. I've shaken the float and it doesn't appear to have any petrol inside, i.e. leak. Are there different sizes of float needle?
The bike idles differently on the prop stand. Faster I think. Is that normal? - Frank Dunning 16/4/2003

Idling faster on the prop stand is normal but can be a sign of being too rich (as is speeding up as the last fuel is used if left running after turning the petrol off). A little increase in idle speed is normal, because the pilot is usually set rich to make idling more reliable, so a slightly weaker mixture is more efficient. But if there is a significant increase then either the pilot mixture is too rich or the float chamber level too high. The effect of leaning should be less on a monobloc than on carbs with a separate float chamber, and should be nil on a concentric. (I adjust the pilot mixture on my TT and GP racing carbs by leaning the bike to left and right and checking the fastest idle is when vertical)
The monobloc float height isn't intended to be adjustable and you've checked the float isn't punctured so the possibilities are either the wrong parts have been fitted (I didn't think there were different length float needles - even those on my tiny Valiant carbs seem to be the same as on my MSS, but I could be wrong, or it's always possible you have an out of spec needle), or maybe a washer has been wrongly put under the union that houses the float needle, or the pilot jet is wrong or has been made larger (typically when clearing a blockage), or an airway is blocked.
I'm afraid you just have to plod through, ideally trying another carb and swapping parts from it until you find the cause. - TW 16/4/2003