Hi Forum,

Maybe someone can explain to me the function of the carb balance tube as found on the (original Amal) inlet manifolds?
Follow on question is why it isn't used on other dual-carb applicationes (Keihin FCR, Mikuni etc)?


Thanks Forum!

If my memory serves me well I think it helps ballance the vacuum between ports at tickover and small throttle openings, but then again it mght just be a cheap way of plugging the vac points. You'll find ballance tubes on a whole host of other bikes cars and carb combinations including those of Japanese origin.

Cash

If your talking about the two tubes that stick up on the intake manifolds, those are for your oil bag vent. At least on my bike that is what they were run from. I had a small brass fitting plugged into the vent on the oil bag and a 1/4" line running to a brass "T" then down to each vent on the intakes.

Chopped you need to consider moving the vent to the tank to feed the air filter, currently you risk a lean mixture as if the easiest route for air to get into the engine is this tube it is fresh air with no fuel and the mixture will be too lean. The vent from the tank on a std 850 goes from the tank to the air fiter before the carb, the engine can only get air from the carb so it must always have fuel included, hence if it jetted correctly it cannot go lean, your setup could go lean even with a correctly jetted carb eg if the air filter got blocked more air would be drawn in from the oil tank leaning out the mixture.

The two tubes emanating from the intakes are to be joined by a single hose, nothing else. The oil tank is to be vented elsewhere. In stock trim it's vented to the air filter. As Kommando stated you run the risk of too lean of a mixture as is. Recalling some history of your bike, I assume the PO kludged the current configuration. In all probability he also jetted to make up for the bastardized intake configuration. In other words, you now have a bike with a big air intake leak.

I agree with JimC, my 850 backfired and ran poorly when the tube blew up and off one side. It was only pushed on there before. I secured it with thin haywire and twisted it with pliers. Hey, you know what they say. Chrome don't get ya home.