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Small hole in the carb manifold
Posted: Thu Apr 16, 2015 7:04 am
by Pike
Hi, does somebody know what function has the small hole on the underside of the carb manifold for 500 Twins (part no. 36605)? I bought the bike with a cracked manifold but this one didn´t have any original hole on the underside, it was gas tight besides the crack. Then I bought a good used manifold but with an original small hole, the manifold fits and works with the bike. But what sense should have a hole in the manifold that I suppose to be normally gas tight? Regards, Pike.
Small hole in the carb manifold
Posted: Thu Apr 16, 2015 7:28 am
by Dennis C
It is there to allow petrol to escape if the tap is left turned on and the carb floods so the cylinders don't fill with petrol.
Small hole in the carb manifold
Posted: Thu Apr 16, 2015 3:42 pm
by Barnes
My 700 twin manifold has this same hole on the underside. Ed
Small hole in the carb manifold
Posted: Thu Apr 16, 2015 8:05 pm
by simon
I imagine that the holes in the manifolds are there for the same reasons as the ones in my Ducati. They are there so you can balance the carbs using a manometer. With the Duc I screw in small brass connectors with .1mm holes in them which stops the vacuum pulses making the manometer unreadable. Makes setting the carb balance a doddle.
Small hole in the carb manifold
Posted: Thu Apr 16, 2015 8:06 pm
by simon
Oh one hole. Nope disregard my previous post.
Small hole in the carb manifold
Posted: Fri Apr 17, 2015 2:57 pm
by geoffro
Dont think thats the reason Dennis as the hole is only about 1/16th, i have one on my Crusader Sport and i have also asked around to see why it's there but nobody seems to know. Come on fellas someone must know.
Small hole in the carb manifold
Posted: Fri Apr 17, 2015 3:10 pm
by Revband
Geffro, Dennis is absolutely correct the bike has a slight downdraft to the carb and if the petrol is left turned on it can overfill the float chamber and the petrol would run into the engine, the flow would only be very small as it would have to pass a closed float valve, bear in mind that the Amal float valve was solid metal when these bikes were made and not the viton tipped ones as supplied now and the seal was quite poor, most bikes with a downdraft cab had this hole.
Small hole in the carb manifold
Posted: Fri Apr 17, 2015 4:25 pm
by Mark M
Just to add to what the others said, if you look inside the manifold you'll see it has been machined to create a small "gutter" to direct fuel towards the hole, further confirming it's purpose. And remember, the carburation as standard, was set up to allow for the slight air leak created by this hole so don't alter it without adjustment! Also, the manifolds are all matched to the size of the original carb fitted so if your bike has had a carb change make sure for best results the hole is the appropriate size.
REgards, Mark