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[Dutto]

Thanks for the comments guys. I'll play around with it today and see how I get on.

[Crusader]

Hello Dutto, Some Amal monoblocks (& concentrics I think) have a semi-circular groove with a fine drilling in it close to where it bolts on to flange. The idea is to give universal fitment of carb. to bikes that may instead of horizontal have inclined or even 'angled' fitment carbs. to prevent fuel leakage into engine as reported in the event of the fuel tap being left on & the float chamber float/pin sticking allowing excess fuel to enter. (some Mikarb/Mikuni carbs have a excess fuel drain tube which exits below the engine.) Question 1 Has the bike never idled properly, in which case perhaps the carb has been wrongly jetted or it needs cleaning. Question 2. If it was previously o.k. have you recently stripped/cleaned the carb in which case you may have inadvertently cleaned out any 'gumming up' of this drilling which a previous owner has compensated for by changing the jet. If so, then you may have answered the question & blocking the hole is the simple solution. Hope that helps.

[Dutto]

Thanks 'crusader'. You're right. Both mono and concentric carbs have this feature. Your comment about cleaning the crud out of the carb is probably correct, although all the jets are standard for the 64 continental. I had to strip and rebuild the engine (unexpected!) but it did tick over. I agree that the easiest thing to do is to block off to the hole. Cheers chris

[geoffro]

Hello Dutto, I asked this question of my 1960 Crusader Sport about four years ago and I was given the same answer as you but that hole is so small there is no way it could divert the amount of petrol coming from a left on petrol tap. On the other hand I've not got a clue what is there for as to me it acts just as an air leak at the Carb flange would in my thoughts which is a strict no no. Would love to find the correct answer to this but in the meantime mine has the hole as I said and it has the most beautiful tickover. Clean carb and check timing, 1/64 BTDC as it says in the book is impossible to find but when you say it as 5 degrees as my engineer friend tells me, it is so easy to find with a timing disc.

[Dennis C]

A petrol drain is it's sole purpose, believe it or not it doesn't really matter but that is why Royal Enfield put it there. Think about it it will never get the full flow from the tap only the bit that leaks through the float valve.

[Dennis C]

When these bikes were made the float needle was solid brass not the plastic or viton tip as they are now and always leaked a bit of petrol if the tap was left on.

[simon]

I think that the hole sounds like a vacuum testing hole which should be blanked off at all times. Normally you only have them on twins so you can balance the carbs with a manometer. Air should only get into the carb via the throat anything else is a wrongun.

[Dennis C]

Sorry Simon, RE fitted these drains to a lot of their bikes because the carb has a slight downdraft, and as I said earlier , it's a drain to stop petrol filling the cylinders, no vacuum gauges to balance carbs back then afaik.

[simon]

It should still be plugged surely as otherwise it will totally screw with the carb as is being described.

[Dennis C]

No that's not the case, in fact blocking it on my Super Meteor screws the carb. When Royal Enfield designed the bikes with this drain the carburetion was adjusted to suit. The hole is only very small but intentional.