My (Brum) carburator is dripping at stand still, not at idle, is that usual?
Yes it is entirely normal, and it happens because of the very steep downdraught angle of the carburettor, ( 38 degrees). This puts the level of the petrol in a full float chamber above the level of the pilot airway, and so the petrol dribbles out until the engine is running, when the air being sucked in through the pilot airway stops the flow.
So many Scott carburettors have their bottom union nuts badly chewed up by unknowing past owners taking bigger and bigger spanners and even monkey wrenches to the nuts in vain attempts to stop the leak.
At least it washes the flywheel clean of oil every time that you run the engine, but it is not a very endearing little quirk of the marque!
Best wishes, Brian Marshall
Thank you, I had rather seen it wash the clutch.
It may be “normal” for fuel to drip but it should not be so and I refuse to believe that they were manufactured to be that way!
I accept that some do, (It’s not just bikes that age!), and by and large we get away with it but it’s not only wasteful it’s also a fire risk. Think not? I have had it happen to me and it has happened to others too, see the bottom of https://www.scotttechnicalities.com.au/technicalities/Chapter%202/2.5%20Carbs+Controls.pdf
I have before me an unused and unmolested 206/151R Amal which, because of a previous post, I had a hard look at. As a result I concluded that the fuel height should be at least ⅛” (3.2mm) below the transverse air holes near the bottom of the main body.
A further point to bear in mind is that due to the steep angle, if the fuel height is permitted to be above this level and with the engine stationary, then there is a likelihood of fuel percolating its way through the pilot jet and then down into the engine which at the very least could upset the mixture ratio and make starting problematic particularly with a hot engine. But then we always to remember to turn the fuel off don’t we? 😉
The fact is that these carburettors are designed to operate with the main jet submerged, so as long as there is a sufficient head above the main jet to prevent air entrainment the fuel height could be made lower. There is a limit of course, fuel height will tend to drop as the power demand rises, (check the line from tank to carb. for flow), and as revolutions rise there will be significant agitation in the float bowel which could make the effective height lower too.
As a reversible experiment it would be a simple matter to make a second float clip groove in the float needle thus lowering the fuel height in the float bowel by that amount.
EFR215’s comments about measuring petrol level seem to have ignored the significance of the position of the float chamber relative to the carburettor body….Swing the float chamber rearwards and the petrol level relative to the carb body is increased, and vice-versa. The correct position of the float chamber is central to the curved cutaway shape in the top edge of the crankcase, provided that it hasn’t been modified. It follows from this that there is no need to start cutting new grooves in float needles….Just swing the float chamber backwards or forwards as necessary.
Please also bear in mind that when starting from cold the float chamber will have been flooded by the use of the tickler button, and this temporarily puts the petrol level well above the pilot airway level.
Amal 206/151 carbs on earlier Flyers are different to the later 206/151R carb as the earlier ones have a totally different primary air system with 4 external holes around the lower part of the mixing chamber. The later carbs have the primary air inlet just inside the bellmouth. The change happened in 1939, when Government buyers of BSA’s, Nortons, etc., quite sensibly insisted that ALL air entering the carb had to be filtered, and of course that wasn’t possible with 4 external holes to be included. Quite when this change filtered down to carbs being made for Scotts, and how long it took for old stock to be used up first, is anybody’s guess.
just a comment that over time the fuel height in the bowl raises itself as the top point on the needle, and the seat in the float bowl cap, both wear. Also worn needles cause the fuel level to run too high by leaking constantly. I’ve seen a few Scott crankcases split around the crank chamber where owners have left the fuel tap on, the needle has leaked fuel past itself, filled up the crankcase on one side with fuel (the side with the piston up) and the owner has given it a stern kick without first rolling it over. The resulting hydraulic lock bursts the crankcase.
If your carb is leaking fuel at standstill and the float bowl is in the correct place I would first check the fuel tap is sealing off correctly, and then replace the needle.
As a further note to lower the fuel height in the bowl you don’t need to cut another groove, you can just place a small light washer or two between the clip and the float.
My only objection to swinging the float bowel fore and aft is that the carburettor is set at an angle so any such repositioning will put the float bowel out of vertical, particularly noticeable on the nearside/offside axis.
The poor old needle valve already has more than enough to cope with on a bike, setting the bowel and thus the needle at an angle is just adding one more possible complication that I’d seek to avoid.
Wear, (I exclude ”adjustments” here!), of the needle valve is less of a problem than might be thought, even with marked evidence of wear it will only result in a hundredth or two of an inch rise in fuel height. If you doubt me do the maths!
For all that wear at the needle valve can be a problem; it is possible that it will not be entirely even leading to incomplete or erratic sealing. Certainly as a land forms the already small pressure per unit area for sealing falls compared with a line-on-line sealing and where there is a land then foreign bodies can get attached.
Mark you there is no need for a new groove, it occurred to me that by far the simplest way to adjust height is to place a lightweight spacer under the needle clip thus raising the needle relative to the float.
“always turn off tap”…
A place perhaps for the application of a modern vacuum tap if thats possible?
hidden of course