As the title says. I have petrol literally pouring from my carb drain valve. It started a while ago so it’s been off the road. I was since offered another set of carbs for it so I have fitted those and the problem persists. I’m at a loss as to what could be the cause. If anyone has any ideas/solutions it’d be much appreciated. I’m already oily today so may as well try and get it solved.
If a second set of carbs doesn’t cure the symptoms, that would suggest to me the problem is elsewhere. Split end to a pipe? Tank leaking? The fuel may be just finally dripping from the drain(s), not coming directly from the drain(s).
It is coming directly from the drain. It is obviously possible that I was unlucky and got another set of carbs with the same issue. Or that there is another problem. I had an issue with my petrol filler drain being blocked, in the 90deg metal bend in the tank. As the fuel return also comes back to one of those if it were blocked could that cause an increase in fuel pressure?
If it is running out of the carb overflows (which is not really the carb drain BTW until you open the screw) then it is down to one of four things. 1) Stuck or worn float needle valves. 2) Overbrimmed tank, sitting on a rear paddock stand (it would also be seen from the tank breather down below). 3) Overbrimmed tank and the tank breather valve (blowing out), is not working or more likely the stub pipe is blocked with rust, paint and bits of rubber. Worse when the weather or the engine is hot. 4) Return to the tank is blocked (when the pump is running).
Of course if the OP really means the carb drains (which are the single screws in the bottom of each float bowl) then there is a damn good chance that they have been overtightened and the float bowls are cracked.
first thing to try is: turn tap off remove plug over main jet/ emulsion tube, or drain screw that drops the float right down, turn fuel on, that allows clearance so anything sitting on the float valve set gets removed to the float chamber and washed out, replace plug, should refill float chamber an not start leaking again, important to check residue bowl and filter, filters frequently disintegrate and jam float needle. add inline filter, to stop rust paint flakes and whatever jamming float needle both types of floats; plastic foam and brass can stop floating or the rubber tip on the float needle can get damaged, or float needle seat is not tight; take float bowl off, gently hold float up on spring in float needle, fuel should stop flowing? remember float height is adjustable, and will mess the mixture up if you bend the tab? turn carb upside down with end with float pivot 45 degrees lower than the gasket surface on the other side of gasket surface; measure and check float height?; holding the float on a angle stops all of the float weight going on to the needle and depressing the needle spring and giving a bad reading, the float should be able to move as you move the carb around; the float needs to be free?
Sorry breakout, Before you post something about carbed SSs you need to know a bit more about them. If anyone wants to know how to set the BDST38s up just ask - but there is plenty already on this forum.
I know plenty, my motor in Ducati Bunburys frame ran high 10secs and was good for105rwhp you know of any other Superlights good for that? reangled valves, bigger valves, straightened inlet, modified squish bands, high comp, race exhaust, Daytona cams, 954cc and FCR41mm Keihen carbs, race exhausts You are right, I was thinking talking of the only carbs i ever had on a 900ss; the only ones i would put on a 900ss but my description holds for all motorcycle carbs from Amals to Mikuni, Keihin, and was only offered as a quick way to diagnose flooding float bowls Tuning them, my method; Fit lambda probe bosses on headers, mount readout where visible disconnect accelerator pump on carbs mark throttle drum into 1/4 turns ride bike in gear required to balance power testing full throttle requires both needle jet and main to be correct test for 3/4 to full open, snap throttle lambdas should go instantly lean, but as the bike accelerates (rpm increases) mixture should strengthen to ideal, reattach the accelerator pump and you avoid lean-out but are assured of best power mixture, my bikes tuned that way cannot be improved on a dyno because the induction system operates in the exact pressure field as exists when riding, which is not provided by a dyno fan this is because: the depression on the jets is equal to the the inverse square of the gas velocity so if you double the throttle area, you halve the velocity, and the mixture goes 1/4 lean. this is why we have carbs that are constant velocity? the metering is independent of throttle position and only depends on the suck, (vacuum) so the vacuum on the jetting system stays the same, (because the air metering is is determined by rpm) the mixture doesnt lean out like a non vaccum compensated carb does; as the vacuum increases with increases in engine rpm the compensating slide opens to maintain gas velocity. this disconnects the inverse square law and the mixture tracks requirements better. The consequence of this is a slower engine rpm response to throttle inputs, Which means the gearbox feels lumpy, because hard to match gear/wheel speeds for down-changing with poor engine blipping, get rid of the CV carbs and you get a much better engine response and gearchanging separate carbs have straighter inlets and produce the most power
not the carbs only the float valves and floats can be the problem, probablt the needle cant seal because your tanks rusting or has rubbish in it, and the fuel filter is not effective, my prefered type is
Breakout - this is so the OP doen't get confused by your posts. The BDST38 carbs do not have plastic foam or brass floats. They have hollow plastic / nylon floats. There is no plug over the end of the main jet / emulsion tube. The float bowl drain is the single screw in the bottom of the float bowl - it also used to set the fuel height - there is a line cast in the float bowl which the fuel level is checked again; however......... .....any dimension for float / fuel height is abitrary (including angle of float). This is because the pilot circuit on these carbs is what controls the revs up to about 2500 - 3000rpm when the diaphragm starts to lift and take over. The float / fuel height affects the pilot circuit and is best set on a dyno..........the height of 14mm which is usually bandied about can be severely out - in my experience it is nearer to 12mm, although I have seen them at 9mm. In turn, the float / fuel height also affects how many turns out the pilot screws need to be. Apart from the float / fuel height possibly being wrong in the OP's carbs; I agree that the float needle tip can be worn, but it is not often that both wear to the same extent. As the OP has the same problem with two pairs of carbs, I reckon he needs to look at something else. I would first recommend he disconnects the tank breather valve and also flips the filler cap to see if the problems stops.
well that is the test for fuel starvation not leaking carbs; if you put a lower pressure in the tank because the breather is blocked; then it lessens the pressure above the float valve, (equivalent fuel column) that has to be overcome by the force provided by the float-buoyancy, so the valve has a higher seating pressure and is less likely to leak. Note these bike from memory have an electric fuel pump? the pressure area of the valve X the pump pressure supplies a force trying to unseat the float valve? Do the quick test as above, if it doesnt work, pull the carbs, check the float, float settings and float valves, you will find the problem to be one of those things or and excessive delivery pump pressure and fit decent fuel filtration, For free; If you ride your bike in the rain the tanks will get damaged by water ingress occurring when you lift a wet fuel cap, to full up. Get some diesel "Fuel-Set" and add 3x tablespoons to tank of fuel this works like a detergent one end attaches to the water the other to the hydrocarbon, (petrol), the result is the water goes into solution is picked up and burnt (increases the octane rating, lol) dries out the tank and stops it rotting LEAKING CARBS NO PROBLEM; REALLY LIKE A LEAKING TAP IN YOUR HOUSE CHANGE THE WASHER; PLEASE DONT TURN THE TV ON TO SEE IF IT STOPS DRIPPING sorry about your dripping carb but i dont want to tell you how to fix it anymore? Just try draining the tank, that will stop it, ahhhhh you have a loose valve cap on the rear tyre??am I right
Camel? Nope. But ive had grit in my needle valve a few times on my lambretta before and had a float develop a hole too. Both returned similar symptoms.
You see Breakout - you really don't know the ins and outs of the fuel system on the carbed SS. You need to do some more research before you comment. The SS tank breather works in two ways: It allows air into the tank easily to avoid a vacuum which as you say can cause fuel starvation - or an imploded tank. But it also breathes out when the internal pressure get higher than it should either by atmospheric pressure or temperature. It releases internal pressure when it reaches about 5 psi (which is why you can hear the tank squeak or fart sometimes). The trouble is that the tank breather blowing out is easily blocked or can become faulty and can allow the pressure to build up above 12psi. When that happens, the fuel system becomes pressurised and if it reaches 12psi it can overcome the fuel's action of holding the float valves shut. (The pump operates at only 3.5psi approx). So when the internal tank pressure increases, the float valves release at around 12psi resulting in the float bowls overfilling and fuel pisses out of the overflow pipes. The overflow will normally stop when the tank pressure drops to normal; however if the tank is brimmed with fuel, once the overflow starts it sets up a siphon and nearly a whole tankful can get emptied onto the floor via the overflow pipes. Another thing the OP should look for - is the breather valve fitted the right way around? The little arrow should face forwards away from the tank. Also note that I had to change the breather valve 3 times on my SS and a mate's one was done twice. Two were actually faulty from new; two were full of rust and crap. Incidentally, when I post something like this, I use punctuation and grammar to makes things as easy as possible for someone to understand. Rambling off into other carb specs doesn't help.
But the OP has tried two sets of carbs and the problem is still there - very unlikely unless all four carb float valves are shagged out. Even so, if the carbs from another bike are fitted, they may not work properly depending on which bike they came from as the float levels could be wildly out. Also note that with the BDST38s, the fuel level is not the same measurement as the float level. The greater the float height when measured means there is less fuel in the float bowls and the smaller the float height means more fuel in the bowls. That's why the pilot circuit can be affected and the pilot screws need different settings. And.....don't expect float heights and pilot screw settings to be identical on both carbs.
Arky, yes I understand that. I am merely eluding to my comments prior to those developments coming to light.
Yep, I know bootsam.......I was just adding onto your bit for the benefit of breakout..........a bit like adding that there is a tiny very fine mesh filter in the fuel system on the carbed SSs (but that wouldn't cause an overflow).
