I have twin Weber 40 IDFs on my t4 2litre engine and the right hand one is over fuelling badly resulting in those two cylinders misfiring at idling speed. At higher revs the misfire disappears but I think the carb is still running rich. Float levels are OK and fuel pressure reg set to 2.5 psi. I have been told that this is quite common and has something to do with the position of the first carb in the fuel supply line in relation to the fuel tank position and can result in siphoning!! Altho I could perhaps think this a reasonable explanation, surely the needle valve should prevent this?? Also was told to reduce fuel pressure regulator from its current 2.5 psi! Has anyone else come across this problem and if so what is the cure?
Not sure what you mean David - where would you put that valve and surely it would allow fuel to flow to the carb as normal, or not at all depending which way round you put it?? At the moment my main fuel feed goes to right hand carb and then on toward the left hand one.
it prevent the fuel siphoning issue, check the filters in the banjos on the carb inlets to see if they are blocked too
i have been considering this very same issue fitting my carbs. The fuel pipe comes out of the tinware on the drivers side to a tee piece. This means the fuel pipe is shorter to the o/s carb so my thought was that the fuel would take the route of least resistance? I was going to use equal lengths of fuel line to try and avoid this issue and just route it differently to the o/s which is easier than making the fuel tee come out of the centre of the tinware.
I'd agree with everything you said. If the float valves are operating properly and the pressure's not excessive, I can't think why there'd be a problem . Fuel pipe lengths shouldn't make any difference. Siphoning - what's doing the siphoning, and where to? Doesn't sound right to me - did that well-known bloke down the pub tell you this ?
Run them at less than 2.5psi??!!?? someone is yanking your chain!! They require a 3.5psi supply, and if one is syphoning then likely as not, it either a badly set float height, or it's time for a service and some new parts in the form of a service kit!!
I'm fairly sure the carbs are Ok both in terms of float level and general condition so am looking elsewhere for answers. Maybe fuel pump or the pressure regulator, or perhaps even in the fuel pipe configuration? First things first, so .... I'm wondering if my electric fuel pump is faulty. Haven't had a look at it yet but thinking about it, when I turn on the ignition I hear it ticking away as one would expect but if I don't start the engine the pump continues to tick. The note does change after a few seconds but the pump continues ticking. If it's a relatively high pressure fuel pump would this force more unwanted fuel into the carb despite the needle valve? I have heard it said that needle valves on Weber carbs are not built to take high pressure (more than 5psi??) and can cause fuel to 'leak' past the valve - true or false?? Could be my regulator causing probs but is there a way of testing it easily without specialist equipment? Would there be a noticeable change in fuel flow from the outlet if I adjust the pressure? I can experiment, I guess! Is it likely that my pipework could cause problems? The feed coming through at the bottom left hand side of the engine bay is routed upwards and then along to the right hand side where the pressure regulator is situated on a similar level to the carbs. The outlet from the pressure reg feeds the right hand carb and there is an outlet from the banjo on that carb which I've used to feed the left hand carb by routing the pipe upward to the top of the engine bay and then along the 'roof' and down to the left hand carb. Is it likely that the weight of fuel in the left hand carb feed could cause back pressure on the right hand fuel feed as a result of gravity - should petrol lines remain on a horizontal plane level with carburettors? Clutching at straws perhaps - but you never know!!
The non return would go in the main tank outlet pipe, but I have only seen it on a t25 where the fuel tank is below carb inlet level and it would empty the fuel lines and cause starting issues. The fuel pump has a pressure relief inside and just stops pumping in effect, the up and over fuel line may cause a problem I suppose, try simply laying it down
I had a similar issue on standard Solexes earlier this week. The float level looked ok, but there was a bit of muck in the lhs needle valve. I discovered this by getting the engine up and running so the carbs were 'full' of petrol. Then stopped the engine, disconnected the fuel hose running to the lh carb and blew down it with my mouth!! My thoery was that the pump should always keep the carbs full during idle, so it you shut off immediately then the float needle shouldn't be letting any fuel through. I tried this a few times and every time the lhs hose let me blow down it. I removed the offending needle valve and there was a bit of muck that was stopping it fully closing. Cleaned it out and problem sloved!
I had another look this evening and although I'm pretty sure there is no siphoning occurring it's quite obvious that the rh carb is running very rich at idle despite the mixture screws being turned in to almost closed. Done a compression test and all OK there. I suppose it could just be that the plugs on those two cylinders aren't firing at low revs for some reason. I have just now swapped the plugs around and will see if that has any effect in the morning - if not I'll swap the leads around as well, just in case. If that resolves nothing I'm taking that carb off and stripping it down so that I can blow air through all the crooks and nannies (well, you know what I mean!!) I shall then try again and if need be perhaps change the idle jets from 55 to 45 just to see if it makes any difference. Any help or advice before I tackle those jobs?
Oh, Yes!! Have now tested the plug swap and found no appreciable difference. So have now removed the rh carb, taken out idle jets, mix screws and emulsion tubes complete and air-blown through all passageways and orifices. Put it back on and ................................................ wait for it - ..................... a definite improvement but this carb just doesn't seem to have the same ooomph as the other i.e. when I start engine and remove plug leads related to cyls 3 or 4 there is a significant drop in revs whereas removing plug leads from cyls 1 or 2 results in some revs drop but not much!! Maybe need new plugs altho' to be honest electrodes look OK and are gapped correctly
Wanna borrow a set of dells for a week ?? You could swop the carbs side to side to see if that makes a difference ??
Hi Steve - are you serious about the loan? Are they spare - I might end up making you an offer for them! I think the next step is to try the side to side swap and maybe I'll get around to that tomorrow. I'm almost totally convinced it's that right hand carb at fault but at least by swapping with the other side it may provide the proof.
