The CX constant velocity carbs can look a bit complex at first but after studying the various circuits for a while i found they arn`t too bad to figure out and are in fact pretty simple. Here`s my interpretation of how they work; there`s three independant jet `circuits` and each one is served by an additional air bleed circuit. Air from the `atmosphere`(high pressure) side of the carb is bled to each jet where it is introduced to the fuel being drawn through that jet before it exits into the venturi as a fuel/air `emulsion` and joins the greater mass of air flowing through the carburetor.The main reason for mixing this extra air from the bleed circuits with the fuel is to pre-atomise the fuel while it goes through the jet which has the effect of breaking up the stream of fuel into tiny droplets and then into a fine mist as it`s drawn into the venturi through the jet by the low-pressure - this is why the main jet holder, primary nozzle and pilot jet have holes in their sides - to allow the air to mix with the fuel. Large globules of fuel would be bad for engine efficiency and performance - fuel droplets would wet the inlet tract walls instead of flowing with the air charge through the intake and would not produce a `clean burn` in the combustion chamber. This air bleed also provides a small amount of positive pressure on the circuit which helps to control the carbs` tendancy to over-fuel at smaller throttle openings - the amount of air bled is proportionally less compared to the main airflow through the carb the further the throttle is opened - the air bleed jets remain the same while the throttle piston and thus airflow through the carb is variable . The 500 Euro`s and GL`s in particular have a terrible induction path route: with the carburetors having to be sited inside the two pressed-steel hollow frame members of the Prolink frame the inlet tracts have to sweep sharply outwards towards the engine`s heads, the fuel/air charge has to negotiate an `s-bend` before entering the cylinder - a bit like a meander on a river,the water (fuel/air charge) would tend to aim for the outer bank of the bend before bouncing off,straightening up again and then having to to the same on the next bend. Not good for engine efficiency! We`ve all read about why Honda supposedly twisted the cylinder heads (to `prevent carburetors interfering with riders legs` and get a straighter shot for the inlet charge etc) but they took a step backwards when they designed the Prolink models. The early twin shock bikes have carburetors placed much further apart than the Euro`s/GL`s with short inlet tracts which site the carbs nearer the head and thus a better inlet path. The design of all the latest sports bikes have concentrated on getting the induction route as efficient as possible; engine canted forwards to lower the c of g and make room for the carbs and airbox directly above, with them now sited under the hollowed-out fuel tank just behind the headstock to give the intake path as short and direct a path as possible. Realising they had fecked up Honda refined the induction system when they made the last of the models - the 650`s, by mounting the carbs in a `V`pointing towards the cylinder heads and with a clever linkage to operate the throttle valves instead of the closely-positioned,parrellel-mounted carbs on the 500`s which meant one less bend for the 650 bike`s charge to negotiate. This makes quite a difference too, probably a major reason out of all the refinements the 650 received that made them better than the smaller CX. They`re much more fuel efficient, too -the 650 gets better mpg than the smaller engine.
Here`s a side by side image of the carbs with the 500 Prolink type on the right showing the angular induction and the original twin-shock and spine-frame model on the left showing a slightly better route, which may be one of the factors (weight being another) why the original Plastic Maggots were better performers than the later `Sports` model !:
......and a comparison pic of the Prolink 500`s versus the 650 versions - 500 on the right again;
Anyway, there are five possible points in the carb where fuel enters the venturi: the pilot port(where the idle mix screw goes), the two off-idle ports which are obscured by the throttle valve and are uncovered when the throttle valve begins to open, the primary jet nozzle, and the needle jet. The pic shows how air is bled to each circuit and is fed around the holder/nozzle/jet. Note that the pilot jet is fed fuel via the primary jet,but the primary jet does not controll the fuelling of that circuit as the primary is so much larger,the fuelling being metered by the more restrictive pilot jet. The two circuits are separate,they only share the primary jet as the source of fuel - the very top pic on this page makes it seem like the two circuits are joined,they are not - its the way the pilot drillings are located `behind` the primary nozzle.
Some more detail on the idle circuit:the idle circuit has the addition of an `air cut-off valve`. The pilot (idle) system is still served with an air bleed but the amount of air fed to the pilot jet is limited in certain running conditions to temporarily enrichen the mix to prevent the characteristic popping and banging in the exhaust system during overrun with a closed throttle.This is caused by the very high inlet vacuum being unable to draw enough fuel through the small pilot port - the idle circuit is only designed for fuelling at idle/very low engine speeds and cannot cope with the engines demand at high revs/vacuum.This leads to an overly lean fuel/air ratio.This mixture is unburnable in these conditions and remains unburnt until it is eventually ignited in the exhaust which is when you can hear the `afterburn`noise in the exhaust pipes.
`A` balancing adjuster screw & locknut, `B` are the two pilot system air bleed jets, `C` is the secondary jet air bleed, `D` the primary jet air bleed,with corresponding jet positions in the venturi;
It`s pretty obvious the route the primary and sedcondary jet air bleeds take - follow the drillings! They can only go in straight lines,so are angled to meet each other,then plugged with a peened-over ball:
`B` shows the path of the pilot circuit air bleeds,there`s two jets & drillings - one feeds the space in the valve housing(the one on the right),the other is subject to being blocked off when the vacuum in the inlet tract pulls the diaphragm,overcomes the spring pressure and the `ball` on the valve blocks off the flow of air in that drilling.Under normal running both jets/drillings supply air to the pilot jet circuit via the port `E`,when the valve operates the centre one is closed thus restricting the ammount of air,enrichening the overall fuel/air ratio enough to prevent `afterburn`.The yellow line indicates the drilling to the vacuum side of the throttle valve.When the cover and diaphragm are in place the two halves of the cut-off assembly are separate - one side subject to low pressure,the other side atmospheric/high pressure.The right hand jet/drilling still provides high pressure to the atmosphere side of the diaphragm to enable it to close and provides a reduced amount of air to the pilot jet bleed circuit during engine conditions when it`s functioning.
I have noticed on occasions during carburation tune-ups that the idle mixture screw is unresponsive, and even when screwed fully -in the cylinder that i am working on continues to run - this is a sign that the air cut-off diaphragm is holed/split. The hole in the diaphragm makes a circuit that should not be there between the float bowl (via the pilot jet) and downstream of the throttle valve (via the vacuum drilling - yellow line in the pic) and draws enough of a fuel and air mixture through it to enable the engine to continue firing - even when the idle mixture screw is fully screwed in when no fuel should flow. This will also cause the idle mixture be be very rich. Normally i`d expect the cylinder to falter or even stall when the idle mixture screw is fully screwed in. Carefully examine the air cut-off diaphragms if there is lots of petrol on them or in the housing - only air should be present in that part of the carb.
As can be seen, a faulty air cut-off valve can give both lean and rich running symptoms:- lean when the engine is revving high with a closed throttle and high induction vacuum (air cut-off valve not working causing an extra-weak mixture and popping in the exhaust) and then rich at idle when extra fuel is drawn into the inlet (via the perforated diaphragm).
To lean the idle mixture the pilot mixture screw is turned in (or up,with the carbs in their normal position on the bike - clockwise),to enrichen,out.Even though the engine only requires a small amount of fuel and air at idle speeds the fuel flowing through the idle circuit still has an air bleed, so it is a mixture (emulsion) of both fuel and air that is exiting the port/jet (as is the case on all three jet circuits in the carb) - this fuel/air emulsion exiting through the idle port is added to the greater mass of air being drawn through the carb via the throttle valve, so altering the pilot screw increases or decreases the proportion of this fuel/air emulsion entering the engine and thus the overall ratio.
To clean the carbs i use basic tools to remove the float bowl, jets, nozzles, vacuum chamber etc and then dump them for as long as possible in a container full of parts washer fluid. After soaking i just use fishing line/monofilament and a squirt-bottle to clean and check the various drillings/passageways around the carb. (Compressed air is the best method however, and i used that a lot during my time at a previous employer, but that isn`t an option now unfortunately.) This can be used on both fuel circuits and airways. To poke out the various passageways i use a piece of fishing line monofilament, it`s stiff enough to dislodge any dirt but soft enough so that it won`t damage anything, and can be bent at an angle at the end and keep this shape which helps to navigate around the sharp bends that are in the carb body that a straight piece could jam on.
Another `quirk` i`ve encountered with these carbs a couple of times is while synchronising them: the adjuster screw would alter the balance one way but be unresponsive in the other direction and the vacuum gauges would not register a fall in vacuum when the screw was turned. The two throttle valves/carbs are joined by a linkage which has an adjusting screw searating each side of this linkage and is forced apart/held in tension by a small spring. When the adjuster screw was turned one way (clockwise i think...) the vac.gauges would respond because the thread of the screw is pulling the two halves of the linkage togther and altering the relationship between one throttle butterfly and the other, but when turned the other way the small spring should force the two sides apart but was unable to because of the resistance which had built up in the linkage due to being stiff from gunge/corrosion etc. I removed the locknut, washer,screw and spring to isolate each side of the linkage and found one side to be rather stiff. After squirting on oil/WD40 and exercising the throttle valve spindle for a few minutes it freed up, and cleaning up the adjuster screw etc for good measure i reassembled it and checked the action - i could now see that turning the screw both ways resulted in proper movement of the linkage. If this quick fix does not resolve the problem you`ll have to resort to a more thorough cleaning of the parts after some dissasembling. If no improvement has been made after soaking the relevant parts for a few days in your favourite penetrating oil a more direct method of freeing off stubborn parts will have to be employed which means the throttle butterfly shaft will have to come out. For the 650 carbs in the pics below, remove: balancing screw, throttle valve screws, throttle valve, shaft circlip & washers, bend up the screw tab, then the ball joint linkage rod fixing screw and then the rod pulls off both ends. Pull shaft out. The shaft is in two parts, one should rotate around the other. The smaller side has an alluminium collar, and it`s under this where the corrosion forms - see the yellow arrows. I had to use a bit af heat to warm the collar, then a drop of oil while twisting/tapping it apart. That did it.. Clean everything up then reassemble with some silicone grease in strategic places. Centre and carefully adjust the position of the throttle valve on the shaft. Peen over the throttle valve screws.
Fast idle setting. When you operate the choke lever/knob two things happen: the choke butterflies close and a small linkage/lever arrangement opens the throttle valves slightly to raise the idle speed to help with the running of a cold engine. I`ve seen on higher milage bikes wear on this lever which means that the throttle valves don`t open quite as far as they should which affects the ease at which the bike will start and run. This wear makes for a rich mixture on start-up (which is what`s required) but with very little extra throttle opening the engine very quickly gets `woolly`, `fluffy` and the fast idle speed difficult to control. Replace the lost travel on the throttle valve bellcrank by bending the tab the lever ot bellcrank - not too much though or the idle will race at full choke. About 2 - 2.5k at full choke setting i find ideal - that`s about 1mm or so of extra throttle opening - not much.. The wear;
Sticky pistons? If you think you have an issue with your carbs that may be related to sticking slides/pistons (and they rarely do in my experience - do this test as soon as you remove your suspect carbs and before any dissasembly or cleaning to find out......) or you just want to check them before installation after a clean/service,there is an easy way to check the operation of the carbs while they are removed from the bike. Take a look at the air filter side of the carb - at the top of the mouth of the carb is a semi-circular/half-moon shaped port that feeds air at atmospheric pressure to the underside of the slide piston.If you apply air pressure at slightly above atmosphere to this port the piston will rise due to the pressure differential.If you have access to a compressed air line apply a gentle blast to this port and you will be able to see the piston rise and fall,if no compressed air is available you can blow with your mouth (if you don`t mind the slight tang of petrol....) and you`ll be able to hear/feel the piston rising until it reaches the top of its travel where you`ll hear it stop, and on releasing air pressure another light thud as the piston return spring pushes it back down. If you want to be able to see the movement of the pistons you can fully open the throttle valve with one hand and then stand in front of a mirror while you blow into the atmospheric air port and look at the reflection to see the slide rise and fall - although you may get some funny looks from other members of your family if you do this! Mine are used to it... If it works with the small pressure differential that you can achieve with this simple test you can be sure that its gonna` work with the higher vacuum and pressure differentials involved when the engine is running. In normal operation the underside of the piston is at atmospheric pressure and the top of the piston at lower pressure - the bottom of the piston slide has three holes in it that are in the venturi airstream and thus at partial vacuum and which are common to the space above the piston - it is this vacuum above the piston which forces atmospheric pressure to `push` the slide up the piston chamber.
The yellow area is the `atmospheric port` which delivers air to the bottom of the piston and the two pilot circuit air bleeds,and is where you want to direct air pressure to when checking the operation of the pistons. The port in the red circle directs air to the primary and secondary air bleed jets/circuits.
Keihin jets are sized in metric ie: a size 112 jet is 1.12mm,and increase in size by .025mm which is .001" (one thou).
However, they do not use the `5` when marking the jets but round up to the second decimal point,so the next size up from the 112 mentioned here would not be a 1145 but 115 (1.15mm), and the original 1125 would of been rounded down to become 1.12 - this saves having to stamp the extra number on the small jet.
A jet size 112 is 1.12mm,and has a hole size of .044" (1.12 x 0.0394 = .044" or 44 `thou)
If you ever need to order main jets for your Keihin VB carburetor they are the 99-101-393-XXX type where XXX would be the jet size,eg: 99-101-393-112
Tuning the needles.
Ever since getting this 650 on the road i was aware of it running a bit rich;doing a plug chop at cruising speed resulted in a plug colour on the dark side,think digestive biscuit rather than rich tea.This is probably a result of the 500 Euro camshaft that this engine runs and the weirdy exhaust system.Carbs in all other respects were good - fuel level,diaphragms,air jets etc,and measuring the diameter of needles from the Ratwing carbs and comparing them resulted in the same readings,so they wern`t worn either.Fuel consumption was about 50,and at first thought that was OK as when i was doing thousands of miles a week on my 500 years ago that one was doing 52 - 55, so slightly less for the larger engine seemed to me to be reasonable.It wasn`t until i put a 650 engine in the RatWing and got 56/57 + that i realised the 650`s are potentially more economical,and the Euro with taller gearing might even better that.This plus the plug colour and fuel prices rising to £5.25 + a gallon made me try and eek a few more MPG out of the World Explorer.Might even learn more about the CX CV carbs,too - s`all about learnin`,see.....
In my experience of using different intakes and exhausts on the CX (such as the K&N`s and straight-through exhaust on the Euratsport) just changing the primary and secondary jets leaves a large range of throttle opening that isn`t affected by the new jets: the midrange. The primary circuit affects the mixture at low speeds/throttle openings (also has a small and progressively smaller effect as the throttle is opened wider) and the secondary only at wide open throttle; when the throttle slide(piston) is at it`s maximun travel and the area of the needle jet that fuel can flow through it is then larger than that of the actual secondary main jet - this is when the secondary main jet takes over the metering of the fuel, because it is then the most restrictive point in that circuit. All this leaves the midrange - where a bike spends a lot of time,such as cruising on motorways etc, with a weak mixture (assuming the exhaust/intake mods will result in a weaker mixture - they usually do..) When i do a plug chop at 70mph a weak mixture at these speeds means the needles will have to be tweaked. Running for 1000`s of miles in this state may cause issues with the top-end. I`ve burn`t out an exhaust valve and seen erosion to the piston crown through this when i ran a car exhaust on my original 500 many years ago. Getting a bit more colour on the plug in the midrange will eliminate any flat spots and bring back throttle response as well as keeping the engine from possible long-term damage. If the mixture is weak the needles need to be raised slightly my shimming underneath and fitting a `spacer/ring` under the retaining screw to provide allowance for the needle to still be able to`float` (if the needles are fixed solidly in the throttle piston they may bind or rub on the side of the needle jet causing wear). If the mixture is rich they need to be lowered. I did this by the mod detailed below.
Anyway,back to the 650. First i tried smaller primary main jets just to see if the rich midrange was in the scope of the primaries.It wasn`t.. I went down to a couple of sizes with no apparent change taking place - apart from a vague feeling the motor was slightly weaker at small throttle openings and sometimes `fluffed` when blipping the throttle on a gear down-change. Main jets only come into play at WOT so wouldn`t affect running at the engine speed where i do most of the miles, which left the needles; what i really needed was a needle with a different taper but that wasn`t an easy option.
The needles are Honda-specified, are not adjustable ,and no alternatives are readily available from Keihin (and i can find no info on which types were fitted to which CX models - they`re stamped with a code letter), so a bit of alternative engineering was required.
I needed to be able to fine-tune the needle position in the throttle slide by lowering them, similar to the clip positions on other carbs. What i did was to remove 3mm or so of the brass at the bottom of the slide `guide tube`.Theres not a lot of room down there and had to be careful not to break into the wall of the tube or go down too far, otherwise it`d separate from the rest of the slide (like it did on my first trial/test one!). When that was done the needle would sit lower down and would decrease the area available for fuel to flow through the needle jet leaning out the mixture. I used shims on top of the needle to take up the space now below the needle retaining screw but still allowed the needle to `float` preventing it potentially rubbing on the side of the needle jet causing wear..The needles in their original positions were measured and using shims above and below the needle `stop`were adjusted and dropped by 2mm. Luckily the carb vacuum chambers on the 650 Euro`s can be removed without taking the carbs off so it was simple to install them and get the bike back together, fill the tank up and see what difference it made...
Three things were noticeable;the engine felt a tad flat when pulling away,needed choke leaving on for slightly longer (usually i could push the choke in after getting underway) and there was a definate flat spot at ¼ throttle/about 70mph. I was interested to see what affect it had on MPG like this though so carried on and ran the tank out over a couple of weekend rides. Usually this bike goes on reserve at about 180 - 190 miles and when i went over 200 i knew that some difference had been made, what i was really surprised at though is that it finally went onto reserve at 235 miles, and i got home after doing 257 miles from that one tankful! That equated to 64 mpg which was impressive, but not with that flat spot. So, off with the carb tops again and raise the needle, this time to about 1mm lower than the standard setting - about halfway in my adjustable range. The next tankful lasted until 215 miles before going on reserve, the engine felt good at all throttle positions,pulled nicely, felt strong, and delivered 60mpg with the next two tank-fills resulting in the same figure. That`ll do. Plug colour was still brownish, but slightly lighter than previously so there may be scope to fine tune a bit more in the future if i am inclined, but i think i`ll leave it at that. Maybe i`ve stumbled on an unusually effecicient engine/camshaft combo?.. The moral of the story though is that if you want hassle-free motorcycling keep your bikes standard, and don`t mix`n`match engine parts that affect it`s running (unless you like tinkering ;-) )
Vacuum fuel tap.These vac fuel taps can be a source of poor running. Apart from the fuel delivery hose there are two other small-bore tubes that connect to the tap; one is the vacuum take-off which connects to the r/h side carburetor and the other one is a vent/drain tube which allows atmospheric pressure on that side of the diaphragm and in the event of a leaking petrol valve drains the fuel away from the fuel tap to the underside of the motorcycle. Originally the vent/drain tube was long enough to reach down and out/underneath in a similar route that the carb float bowl overflow tubes take, but on all the CX`s/GL i`ve owned it`s usual to see just a small length, say 6", connected to the tap that just rests somewhere under the fuel tank area. Even if no hose is on there it`ll still work, the other end of this vent/drain hose does not connect to anything. Inside the tap there are two diaphragms (Actually there are three,but the third one that is inside the vac hose stub/end cover plate is really only a one-way valve - try sucking on the vac stub on the fuel tap cover when it`s dissasembled, you should be able to suck but not blow through the stub.This is probably to prevent possible vacuum diaphragm damage in the event of an engine backfire through the inlet) - one is the larger vacuum diaphragm and it is connected by a small metal cylinder to the fuel diaphragm.The round plastic part is the spring seat.The spring pushes both diaphragms to the closed position preventing fuel flow until the engine is spinning over where the vacuum generated pulls the vac diaphragm over against spring pressure,and lifts the fuel diaphragm off the fuel delivery port with it enabling fuel to flow.
Normal assembly view of tap components;
It`s important to connect the tubes the right way round to ensure that engine vacuum is applied to the correct side of the vacuum diaphragm.ManyCX`ers have bought a CX/GL with engine running problems only to find that the hoses have been connected the wrong way round by previous owners.They do sometimes run with the hoses connected the wrong way round,just not very well.....
Faults that can occur with the tap are perished/holed diapragms. If the vac diaphragm leaks it may not create enough vacuum to open fully and could cause fuel starvation. The engine`ll also be drawing in a small amount of extra air giving the same symptoms as an air leak on another part of the inlet - probably most noticeable as an uneven/poor idle with a bit of misfiring/`fluffing`/unsteady idle speed. If the fuel diaphragm has holed you`d get a small leak/whiff of petrol around the tap and the vent/drain hose may produce a drip or two of fuel.
These two diaphragm are unservicable - note the pin-prick hole in the fuel valve and the rubber has perished around the middle of the vacuum one - this is the usual place for the vac.diaphragm to leak;
There are a couple of ways to convert the vac tap to manual operation which offers a few advantages. If you want to eliminate potential problems,have discovered a fault on the tap and want a quick/emergency or semi-permanent repair or just want to save time sourcing parts and about £25 on a repair kit see this comprehensive web page for directions. The fuel daiphragm must be in good condition for this modification or the tap may leak fuel.
The other way to convert the tap to manual is to re-locate the spring. Take the tap apart and simply re-assemble with the spring in its new position in the main body of the tap as in the pic between the fuel delivery port and the fuel diaphragm - it fits so well it is like it was designeed to be there! This will have the effect of permanently opening the fuel valve irrespective of whether the engine is running or not. Of course in both cases the fuel diaphragm has to be in servicable condition with no holes or leaks and the vacuum hose to the carb has to be plugged to prevent the engine sucking in unmetered air. The vacuum side of the fuel tap is now redundant so the stub where the vac hose connected to can be blocked off or left unconnected.
Here`s another method of fixing a leaking vac.tap and converting to manual at the same time that is useful when the fuel diaphragm has perforated.
During the revival of the Ratwing after a near two-year lay-up there was fuel dripping out of the vent/drain hose. When i took the tap apart the fuel diaphragm was holed. It was knackered. This tap was being used as a regular vacuum type before so this was the perfect excuse to convert it to manual. I fixed it by removing the fuel diaphragm and replacing with a rubber washer i cut from a larger sheet of 1.7mm thick petrol-proof rubber (viton - look on ebay for viton washers you can make them out of, you can get `em for a few pence each. Even if they`re not exactly the right size you can trim them down to fit). This washer is clamped between the main tap body and the middle section and forms a seal between the two and blanks off the two halves of the vac tap isolating the fuel and air sides and lets fuel flow whenever the tap is set to `on` or `res`. Esentially the same as relocating the spring. I left the vacuum diaphragm in just to seal that side of the tap (not really necessary though) and joined the two pipe stubs together with a short piece of hose `just in case`. This is now my prefered method of converting to a manual type fuel tap because it eliminates completely the old, original parts inside the tap which could still be a potential source of failure (fuel valve leak). Of course the vac.take-off on the r/h carb still has to be plugged;
In this case i could of just relocated the spring to hold open the fuel diaphragm, and then joined the tap vacuum and vent/drain stubs together with a short length of hose to prevent any leakage, but the leaking fuel diaphragm would of let fuel leak into the vac side of the tap and into this hose with it going nowhere (where fuel was never designed to be) not flowing or moving; not very neat and a bit of an untidy solution. The fuel may `go off` or stale over time and cause further problems. Dunno whether the material the vac diaphragm is made with is suitable to be in permanent contact with petrol, either.. The viton washer method is more positive,effective and secure, and keeps the fuel on the fuel side of the tap.
You can repair/service/modify the vacuum tap with it still attatched to the tank - with the tap set to `off` no fuel will flow even with the vacuum side of the tap taken off and both diaphragms removed.
If you do find it necessary to remove the tank and want to extract the fuel strainer on a Euro or GL, you may find it difficult to remove due to a build-up of corrosion inside the threaded boss the tap fits into. The tap usually comes off OK with the main fuel tank `straw` attatched with it but leaves the filter/strainer still in the tank. To get it out i use one of those spiral stud extractors - it`s about the only use they are suitable for as they`re absolutely useless at extracting a broken off stud! I screw it in just enough to grip the plastic filter and then twist/wiggle to ease it out. Don`t be tempted to get a pair of pliers on what`s sticking out of the tank because you`ll just mangle/chew it up and deform the part which is supposed to be a nice,snug fit in the fuel tap body. To replace the tap/filter assembly the filter fits onto the tap first (over the reserve `straw`) and it is then inserted into the tank together;
Extra vigilance must be used after converting to manual operation because of the risks of overflowing fuel which in certain conditions could result in damage to either you or the motorcycle: turn the tap to `off` when the bike is not being used - something which i have always done anyway due to my general mistrust of the vacuum taps..
A manual tap is useful in some way to help with the common quirk of CX`s only firing on one cylinder when left unused for a few days/weeks due to the evaporation of fuel/low fuel levels in the float bowl. The carbs are `primed` as soon as the tap is turned on which fills the bowls to the correct level before an attempt at starting the engine is made, rather than fuel only starting to flow through the tap while cranking the engine with the starter motor, which can result in the engine running on only one pot for a while until both carbs are fully filled with fuel or the stale fuel is replaced with new. I have a clear-bodied inline filter on two of my bikes and after the bike has been stood for a week or so i blow into the vent/drain tube (which is long enough to be placed easily to hand/mouth!) to operate the diaphragm and fuel can be seen to flow for about 8 - 10 seconds before the bowls fill completely and the float needle valve stops flow. With the two bikes which have been converted to manual taps i turn the fuel on a few minutes before attempting starting and don`t experience the one-pot firing thing...