TODAY'S TIP :-)

[John Orchard]

To everyone that sits on a start line or in general that lets go/snaps the throttle closed when revving their engines.....
Next time you want me to work on your carby, I'll shove it up your clacker and snap that cable fully open and let it go!
ROLL THE FRIGGIN' THROTTLE OFF!!!!!!

Why does it upset you so?

[John Orchard]

Cutting vibration through the handlebars;

I like to run mild steel handlebars, for one it's 'period' so I prefer the look, if I crash and bend them, I can just straighten them on the spot. My old hands don't handle the vibration like they used to, I now fill them with silicone, one squeeze gun cartridge easily fills 7/8" steel bars.

When applying the silicon, ONLY squeeze in from one end, hold the gun tight against the bar end and pump away until it starts coming out the other end. Before fitting the grips, wipe out a little from each end, about 15mm, this stops it expanding and oozing out the end (and jamming the throttle) on hot days.

This application cuts down handlebar vibration by at least 50%, well worth doing.

[pokey]

Interesting John. Ive never tried silicone but i have used sand filled corked bars that did work in not crimping the mild steel bars when it came to all out of plans time. The vibrations were less but you could feel the additional weight and a bit off putting.

According to Newton, to counter any vibration a similar mass in an opposing motion is required and proven when Mitsubishi patented the balance shaft that all engine manufacturers who make over square engines now pay a premium to Mitsubishi for. The other method is to increase the mass which the initial source has to vibrate and this is why bar weights on road bikes are used.

[John Orchard]

Big bore twostrokes, ping or blubber,

After reading all the pinging/rich blubber issues that mid '80's 500/490's had, and owning a few myself, I have investigated the issue and come to the following conclusion, which I have proved correct in my experience.

It has been said that the Honda CR500 was so much worse (ping king) than the CR480, even with a similar carb, in this instance, the stroke was increased in the 500, which meant it vibrated more. Many have said that fitting a PWK carb to the CR500 fixed it, it was because the pilot pick up was lower in the bowl.

The problem relates predominantly to VM Mikuni carbs and round slide (PE) Keihins, on bikes that vibrate a lot. With the fuel bowl off these carbs, you notice the pilot jet is mounted quite high in the bowl, when a bike vibrates a lot (big bore twostrokes) the fuel in bowl becomes quite aerated, hence while the pilot jet draws the aerated mixture obviously the bike will run lean. Richening the mixture to cure the lean mixture during the max vibrating period will make the carburation too rich when not in the bad vibrating range.

The fix (and proved to myself many times) is to extend the pilot pick-up point lower in the fuel bowl, I did it with a piece of rubber fuel hose in my CR500 and with a snug fitting piece of aluminium tube in the KX400. Caution; extending to the very bottom of the bowl will allow the small (read; easily blocked) pilot to get blocked more than usual (could be why carb manufacturers mounted it high in the bowl?) Also be careful that a rubber hose extension doesn't seal itself on the bowl floor, I angle cut the bottom to eliminate that issue.

Enjoy :-)

[pokey]

Yes poor air/fuel ratio will cause that condition  and so will heat.  I have heard the big yams benefited from an extra head gasket (lower comp) and or changing to a better head design to assist in even heat dissipation plus better flame control.

Aeration of the fuel as you describe is not a good thing  and the rubber tube may have merits in getting below the wash. Thinking out aloud and havnt really given it a lot of thought.....perhaps a metal sleave and or a spacer for a deeper bowl or  better yet a mesh device around the jet to work as a baffle could be worth investigation as baffles are used extensively to limit aeration though cavitation from baffles is possible . With a longer effective tube the venturi needs more suction as it has to lift longer or it results in a lean condition.

The usual fix for thinner air is a steeper taper needle  and vice versa so perhaps the answer is in the needle. Worth a thought.

[John Orchard]

The only way to counter the pilot sucking aerated fuel, in a VM or PE carb, on a badly vibrating bike, is for the pilot to draw the fuel from under the bubbling froth that sits on the top of the fuel in the bowl.

[pokey]

A fine mesh screen attached to the stock length pilot aperture would allow all fluids to pass while inhibiting the passage of bubbles/aeration. A carb is a simple pump and follows all the same rules and if Cavitation or vortexing is present in a pump a Cavitation plate is attached to the inlet or the inlet is lowered beyond the Cavitation/Vortexing point but less lift is achieved if lowered.. If aeration is present a baffle screen is installed..

Vortexing is a tornado that sucks air from the surface. Cavitation is air being pulled out of a liquid usually at a high temp or velocity.. With a foaming surface and a constantly changing surface height vortexing should be considered as a high possibility as its easier to suck air than liquid. Your extended tube as mentioned will relieve vortexing but at a cost of less lift via that jet.

The question is ... Is it Aeration or Vortexing or even Cavitation?

[John Orchard]

An extension to the pilot pick-up is not really an 'extension of the pilot jet', as the inside diameter of the hose/tube is many times larger than the pilot jet, consider it as 'baffle tube', the fact that it is drawing (read; using) the fuel from lower in the bowl does not mean there is any increase resistance to be drawn upwards, as the weight of fuel level higher than the tube pick-up point counters the extra distance to travel, did that make sense?

[pokey]

An extension to the pilot pick-up is not really an 'extension of the pilot jet', as the inside diameter of the hose/tube is many times larger than the pilot jet, consider it as 'baffle tube', the fact that it is drawing (read; using) the fuel from lower in the bowl does not mean there is any increase resistance to be drawn upwards, as the weight of fuel level higher than the tube pick-up point counters the extra distance to travel, did that make sense?

Not according to the Bernoulli equation or my irrigation and subsurface water design certifications.. The actual jet is only one part of the device and the entire length of the system needs to be calculated for friction and depth as the lower you go the more weight above and the longer the tube the more friction involved in calculating a jet size fora specific NPSH as its calculated at the end of the tube not where the jet lives or at surface level.. If friction overcomes the negative pressure it just wont lift and the smaller the tube the more friction generated. Different materials have a different friction coefficient.
 If a tube even laying horizontal to a positive or negative pressure generator is long enough you wont get any flow and the closer you are to the limits of the system the more new issues arise like cavitation and heat.. We use a few set formula and material friction charts  to calculate the Net Positive Suction Head. carbs work on. Carbs are smaller volumes but engineers use the exact same tools to design the carb and where and why they put things.

The reason why we need to set the float height correctly is to ensure the main and other jets are subject to the conditions calculated when the  jets were specified. I know it only seems like a small change though it will have a big effect and should be calculated for. I hope that makes sense.

[John Orchard]

But the float-height level in the tube is exactly the same as outside the tube, let's call the tube a 'mini fuel bowl' :-)

[pokey]

But the float-height level in the tube is exactly the same as outside the tube, let's call the tube a 'mini fuel bowl' :-)

lets not call it that as that point is gone as soon as the engine starts, hopefully bringing the fuel to the top to be atomized.

 Its the actual depth of the orifice which has an effect and the reason the floats are set, its the weight of the fluid above the aperture and the total length above and below the "jet" and the jet itself that requires a friction loss calculation. suction and delivery. Bends and valves also have a friction rating.

As a kid we had a pool and I tried using the garden hose as a snorkel. Worked sort of okay but I was sucking hard so i tried the pool vacuum hose and it was fine until I swam around the bottom and my lungs couldnt suck air through a 2 inch tube at a depth of only 4 feet. We have all syphoned fuel and this is the same. A short hose and a short lift you get a mouth full hard and fast, a longer hose or a higher lift and you have to suck your eyes out and it doesnt get any easier knowing that level in the jerry can starts at the same level as whats inside your hose and it will always a trickle and often fails and you have to start sucking again. Even our pressurized house water lines need a large feed line and the longer and smaller the distribution lines the less flow.

With a suction pump the theoretical maximum suction lift for a product the density of water at sea level is 10.33 metres before cavitation and total failure from loss of prime occurs and thats under absolute ideal situations. Less if the inlet is longer and even less than that if the inlet line is decreased in size and thats just to get a dribble and why most installs are limited to a maximum of 8 metres with a 3 inch line. if the situation is beyond those specs a submersible pump or a deepwell pump that pumps water down and past an injector and then circulates to assist the lift is installed and that injector is just a jet and also needs to be sized for the required flow which effectively push water up not suck.

In short, the longer the line the fatter it has to be just to achieve near the same desired flow. if the line sucks from a deeper point an even larger diameter line is required and as you have guessed the calculation is multiplied making a fairly large inlet.

[John Orchard]

I agree with what you are saying, but in this regard, the jet is the same (stock) distance (depth) from the venturi, so with all measurable related factors being taken into consideration, mixtures will be as per stock, when not in the max vibration range, and with aeration removed from the equation, the mixtures will still be correct at point the of max vibration.

[Tomas]

Hello
From what i understand I do agree with John's idea. Basicaly by using a piece of hose you are lowering the point(by maybe a few milimeters) from which fuel is sucked into jet (avoiding foamy fuel near top) therefore no(or much less) air bubbles are present and mixture is more consistent. Bit like when you drink freshly poured glass of beer you burp lot more but when beer is flat you burp less

[pokey]

With more overall length and greater depth comes more friction so in a perfect world a leaner situation occurs.

However we dont have a perfect world if indeed  as you describe its ingesting air at calculated dimensions its already in a lean state from factory and the addition to the circuit corrects the  air issue while still limiting flow and the test results produce favourable results it seems it was jetted incorrectly at the start ..perhaps to compensate for an undocumented issue as 2 wrongs yadda yadda and so forth.

[John Orchard]

Sometimes I have found a small amount of up & down play in carb jet needles, maybe a slightly bent retention plate or the recess for the needle clip in the throttle valve is a touch too deep? Anyways, with the retention plate out of the carb and on the bench, I create a depression around the hole for the needle, with a centrepunch, this keeps constant pressure on the needle clip, confirming no unwanted movement of the needle.