Carb tuning and colour tunes

After fiddling ages trying to set the slow running mix with a colour tune i give up..Why...because its nign on impossible!
Reason is and its taken be 40 years to reason this at tick over the engine runs full retard .and attempting to adjust mixture colour at tickover at full retard is a wasted of time.
Fuel will not combust correctly with a retarded engine , the flame burn rate is much slower, happening when the pistion is de-compressing ..well thats what i think....anyone have other ideas.?

I've used the Colourtune kit with some success but it's debatable how much more effective it is that the old fashioned way of turning the airscrws to get the fastest/smoothest idle. I'm pretty convinced that the combination of modern petrol & Amal carbs is a bad one, particularly where reliable idle speed is concerned. I have to have my idle speed set higher than I would like to make it reliable. Even then sometimes I stop at lights & it idles around 1K revs & then other times it races away around 1.3K! It's annoying but I just live with it.

Ian

I think the Colortune can get you an ideal mixture for idling.

But- that is not always the best mixture to have it set at. Sometimes you need a richer idle mixture for best throttle response off idle.

I am not sure fuel levels with carbs is reliable , will full fuel tank compaired to nealy empty have an effect/ my mikuni as an anoying habit of idling fine on starting then after 10-15 seconds slows and as a spit back . guessing the level falls before the fuel needle opens? Then slightly over fills .

You are right , "just live with" the stupid things..they are just a compremise ..set right today..weather turns fron high to low depression and the mixture changes...that why Triumphs fuel injection constantly adjusts thousands of times a minute.

john robert bould said:

I am not sure fuel levels with carbs is reliable , will full fuel tank compaired to nealy empty have an effect/ my mikuni as an anoying habit of idling fine on starting then after 10-15 seconds slows and as a spit back . guessing the level falls before the fuel needle opens? Then slightly over fills .

You are right , "just live with" the stupid things..they are just a compremise ..set right today..weather turns fron high to low depression and the mixture changes...that why Triumphs fuel injection constantly adjusts thousands of times a minute.

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Fuel tank level / volume should not effect carburation as the tank only fills the float bowl, the carb draws fuel from the float bowl. The float bowl effectively isolates the tank related feed 'pressure' from the carb jets. In essence, the float bowl is the 'neural point' in the system.

Ya can judge float level by where pilot screw lands for best idle though I found pilot can compensate for a good big off float level as long as it starts and don't flood. Slow one side down to barely running then diddle other side pilot for nice idle then visa versa, then equalize the exhaust puffs so each side doing its share. Helps to have thumb screw slide adjusters to diddle from saddle for conditons, showing off the pleasantness of slow but damaging idle or back up for non stall and charging security of real use. After reading years of Colortune funny business on other forums I lost the desire. If bored can juggle float level till each pilot likes 1.5 turns out as Amal baseline advertised. Can youtube Colortune to see them in action.

Eddie, in essence yes, you are correct.
But the theory falls down when the carb is subjected to the shaking it gets ? I did'nt suggest the float bowl gets pressurized from a bigger head .but as the old Wal Philips fuel injectors ran rich on full tanks and the hole was static empty or full , make me think the fuel hight as an effect. The float as a leverage against the needle,the needle offers a "Blocking ressitance" against the pressure..i imply the less head will seal better... ?

Fuel head matters in gravity feed old school no pump fuel injectors but in carbs it only affects the speed of filling float bowels not the sealing nor the float level ease to feed out spray tube in the venturi. Carbs depend on speed of air flow and air density to suck up fuel while the gravity feed injectors only depend on the mass of air flow and self compensate for attitude and barometer so very popular with aviation use. Concentric bowls with pick up in the middle compensated for the fuel level tipping on turns.

john robert bould said:

After fiddling ages trying to set the slow running mix with a colour tune i give up..Why...because its nign on impossible!
Reason is and its taken be 40 years to reason this at tick over the engine runs full retard .and attempting to adjust mixture colour at tickover at full retard is a wasted of time.
Fuel will not combust correctly with a retarded engine , the flame burn rate is much slower, happening when the pistion is de-compressing ..well thats what i think....anyone have other ideas.?

Click to expand...

I have used these ColorTune kits and find them very good, provided you only use them in the dark.
You need to do a compression check on a hot engine, and the engine should preferably have a t least 100 psi and the two cylinders within 10 lbs of each other.
You might have the wrong amount of retard on your ignition if it is set to low then it is not suitable for the engine.
Suppresser caps , H.T , low tension leads should be in first class condition, by that I mean there should be NO black copper hidden under the insulating cover ANYWHERE, the connections should also be clean, including the ignition switch, cut out switch, all are relevant .
Carb pilot jet & holes in the carb body should be clear, and the slide should not be so loose in the body that it flaps like cans in a dustbin :!:

In the past when using Mikuni carbs, before re-jetting them I've always set the float level by measuring from the gasket face to the float or lever arm as specified in the relevent manual. Is there a measurement and way of setting the float level in 34mm Mk2 Amals ? It is something I've never bothered to check.

You can check the float level by running a clear tube from the drain hole to side of the carb bowl.

Bernhard said:

You can check the float level by running a clear tube from the drain hole to side of the carb bowl.

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It still needs to be set to specification, where are the numbers ?

A clumsy dummy like me just gets floats good enough to start then set air screws to best idle then move floats so best idle occurs on what the factory manual says they should be set too as set best baseline fuel level for the rest of the circuits but mostly just for mental statisfaction as my current Combat runs great with float set a bit too high so air screws both below 1 turn out but being lazy and enjoying the off idle tire spin steering help off pavement I've left it that way a couple years now. Also notched Amal spray tubes for a bit more off idle reaction. Here ya go to see scope of Colortuning...
http://www.youtube.com/results?search_query=color+tune

Several articles have been published about setting the float level on Concentric Carburetors and this is a summary of the important points from people in the know:

The actual specification, from the engineer who designed the Concentric Barry Johnston, gives a gas level of between .170" to .240" below the top edge of the float bow, 170” being rich and .240" being lean. This level has a dramatic effect on the running of a properly tuned motorcycle. It pays testaments to the design of this carburetor that the range of miss-adjustment it can tolerate and still have the motorcycle run, if only poorly. It is suggested that you set the float level first before doing any carb adjustment.

The level can be estimated by placing the round edge of the plastic float between .060" and .090" below the edge of the bowl when the needle is depressed by a slight force on one of the float's needle tangs. You can turn the bowl upside down to establish this. With the float above the top of the bowl, as described in the Norton Tech manual, the float can contact the bottom of the body and not allow the float needle to seat firmly. It also makes the carb rich through the jetting range. With the float, level with the top of the bowl, although not as rich as when it is above the top, it is still out of the specified range.

The brass needle seats are installed and adjusted cold by the factory. Using a proper sized drift, you can adjust the brass seat on a solid surface.

http://www.oldbritts.com/amal_tun.html