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[nigelphoto]

Jefrs - I've reduced my popping and banging somewhat but not eliminated it. I replaced the flat biscuit manifold ring with the round 535 one, torquing up cold to 14lbs ft. - there was also a slight leak in the front pipe to silencer joint (and a broken bracket) and thats now been tightened up. I have also noticed that the symptom depends on riding style. When approaching a hazard, if one shuts off completely and immediately blips for the next downshift (i.e. approaching a corner fairly fast) it will induce explosive flatus but if one blips the throttle to change down and then holds it open a little to 'carry speed' into the corner then the condition is more akin to gentle borborygmus. Regarding the plug colour, ignore the soot around the outside thread and look only at the electrode. It is a lot more difficult to do a plug chop with an iridium plug as its less easy to see a colour on the tiny electrodes but the insulation core should be biscuit brown. When you remove the exhaust front pipe there will be plenty of soot in the tract but no hard carbon deposit. If you go to a showroom and run your finger around the inside of any silencer outlet on a secondhand EFI model you will see soot on your finger - a bit more pronounced on the non-cat silencers like my 2011 C5. That doesn't mean an immediate decoke - some of it is due to the residue of modern gasoline by-products exacerbated by the basic fuelling inefficiencies of the antiquated Honda system on the Royal Enfield. As I wrote in my Real Classic report a few months ago, it starts first press of the button, I get a lovely slow tonk tonk tonk tickover 30 seconds after start up, instant throttle response throughout the range with not a hint of flat spot and clean running at full throttle. The down pipe is a nice golden colour (not blue) so all in all I can forgive some farting on the overrun (we all do it as we get older!) and I sleep soundly not worrying about backfiring or soot any more.

[Presto]

Thack thanks very much for taking the trouble in giving me an answer. I take your point about the nature of multi-grade oils. But I’m still not satisfied on other matters. I can find no evidence of this ‘throttle cracking’ function on carbs with a ‘starter jet’, instead of a tickler. The cold start circuit on Mikuni’s, Mikcarbs, and Mk 2 Amal Concentrics, as I understand it from observation, simply open an auxiliary fuel jet, to richen the mixture for starting – they have no mechanical effect on the position of the throttle butterfly or slide (as the case may be), they do not, to the best of my knowledge, open the throttle, not even by a ‘crack’. All of which means to say that I still have no convinced idea as to what this Bi-starter thing is doing in the injector body.

[Presto]

What I mean is, if the function of the Bi-starter is to give an increased tick-over period, while the oil thins, then it’s sprung design to close instantly indicates that it is useless for its intended purpose. The possible conclusions to draw from this seem to me to be only two: either the designers are idiotic or the function of this contraption is other than to give an increased tickover while the oil thins. Now, who says which of these is the right deduction?!

[Presto]

Here we go again - but this will be my last word on this - thanks to the ‘Owner’s Handbook’! The advice is ‘depress and hold the manual bistarter for a minimum of one minute … to keep engine idling rpm steady.’ The (maybe unnecessary) spring loading of the lever is just to avoid the thing remaining open once it’s served its purpose - not that it is to be released as soon as the engine has fired up. Many thanks Thack – your analysis has been on the ball from the start!

[ric]

Brucehur, when the dealer mentions an automatic choke he is referring to the automatic cold fueling adjustment made by the ECU which occurs below a predetermined oil/engine temperature.




A richer starting mixture is automatically provided when the return voltage from the engine temperature sensor informs the ECU the engine is below a certain temperature. As the sensor heats up the value changes and the amount of additional fuelling decreases until the engine exceeds a minimum predetermined operating temperature. On my previous bikes utilising EFI this was always around 60 degrees.



Our basic setup does not have any additional input from an IAT (Intake Ambient Temperature) sensor so if the engine temperature is say -20*c the engine would electronically receive exactly the same fueling as if it were for example at freezing point. Consequently at lower temperatures the bike could be problematic in starting so this is where having the option of increasing the preset fuelling manually from say a Bi-Starting??? device would be beneficial.



When I started my EFI this morning ambient temperature was 3* and did not require any additional help whatsoever from the BI-Starter. My guess is that if you using yours above freezing you could indeed be responsible for fouling the plug. To alleviate your plug problem it would be far better to control the initial startup period by only using the throttle.

[Presto]

A final last word!!! ric, as Thack has been telling us, the Bistarter does not affect mixture strength, only tickover speed. Mixture cannot be regulated manually.

[Rattlebattle]

I give up! Thack, you are right RE the fuelling; what I meant was it passes a greater volume of mixture, not the same thing. My bad. Perhaps you should do a sticky explaining how it works to stop the same questions being asked....

[Thack]

Hey, Presto, just one last thing:



"The cold start circuit on Mikuni’s, Mikcarbs, and Mk 2 Amal Concentrics, as I understand it from observation, simply open an auxiliary fuel jet, to richen the mixture for starting – they have no mechanical effect on the position of the throttle butterfly or slide (as the case may be), they do not, to the best of my knowledge, open the throttle, not even by a ‘crack’."



If you look inside a Mikuni you will find the choke also opens an auxiliary air passage, thus giving you the equivalent of opening the throttle a bit. In fact, it's almost like a miniature carburettor alongside the main one.



I don't know of any bike carburettors which literally "crack" the throttle plate to achieve a fast idle. However, it was quite common on car carburettors. For instance, the choke on the SU HS-series would drop the main jet away from the needle (thus letting more fuel through) and also rotate a cam which acted on an adjustable screw, which lifted the throttle plate open. For what it's worth, my Kawasaki Versys uses a vaguely similar system to achieve a fast idle - it does actually crack the throttle plate open.



By the way, I cannot say for sure that every Mikuni opens an air passage as well as the auxiliary fuel jet, but the ones I've studied have. I think that is how the Amal Concentric Mk2 works, too (which is, I'm told pretty much a copy of a Mikuni anyway). The choke on the Mk1 Concentric is totally different, and works by restricting the air intake.



"Works" in the sentence above is sarcastic - it's an appallingly bad system and I don't think anyone actually used it. We just used to operate the tickler and then hold the engine at a fast idle with the throttle.

[ric]

Yes, the Bi-Starter opens an air way and the change in pressure detected by the MAP sensor informs the ECU which compensates for this by adjusting the fueling. I know what's been said and am not in disagreement with this...
But my engine has given symptoms of eight stroking whenever I've played with the Bi-Starter on a hot day which does seem indicative of it having some fuelling input even though it shouldn't. Perhaps the ECU can recognise an out of range input from the MAP sensor and make a further fueling adjustment as a result of non-load data in addition to its primary function of increasing the engine speed.

[Thack]

Ric: the reason I don't think the bi-starter has such an effect (albeit via the MAP) is that I've measured the fuel injection pulse duration and it doesn't change (or not detectably) when you open the bi-starter. On the other hand, when the engine temperature is low, the injection pulses are much, much longer. They can exceed 12ms immediately after starting, whereas they are typically around 3.5 to 4ms on a hot idle. That's why I'm certain the bi-starter doesn't cause enrichment, neither by design nor by accident.



Incidentally, I think (but don't know for sure) that the ECU does a small enrichment for start-up even when hot. I will get around to investigating this in more detail when the weather improves.



I've done enough measurements to satisfy myself that the system works as I described, but of course new data is always welcome. It's got to be objective, instrumented data, though - subjective data doesn't count!