Dyno run (2017)

SteveA said:

Comnoz gave Fullauto the advice to try a different air jet.

This is interesting because as far as I can see, most people who use Mikunis don't even know there is one, let alone change it from the size that most 32 to 36 Mikunis seem to be supplied with. What I have been able to read so far says that the air jet has an effect in combination with the main jet at WOT, and that incorrect sizing can cause weakening or enrichment at the revs built toward maximum rpm.

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Have a few thoughts to pass along on the air correction circuit that might clarify a little more about its function.

When an engine is operated under load, the carburetor should deliver a constant fuel/air ratio to the engine, regardless of the engine speed (air speed through the carburetor). To achieve a constant fuel/air ratio, carburetors employ an air correction circuit as part of the main circuit, or so-called high speed circuit. The air correction circuit is a fine tuning feature of the main circuit whereas the main jet is a coarse tuning feature of the main circuit. The fundamental physics underlying the need for air correction of the main circuit are the following.

The pressure drop in the venturi is not linearly proportional to the air flow through the venturi, i.e., the pressure in the venturi decreases at a higher rate than the air flow increases. This causes the fuel/air ratio to increase disproportionately with increasing air flow and results in mixture strength becoming increasingly richer with increasing air speed.

The purpose of air correction in the main circuit is to compensate for this non-linearity that exists between air speed and pressure in the venturi. The air correction feature of the main circuit typically introduces air into an annular area surrounding the main nozzle, which in turn proportionately dilutes the fuel/air mixture to maintain the desired mixture strength under all air flow regimes.

Air from the air correction circuit is commonly introduced to the main nozzle by 2 different means. The 1st means, as shown in Figure 1 (from the Two-stroke Tuners Handbook), involves delivering air directly from the air correction circuit to a region near the top of the main nozzle, where it enters an annular area surrounding the main nozzle and "dilutes" the main nozzle discharge to correct the fuel/air ratio (this is typical on smaller Mikuni VM round slide carbs). The 2nd means of introducing air from the air correction circuit to the main nozzle is shown in Figure 2 (from the Two-stroke Tuners Handbook), where air is introduced to the main nozzle well and fed, via emulsion holes in the main nozzle, into the liquid fuel to dilute it with air. As air speed through the venturi increases, causing "air correction" flow to likewise increase, the fuel level in the main nozzle well decreases, revealing more emulsion holes and introducing more air to maintain a constant fuel/air ratio. This 2nd design has the added benefit of assisting in emulsifying the fuel/air charge by breaking the surface tension of the fuel and aerating it via the tiny streams of air entering the fuel through the emulsion holes.





A wide variety of emulsion tubes having various hole patterns and hole sizes are employed by tuners to provide proper fuel/air mixtures for any engine, regardless of the configuration or modifications performed on said engine. On some carburetors the diameter of the air correction inlet orifice is fixed, but on more sophisticated carburetors, air correction jets are provided in a variety of sizes, much like main jets.

Myriad main nozzle designs can be viewed at the following link.

http://tinyurl.com/cxyq4yv

Regarding the misfire you encounter at high engine speed under only the highest load (top gear), sounds probable that it could be lean back-firing. Might a slightly larger main jet resolve the issue, especially in light of your comment that the main might be on the small side (relative to the book recommendation)? In the alternative, a smaller air correction jet would slightly enrichen as well as delay full recruitment of the main circuit. After all, the air correction jet is simply a controlled air bleed we introduce to the main circuit.

edit - change backfiring to misfire

Right!

'Regarding the backfiring you encounter at high engine speed under only the highest load (top gear), sounds probable that it could be lean back-firing. Might a slightly larger main jet resolve the issue, especially in light of your comment that the main might be on the small side (relative to the book recommendation)? In the alternative, a smaller air correction jet would slightly enrichen as well as delay full recruitment of the main circuit.'

So, to clarify, I increased the size of the main jet, and the problem was a little worse! Hence my thinking that a slightly smaller air jet might help?

I have a book that suggests that you get the main jet right at the lower end of its scale and then use the air jet to tune it at full throttle & use the biggest air jet you can as it will give better atomization .The problem for me with that idea is when is the needle system having no more influence on the main jet.
An O2 & EX gas temp sensor would assist you a lot.

I have never changed slide cutaway, air jet or pilot jet in a carb on a four-stroke motor in my life, even when using methanol. With a two stroke, the situation is different, even the slightest bit too rich can stop the motor, and the slightest bit to lean leads to destructive detonation. Tuning for petrol in a Commando motor would be difficult, but there is no need to create problems where there are none. In a two stroke, the pilot jet is used to provide fuel when the throttle is slammed shut - to stop seizures. My brother has had to change air jets and pilot jets in his H2 Kawasaki speedway sidecar engine which is on methanol, but that has been the only time we have had to change that part of the jetting.

VM Mikuni carbs are designed for use on two-stroke motors and Mk2 Amals are basically copies of VM Mikunis. The pilot circuit does virtually nothing when those carbs are used on a four-stroke motor.

Total BS! The pilot circuit on Amal MK 2 has everything to do with how the bike idles and transitions off idle. This is based on personal experience on my 78 Bonneville with Mark II's. Also, unlike concentrics the pilot jet on the Mark II is removable and easy to change to dial in the idle and transition.
As a further note the Mark II's have an air jet that is easy to change to affect draw on the main jet. Something the concentrics don't have.

htown16 said:

Total BS! The pilot circuit on Amal MK 2 has everything to do with how the bike idles and transistions off idle. This is based on personal experience on my 78 Bonneville with Mark II's. Also, spare me how they run on methanol. I don't care and I doubt too many others do either.

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Now now stay calm, with 25 & 7/8 degree head rake your pilot circuit has no effect on your primary chain tension what so ever

SteveA said:

Right!

'Regarding the backfiring you encounter at high engine speed under only the highest load (top gear), sounds probable that it could be lean back-firing. Might a slightly larger main jet resolve the issue, especially in light of your comment that the main might be on the small side (relative to the book recommendation)? In the alternative, a smaller air correction jet would slightly enrichen as well as delay full recruitment of the main circuit.'

So, to clarify, I increased the size of the main jet, and the problem was a little worse! Hence my thinking that a slightly smaller air jet might help?

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If the larger main jet made it worse, then making the air jet smaller would be heading further in the same direction (less air leak = more fuel) so likely not where you'd want to go. If the larger main jet made it worse, my logic would direct me in the opposite direction to make it better.

Any possibility that we're confronted with the old carb conundrum - "95% of all carburetion problems are ignition issues"? In my mind an ignition issue fits your criteria perfectly, i.e., it pulls fine until the ultimate load is applied (full throttle, high rpm, highest gear) which is the highest stress condition under which to fire the plug. Of course there are myriad other issues that could be in play, but certainly ignition fits the scenario too. Will be very interesting to hear how the issue is ultimately ironed out.

Great information! The Sudco Mikuni tuning book doesn't even mention what the air jet does and I was under the impression, before now, that it was a low speed circuit thing. Gppd stuff. Keep it coming. I got my O2 sensor fitted yesterday. If nothing else it looks pretty! I can tell people it's a boost gauge! Seriously, it's already pointed me in a couple of directions that I didn't expect.

splatt said:

I have a book that suggests that you get the main jet right at the lower end of its scale and the use the air jet to tune it at full throttle & use the biggest air jet you can as it will give better atomization. The problem for me with that idea is when is the needle system having no more influence on the main jet.
An O2 & EX gas temp sensor would assist you a lot.

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When the needle is withdrawn from the needle jet to a point where the annular area between the needle and the needle jet exceeds the x-sectional area of the main jet orifice, the main jet becomes the sole restriction limiting fuel to the main circuit.

Due to the Photobucket snaffu (disappearing images) a graphical image posted a few years ago in a different thread, showing the ratio of needle annular area to main jet area as a function of throttle position for a specific carb set-up, has disappeared. The subject image may help explain the foregoing so have included it here again. Note that in the specific example shown here the main jet becomes the controlling feature of the main circuit at slightly under 3/4 throttle opening.

Backfiring is different from spitting back through the carb or a cough/flat spot due to carburation lean-ness ? Is it backfiring through the carb or through the exhaust ?
If it is backfiring through the exhaust at high revs under load, sounds like ignition problem. The fact that you have already tried richening the mixture would indicate that. Have made sure there is no water in your fuel bowls which might cause an intermittent blockage ? No beads of aluminium on your spark plugs ?

Are you mounting your O2 sensor as a permanent fixture or is it just for a period of tuning?
A while ago jimc posted some 02 numbers that are pretty spot on

splatt said:

Are you mounting your O2 sensor as a permanent fixture or is it just for a period of tuning?
A while ago jimc posted some 02 numbers that are pretty spot on

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I intend to get the VM spot on, dyno the results and then fit the TM 34, get that right, then dyno again. Then, the twin carbs!

WZ507 said:

If the larger main jet made it worse, then making the air jet smaller would be heading further in the same direction (less air leak = more fuel) so likely not where you'd want to go. If the larger main jet made it worse, my logic would direct me in the opposite direction to make it better.

Any possibility that we're confronted with the old carb conundrum - "95% of all carburetion problems are ignition issues"? In my mind an ignition issue fits your criteria perfectly, i.e., it pulls fine until the ultimate load is applied (full throttle, high rpm, highest gear) which is the highest stress condition under which to fire the plug. Of course there are myriad other issues that could be in play, but certainly ignition fits the scenario too. Will be very interesting to hear how the issue is ultimately ironed out.

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I think we have a divergence in terms here, I said misfiring, you have said 'backfiring' which someone else now assumes is my problem. It is a slight misfire and I suspect no one but me was aware it was happening even though it was on the front straight (only) and past the pits. (oh! and my little '70s 750 twin was hanging on to an '80s 1100cc Yamaha 4cyl FJ at the time) It didn't happen in any other gear than top or anywhere else on the circuit. But I guess it could be relevant that the straight climbs, so yes, highest load.

Unfortunately it won't be fixed overnight since it is a race bike and probably won't be run again until spring. Dyno run would be good, but need to find one first.

The air jet fitted is a 2.0. there isn't a bigger one made, I would have to drill it larger! or leave it out. Will continue to think this through a bit.

Could be ignition?, well no reason to say it can't be, check and check again. And it was running fine with 34mm carbs on, also fitted with 2.0 air jets, on a circuit with more climbing and longer straights and using higher final drive gearing.

It is a new Pazon ignition working with previously good coils and plugs. Could be the battery discharged a little (total loss), but in general terms it did far less over the weekend in question than it usually does from a single charge (large and heavy battery for the application, AGM not Lithium) so that wouldn't be my first thought. But I will check out the battery and all the ignition connections.

Unless your battery is old, it should hold enough charge for a few races - more likely to be a loose connection somewhere. If you got it flat out past the pits with the throttle fully open, the plug reading should show whether it is too lean. The ride back to the pits after the race should not mask the plug reading too much that you could not see whether the mains are too lean. I always run them too rich and don't worry if the bike loses a little bit of power. If I get a miss, I know it is the ignition system. The mid-range jetting is different. If the bike coughs while I'm riding it around corners, I know I have to lift the needles.
It sounds as though your race circuit is like most other small circuits, you are not on full throttle for very long.

Steve,
This sounds very familiar to me, rev out in 2nd , change into top at about 5k in third and then misfire , nearly always at the same point in the rev band in top gear , if not too vicious , its usually due to float issues either flooding or starvation , some times one can back off and then go through the 'patch' . Difficult to do on a race circuit .

oldmikew said:

Steve,
This sounds very familiar to me, rev out in 2nd , change into top at about 5k in third and then misfire , nearly always at the same point in the rev band in top gear , if not too vicious , its usually due to float issues either flooding or starvation , some times one can back off and then go through the 'patch' . Difficult to do on a race circuit .

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I won't discount that possibility. Because I have had that before! on the 34mms, it was flooding, which I could also see with fuel in the drain pipes. I changed the float needles, seats and the floats and checked the float heights then, used the carbs for 2 meetings, fine.

I also made the carb mounting more rigids with some rubber strap supports as I felt 'vibration' could have been a contributor'. The 36mms are heavier so I made sure I did the same again to keep them rigid.

I have the 36mm carbs apart and am changing float needles and seats and checking float height anyway. Problem doesn't feel quite the same this time, nor did I notice much in the way of fuel coming out of drains, but agreed this needs to be eliminated as well and is on my agenda.

Fullauto said:

I intend to get the VM spot on, dyno the results and then fit the TM 34, get that right, then dyno again. Then, the twin carbs!

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Now you're talking , but what happened?
You were always firmly in the "single carb is best " camp!
Wise decision though. With that cam & head twin carbs will make it fly.
And I don't think they need to be some expensive designer carbs, just a pair of 32 mm Amal Premieres.


Glen

Anybody else interested in the air jet info might want to read this that I have received from a classic race only forum.....Makes interesting reading for me:

'Technically, the air jet is only really functional at high engine speeds and large throttle openings. It has the effect of changing the slope of the fuel curve - the carbs tend to richen up as rpm increases for the same throttle opening. The air jet can compensate for this - a small jet will make the jetting richer at the higher rpm (bigger jet make things leaner), though the changes with air jet size are not linear. There will be different combinations of air jet and main jet that produce the same apparent mixture strength, though peak power may not be the same (as a result of different fuel droplet size): this can be useful as a cooling effect on some engine.

Typical 4 stroke applications can be confusing: 26-28mm carbs often like a 0.5, 30-34mm around 1.0 and 36-38mm back to 0.5'

No mention of the 2.0 size these carbs are often supplied with for Commando application! And somewhat in line with Victory Library VM manual reccommendations for air jets.

worntorn said:

Now you're talking , but what happened?
You were always firmly in the "single carb is best " camp!
Wise decision though. With that cam & head twin carbs will make it fly.
And I don't think they need to be some expensive designer carbs, just a pair of 32 mm Amal Premieres.


Glen

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Well, Glen, never say "never"! My experience with Anals, sorry, Amals, was in the 70s on my original MKII. They were a pain in the arse then, one of the things that stuck in my memory! When I bought my current 850 MKI, it already had a badly tuned VM34 on it. I hadn't played with one before, to the extent that I travelled for some miles with the choke on! As my familiarity grew, I realised what a lovely, economical, trouble free item it was. And so it has been for 60,000 miles. Which, as I am fond of saying is three sets of Amals with a rebuild each! No running or idle issues and up to 165 miles from a Roadster tank. That is something like 65 miles per Imperial gallon. Back in the day, with the Amals, it was more like 42mpg. A big difference when you ride on a daily basis. The list of Amal war crimes is quite extensive. Worn out after the warranty period has expired, broken slides being ingested, idle circuits blocked, the need for constant balancing due to the crap cable arrangement, etc.

I appreciate the improvements that have happened over the years, with the work done by various people, like Don Pender with his bits and pieces, which seem to make them usable and reliable after only 40 years of development! However, I have always leaned towards the Mikuni or Keihin way of doing things. The Keihin CRs are just about "in period", and a really good carb, but the Keihin FCRs look just a bit too modern for my tastes, just like most of the brake kits available. The Production Racer front end that I have on actually pre-dates the Norton Lockheed brake on the Commandos by two years and works superbly. But, I digress.

If I were to go the Amal route, I would have to shell out on Don's bits and perhaps some bits from a previous member on here called Needing, who has spent his life on finding ways to make Amals useful, apart from as fishing sinkers. With the CRs or the FCRs, it seems they work with no other issues, just jet and forget.

I did a Youtube video the other day of me riding from the hills back to my house. If you watched it. you would think it was a romp in the country, with no laws being broken (ahem). Until you look at the speedo. I'm moving along quite rapidly whilst holding corner speeds and really straightlining the curves, with emphasis on maintaining as much forward motion as possible. Let's just say that using my MPH speedo on the same number as the KMH speed limit signs was pretty easy and smooth, if you get my drift. However, looking at the tacho, you would be surprised that it never read much more than 3000rpm! I have 22 tooth gearing as well,so it seems fairly tame.

This is the strength and beauty of my motor. Quite subtle. A guy with a 750 rode it the other day, and when asked about the power, he said "it goes about the same as mine". I smiled at that one. His was on standard 19 tooth 750 gearing and he wasn't taking in all the inputs. I'm sure that it revved pretty much the same, but forward motion was way ahead of his. If you look at any other video on Youtube, when the riding gets spirited, you see the revs rising and the nuts being flogged off the bike. No good for any distance. Too tiring as well as having no mechanical sympathy for the bike. On mine, you can maintain a good pace without the fuss, while maintaining excellent fuel economy.

So, the question is, what will twin carbs do to materially help my style of riding?