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Main or air-correction jet? #436584
05/24/12 11:30 pm
05/24/12 11:30 pm
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Gnashville
DavidP Offline OP

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Hello folks.
Still tweeking my Mikuni VM's on my A65L. She runs great through the lower throttle openings and pulls well at WOT.
However, it starts 8-stroking around 6500rpm.
I was wondering if it would be better to go down one size on the main jet or up a size on the air-correction jet.
From my understanding the air-correction has more effect at higher rpm, while the main jet can at least slightly change the mixture across the rev range.
Thanks


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
72 T120V cafe project "Mr. Jim"
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Re: Main or air-correction jet? [Re: DavidP] #436602
05/25/12 2:36 am
05/25/12 2:36 am
Joined: Mar 2005
Posts: 10,157
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John Healy Offline
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David: You don't tune these carburetors using rpm as a reference. You tune these carburetors by referencing the position of the slide in the body.

Mark you throttle so you can see it as you ride. Mark closed and full throttle and divide the marks at 1/4, 1/2 and 3/4 throttle opening. Then ride the motorcycle and when the bike starts to 8 stroke look at the position of the throttle. This will reference the function of the carburetor that is causing your problem.
John


Re: Main or air-correction jet? [Re: DavidP] #436613
05/25/12 3:36 am
05/25/12 3:36 am
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Gnashville
DavidP Offline OP

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Thanks, I understand that, and my throttle is marked. As I said, at WOT the engine accelerates without complaint, but starts to 8-stroke at 6500rpm. When using the next larger size main jets it would begin to 8 stroke sooner at the same 100% throttle setting.
I only wish to know if using a larger air-correction jet would clear this up without affecting the mixture at lower throttle openings.


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
72 T120V cafe project "Mr. Jim"
Re: Main or air-correction jet? [Re: DavidP] #436620
05/25/12 4:12 am
05/25/12 4:12 am
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Ron - in California R.I.P. Offline
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Hi David... you are on the right track, sort of... but you need more information to make any decisions.

In the olde days you would just go to a smaller main jet. But in doing so, you could have a very lean spot at a lower or even higher RPM. Fuel curves on carburetos are difficult to get very linear... Modern engines with fuel injection is a lot easier... just dial it in and go..

You need to be able to measure the A/F ratio or O2 content at WOT at each RPM and graph that (a good dyno can do that real easy as part of their testing). Once you know what the curve is you can then see what needs to be done to lean it out where needed and not run too lean in other spots... tricky..! Sometimes the holes in the needle jet need to be modified...

But get it right and you have a real happy engine that will make more power both above and below the power peak...

Good Luck..!

Ron

Re: Main or air-correction jet? [Re: DavidP] #436665
05/25/12 12:57 pm
05/25/12 12:57 pm
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pushrod tom Offline
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A couple of pulls on the dyno will tell you a lot. If you cant do that just go smaller on the main one step at a time and see what happens. PRT

Re: Main or air-correction jet? [Re: DavidP] #436669
05/25/12 1:43 pm
05/25/12 1:43 pm
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Posts: 10,157
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John Healy Offline
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David:
Unlike an AMAL, where the needle fully clears the needle jet orifice long before full throttle, the Mikuni needle never leaves the needle jet. This puts the needle in play through full throttle. While the needle is listed for tuning from 1/4 to 3/4, with the Mikuni it can be used to make subtle changes above 3/4 throttle. So you have the air bleed, main jet, and diameter of the tip of the needle to work with.

Using the Sudco Mikuni catalog go to the page listing all of the needles. Determine which series your carburetor uses #4, #5, #6, etc. Determine if you have a single (typically used for 4 stroke applications) or double taper needle (there are three taper needles for some models). If the needle has one letter after the series number it is a single taper. If it has two letters after the series number it is a double taper, etc..

Example: 6DP would be a series #6 and double taper. The Letters refer to the angle of the taper: A=0.15', B=0.30', C=0.45', D=1.0', E=1.15' etc through Z=6.30'. More information on this can be found on line when you down load Gordon Jennings Two-Stroke Tuner's Handbook.

Look at Sudco's chart giving the diameters of the needle and select one that has a larger diameter at the tip or D5-D6 area. For example compare a #6L1 with a #6N1 The straight portion of the needles (A) are the same (A = 62.3). The taper starts at the same place (B = 37.0). D-1 through D-3 are virtually the same. D-4 through D-6 show marked differences in diameter (#6L1's D-6 = 1.313 while #6N1's D-6 = 1.058).
This should allow you to make subtle changes at full throttle without changing the jetting lower down.
HTH
John


Re: Main or air-correction jet? [Re: John Healy] #436674
05/25/12 2:15 pm
05/25/12 2:15 pm
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Magnetoman Offline

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Originally Posted By: John Healy
Look at Sudco's chart giving the diameters of the needle and select one that has a larger diameter at the tip or D5-D6 area.
Unfortunately, what you are going to find when you think you've identified the perfect needle from the catalog is not all of them are manufactured anymore (and, not all needles that are manufactured are in the Sudco catalog). The almost total move from carburetors to fuel injection over the last decade has resulted in a decreased selection being available, with the catalog not having been updated to represent the current supply situation. I've gone through this twice recently, and for over a year have had one saved search on eBay hoping a particular needle from old stock turns up.

Re: Main or air-correction jet? [Re: DavidP] #436678
05/25/12 2:52 pm
05/25/12 2:52 pm
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John Healy Offline
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Well, I suppose if it was easy anyone could do it...

Speaking of easy, why don't you bolt on an AMAL and get on with life? bigt

Is your problem really carburetion? As it seems to be rpm related (where a harmonic vibration is upsetting a connection) could it be a loose connection on say the terminal on the back of the ignition switch?


Re: Main or air-correction jet? [Re: John Healy] #436683
05/25/12 3:47 pm
05/25/12 3:47 pm
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Magnetoman Offline

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Originally Posted By: John Healy
Speaking of easy, why don't you bolt on an Amal and get on with life?
Mikunis used to have two advantages: better materials, and more tuning choices. Now they have only the former. And, with the introduction of hardened slides and such by AMAL, it's not even clear they still have that. If I were in the market today for a new carburetor for one of my British bikes that I needed to function a long time with the minimum of maintenance, I would seriously investigate the Amals being produced by Burlen rather than the VMs of Mikuni. Not that there's anything wrong with Mikunis, but an AMAL would be better, all other things being equal.

The new Wassell clones, which I assume are made possible by the expiration of patents, are a disturbing development. It's hard enough to understand how one company can make a business out of supplying replacement carburetors for bikes that haven't been made for 30 years, but it is really beyond my understanding that there are now *two* companies. Unfortunately, motorcyclists are cheap, so even if Wassell makes an inferior product (which I have no way of knowing at the moment), they're going to grab a good share of the market. If that drives Burlen out of the carburetor business just as they seem to have stepped up to Mikuni-level quality, everyone will lose.

Re: Main or air-correction jet? [Re: DavidP] #436691
05/25/12 5:39 pm
05/25/12 5:39 pm
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John Healy Offline
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Further to what Magnetoman illuded to, when you really get down to it the actual selection of needles available for a particular series carb, to be used on a 4 stroke application, there is not many more needles available than a similar offering by AMAL.

Sure, there is a couple of pages of various needles listed in the Sudco catalog, but only a few are applicable for a given series and fewer for a given application. For example, in the 6 series there are a dozen odd variations listed, but you get right down to it only 3 or 4 are applicable for each application. More for two strokes than 4 strokes. There are also odd balls listed that would only be used for alcohol or oxygenated fuels.

As far as selection of needle jets. While you can buy a bigger selection of needle jet diameters, AMAL tuners Have been reaming needle jets to half, or one quarter sizes since the carburetor was introduced. So I have never got caught up in the tune ability sales pitch. The MKII, with a brass slide is every bit as useable as Mikuni VM, and will flow more air for a given size. The AMAL 40mm, which is actually 39mm, will flow more air than the equivalent Mikuni 40mm VM.

Although you can forget the Smoothbore unless your idea of riding is the Isle of Man. Because of the tea cup design of the slide (like a GP) the throttle response/acceleration time is poor.


Re: Main or air-correction jet? [Re: DavidP] #436692
05/25/12 5:58 pm
05/25/12 5:58 pm
Joined: Oct 2008
Posts: 5,014
Gnashville
DavidP Offline OP

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Thank you John and Magnetoman.
Of course, you are correct about the jet needles. After testing, and with the aid of the Victory manual, I have settled on two jet needles, which I change depending on atmospheric conditions. I've found that the 6DH3 or 6DH2 work best with my particular engine/exhaust combination. Takes all of ten minutes to change with the carbs on the bike.
Good point about the needles never clearing the jet. I might just try raising the clips.
In any case, the BIBR rally is approaching. That adds about 2000' to my location's altitude, so smaller mains are in order.

Magnetoman: what needles do you need? I have some 6F4 and some 6DP17 which don't work for me.


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
72 T120V cafe project "Mr. Jim"
Re: Main or air-correction jet? [Re: DavidP] #436696
05/25/12 6:27 pm
05/25/12 6:27 pm
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Magnetoman Offline

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Originally Posted By: DavidP
what needles do you need? I have some 6F4 and some 6DP17 which don't work for me.
Thanks for the offer, but the carburetor is a smaller one that takes 5-series needles. Actually, it runs fine right now, so this is a case of not leaving well enough alone. If I knew there wasn't another needle available ("available," if only in the catalog) I would be perfectly happy with how it runs.

Re: Main or air-correction jet? [Re: DavidP] #436770
05/26/12 7:37 am
05/26/12 7:37 am
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Gnashville
DavidP Offline OP

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http://www.psep.biz/store/mikuni_vm_type_parts.htm

Have you tried these folks?
Sometimes you must turn off "styles" when viewing their site, but I've always found that they have what I need.


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
72 T120V cafe project "Mr. Jim"
Re: Main or air-correction jet? [Re: DavidP] #436787
05/26/12 10:48 am
05/26/12 10:48 am
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Magnetoman Offline

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Originally Posted By: DavidP
Have you tried these folks?
Yes, they're one of many I tried, hoping they'd have an old one on the shelves even though it isn't listed on their web page. Note that they have only seven of the 5 series needle jets listed (as opposed to the number in an old Sudco catalog), which illustrates the problem of Mikuni dropping parts from production. Retailers can't stock what factories no longer make.

Re: Main or air-correction jet? [Re: DavidP] #436811
05/26/12 3:43 pm
05/26/12 3:43 pm
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California
Ron - in California R.I.P. Offline
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David, I will be polite, just ignore some of the mis-information above... you are on the right track...

Ron

Re: Main or air-correction jet? [Re: DavidP] #436822
05/26/12 4:55 pm
05/26/12 4:55 pm
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John Healy Offline
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Ron, please don't be polite!
John


Re: Main or air-correction jet? [Re: John Healy] #436826
05/26/12 5:38 pm
05/26/12 5:38 pm
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Gnashville
DavidP Offline OP

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Originally Posted By: John Healy

Speaking of easy, why don't you bolt on an AMAL and get on with life? bigt

Well, for about $300 I could get a new pair of 930 Amals.
But, then I couldn't afford to put gas in the tank.
THAT would certainly simplify things. laughing


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
72 T120V cafe project "Mr. Jim"
Re: Main or air-correction jet? [Re: John Healy] #436829
05/26/12 6:48 pm
05/26/12 6:48 pm
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Magnetoman Offline

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Originally Posted By: John Healy
... Amal tuners Have been reaming needle jets to half, or one quarter sizes since the carburetor was introduced....
It's one thing to run a reamer through a hole, but it's quite another to actually end up with a hole of the precise size of that reamer. I suspect most of the tuners who were reaming jets did not have the gauges that are needed to check their work, and what they got wasn't necessarily what they thought they had.

I have a set of quality reamers in half-thou. steps covering the range of interest for AMAL needle jets. I also have a Starrett bore micrometer that covers the range 0.100"-0.120" to a ten-thou. using a split cylinder of length 0.15", and a Diatest set that uses split balls that make contact with the walls of the hole over a length ~0.004". Where the Starrett and Diatest overlap, and where I have two ring gauges to set/check them, they agree to better than the smallest division on each gauge (i.e. better than 0.0001"). The holder for my Diatest gauges lifts a platform straight up to insert the gauge in the hole. Since they are ball gages, only the widest circumference makes contact, so this serves as both a roughness and taper meter for the inner surface of the holes at a length scale of ~0.004", whereas the Starrett averages over the longer scale of 0.15", and a plug gauge over the full ~0.5".

My experience is that manually reaming precision, smooth bores ~0.5" long of dia. ~0.1" that have no taper or bellmouthing at a scale of a few ten-thou. is far from trivial. However, although I think my skills as a machinist are pretty fair, maybe I just don't have the talent to do this. Anyway, because of this, Mikuni's finer steps of needle jet diameters is a real advantage to the tuner.

Re: Main or air-correction jet? [Re: DavidP] #436847
05/26/12 10:19 pm
05/26/12 10:19 pm
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pushrod tom Offline
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Have things gotten off the track some here? I thought the problem was WOT. How does needle taper and miniscule differences in needle jet diameter affect your mixture at WOT? Barring any electrical glitch (easy enough to double check)why not just go down on the mains to see what happens? I will admit though that fuel related 8 stroking is a bit unusual at 6,500. Almost all of the super rich symptoms caused by 'reversion ' have cleared up by that point. Example is my current set up. Max 'rich dip' is about 4,800 and goes up from there. So, it runs from about 10.8 - 13.2 at the top of 7,300. That was recorded on a very 'dense air' day with Standard AMAL GP carbs and 220 jets. The stock 250's were way rich and when we actually ran the bike on the track we changed to 210's. It ran well. With that said, it doesn't 8 stroke even at 10.8. Changing the jets is a lot easier than buying a set of reamers,eh? PRT

Re: Main or air-correction jet? [Re: pushrod tom] #436851
05/26/12 11:21 pm
05/26/12 11:21 pm
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Magnetoman Offline

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Originally Posted By: pushrod tom
Changing the jets is a lot easier than buying a set of reamers,eh?
And more likely to give you the results you expect.

Re: Main or air-correction jet? [Re: DavidP] #436868
05/27/12 2:22 am
05/27/12 2:22 am
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Ron - in California R.I.P. Offline
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OK, I will attempt to help you all... but unless you are open minded you won't get it.. There are a lot of people that think like the olde school folks.. Jetting or should I say tuning needs to be done at speciffic throttle openings, and run through the useable RPM range... yes you tune for RPM at ONE throttle opening... or else you can have a big piston bill... Air Correction is used to change the fuel curve... this means it (almosts always**) will tend to swing the curve, kind of like a seesaw... it will richen one end of the curve (by RPM)..or lean it depending on which way you go on the air jet.. The pivot of the seesaw is usually right at or near peak power... And of course it rarely is linear on a carbureted engine...

Check out this link.. this is the dyno I use, and the example listed is the best example Derek has on his WEB site, not a Brit Bike, but the same tuning principle applies. It may or may not be possible with an AMAL, but it would likely need a lot of modifications... forget trying to use a MkII AMAL. To do proper tuning you need a carb that has some method of tuning the air correction, this means not only the air jet, but also a needle jet that has emulsion holes in them... sometimes you need to change or modify the holes in the emuslion jet.. usually more of them, and make them all smaller, depending on what you started with. So a Mikuni or Del Orto is a far better choice than any AMAL...

http://www.moto-lab.com/gallery/hiresview?itemid=267

The HP gain is a result of the tuning the fuel curve. And in this case, WOT was not the biggest issue on this bike. The bottom two lines are the fuel curve at WOT.. Notice at WOT by leaning down the mixture the bike gained power at every RPM... seems simple right..? This is likely about the equavalent of 2 to 3 main jet sizes. Looks good right..?? And at WOT this bike would be just happy..

Now look at the bottom and click on any one of the part throttle runs... At 25% throttle, this engine would be happy as long as it was within about 3,500 to about 6,000, but at any other RPM it would certainly burn up...or at least run crispy hot...

At 12.5% this engine must of wanted to lean misfire or surge like crazy, and it would be running very hot.. and likely burn up..

But, by altering the air correction, (the red line) the engine is now on the rich side at part throttle... which is far better than too lean... can you imagine "modulating" the throttle in a turn only to have it go boom..!

OK, ao once the air correction was correct, then the main jet could be finalized if needed... probably had to go leaner yet on the main.... thus this engine now makes more power at every (useable) RPM regardless of throttle opening... Now you can see that proper use of air correction combined affects everything... While the main jet alone would not of helped this engine at all..

I wish I could find an example of a bad WOT fuel curve, but none were on the WEB site that were that clear cut to see... but I can clearly tell you that my racer with the AMAL was dripping rich (megaphonitis / major misfire) from between 2,500 and 5,500 RPM... Now with tuning the air correction, my engine will pull WOT at 3,500 all the way up.. And the power went up way more than the example bike... mostly in the 5,00 to 6,500 RPM range.. where I got as much as 6 HP gain..! That is what gets a bike off the corners, far more than peak power...

Re: Main or air-correction jet? [Re: DavidP] #436869
05/27/12 2:29 am
05/27/12 2:29 am
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Ron - in California R.I.P. Offline
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OK, that last one was long.. sorry... but I need to add a few things...

There are more "things" that affect the fuel curve to some degree... this includes intake length, exhaust length, exhaust type (megaphone, straight pipe, or silenced) and the type of air intake (velocity stack, air box, etc)...

There is one example where just lengthening the exhaust improved the fuel curve and increased the power a bunch... your typical Harley needs longer pipes...

I also found that small changes can do a lot..! A 1/4" change in intake length got a 2 HP gain with no loss of midrange..!! (ran out of room, so could not go any longer...then..). Same with the exhaust... I am running a much longer exhaust than the original Gold Star specs... the dyno does not lie..! Your butt will lie....

Hope this helps...

Ron

Re: Main or air-correction jet? [Re: Ron - in California R.I.P.] #436876
05/27/12 3:56 am
05/27/12 3:56 am
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Magnetoman Offline

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Originally Posted By: Magnetoman
If I were in the market today for a new carburetor for one of my British bikes that I needed to function a long time with the minimum of maintenance, I would seriously investigate the Amals being produced by Burlen rather than the VMs of Mikuni. Not that there's anything wrong with Mikunis, but an Amal would be better, all other things being equal.
Originally Posted By: Ron - in California
So a Mikuni or Del Orto is a far better choice than any AMAL...
Point of clarification: I lost sight of the fact I was posting to the competition board when I wrote the above. As the "function a long time" and "minimum of maintenance" indicates, my comment applies to most uses of British bikes other than racing or extracting the max. possible h.p. from them. "All other things [are not] equal" for racing, where the tuning options of a Mikuni give it advantages over an AMAL, even for tuners who can ream a jet to the nearest ten-thou.

Re: Main or air-correction jet? [Re: DavidP] #436883
05/27/12 4:46 am
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Thanks all for another interesting discussion. The machine in question does not get raced. I only posted here thinking that you racers would have more experience with Mikuni tuning, particularly with the relationship between main jet and air-correction jet.
From another site: "For a given air correction jet, a larger main jet will flow more fuel across the RPM range.
For a given main jet, a larger air correction jet will reduce fuel flow in the higher RPM range.

(shamelessly copied from an article on Webber DCOE carbs)"

This agrees with what I think the Victory manual says, and is the only reason I mentioned RPM in my original question. It also agrees with what I know of Solex carbs on old VWs.
Anyway, I have some larger air-correction jets. I shall try them first. Mikuni jets are tough to find locally, and it seems a waste to pay $8 for shipping $6 worth of brass.
Thanks again


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
92 BMW K100rs "Gustav"
72 T120V cafe project "Mr. Jim"
Re: Main or air-correction jet? [Re: DavidP] #436923
05/27/12 2:01 pm
05/27/12 2:01 pm
Joined: Mar 2005
Posts: 10,157
Boston, Massachusetts
J
John Healy Offline
BritBike Forum member
John Healy  Offline
BritBike Forum member
J
Joined: Mar 2005
Posts: 10,157
Boston, Massachusetts
Ron: Fair enough, except most of the Mikuni kits sold for British twins are supplied with Primary, not Bleed needle jets and tuning the holes in the emulsion tube is not an option.

Just for an example look at the popular Mikuni kits offered in the MAP catalogue where they list spare parts for their kits. Their offering is a 159 which is a Primary needle jet. Also, most of the kits that have been offered to us for the past 30 years were supplied to us with Primary needle jets.

So while we are aware of the seesaw effect, David's concern is at one end of the swing. If he uses a larger air correction jet, what will he be doing to the other end? Given his measurements are limited to "seat of the pants" observations, it seems restricting flow with a D-5-D6 position leaner needle or a smaller main jet would make sense.

As aside, making such swing when using a megaphone makes tuning out as much megaphonitis as possible problematic. Solve one problem create another. As you said, "this means it (almosts always**) will tend to swing the curve, kind of like a seesaw... it will richen one end of the curve (by RPM)..or lean it depending on which way you go on the air jet."

Now, you are not the first person to have to deal with megaphonits. I too had problems with our 500 and our MKII Amals. But with the help of Tom Ullman, carburetor engineer at Grosvenor Works with help of one of his "wet" flow benches, he taught me how to work around this.


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