[basanti]

I'm still struggling to dial in the 30mm performance carb, so....

1) Is it a 100% fact that at  WOT the needle position has no effect on fueling?

2) Is anyone using jet size larger than 142 for the AVL 2008 model?

I'm not even using the straight through "Goldie" exhaust. i have the shorty exhaust and the S&B filter.

At WOT the bike would start to hesitate at around 105 km, regardless of jet size (tried 132, 138, 140 & 142). I checked the float level and it was low. So that was adjusted and all jet sizes were tried again. This time i get  the same symptoms start again but a little later when I'm cruising at 120-125kms ... plugs are sparkling clean/white. The 142 was boggy and way too rich (by feel) while getting up to WOT, but after that again the same hesitation. The 132 felt the most responsive up to 1/2 throttle. Needle position remained on the center for all jets... figured i have to get the main jet correct at WOT before i start to mess with anything else, correct?

Before i go and order jet sizes larger than 142 i just want to make sure I'm barking up the right tree.

What it seems like to my untrained eye is that the float chamber is emptying out quicker than its filling.

Anyone had problem with this? What can i do to check/rectify this?

I'm reaching a point where I'm thinking of binning this carb. Please help!!!

Thanks in advance...

[Rick Sperko]

What it seems like to my untrained eye is that the float chamber is emptying out quicker than its filling.

How are you determining this? Do you have a clear bowl?

My understanding is the needle is not a part of the main jet circuit at WOT. If you have a white plug, then you need a larger jet. One thing I noticed while dialing in was that my shorty exhaust had a big hole on the bottom where it joined the header pipe. Closing this hole would obviously affect jetting. I believe making it more rich.

I actually appreciate this post, because I have been struggling with a 32mm flat slide (on a iron barrel) and was kicking myself for not ordering the CMW 30mm.

Good luck,
-Rick

[ace.cafe]

A couple of observations.

First, a clean white plug is too lean, no matter what you think it "feels like".
You need to get some kind of color on that plug.
Richer main jet.

Second, 125 kph is about top speed for that motorcycle, if you are getting true speedo readings. Maybe 130 kph might be possible, but that's about it..
Have you contemplated the idea that that's as fast as it will go?

The needle will have an effect, and I'd put the needle at its highest setting(in other words the needle should be raised, by lowering the position of the clip), and see what that does for you.

You're getting to nearly top speed, but you have no plug color, so you are lean.
You can probably get where you want it with the next jet size up, and playing around with the needle position.

[Nacho]

basanti,  I had the same problem with that carb.  I think you are correct about the float bowl emptying faster than it is filling, or maybe the float bowl is too small, or maybe the fuel tap is too small..... I worked this out because if the tank was full then it would be ok at WOT and behave as expected.  If the tank wasn't full it would miss and run lean at WOT no matter what main jet I fitted.

I recommend the Mikuni VM30mm - it works great!!  I'm actually about to try a rare VM28 and see if I can still pull 80mph.

PS I have a flatslide for sale!!

[basanti]

After spending 2 days on a deserted road outside Bombay, I'm about to throw in the towel. I've rechecked using all jets with various needle positions and i keep on getting lean.

That is with size 142 main, 38 pilot jet and  air screw anything between 1 1/2 and 2 1/4. I found carbon build up on the edge of the spark plus with the air screw at 1 1/2 but better idle, and no carbon with air screw at 2 1/4 but idling fast at stand still.

The 132 main is much more strong and peppy than anything else, but the spark plug was a brilliant white and a bit of the electrode blue. 

So as i said #%*%##$@!!!! and was packing up to go home and on the way flinging the carb in the creek, I see a tiny leak between the bend pipe and exhaust. Do you think this leak is enough cause to make a 142 jet show so lean? Theoretically may be, but all you guys with experience, actually will it make that much of a difference?

[ace.cafe]

After spending 2 days on a deserted road outside Bombay, I'm about to throw in the towel. I've rechecked using all jets with various needle positions and i keep on getting lean.

That is with size 142 main, 38 pilot jet and  air screw anything between 1 1/2 and 2 1/4. I found carbon build up on the edge of the spark plus with the air screw at 1 1/2 but better idle, and no carbon with air screw at 2 1/4 but idling fast at stand still.

The 132 main is much more strong and peppy than anything else, but the spark plug was a brilliant white and a bit of the electrode blue. 

So as i said #%*%##$@!!!! and was packing up to go home and on the way flinging the carb in the creek, I see a tiny leak between the bend pipe and exhaust. Do you think this leak is enough cause to make a 142 jet show so lean? Theoretically may be, but all you guys with experience, actually will it make that much of a difference?

It's possible for an exhaust leak to make the bike run lean.
I don't know if your particular exhaust leak is severe enough to cause your problem, but it certainly isn't helping.

I'd also have a good look at all your intake manifold joints, for air leaks there. Some of your symptoms sound like an air leak at the manifold, to me.

If you have a float-bowl filling issue, you can solve that with a fairly large-sized fuel filter put into the fuel-feed line to the carb, at a level higher than the float bowl,  so it can gravity-feed the float bowl like the tank does. Get a see-thru clear one if you can.
The filter will act as a fuel "buffer" in the line, and even if the petcock can't flow enough fuel, the filter has a small reserve capacity in there that can flow. Let the fuel filter fill up before you start up the bike, and it should stay full during warm up.
Then you can try your wide-open throttle tests.
If you find that the filter is being emptied when you run at large throttle openings, that shows that your petcock tap is not flowing sufficiently to keep up with the engine.
The fuel filter can overcome that for short runs at higher speeds, and may be ok for your use. If not, then you need to get a petcock with more flow capacity.

These are just guesses, with potential solutions. I don't claim to actually know what your issue is, because I'm not there to troubleshoot it. I'm just guessing some things that it "might be", and giving some possible answers to them.

[Nacho]

Ace, thats a great idea... the one about the fuel filter "buffer"

(I still recommend a Mikuni VM  )

[basanti]

Ace, Nacho... appriciate the replies. My level of expertise in carbs is zero so took the carb to a mechanic and he checked the float level and said it was low so he adjusted it. Also had him check for air leaks between carb and inlet port. All was ok there. That was before this mornings experimentation.

Gonna seal the leak in between the bend pipe and exhaust before trying again.

I don't see how i can go bigger than the 142 main, cuz the needle is at the leanest setting and it works ok. Changed it to one position richer and the pick up suffered and was very boggy with the BoooWWaaangg sound from the engine. But still a white spark plug at WOT.

The engine runs much cleaner stronger with the 132 jet and needle set at richest setting. But spark plug was way way too lean (blue electrode).

Top speed on all jets was 120km, except for the 132 which was a bit more.... again can't say for sure because not that much difference.

This all might sound like rambling but its the results i got and i don't know how to decipher it:(

Will post my result as and when i get time to try again.

(I still recommend a Mikuni VM  )

Gotta try everything before i abandon ship.

[Rick Sperko]

I feel your pain. From what I can tell, I am the first person on earth to put a Mikuni TM32 on an Iron Bullet. My experience is much like yours. I am starting over after closing my exhaust leak.

I contacted Nacho about his carb because even though you and he had trouble, there are others that have succeeded, so at least it is possible and some have been happy with the 30mm. I am getting to the point where I am afraid of stripping my spark plug hole because of all the plug chops.

Good luck,
-Rick

[Joe28]

There's not a whole lot I can add but I'll give some tips I've learned over the years.
Carb circuits:
pilot/air screw- idle to 1/8 throttle
Needle- 1/8 to 3/4 throttle
Main jet- 3/4 to WOT
Now, the main effects the needle a bit, the needle affects the pilot a bit.
They all over lap.
To rich en the needle put the clip CLOSER to the pointed end, (this RAISES the needle out of the main jet, exposing MORE of the hole, thus richer.
Usually if the needle is wrong, you get a hesitation at 1/8 throttle.
Good rule of thumb is start at the size you had in the old carb.
You can tell if you off by reving the engine, (now this ISN"T scientific, just thumb ball park).
It should rev nice and smooth, no stumble, flat spots.
The air screw should be @ 1 1/2 out .If it's more, leaner pilot, more than 1 1/2- richer, this way you always have adjusting room. IDEALY you want it there, but as long as there is some adjusting room your go to go.
A lean main will cause the bike to "SURGE", or fall on it's face at WOT.
A exhaust leak WILL cause a jetting problem.(you get a backfire through the hole when you chop the throttle. Just cover the tail pipe outlet and you'll hear it)
A newly oiled air filter or dirty will cause it to run rich.
A air filter leak- lean
If you think the fuel is draining out of you bowl faster than coming in, check the tank flow, (very rare, you gotta suck A LOT of fuel to empty it out. Take the line off and see what type of flow you got. I've seem the outlet just about closed with rust ).
 (the fuel filter trick is an EXCELLENT thing to try, plus it keeps the carb cleaner. I only use clear fuel line and put fuel filters on all my bikes)!
I have had 2 off road bikes vent tube clog, that made a vacuum and it stalled.
Try running with you gas cap loose to let air in.
Don't look at the jet size, keep going up till the plug looks tan, (although 2-3 steps up is a  LOT. if your going up that much,hmmmmmm)
Check the slide in the bore, a loose slide will cause an air leak, making jetting a pain.
I found on my '01 iron barrel, EVERY TIME I tried to make it better, I'd spend more hours screwing with it, only to go back to base settings.
It's like my old ATK motorcycle, it didn't like to be modified.
Good luck
Just an old guys thoughts.
Joe
It's running great, let's try, (fill in your favorite mod here).
later, "Man this is running like my grand mother! what's wrong with this"!

[Nacho]

I have posted this before..... its just what I do - I have never known anyone else to do it (or even agree with it!!):

Choose the Main Jet based on which one gives the max top speed.  There will be 2 or 3 (or 4) that give the you the same max top speed so choose the smallest.

Needle position, cutaway etc... are all chosen based on seat of pants

Works for me.  I have set up numerous bikes like this and always have a good plug colour, when I get arround to looking at it.  I even set up a Suzuki GS1000 like this and got better 1/4 miles times than the magazines of '78 said they got when the bike was new.

BTW.... I think plug chops are dangerous, bad for the engine and can be very missleading - but thats just me 

[Nacho]

haha this reminds me of the blank stare I get when I tell someone about this method of carb tuning.  Thats cool I just wanted people to know of an alternative method that seems to work well.  I'm sure I read it in a book once but maybe I just dreamt it up one day.

Although I will say I have tried every other method of carb tuning under the sun (or so it seems) (including lambda sonds AND dynos) and this is by far the best, in my opinion

 

[PhilJ]

BTW.... I think plug chops are dangerous, bad for the engine and can be very missleading - but thats just me 

Elaborate please. I've never heard that before. I'd be interested to know your reasons.

[Joe28]

They'l work, the idea being if it got ENOUGH gas, you'll have the engine working, too much, it'll load up and slow down.
The needle and slide, it's easy for those, a burble jjjuuuussssttt off idle you can't clean up, a bigger cut away,
The needle will give you a hesitation off the pilot if it's too low, a bog if it's too high.
I never look at one circuit.
I get it close, run it and look at the over all pic of my plug.
You don't, (well most people don't) run WOT or 1/2 throttle or idle, so, it's a combo of all.
Could the stumble be that's as far as the timing will advance and it's retarding to keep the engine together??
Joe
Fix it? Fix it? Why?

[Nacho]

PhilJ,
I think plug chops are dangerous because you have to ride along a public road and just kill the engine.... so you are stopping at a random place on a road - this in itself is dangerous.  You then have to stand at the side of the road.... obviously a dangerous place to be.  If you have a private road where you can ride at 80mph then you are very lucky!!
I think Plug chops are bad for the engine because you are removing and installing the plug in a hot aluminium head so you are more likely to damage the thread than if it was cold.  Also, I never liked just killing the engine and imediatly stopping... because at high throttle openings there is lots of air to cool your hard working engine - then you just stop dead!!  Normally you would actually travel at lower throttle openings for some distance before stopping, at which point your engine has cooled somewhat.
I have found plug chops to be inconsitant.... maybe thats just me....  maybe you have to ride at the same speed and throttle opening for a certain distance to get a valid reading...  which would be difficult on a public road, although a bloke in a pub once told me  that it takes just one spark to change the colour of a plug.... either way I have not had much luck with consitant plug reading when doing chops.  I just have a good look at the plug when I replace it.  And I look inside the end of my 'pipe on long journeys to make sure its not too rich.
This is obviously just my experience and opinion... I'm always open to hearing information about carb settings etc...

[PhilJ]

Interesting Nacho. You have some very valid points about safety. Where I likve I have more options as to where I stop and as matter of self preservation, pick my place well.

As to hurting the bike I guess arguements could be made either way.

If a person doesn't understand expansion and  contraction, plug chops should probably be avoided.

Anyway I was just courious. Thanks for the reply.