need CV carb exPERT

[Joe Nardy]

The CV carbs don't have adjustable needle positions.
I will try to figure out how to post photos and text for the carb mod, or i can assemble a .pdf and email to you.

I think you can raise the needles by shimming them with washers. I don't know how you'd lower them.

If you need somewhere to post pictures I can do that. You need a web host that allows linking to pictures. I think Photobucket does and it is free.

Thanks,
Joe

 

[KEITH KRAUSE]

Joe, I didn't mention raising the jet needles because his bike is totally stock except for the K&N filter. Usually richer mixture screw settings will work well enough. But since his bike has always done this since he got it, it's always possible a jet needle change could help things. I did mention his problem is where the pilot circuit and jet needle circuit overlap and both/either could be causing his problem.
I was a little curious too about his throttle adjuster mod' but was concentrating on his problem. I've never really had a problem with losing my setting while torquing the holder nuts. Sure, I have a little trouble at times. I just follow the factory torque setting, which is generally 3-4 ft/lb. Over-tightening them is where you have problems.

 

[Road_Clam]

As for your mixture screw question, if the pilot circuit is clean/correct jet size/correct float level, turning the screw out will have a normal effect and allow more mixture. As for if the pilot circuit is to blame, there is an overlap of the pilot and jet needle circuits at the throttle positions you say the problem presents itself. Mostly the jet needle in my opinion.
What I find interesting is you previously said to get a good idle you must set the mixture screws to 1/2 turn out. Anything else and idle quality suffers. You now have them set at 2 turns out (which is much closer to normal) and performance has improved. The bike is idling well at 950 rpm's. Unless you made some change we don't know about, what do you think has happened here that the bike now accepts the screws at 2 turns out?

air screw adjustment vs. pilot jet size needs to be "in sych" with each other also. Where the Mikuni BS mixture screws meter fuel flow, the generall rule of thumb is if you need to adjust the mixture screw less then 1 turn, you need a leaner pilot jet. If you need to adjust the mixture screw beyond 2.5 turns you need a richer pilot jet. Basically mixture screw adjustment between 1 and 2.5 turns out, is a good sign you have chosen the correct size pilot jet. Every bike will run a little different, based upon many variables, ie. humidity, ambient air temps, mods, and so on.
Rich

 

[toymechanic.]

nert wrote:I Sync the carbs (1) a year, begining the start of spring riding season. I have a Morgan CarbTune. The most difficult part of the sync is the set screw and lock nut, as it it easy to foul the setting once the lock nut is tighten. I have developed a pretty simple carburetor modification to provide tension to the Synchronizer screw, eliminating the jam nut for the Synchronizer screw. Adjustment is performed more easily and more accuratly. I have retested the sync to be sure my design has held up. ( i am most happy with the results). I have modified several carb sets on other Mikuni banks with successful results.

Would you care to share to mod's you made, because I hate the lock nuts as well. I was thinking about using a teflon inserted set screw.

Also to answer a question that I don't think was answered. The fuel/air mixture screw uses more fuel the farther out it is. Secondly the fuel level high or low will not effect the fuel/air mixture screw circuit, unless the float level is way to high or way to low. Also if your carbs have those little black plugs, they need to sealing properly or they will effect the f/a circuit big time.

Lastly post the mod's, because allot of us have inquiring minds. :lol: :lol:

 

[nert]

HEY, you have the same bike i have!! mine is the blue

http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

According to the CV Carb Tuning papers i have. Fuel level, even within range will make a very big differance in performance. And i can say, without reserve, it does. The little black plugs are new and seal well.

Thank you for the answer on the pilot screw question.

Yes, i would like to share the mod. I put together a .pdf with directions and photos. Don't know how to make available other then send by email.

 

[KEITH KRAUSE]

I answered your question earlier about the mixture screw and how to adjust it richer/leaner.
Also, road_clam has the "rule of thumb" regarding mixture screws reversed.
It's true the basic range of adjustment should be around 1 to 2 1/2 turns, but if best idle is achieved at less than 1 turn, this suggests you need a larger pilot jet or the fuel supply portion of the pilot jet circuit is clogged somewhere. If best idle is achieved beyond 2 1/2-3 turns, this suggests the pilot jet is too rich or the air supply portion of the pilot circuit is clogged somewhere.
You can visit the Mikuni carb tuning site, such as their HSR section, or K&N's site. They'll tell you the same thing.

 

[Road_Clam]

I answered your question earlier about the mixture screw and how to adjust it richer/leaner.
Also, road_clam has the "rule of thumb" regarding mixture screws reversed.
It's true the basic range of adjustment should be around 1 to 2 1/2 turns, but if best idle is achieved at less than 1 turn, this suggests you need a larger pilot jet or the fuel supply portion of the pilot jet circuit is clogged somewhere. If best idle is achieved beyond 2 1/2-3 turns, this suggests the pilot jet is too rich or the air supply portion of the pilot circuit is clogged somewhere.
You can visit the Mikuni carb tuning site, such as their HSR section, or K&N's site. They'll tell you the same thing.

OK, not to start arguing, but I am confused. The BS mixture screw meters fuel. Turning in the mixture screw CW, is effectively leaning the idle circut, correct?(or do I have this specific comment backwards?)

 

[nert]

As for your mixture screw question, if the pilot circuit is clean/correct jet size/correct float level, turning the screw out will have a normal effect and allow more mixture.

I did read your answer, i was being courteous to "toymechanic." But after reading your last post, i am confused as well since.

less than 1 turn, this suggests you need a larger pilot jet or the fuel supply portion of the pilot jet circuit is clogged somewhere. If best idle is achieved beyond 2 1/2-3 turns, this suggests the pilot jet is too rich or the air supply portion of the pilot circuit is clogged somewhere.

that appears to be contradictory, but at this point i have to accept it on faith.
after more thought!!! I think i got it. IF the fuel portion is clogged, then as the screw is turned out, it only allows more air in (not fuel) and the mixture ends up lean so the screw is turned in for better idle quality by mixing the restricted amount of fuel in correct balance with the air that can still be metered by the screw. Correct? Or am i still confused?

 

[KEITH KRAUSE]

I answered your question earlier about the mixture screw and how to adjust it richer/leaner.
Also, road_clam has the "rule of thumb" regarding mixture screws reversed.
It's true the basic range of adjustment should be around 1 to 2 1/2 turns, but if best idle is achieved at less than 1 turn, this suggests you need a larger pilot jet or the fuel supply portion of the pilot jet circuit is clogged somewhere. If best idle is achieved beyond 2 1/2-3 turns, this suggests the pilot jet is too rich or the air supply portion of the pilot circuit is clogged somewhere.
You can visit the Mikuni carb tuning site, such as their HSR section, or K&N's site. They'll tell you the same thing.

OK, not to start arguing, but I am confused. The BS mixture screw meters fuel. Turning in the mixture screw CW, is effectively leaning the idle circut, correct?(or do I have this specific comment backwards?)

Turning in the mixture screw on a CV carb does effectively lean the circuit, in a CORRECTLY operating, correctly jetted pilot circuit. But, the mixture screw also meters air, not just fuel. The operation of the screw can actually give reversed results from what's expected if the pilot circuit has a blockage of air or fuel. A "blockage" can also include a too small jet, or poor vacuum. A too large jet will also reverse the expected operation of the screw.
I'm not arguing either. See? Lots of smilies. I just share my experience/opinions, but mostly experience. Just because I say or read something, doesn't make it fact. I suggest asking Mikuni or K&N or one of the better jetting/tuning sites if you need more info. They can explain better or more clearly than me?

 

[KEITH KRAUSE]

nert, the last part of your last post is a good scenario.
The thing here is a CORRECTLY operating, correctly jetted pilot circuit will allow the mixture screw to operate in its designed and expected way.
If anything causes a lack of fuel or air, or an unequal abundance of fuel or air, the screws operation will not be normal or as expected.

 

[KEITH KRAUSE]

Another thought after reading back. May not matter and the new mixture screws you installed may be just fine, but unless there's obvious damage to any screw, I would never substitute aftermarket screws for genuine Mikuni.
They are precision made and any difference in length or shape will throw normal adjustment off. Brass is generally good for continued use, unless the screws were damaged.
Same thing goes, even more so for for any fuel or air jet. Keep Mikuni parts in a Mikuni carb if at all possible. An aftermarket jet kit is something else but swapping jets/screws just because they're new can create problems.

 

[Road_Clam]

This thread is making my brain hurt. LOL

I only have a fair amount of Mikuni CV experience. My main expertise in Mikuni VM's. I can tell you from 22 years of jetting motocross bikes, and snowmobiles, my above mentioned thought process as far as tuning the idle/low speed circut always produced my desired outcome. Mikuni VM's however work the opposite of CV's. Closing the mixture needle richens the mixture, and opening the screw leans the mixture. I'm here to learn, not cause trouble, so I am simply going to agree, and leave this one alone.
Rich

 

[nert]

keith
at the time, the back of mind was echoing that same thought. The front of my mind wasn't listening. (some times i feel schizophrenic).
Anyway, i neatly saved all the old jets, even the gasket and o-rings. Some parts of me are smarter than others. Hell, i think i still have some needles and slides from the old BSA Amals.

 

[KEITH KRAUSE]

This thread is making my brain hurt. LOL

I only have a fair amount of Mikuni CV experience. My main expertise in Mikuni VM's. I can tell you from 22 years of jetting motocross bikes, and snowmobiles, my above mentioned thought process as far as tuning the idle/low speed circut always produced my desired outcome. Mikuni VM's however work the opposite of CV's. Closing the mixture needle richens the mixture, and opening the screw leans the mixture. I'm here to learn, not cause trouble, so I am simply going to agree, and leave this one alone.
Rich

This specific subject has caused me trouble too. I also understand the VM carbs well and know their side pilot air and pilot fuel screw functions.
It's when there's a problem with the circuit that "unusual" things happen that I found hard to understand at first. I'm learning too.

 

[KEITH KRAUSE]

keith
at the time, the back of mind was echoing that same thought. The front of my mind wasn't listening. (some times i feel schizophrenic).
Anyway, i neatly saved all the old jets, even the gasket and o-rings. Some parts of me are smarter than others. Hell, i think i still have some needles and slides from the old BSA Amals.

Well, as I said, your new jets/screws may have nothing to do with the problem but it's a possibility. Using genuine parts, when precision matters, just gives you one less possible problem.

 

[nert]

If any one should want a .pdf file of the sync screw mods. Send me a personal message with your e-mail address. i can e-mail it. Don't know how to post it.

 

[toymechanic.]

HEY, you have the same bike i have!! mine is the blue

http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

According to the CV Carb Tuning papers i have. Fuel level, even within range will make a very big differance in performance. And i can say, without reserve, it does. The little black plugs are new and seal well.

Thank you for the answer on the pilot screw question.

Yes, i would like to share the mod. I put together a .pdf with directions and photos. Don't know how to make available other then send by email.

Nert,

My bike is soon to be two-tone dark blue and light blue.

Gary