Air/fuel mix settings

[kalbo]

I have 'full power' Vmax, The brass plugs that covered the air/fuel mix screws have been removed and they have been replaced with rubber plugs (to keep out the dirt and water). The bike has an aftermarket exhaust also, so some alterations to the carbs settings has obviously been done in the past. I think the mix has been made a little richer, to compensate. I think it may be set a little too rich. I would like to check how many 'turns out' the air/fuel screws are set at, but I checked my tool box this morning and I don't have a narrow enough screwdriver to get at them. Actually, I am a little confused, as I am not sure whether all 4 carbs should be set the same? or whether the front cylinder and rear cylinder are set differently. I believe that under 5,700 rpm only 2 of the carbs are feeding the 4 cylinders, but I am also confused as to which 2 carbs that is? I have had a little trouble getting the bike to idle consistently and wanted to check the mix setting on the relevant carbs, which ever they are? Can anybody throw some light on it for me. Thanks.

 

[Zeus36]

Get the right screwdriver for the A/F screws. Get a carb sync tool as well (even a cheap dial type will work). Sync the carbs first.

The fuel mixture screws ONLY deal w/ mixture at idle and a little bit above idle. They will determine off idle throttle response and extremely low idle cruising.

AGAIN ----off idle throttle response and extremely low idle cruising

To adjust your mixture, run the motor until warm and idle is stable. Turn each screw in, one at a time, until the engine stumbles, then back the screw out until just rich of peak idle (idle drops again). Then turn them back in to peak idle. Repeat for each carb. If your screws are out more than 3 turns, the jet probably needs to be swapped to the next larger. The final position should be around 2.5 turns out on a stock set-up bike, so you may be out a bit richer for your aftermarket exhaust. From my readings here on the site, stay to the lean side.



Lots of info here on the site if you search for "mixture" or check the stickies.

These will get you started:

http://www.vmaxforum.net/forumdisplay.php?f=6

http://vmaxoutlaw.com/tech/low_speed_carb_tuning.htm

http://vmaxoutlaw.com/tech/cv_carb_tuning.htm

 

[Zeus36]

V-Boost:

Each cylinder has it's own carb.
Four downdraft carburetors feed the cylinders through individual inlet tracts. However, the tracts between cylinders one and two and cylinders three and four are separated by butterfly valves. The valves are controlled by servo motors that measure engine rpm. At approximately 5,750 rpm the servos start to open the butterfly valves until they are completely open at 8000 rpm. With the butterfly valves open each cylinder on an intake stroke is fed by two carburetors, rather than a more conventional single unit. Horsepower is considerably increased especially in the mid-and high-rpm range.

The cylinder intake timing is different which can allow two carbs to feed each cylinder during the intake stroke under V-Boost.

 

[caseyjones955]

One thing I have learned is never to trust the guy who slapped on a full exhaust to make correct carb adjustments. Mine was done by a reputable powersports shop and it was pretty far from ideal. I got help here to get it right.

Each carb feeds its respective cyl until vboost kicks in, then both right carbs open up to both right cyls, same for left. It happens via a servo and butterfly valves on the intakes. lots of info on its operation in the fuel and tuning section. I adjust my af s rews much the way Zeus said. I keep that little pocket screwdriver on me when I ride and make little tweaks until idle and off idle are crisp. X2 for getting a synch tool, like many others I use the Morgan Carbtune and love it. Having one of these is almost requisite for having a multi carbed bike. Trying to dial in the AF screws with carbs out of synch is an exercise in frustration. I synch carbs at least twice a season and reset AF. Its fairly quick now that Im used to it and she stays crisp and snappy all the time.

 

[caseyjones955]

V-Boost:

Each cylinder has it's own carb.
Four downdraft carburetors feed the cylinders through individual inlet tracts. However, the tracts between cylinders one and two and cylinders three and four are separated by butterfly valves. The valves are controlled by servo motors that measure engine rpm. At approximately 5,750 rpm the servos start to open the butterfly valves until they are completely open at 8000 rpm. With the butterfly valves open each cylinder on an intake stroke is fed by two carburetors, rather than a more conventional single unit. Horsepower is considerably increased especially in the mid-and high-rpm range.

The cylinder intake timing is different which can allow two carbs to feed each cylinder during the intake stroke under V-Boost.

If the jetting and tuning is close to right, and vboost is functioning correctly, its a very noticeable boost in power. Since I got mine sorted out it pulls very clean and hard from about 6k to redline, and past redline. Noteworthy considering the absence of a rev limiter, gotta watch that.

 

[kalbo]

Just checked the air/fuel screws. Both front carbs were set at 1.5 turns out. Both rear carbs were well out. Right was 8 turns out !!!(don't know how it was in?) and left was 4 turns out. Set them all to 1.5 and it now idles steady at 1000 rpm. I will however now clean the carbs and have them properly balanced. Thanks to everybody for the input.

 

[Barry barker]

Not sure what exhaust you have, but if you are going to clean the carbs its a good time to investigate the main jets. Many previous owners would upgrade the exhaust and install larger main jets. That truth is better exhaust runs richer and usually needs to be leaned out. The guys on this site recommended i go with 147.5 main jets with my Marks system. Glad i listened, the bike runs perfectly. Stock is 152.5.

 

[caseyjones955]

Just checked the air/fuel screws. Both front carbs were set at 1.5 turns out. Both rear carbs were well out. Right was 8 turns out !!!(don't know how it was in?) and left was 4 turns out. Set them all to 1.5 and it now idles steady at 1000 rpm. I will however now clean the carbs and have them properly balanced. Thanks to everybody for the input.

Glad you got it sorted. A good carb synch tool pays for itself after a couple uses, i throw it on anytime I touch the carbs for any reason, or when I feel responsiveness wane ever so slightly. It takes 10 minutes but shops tend to charge .5-1 hr as a minimum.

When you check your main jet size also check if the jets are MK or or DJ (dynajet) by the markings. they are not the same but there is a conversion chart here. I bought mine with full kerker 4-2-1 exhaust and a DJ stage one jet kit and it was very rich. I leaned it out by gradually dropping several main jet sizes and returning the pilot air jet (behind diaphragm) to stock. Like Barry, I ended up with a 147.5 MK main jet. It was quick before but now with correct jetting and a good synch not only does it idle rock solid with crisp low rpm ridability and clean strong midrange but once vboost kicks open ya best ba hanging on.

 

[kalbo]

Just checked the air/fuel screws. Both front carbs were set at 1.5 turns out. Both rear carbs were well out. Right was 8 turns out !!!(don't know how it was in?) and left was 4 turns out. Set them all to 1.5 and it now idles steady at 1000 rpm. I will however now clean the carbs and have them properly balanced. Thanks to everybody for the input.

Although I set the A/F mix at 1.5 turns out and the bike idled and ran well, I thought it may be too lean, as everyone is saying 2.5 is stock? I opened them a little more to 2.0 turns. The bike ran ok, but once it was hot (after a good blast on the freeway) the result was a very poor idle (again). So, I put them back to 1.5
Unfortunately, I have no idea whether my bike has been re-jetted in the past? The guy I bought it from was a Korean and spoke very bad English and couldn't help. I am no mechanic, but I understand that the jetting would have no effect on the idle, as they don't get used until the higher rpm's.
I am confused as to why only 1.5 seems to give the best result?
The bike has an aftermarket exhaust with almost zero air flow restriction (baffles) I understood that this would require a slightly richer setting, but the exact opposite seems to be the case.
Forgive my ignorance, and it may sound like a stupid question, but when I close the a/f screw I am reducing the amount of fuel in the mix or the amount of air? I assumed it was fuel. Which makes no sense why it runs better with less? Sorry..but I am picking this up as i go along. There are no good mechanics where I live in the Philippines, so i am having to teach myself.

 

[caseyjones955]

Turning the screws in leans out the idle. 2.5 is just a jumping off point anyway and could go either way from there. At one point I had mine at 1.75 turns out but have made lots of changes and tried several things. There is some great reading on this but I cant link it with my phone. There is a downloadable service manual here also that should help as you start getting into it.

I wanted to bring my vmax to Iloilo but the cost of shipping/ registering is insane and parts not quite as available. Im not sure what ill ride while im there but I think one of my brother in law has a broken CR500 in Cebu. That might be fun if I can find any parts at all.

 

[kalbo]

So, your saying that turning the screw in leans out fuel, not the air. That's what I thought. I will continue to run it at 1.50 and see how it goes. I know an american guy who is a 1st class motorcycle mechanic. He is in the States at the moment but will be coming to the Philippines in February, he is going to clean and set up the carbs for me properly. As long as I can ride it until then as it is. Yes, importing bikes here is expensive, import duty of 60% value of your bike, plus shipping costs and whatever other charges customs can think of. Honda parts are easy to obtain though.

 

[Traumahawk]

Here is why you have to lean it out with a better flowing exhaust. The carbs on the Vmax are CV carbs.....constant velocity carbs. So, with a set amount of air, a set amount of fuel will flow. If the carbs are flowing more air......more fuel will flow.

So, the exhaust flows more air out of the engine, so more air will flow into the engine. With more air flowing INTO the engine, more fuel will be pulled INTO the carbs and into the engine. With more fuel flowing into the engine, you will need to lean out the carbs by changing out the main jets.

So there are some really good "stickies" (How to's in the carb section of the forum) that show you how to sync the carbs, how to change the main jets...etc.

These bikes really are easy to work on.....and this forum has some great information on how to do ANYTHING to a Vmax.

So, what i am wondering ...if the main jets are rich, and so to make the bike more ride-able, you have had to lean out the A/F screws.....AKA idle mixture screws.

 

[kalbo]

Thanks Traumahawk. That was a very clear and simple explanation of why my bike is running better lean. I could also add some baffling to the exhaust to restrict the air flow slightly. It would also make the neighbors happy !