Carb to turn or not to turn that is the question.

[The Nutty Professor]

X19soysauce asked me a very good question the other day and it took me a while to come up with a way to explain it to him, but once I did he told me it was a great answer. The great answer part is for you to decide but here it goes. X19 asked me why turning the air/fuel mixture screw on his carb won't make the bike faster. Some people might cluck their teeth or shake their head but on the face of it it's a valid question if you look at it from the point of view of someone who is new to the motor modification deal. We always tell the newbie's that adjusting the air mixture screws will get the carb right and in turn that will get them going faster. Well if you add a little more why won't it make the bike go even faster?

I explained it to him like this...Let's turn your carb into a guitar. It's out of tune and you have to adjust it to get your jam-on. Well to tune up the guitar you have to adjust the string tension so you turn the adjustment knob slowly as you play. When you get it just right you can tell because the pitch is just right. Well you want it to sound better so you tighten it up even more but all you've done it screw the sound. Maybe buying better quality strings (Quality carb or a larger carb) may get the results you're looking for maybe not but you're dead certain tightening the crap out of the strings ain't doing your bleeding ears any justice. If any of you motorheads out there can add to this simple explanation it would do a world of good.

One last thing:Swheels mentioned this to me. I already knew it but it's one of those things you don't think about because you know about it and you're not making the mistake. To many people are buying carbs that are to big for their motors. Dumping more fuel into the motor than it can use is asking to burn the motor into a slagheap. Excess fuel in the chamber will not burn. That unburnt fuel has to go somewhere and it does. Some of it goes out the exhaust but some of it sticks to the walls of the cylinder and thins the oil film. As the rings scrap it down it thins the oil even further down the walls until it leaks past the rings and into the oil. Now you have two problems, oil that is thinned out because it's diluted with gas and scored cylinder walls because they can't hold a oil film for the rings to ride on. remember to much of anything can be bad.

[Blitz$M.Inc.$]

just saw this nutty and as a guitar player i can say that everything must right for it to sound the best
you can relate the perfect pitch of a note to the perfect air/fuel ratio of your mix
i use ernie ball super slinky hybrid strings because the top three strings are of a heavier gauge and have a deeper sound the the standard size, but i still have to tune them in same as the other size strings
carbs are the same, you can run it not perfect but the performance will suffer
turning the air fuel screw WILL make your bike faster, just not so much that you would notice
the end result is 'this thing sounds/runs great'

[redryderaus]

Ok. I see there is still some misunderstanding about carby selection and tuning.

First of all, the mixture screw only affects idle mixture and maybe up to 10% throttle opening. It is there to fine tune the mixture supplied by the pilot jet for reliable idle and clean acceleration from a closed throttle. It has no effect on top end! Therefore it will not make your bike any faster. To adjust your WOT mixture means changing jets. To get the mixture right throughout the midrange you change needle height and even change to a different tapered needle.

Second, running a carby too large for the engine will not result in excess fuel in the cylinder (I wish I had a dollar for every time I've heard that particular misconception rolleye0010). The engine will only draw in as much as it can, no more. What running too large a carby will do is reduce the velocity of air through the carby which will result in poorly atomised fuel ie larger droplets. These larger droplets do not burn as well as fine droplets, resulting in a loss of power and unburnt fuel simply blowing out the exhaust. You will also suffer from a lag in acceleration each time you open the throttle as the velocity is too low to draw fuel through the main jet and the mixture goes lean until the revs pick up and the air velocity is high enough to draw fuel through the main jet.

Sorry to rain on you Nutty,

red ausflag

[The Nutty Professor]

Ok. I see there is still some misunderstanding about carby selection and tuning.

First of all, the mixture screw only affects idle mixture and maybe up to 10% throttle opening. It is there to fine tune the mixture supplied by the pilot jet for reliable idle and clean acceleration from a closed throttle. It has no effect on top end! Therefore it will not make your bike any faster. To adjust your WOT mixture means changing jets. To get the mixture right throughout the midrange you change needle height and even change to a different tapered needle.

Second, running a carby too large for the engine will not result in excess fuel in the cylinder (I wish I had a dollar for every time I've heard that particular misconception rolleye0010). The engine will only draw in as much as it can, no more. What running too large a carby will do is reduce the velocity of air through the carby which will result in poorly atomised fuel ie larger droplets. These larger droplets do not burn as well as fine droplets, resulting in a loss of power and unburnt fuel simply blowing out the exhaust. You will also suffer from a lag in acceleration each time you open the throttle as the velocity is too low to draw fuel through the main jet and the mixture goes lean until the revs pick up and the air velocity is high enough to draw fuel through the main jet.

Sorry to rain on you Nutty,

red ausflag

bluelaugh No rain Red. To put a fine point on it I'm not nearly as good with motors as I would like to be. With this explanation I was trying to put it as simply as I could. The day I know everything is the day they close the lid.

[redryderaus]

bluelaugh No rain Red. To put a fine point on it I'm not nearly as good with motors as I would like to be. With this explanation I was trying to put it as simply as I could. The day I know everything is the day they close the lid.

I'll still be learning the day they close the lid. Wherever I end up going I'll be driving 'em nuts with endless questions bluelaugh

Cheers,

red ausflag

[The Nutty Professor]

I'll still be learning the day they close the lid. Wherever I end up going I'll be driving 'em nuts with endless questions bluelaugh

Cheers,

red ausflag

I had to laugh at that one Red. I'm probably older so when you get there don't be surprised if they ask you "Ah do you happen to know a guy who likes to call himself The Nutty Professor?" bluelaugh Just say no and they'll probably let you in the pearly gates LOLOLOL

[Blitz$M.Inc.$]

i ment in the range that the air/fuel is effecting, you would get a better rev and more performance in that range only
i didnt mean like top end or anything
:)

[redryderaus]

I had to laugh at that one Red. I'm probably older so when you get there don't be surprised if they ask you "Ah do you happen to know a guy who likes to call himself The Nutty Professor?" bluelaugh Just say no and they'll probably let you in the pearly gates LOLOLOL

No way! If you're already there you'll have the perfect racetrack already setup for us! bluelaugh

Cheers,

red ausflag

[Kurlon]

Seeing as we're getting technical...

The air or fuel bleed screw (or both, depending on your carb) and associated pilot jet DO have an effect at all throttle positions. It's just minor once you get past just cracked open. : ) They essentially establish the baseline amount of fuel being tossed into the mix. WFO, or at idle, that circuit will pump X amount of gas in, no matter what.

This is why you should recheck your main jet and needle settings after you play with the pilot circuit. You could have leaned or richened things enough to warrant going up or down one step on the needle, or in some cases moving one size on the main.

While it's nice to think of each of the metering portions of the carb as operating in one area independent of the others, it's not quite that nice and simple. Playing with the needle for example is supposed to control partial throttle mixture, but you're also tweaking the effective main jet size at WFO. You make even more drastic changes to the effective main jet size if you change tapers or lengths.

[Blitz$M.Inc.$]

i have a 190 main jet that didnt run right till i dropped the needle clip to the bottom, which in effect allowed more fuel in those ranges, it still could use some tweaking, im thinking of getting a smaller taper needle to see if i can get it even better