Mixture Screw Settings

[V7Goose]

I have posted on this in the past, but thought I would update the info with a fresh post since I just revisited the settings on QuickSilver.

 

Factory settings on the mixture screws seem to always be extra lean - somewhere around 2.5 - 3 turns out, if I remember correctly (I always check this and write it down for the carbs I work on, but then I give the notes to the owner and do not keep them myself for reference!). This lean setting is not too surprising with the current state of emission regulations, but I do find it strange that Yamaha's published spec for CO on this engine is MUCH richer than they come set from the factory. I have a tendancy to believe that a manufacturer should actually set up the machine to match their own published specs, but Yamahaha both sets their floats WAY higher than the spec and the mixture leaner than the spec.

 

Although I do have an exhaust gas CO analyzer, I rarely use it because it is tedious and a PIA. I have found that I can get excellent results by simply tuning each carb for max RPM. I have also tried the Colortune, but I find that tool a sad joke.

 

The published CO% for this engine is:

#1 & #3 = 2.5-3.5%

#2 & #4 = 3.5-4.5%

 

For reference, here are the settings I have found on my RSV - Using the max RPM method, I still end up with settings that are slightly too lean, especially for the front cylinders. Typically this method leaves the screw about 3.5 turns out, and that is what I would recommend as a starting point for someone looking for that info.

 

Using the CO analyzer, I get about 3% with the screws set somewhere between 4 and 5 turns out, and pushing the front cylinders all the way up to 4% CO requires a full SIX turns out!

 

Six turns out seems like a huge amount, but I have used the analyzer on this bike twice (two years apart), and the settings I ended up with are the same today as they were back at the beginning of 2009, so I am getting consistent results. Although the bike has been running perfectly for the past two years at the midpoint CO settings, I think I'll probably reset the front two jugs for the minimum of 3.5%, just so I'll feel better about the screws not being so danged far out!

 

But for the majority of carb setup and tuning for 2nd gens, I think I will continue just setting for max RPM + 1/4 turn on each carb. Even without a tach on every bike, I find it pretty simple to do it by ear. I used to actually leave the screws a tad more IN from max, but in light of the settings required to reach the published CO numbers, I've decided to set them 1/4 turn richer.

 

If you feel the need to adjust your carbs for any reason, here is the process I think will produce good results for just about anyone.

 

First, sync the carbs and make sure the idle RPM is set properly. If you do not have a tach, just realize the idle spec of 1,000 RPM is probably a little faster than the idle sound you prefer.

 

Just leave the sync gauges connected while you do the mixture screws, since you will want to double check the sync setting when you get done.

 

I'd start with each mixture screw about 3.5 out, then the first thing is to start turning it IN to verify you get a drop in RPM (which proves your idle jets and circuit in that carb are actually working) - no point in trying to tune a carb that already has problems with plugged idle jets, right? Now slowly turn the screw out to the point where the RPM no longer increase. Select the final setting by turning the screw back in to just where you first detect any drop in RPM, then back out 1/4 turn.

 

Repeat that with each carb, then verify the sync is good, and you are done. If you have problems getting a good setting on any specific cylinder, I'd start looking for other carb issues there, such as plugged jets, bad o-rings on the mixture screw, diaphragm holes, etc.

Goose

[BigBear]

Hey V7Goose!

 

Thanks for this very informative post!

 

You got pics of where the mixture screws are located?

 

Thanks!

[Seaking]

I'm going to have to dig out the papers we did from when you did the carbs on my bike..

 

Thanks again for the very detailed and comprehensive what and the 'why' the what does.

 

Makes more sense to me now.

[Yammer Dan]

Most will find that each bike has its own quirks and this setting is not a good starting point. I set mine at 2.5 and go from there.

Edited December 3, 2010 by Yammer Dan