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Carby adjusting...


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Thought I'd share this because I see a lot of people wondering how to adjust their idle mixture. On some bikes it's really hard to get at the idle mixture screw and near impossible once the engine's hot. I'm a mechanic and need to adjust mixtures accurately and so I made this tool to do it. It's a piece of wood about 12mm square, with a screwdriver made out of a small bolt, and the drive belt, gear and tensioner out of an old printer. It cost nothing and only took about a half hour to make once I'd decided how to do it. Anyone that works on a suzuki quad needs one. The standard setting as recommended in the manual is only a starting point, and they always run nicer with a proper adjustment. The gear at the screwdriver end is pressed on there and is tight enough to turn mixture screws if they are not seized.

The adjustment should be done warm, and it's supposed to be the highest revs you can get with the least throttle opening, then you wind the mixture screw in until the revs just start to drop. That settings called "best lean", and it's what they run best at when hot. Sometimes though if the bikes doing a lot of stop starting I leave them a fraction richer than that.

20220202_095814.jpg

20220202_095804.jpg

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  • 1 month later...

You just poke the screwdriver end in from the right side of the bike on a suzuki, (after taking off a small bit of black plastic if it has one in the way), then raise/lever the screw driver up into the idle mixture screw's recess, turn the pulley on the outer end till the screwdriver engages, and then turn the outer gear to adjust the mixture. I'm always careful to keep the belt from touching the hot motor,(it's pretty close), but it has touched a few times and seems reasonable heat resistant.

That's the third design of tool I've made and far and away the simplest and best to use. From one pulley to the other is about two-hundred mills, the screwdriver length is a bit critical because if it's too long the pulley won't fit above a cast bit of the bowl, and if it's too short it won't engage with the screw. I left the screwdriver a bit long until it had the gear pressed on, then held the bolt head in the vise while I cut it to the right length and filed it to a screwdriver end. I think from memory there's about five mills of leeway on the length of screwdriver, perhaps a little less, three mills perhaps. The printer I pulled apart had several gears I could have used, but I could see that a small diameter would give more room.. As it happened a gear any bigger than the one I used would have touched the carby and caused the screwdriver to be at an angle. I can measure the diameter if you want.

 

That belt by the way is like a cam belt, it's got teeth on it so it's a solid drive.

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  • 4 weeks later...
On 2/1/2022 at 4:36 PM, Mech said:

I made this tool to do it.

That is genius, I gotta say.  Thanks for sharing!

On 2/1/2022 at 4:36 PM, Mech said:

The adjustment should be done warm, and it's supposed to be the highest revs you can get with the least throttle opening, then you wind the mixture screw in until the revs just start to drop.

Often the problem is when I tune it like you say, then I get a stumble just off closed throttle indicating the needle and slide cutout interactions aren't able to compensate quickly enough.  So I like to tune it such that it idles a little more on the needle/slide circuit rather than fully closing the slide and adjusting the pilot/airbleed circuit, that way there is no big change to compensate for.  The downside is it may be harder to start the engine manually because the more the slide is open the more vacuum must be generated to draw enough fuel through the needle-jet to mix properly with the air which means the piston has to move pretty fast, but with electric start it's no big deal.

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  • 4 months later...

Yeah well the reason you have that problem Randy is because the way you do it, you are using both the idle discharge holes as one.

There are two discharge holes into the venturi right by the butterfly, one on the engine side and one on the aircleaner side of the butterfly, when it's closed properly.

When the butterfly is closed off the idle mixture is supposed to come up from the adjustment needle and get sucked out the hole nearest the engine, but, the mixture is getting diluted by an extra measure of air getting drawn in through the second hole, the one away from the engine. Because of that extra air, we end up adjusting the idle mixture slightly richer to get it right. As the butterfly starts to open, the second hole starts to have vacuum applied to it and it starts discharging fuel as well, and, at the same time, the first hole doesn't get extra air anymore.That second hole discharging combined with the now undiluted mixture out the first hole is what stops the flat spot you have when you adjust it your way.

The way you do it Randy, you have both holes discharging fuel at idle, and because of that, you have the idle mixture screw wound in more than it should be. It gives the right mixture at idle, but when the butterfly opens there is a shortage of fuel.. it's being restricted by the idle screw.

If we want our bikes to run right, it's important to set the idle mixture with the least possible amount of throttle opening. Once the idle mixture is right, at a slow idle, we can wind the speed up a little if we want.. but don't start adjusting the mixture again or you will start going around in a vicious circle of mixture/speed/mixture/speed, which will lead to the symptom Randy describes.. a flat spot.

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Both ways have the right mixture at idle, but my way, as the throttle is opened the mixture gets richer, your way Randy, it gets leaner as the throttle gets opened.. 

And, just for the record.. It's not just my way, it's the way the carby was designed, and jetted, to operate.

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11 hours ago, Mech said:

The way you do it Randy, you have both holes discharging fuel at idle, and because of that, you have the idle mixture screw wound in more than it should be. It gives the right mixture at idle, but when the butterfly opens there is a shortage of fuel.. it's being restricted by the idle screw.

If we take a carb that is tuned properly to run from just off idle to WOT then there will never be a shortage of fuel at any opening above idle.  Then if we tune the pilot circuit such that the carb idles with the slide open a bit then there will still never be a point where there is a shortage of fuel.  The only issue with tuning it like that is: because the slide is open a hair the engine needs to develop more vacuum to draw gas out of the needle jet which makes it harder to start manually, but it's not an issue with electric start.  The advantage is the tuning procedure is easier because the pilot circuit is less relevant and the throttle response is crisp with no stumbles.

Your way requires everything in the carb be absolutely perfect (slide cutout, pilot jet, air jet) in order to have seamless transition from idle to off-idle, and that's often not the case, especially with aftermarket carbs or switching carbs to different engines or tuning stock carbs after modifying engines.

There are a range of slide cutouts available and you can't just pick one and make it work.  There are a range of jet needles and needle jets and pilot jets and air jets.  It's a puzzle with at least 5 variables to tune to make work right just to transition from idle to off-idle.  So to avoid that nightmare I just tune it with the slide open a bit then there is no transition to worry about.

The downside is it's hard to use a pull cord or kick starter to generate the vacuum necessary to draw fuel out of the needle jet with the slide open a hair.  If not for the issue of starting, a pilot circuit wouldn't be necessary at all.

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I bought one of these Ratchet Wrench, Mini 36 Teeth 1/4in Drill Bit Socket Ratchet Wrench Screwdriver Sleeve Spanner Car Hand Tools(Orange), Socket Wrenches - Amazon Canada

and I still had to grind half the hex shaft off a slotted bit before it would fit under the carb on my quadrunner.  And then turned out there wasn't enough room sideways to swing the ratchet!  So I have to stick my fingers in there and spin the teeth....

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My previous design Bender was a bit simpler. It was like that tool I've shown above, but without the belt of outer pulley. It was used by holding and engaging the screwdriver with one hand, then reaching in with one finger and pressing on the gear hard enough to turn the screwdriver by the gear..

Yes I know Randy, you've already told me you come here because you  enjoy arguing.. Which is why I've refrained for so long from responding.

I'm familiar with the problems with old bikes, and carburetors.

I'll leave it to the readers to make up their own minds how to do things.

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Here is a graphical representation.

carb.thumb.jpg.379cb62ab31592dc8ac7afe1e653dfbd.jpg

When the slide is closed, a vacuum is easily generated which draws fuel and air mixture up through the pilot opening (3 in the pic).  If the turns on the mixture screw are correct then the engine will idle fine.

When the slide is opened just a hair, suddenly air rushes in and the vacuum drops.  That's where problems start, and in this picture they remedied that with a bypass outlet (4 in the pic) which draws fuel directly through the pilot jet to help make the transition from closed slide (high vacuum) to open slide (low vacuum).

On larger carbs the problem is remedied with an accelerator pump that squirts fuel into the venturi.

That transition from high to low vacuum is the most difficult part of a carb to tune and my idea is to eliminate it altogether by never closing the slide and relying totally on the pilot circuit.  The only downside is it's difficult to generate enough vacuum to draw fuel up the needle jet to start the engine with a pull cord or kicker if the slide is still open.  The upside is there is no transition to worry about and the throttle response will always be crisp.

carb2.thumb.jpg.8024ea5e6b14f58136485cb599032953.jpg

If you have a stock bike and stock carb, great!  You can easily set things by the book and not worry about any of this.

If not, then you're certainly welcome go through this tuning nightmare all you want.

2 hours ago, Mech said:

you've already told me you come here because you  enjoy arguing.. Which is why I've refrained for so long from responding.

I complimented you on a good idea and offered an alternative way of doing things which for some reason caused you to get bent out of shape.

All you had to say was "Thanks!" and "Good idea!".

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But but but... neither myself or any of the carby manufacturers think it is a good idea !!

But what would we know...

We all even think the slide has the cutaway on the other side..  Foolish us..

You're so smart Randy, you should run for President.. 

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How do you know what carb manufacturers think?

Turn the mixture screw until rpms are at a maximum then turn the idle speed screw down to reduce rpms then turn the mixture screw until rpms are a maximum then turn the idle speed screw down and at the end of that process the engine idles great but you stab the throttle and the engine dies.  Now what?

Option 1: Tear the carb apart and spend lots of money on jets, needles, slides and spend hours and days and weeks struggling to tune it like Mech says you should.

Option 2: Turn the idle speed screw back up and turn the mixture screw until the rpms are where you want them and verify the engine doesn't die when the throttle is stabbed.  Done.  Easy.

Which is the good idea?

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I'm a mechanic Randy, trained and educated in these things, with fifty plus years of bike owning, riding, and repairing, working on bikes from every continent and models from the forties onwards. I've repaired, restored, modified and customized hundreds of bikes for a living..

Nightmares, days and weeks struggling to tune it...  These are not problems I have..

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Well I do want advice and help because most carbs I encounter that need tuning ultimately have that problem.  I have 3 atvs now with that problem.  The easiest thing I know to do is turn the idle speed back up and readjust the mixture until the stumble goes away.  Other than that I'd have to spend hours, days, weeks as I always have struggling to get it tuned.  I've ground slide cutouts with rotary tools and have sanded needles in addition to drilling jets trying to get aftermarket carbs to work on modded engines.  It was fun the first few times as a learning experience but now it's just too much work, so if you know an easier way I'd appreciate it.

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Ha.. In all my years I've never needed to modify a slide, or needle..  But, we are all at low altitude and a small market, so we don't have many variables. I have tuned bikes for competition though, and made up carbies and tuned them, worn parts and all.

I'd recommend getting the manual for your bike, checking you have the correct carb, that all of it's jets and settings are right for that carb for that market, and then tune it as the manufacturer recommend.. which is the way I've been recommending.

If it's not the correct carby for your bike, get a manual that has the specs for that carby, exactly that carby, for the market the carby was intended for, with the slide and needle it has, and then check/change jets from there till it's running right.

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Often, a model of bike comes out with the same carby model, but with different specs, (and part number), which have different jets, slide, needle etc..   The details are found in the supplement section of the manual.

Almost any of the variations will run pretty well on any of the year/version of bike they were made for, as long as everything in the carby is a matched set, as designed by the manufacturer and spelled out in the supplement section.

Start thinking you can improve on their design work, and mixing and changing things, is not a good idea.

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Some more thoughts, for those that are having problems...

One thing that does get put together wrong is.. the slide needle has nearly always got a washer/spacer above and below the clip, and sometimes those spacers, especially plastic ones, can be different thickness, by up to about one mill. The manuals hardly ever explain which is which. One mill can be adjusted with the c clip on the needle, but it's always better to check/try which way the spacers are if the needle seem to be set wrong. Needles do get bent, and that can cause rough running at certain speeds. I've replaced needles for that reason.

Slides wear, but I've got plenty of bikes running fine with really worn slides. Slide needles wear slightly after a long time, but I've got bikes running fine, with wear on the needle that could be seen with the naked eye. Jets, they say they wear, but we can buy jet measuring tools, which I've used, and the wear is minute even after a lot of use. To all intents and purposes, jets don't wear.

Hope that's useful..

 

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