Jetting, re: throttle position vs rpm's.

[KEITH KRAUSE]

This could go in the "Tips" section or "General Discussion" section, but I thought it should go here.
I keep seeing people asking for jetting help. They usually describe the problem at a certain rpm. While rpm's can sometimes help describe what's happening, it's the THROTTLE POSITION that's most important.
Yes, rpm's matter in some cases, for example: if you're testing the main circuit and do a 5th gear roll on starting at 2,500 rpm's, you can't expect the carburetion to be good. The bike needs to be near or at the start of its powerband for an accurate/fair test. Obviously, rpm's and throttle position are linked in certain situations, but if you use throttle position as a guide, you'll have a much better idea of what jetting circuit the motor is on during your problem or testing.
Using tape and a marker, mark your throttle housing and grip at different points, such as 1/4, 1/3, 3/4. Be sure the cable(s) are adjusted and remove any slack by turning the grip slightly to remove any slack. Then make your marks. Make the marks so you'll see them as you would when actually riding/testing the bike.
By saying "the bike bogs at 1/2 throttle" instead of "the bike bogs at 4,000 rpm's" will give a much more accurate idea of what's wrong and what it will take to fix the problem. Keep in mind the bike needs to be near or at its powerband as I described above when testing. This is true for the main and needle circuits. For the pilot circuit, powerband doesn't factor in. For the pilot circuit, you generally test at the minimal throttle position that's necessary to ride the bike in the higher gears. The needle circuit quickly overlaps the pilot circuit even at mimimal throttle openings. You can of course do some closed throttle tests to HELP determine correct pilot jet size/pilot circuit adjustments. Excessive "popping" during closed throttle deceleration being a common problem.
You don't have to take my word for it, just visit a GOOD website, such as Mikuni, and you'll see how the 3 jetting circuits work and how rpm's are a secondary consideration.
I'm not trying to be "Mr. Carburetor" here, or get into debates, I'm just trying to share info that will make jetting a little easier for the owner and easier for those who try to reply. THROTTLE POSITION.

[Hoomgar]

hmm, didn't realsize that but followed your throttle position suggestion first time you said it as noted in my tech thread.

Thx for posting Keith. And um, you ARE Mr. Carburator so just except it

[lhanscom]

With the CV style carbs RPM plays a bigger role then on VMs doesn't it? If you wack open your thottle at 2500 rpm the slide won't open all the way, giving you false feed back, until the rpms go up enough to make the slide go to full throttle. Some of this problem may go away if you're doing the testing in the power band, I'm not sure where CV carbs get to true full throttle in the rpm band.

I wish all carborators just had a mechanical slide, I like being the one controlling the throttle, not a spring and some air pressure.

[Anonymous]

I keep seeing people asking for jetting help. They usually describe the problem at a certain rpm. While rpm's can sometimes help describe what's happening, it's the THROTTLE POSITION that's most important.

Words to live by...

With the CV style carbs RPM plays a bigger role then on VMs doesn't it? If you wack open your thottle at 2500 rpm the slide won't open all the way, giving you false feed back, until the rpms go up enough to make the slide go to full throttle.

Also true. This is what was giving me a rich spot at 6000rpm on a full throttle roll-on started at lower rpm. It would pull good until 6K, then bog. I expect this was where the slides finally got fully open and the rich needle setting combined with the mains to overload the engine with fuel. Lowering the needle solved the problem.

Yes, rpm's matter in some cases, for example: if you're testing the main circuit and do a 5th gear roll on starting at 2,500 rpm's, you can't expect the carburetion to be good.

That is one of the strong points of the CV carbs, they slowly open the slides to suit the engine demands as the revs rise fo rsmooth response to sudden throttle changes. I can nail full throttle from 2000rpm or lower and it simply pulls through to redline, even in 5th now. But it is true that you must be aware of where the operating conditions are less than optimal and expect some strangeness there.

Also, the best order to do things in is:
1) Pilot circuit
2) Needles
3) Mains

This is because the pilot and needle settings will affect the mains, but the mains do not affect the lower circuits. So if you set your mains first, then adjust your pilot and needle circuits, you may have to go back and redo the main jets again. If you start at the bottom and work your way up you will not have to redo things just because you changed the next circuit along.

Great advice, Keith.


Mark

[KEITH KRAUSE]

With the CV style carbs RPM plays a bigger role then on VMs doesn't it? If you wack open your thottle at 2500 rpm the slide won't open all the way, giving you false feed back, until the rpms go up enough to make the slide go to full throttle. Some of this problem may go away if you're doing the testing in the power band, I'm not sure where CV carbs get to true full throttle in the rpm band.

I wish all carborators just had a mechanical slide, I like being the one controlling the throttle, not a spring and some air pressure.

CV and VM carbs do react a little different to sudden throttle openings.
I prefer the way VM's operate over the CV's, but both carbs can be "gagged" when suddenly opened. Poor jetting or other conditions magnify this.
When re-jetting or troubleshooting, you have to factor in the characteristics of your carbs. At least we can do that by having "general knowledge" of the carbs. We also need to factor in other things, such as the exhaust design or cams you've chosen. Most of us can't figure that part out. We have a flat spot at a certain throttle opening or point in the powerband and we blame the carburetion, when in fact it's the exhaust or cam design. That's why we sometimes NEVER get the jetting to our liking, because we aren't going to toss the pipe or change the cam.
But both carbs still work basically the same. Knowing the throttle position and what jet circuit(s) is operating during the problem will better serve you than going by rpm's. Throttle position is something you can see and understand what's happening inside the carb. RPM's can help you describe a problem, but they're still secondary in solving the problem.

[KEITH KRAUSE]

mark m, you're the FIRST person (besides me) that says to get the needle circuit correct BEFORE the main circuit. 8O
I've suggested this since I've found this site and for many years before that to people I meet. I find re-jetting quicker and easier that way. I get the needle circuit right, because it's more important to everyday riding than the main circuit. If the needle circuit is perfect at a small expense to the main circuit, such as a loss of a couple MPH's... oh well. I'll take good roll on/passing performance any day, instead of a bog or flat spot by a slightly rich main.
After getting the needle right, I go with the largest main I can that doesn't effect the needle in a negative way.
If you read many websites, they always say to change the mains first.
Guess it's you and me against the world!

[Anonymous]

If you read many websites, they always say to change the mains first.
Guess it's you and me against the world!

That is an old two stroke tuning method. I have heard of guys tuning the bottom circuits with no main jet in place at all, just to avoid a lean condition and cylinder damage. It will certainly run with no main jet in place.

I get the needle circuit right, because it's more important to everyday riding than the main circuit. If the needle circuit is perfect at a small expense to the main circuit, such as a loss of a couple MPH's... oh well. I'll take good roll on/passing performance any day, instead of a bog or flat spot by a slightly rich main.

Absolutely. You spend almost no time at all at full throttle on a liter bike, but spend all your time at small throttle openeings. I want perfect response on the pilot and needle for smooth operation in traffic and around town. Having a carb hiccup happen at 3000rpm in 1st gear with 50ft-lb of torque available is a sure fire way to have a problem in a hurry. I also try to set the pilot and low throttle needle on the lean side for better mileage and crisp response, with it transitioning to slightly rich from the mid needle onto the mains.


Mark

[KEITH KRAUSE]

Thx for posting Keith. And um, you ARE Mr. Carburator so just except it

I've been called a lot worse Mark!