Carbtune at different RPMs

Trying to decide what RPM to use the carbtune at; If I have the engine at 1100 RPM and balance them level it changes as I increase RPMs. If I set them level at 1100 RPM they become asymmetric till 3K or so and then are level.

If I set them level at 2K RPM where it's closer to where I'm usually riding at in average around town traffic, they are way off when back down at idle.

However, If I set them level at 2K, they stay level up to the upper RPMs

Seems like I should set them at 2K, closer to where the bike is running when in operation, the only time the bike is at 1100 RPM is when I'm not moving and I don't keep it there for long with the engine running.
What's the consensus?

 

The problem here is that when you're looking at them at anything above idle, it's NOT the same as when the bike is riding down the road at the increased RPM. The engine's not under load.

We're measuring manifold vacuum. Manifold vacuum on a motor revving 2K or 3K with NO LOAD is going to be way different than manifold vacuum at 2K or 3K when the motor is under load.

Sync them at 1100 like the book says.

 

Will do. Thanks!

 

Hey, fitz, how does that work? I read this before and didn't say anything because I was 100%, but that was what I imagine based on what I'd read.

How is it different?

 

Why is what different?

Manifold vacuum produced at say, 3k on a loaded-down motor vs. manifold vacuum produced by a motor running "free" at 3K?

Because when you load the motor, you create resistance to the pistons' pushing on the crank that isn't there when there's no load.

 

You put different resistance on them, sure, but shouldn't they create the same amount of vacuum regardless, and thus draw the same mixture, have the same timing, and burn the same color, etc.? I mean, intuitively it makes sense that it is different, I 'get' that, but I can't explain it to myself or someone else who asks me.

 

Not if they're not doing the same amount of "work."

Do you know the actual definition of horsepower? (Look it up. I gotta explain manifold vacuum.)

Air flow through the engine is determined by the rotation of the engine, multiplied by the displacement, and the density of the intake stream in the intake manifold. In other words, more of any of the above=more air flow. In this case, our only real variable is RPM (the "rotation rate.")

During the intake stroke, the piston descends in the cylinder with the intake valve open. As the piston descends it effectively increases the volume in the cylinder above it, setting up low pressure. Atmospheric pressure pushes air through the intake manifold and carburetor (where it is mixed with fuel) on its way to filling the cylinder.

When the throttle is opened, air rushes in to fill the intake manifold, increasing the pressure and filling the vacuum brought about by the descending piston.

If the engine is operating under light or no load and low throttle, there is high manifold vacuum. Lots of "suck" compared to atmospheric pressure; nothing "holding the motor back."

If the engine is operating under heavy load at wide throttle openings (such as accelerating from a stop or riding up a hill) then engine speed is limited by the load rather than its ability to flow air and minimal vacuum will be created.

You wanna know how I learned this?

Back when I was starting to learn about motors, my Grandma, bless her heart, had a (Candy Root-Beer) 1949 Ford sedan. It had VACUUM-OPERATED windshield wipers. My Grandma drove like, well, a grandma. Anytime I rode with her and it was raining out, I got a first-hand lesson in manifold vacuum and how it varies by load and speed. After my Dad explained about the reasons for the erratic wiper behavior, I started paying attention. Lessons learned early stick with you.

This is the same principle, by the way, that makes the manometer reading increase as you close the throttle during synchronization.

Haven't you done a vac sync yet? I would have thought you'd noticed this.

 

The ColorTune Plug isn't a Multi-function Instrument.
On our Bikes, where there is virtually only one Mixture Adjustment
:: Unless you Rejet ::
The ColorTune is ONLY effective in Achieving the Optimal Mixture for IDLE.

There's NOTHING ELSE Tunable that effects Mixture.
NO Computer.
NO Sensor
NO Mass Air Flow

Only Pilot Mixture, ... and that is Limited to a window of Fine Tuning that, ... when the all the right Jets are in place and the Passages clean and clear, ... is very often found to be within a few degrees either way of too lean to too rich.

The Main Jet Size is FIXED.
In order to accomplish any real alterations to OFF-Idle Mixtures --- The Carbs need to be ALTERED ...

Not adjusted!

Therefore, ... the Colortuning of our Bikes is really a matter of using the tool too FIND the Optimum IDLE Mixture.

Once the Bike is revved-up ... Off Idle ... The ONLY Adjustment to be made is for the Pilot Mixtures "Supplementary Richness".

The Rich-end of BLUE plus additional Richness beyond BLUE ... which the Plug is not reliably able to indicate once the Mixture Reddens and Darkens.

 

@KA1J: Have you gone through the carbs, checked valves, and done a compression test on this bike?

 

Heh, 100% new rubber on the carbs, tubes in carbs are immaculate and the carbs were taken down as far as possible, new throttle shaft seals; the whole wazoo. Rebuilt the engine last year with valve job & new rings. The engine runs very powerfully and idles like a kitten purring.

 

Then sync it at idle and ride it lol

 

Yes indeed!

I'm obsessive about my tuning and check the sync every 400-500 miles or so and since it matters this much to me, how accurately I'm doing it is important.

The reason for my original question was to know which is the best way to sync seeing as doing so at 2K RPM appeared to be the best way to have linearity of manifold pressure from 2K up to 9K. With the bike rarely at idle, I care more about linear results of the 2K & up RPMs. Seeing as
adjusting at both 1.1K and 2K gave different results with the manifold pressure AND that the results were repeatable, I had to address this discrepancy to find out why & then what to do.

This is a great forum for these answers & I'm not asking the local bike shop because they don't know these bikes and worse, they think on their feet rather than know the answers. I've come to understand most everything I need to know, I can get the answers to that here. I trust Fitz & Rick to give solid advice and many others here know the answers as well. If someone offers advice that could be better, it gets corrected in follow-up posts right away.

This forum is like Snopes for XJ hearsay!

 

You can not use a color tune at anything over idle. Running the engine at higher RPM's will overheat the colortune and crack the glass and ruin it! READ the instructions and warnings that come with it!!!

 

I'm lost! We started with the Carbtune and ended up on the Colortune.

Very good explanation Fitz. You must like typing. Maybe an easier way to look at it is a motor under load at any given RPM will have a larger throttle opening then the motor not under load at the same RPM, hence different vacuum levels.

 

a BMW K100 calls for a sync at 1K and 3K but it has FI and a "air bleed screw" on the throttle bodies plus the regular butterfly adjustments. like they said without the extra adjustment there's no way to do it

 

I forget about the CarbTune System, all the time.

I confuse CarbTune for ColorTune all the time.

Does CarbTune come with a set of Vacuum Restrictions?

 

Hi Rick,

Yes, there are tiny tubules that go inside the inner lumen of the vacuum lines. They're located maybe 8" from the carb end of the lines.

 

Mine did...

 

Since you have the Restrictions, ... why not Sync at at 18-Hundred and call it a day.

If you have the Carbs pulling all the same at 18-Hundred, ... you got the whole range covered.
Synced!
All dancing on the same line.

Then you'll have enough pull to rip the dogs right off the side of Second Gear!!!

 

Noooooooooooooooo! I have clutch slip at the end of 2nd gear. My doggies be treated well! No popping out of gear allowed!

 

Fix your clutch.

I love how everybody seemingly forgets that clutch friction plate "pads" are like tiny brake pads and expects that after 30+ years of soaking in oil/drying out/soaking in oil that they'll just work fine. You wouldn't trust 30 year old brake pads.

I think it's because you can't see them.

but I digress...

Sync'ing at 1800rpm would mean that the throttles are open quite a bit, and we're still not under load.

It might be interesting to see if the settings that work at 1100 are still good to 1800; but I wouldn't adjust at 1800.

 

LOL! Everything in time, just can't do it all at once. It only slips at WOT at the upper end of RPMS in 2nd gear. I just run the bike nice and it acts nice. I'll replace the pads sometime this summer.

 

In the meantime, try to minimize slippage. The more it slips, the better chance you'll need new plain plates too.

 

Synching at 1100 rpm is important for starting, warm up, and smooth idle. Even though you can synch at high rpm it really doesn't do much for you. When the the engine is at idle the butteryflys are almost totally closed and a given change in butteryfly position or even pilot screw will have a large effect. Now at high rpm that "change" will have very little effect because it is dwarfed by the massive air and fuel coming out of the main system. Looking at it from a butterfly perspective at idle moving a butterfly a .001 will have a big effect, at 3000 rpms and under load .001 butterfly movement can't be felt.

It gets to be a lot of work but if you want to fine tune for high rpms and loaded conditions make very small changed to the float level.

Sometimes you have to ask yourself, "Is the view worth the climb?"

I'd be riding right now but it is 106 deg outside.
Cheers!

 

Synching at 1100 rpm is important for starting, warm up, and smooth idle. Even though you can synch at high rpm it really doesn't do much for you. When the the engine is at idle the butteryflys are almost totally closed and a given change in butteryfly position or even pilot screw will have a large effect. Now at high rpm that "change" will have very little effect because it is dwarfed by the massive air and fuel coming out of the main system. Looking at it from a butterfly perspective at idle moving a butterfly a .001 will have a big effect, at 3000 rpms and under load .001 butterfly movement can't be felt.

It gets to be a lot of work but if you want to fine tune for high rpms and loaded conditions make very small changed to the float level.

Sometimes you have to ask yourself, "Is the view worth the climb?"

I'd be riding right now but it is 106 deg outside.
Cheers!