Use of Carbtune(2008)

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Apr 7, 2008
Encouraged by comments on this forum I have ordered a Carbtune and it arrived today :D
I think I understand the basics but I would most certainly appreciate if any of the forum members would like to share their experience with this equipment. For example, what procedure is used to tune your bike with Carbtune.
Any input would be appreciated!

Best regards,
A tool useful for setting of the idle circuit. First step is to understand it's limitations. Some rules I have found helpful: A bike that will start cold with no choke is too rich. All carb tuning starts with the setting of the float (floats) because all the circuits run off this setting. Be sure you have the right parts inside the carb very close attention to the needle jet because of bad ones that are out there. Norton's had a lot of running changes to their jetting set ups be sure your part set ups all match.
Norbsa, your reply would seems to describe tips for using a Gunson's 'Colortune'?:

A (Morgan) 'Carbtune' is a device for carb balancing:

Per G said:
what procedure is used to tune your bike with Carbtune.

The basic procedure is fairly straightforward (obviously read the Carbtune instructions first). The object being to synchronise the slides so the intake vacuum readings of both inlets are (more or less) identical at idle (throttle stop adjustment) and that both throttles rise by equal amounts off idle, the result being that (theoretically) both cylinders should perform equally.

So, (after you've read the instructions) you will need to hang the Carbtune from a convenient position where you will be able to see it while doing the adjustments (L/H handlebar grip)?) using the re-usable ty-wrap supplied, so that the hoses can easily be connected to (in the case of a Commando) the balance pipe stubs, the Carbtune adaptors (M5 & M6) in the kit are not normally required, because the rubber pipes should push straight onto the balance pipe inlet stubs (a little lubrication will help).


For Amal carbs you will probably need to use the Carbtune in the 'upside-down' position (this method is mentioned in the instructions) as some engines don't give very high vacuum readings, so the Carbtune needs to be inverted to show a reading, so you will be reading it upside down.

Make sure there is some cable slack at each carb.

Start up, and allow the engine to warm up.

Once the engine has warmed up adjust each throttle stop screw until the two indicator rods are showing equal readings, and also set the screws so the idle speed is correct.
Remember, the carb with the lowest vacuum reading has its throttle open more than the other one.

Then open the throttle slightly, while observing the rods, the rods should move more or less together?
With the the throttle held steady off idle the rods should show equal readings?
If not, then the use the cable adjusters to set them so both are equal.

Recheck the whole thing by blipping the throttle, you may find that if the carbs are a bit worn that the readings will tend to vary a bit each time, hopefully it won't be by too much?
Yep, you nailed the cold starting jetting reference, I've got it with a totally rebuilt engine with new "properly" (by the book) jetted Amal concentrics. Just a tickle and off it goes first thing in the morning - no chokes. Oh, yes, sooty plugs.
Thanks guys!

Since I have my carbs re-sleeved and "tight" I might not need to use it "upside-down"?
Hope to get a chance to use it this weekend but work and family might "interfer" with my plans :)

Per G said:
Since I have my carbs re-sleeved and "tight" I might not need to use it "upside-down"?

Some carbs just don't work at a high enough vacuum level to be within the standard operating range of the Carbtune.
That isn't really to do with carb wear, but is just a feature of the type of carbs (or engine?).
You are likely to get much clearer rod readings with the unit in the inverted position for Amals.

I also have to use my Carbtune in the inverted position when checking BMW flat twin butterfly Bing carbs, as the Bings don't give particularly high vacuum readings either.

Quote from the Carbtune instructions:
"The manometer must be used vertically for a scale starting at 8cmHg (centimetres of mercury. 1 inch=2.5cm 1cm=0.4inches) and going to 42cmHg. Most bikes have readings higher than 8cmHg, but some bikes such as R-series BMWs and two strokes can have readings lower than this.

To get a reading below 8cmHg remove the plastic cable clip from the slot at top of gauge and insert it into the slot at the bottom. Push cable clip into slot from front of gauge. Hang the Carbtune Pro upside-down."

I need to strobe the bike first, have a friend coming over to give me a helping hand. If we don't have too many beers while doing the strobe I'll probably try the Carbtune too :D

To check the calibration of the Carbtune or indeed any multi gauge vacuum tester. As soon as you're happy with the readings swop the tubes over and note any changes if they are significant you'll have to go half way or recalibrate them.

This forum is great, thanks for all your inputs, it's appreciated!

cash said:
To check the calibration of the Carbtune

Due to the relative simplicity of the Carbtune design, it doesn't seem to go out of sync by much, -if at all?
My Carbtune II must now be at least 20 years old, the rods need an occasional wipe to keep them running smoothly in their guides, but that's it, really.
I just received my Carbtune and expect to be using it this weekend for the fine adjustments on my carb setup.

Quick question...I realize that this is a tool for synchronizing the carbs but can you also use this for dialing in the air pilot jet settings? It seems that as you adjust the pilot jet for the best idle, you would see changes in the column levels with slight changes in engine speed.

mgrant said:
I realize that this is a tool for synchronizing the carbs but can you also use this for dialing in the air pilot jet settings? It seems that as you adjust the pilot jet for the best idle, you would see changes in the column levels with slight changes in engine speed.

Yes, I've noticed that pilot screw ajustment alters the vacuum readings.
Tried the carbtune today, great tool!
Got the AA unit serviced, set the ignition by strobe and took a test run. Rock solid idle and great high rpm power but low rpm response was no good. I have synked the carbs by removing the float, main jet /holder and by watching the needles lift at the same time I thought I got the synk perfect so when the bike got better low rpm response with the choke on I adjusted the pilot air screws to get a richer mixture but it did not help much. Now I was confused :?

Using the Carbtune at idle it needed a minor trottlestop adjustement to be perfect, so far so good. Raising the RPM to 2000 the Carbtune showed a MAJOR difference indicating that the sync was completely off! Had my neighbour to come over blipping the trottle while I checked the exhaust pulses and yes, it was a big difference. Now I was confused, how could the carb sync I did by watching the trottle needles go wrong?
Anyway, I started to adjust the carb trottle wires and the better Carbtune reading I got the better low rpm trottle response I got!

Since I already adjusted the pilot air screw down to only a 3/4 turn open I now need to get back to approximately 1 1/4 turn and start doing all adjustments over again. Hope it's not gonna be snowing tomorrow!

The old well documented way;
Get two drills or round bar say 6mm diameter, trap one under the cut-away of each slide and gently turn the throttle and watch which moves first. Adjust the cables untill they both move at the same time. Then check the cut-aways clear the top of the carb bore at the same time, feel for the step with your finger.

cash said:
The old well documented way;

That method is probably accurate enough, to a certain extent, a set of vacuum gauges will balance gas flow, which I believe is more important than setting both (for a twin) throttle slides to exactly the same height? As the gas flow characteristics of individual cylinders can vary due to production tolerances of the inlet and exhaust ports (unless they have been closely matched by porting work?) the result being that one throttle and idle stop may need to be set slightly higher or lower than the other in order to create an equal gas flow to each individual cylinder?

And I would use the drill bit method myself when rebuilding carbs (for twins, 3s or 4s), but prefer to use gauges to do the fine tuning, because they will normally be slightly out of vac. sync. otherwise, engines appearing to run more smoothly when set for equal vacuum, rather than equal throttle heights.
Here is carb sync device that can be made from stuff you may already have in the garage. It comprises a single vacuum gage, an aquarium valve manifold and some rubber tubing. The valve manifold serves as a snubber to dampen gage needle fluctuations. Also, the valve manifold permits you take readings from multiple carburetors with only a single gage. I built this to sync the four carbs on my old KZ 1000, but found it works great for my Norton, too.

Obviously for a Norton, you only hook up two of the rubber tubes: one tube connects to each of the two intake manifolds. After the engine has been brought up to temperature, take a vacuum reading from one carb by opening the respective valve. Then close that valve and open the other one to read the vacuum on the other carb; make carb adjustments as required. Repeat the process until the two carbs are balanced.
Use of Carbtune(2008)
Continued working with the Carbtune today and got the carbs vacuum nicely "lined up" and following side-by-side moving the trottle up and down. It was used in the upside-down position.
I must say that I was surprised by the highly improved trottle response achieved by tuning with Carbtune.
It's a quick and exact method, provides a great result and most certainly well worth the money spent getting the tool :D
I have to add a further comment on the trottle response.
Not that I have much experience with older twins but I can't recall an old twin having an impressive trottle response but now blipping the trottle the rear end of the bike "sits down" nicely :lol:
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