Carb Synch Question

Hi guys -

So I bought a digi sync carb tuner - it was pretty cheap and frankly, I've balanced carbs the old fashioned way but I'm kind of tired of it. This was similar to biasing tube guitar amplifiers - get the process but wouldn't mind speeding it up. I find when I have the carbs on my 73' 850 perfectly balanced it vibrates way less at all rpms ranges and rides much smoother. Anyway i hooked it up in place of the balance tube and dialed everything in at around 1,000 rpms (after a 20 mile ride so warmed up). Pretty slick me thinks. Then I put the balance tube back on and fire it up and it runs beautifully but is idling at around 500 rpm?? So obviously the balance tube is effecting. So i crank the idle back up to 1,000 as evenly as possible but it's still rougher than it was. Then I connected the machine again and it's idling at 1,350 which makes sense but way too high clearly. I then resynched them at that speed, put the balance tube back on and in business at 1,000 rpm idle. Can someone explain to me what happened. Is there a better way to go about it? Thank you

http://thedigisync.com/digi-sync-top-page.html

If you are balancing your carbs at 1000 rpm all you are doing is balancing the idle IE the throttle slide adjustment screws
Do you ride the bike with the guage fitted?
And adjust the cables accordingly?

baz said:

Do you ride the bike with the guage fitted?
And adjust the cables accordingly?

Click to expand...

There you have the real solution. Ride the bike at cruise and have a passanger read the gauge or (they drive and you read the gauge).

One even better is a brake dyno, like this one out pictured in my old garage.
0-100mph and 0-100rwhp adjust at cruise doing about 15 hp and you're golden.
I'm hosting our local club- Start your engines and dynoday second saturday of april.
Heavens forbid, I only use mercury carb stix.

See you there:
Where my dyno now lives, down stairs center bay and I live on the 2nd and 3rd floor.

That's a nice offer from Dave, if you can take him up on it.
I'm no guru but it seems odd the RPMs drop.
Could there be a vacuum leak in your instruments?
Try pulling the tube and cap the ports and see where the RPM's are.
If it drops then, I'd suspect a leak in the instruments.

I understand I am only balancing at idle, the bike runs fine throughout the revs and is rough at lower speeds / idle which is what I'm trying to fix. I guess I am asking since the balance tube is removed to test - i.e. the intrument replaces it, something must be different when the balance tube is put back on. I wish I could dyno it but that is much too far from me.

FWIW
always set your timing first,
adjust you valves,
always adjust carbs last

This procedure follows a skilled thorough cable setup...
The whole point of the tiny balance tube is to allow a little idle vacuum cross back and forth between sides to keep the idle mixture circuit have a more consistent flow at the lowest engine pulse speeds. Without it would allow the idle circuit fuel column to drain back a little bit between pulses.
I only adjust the idle mix and speed with the balance line hooked up. My measure is to stand behind the bike and listen for equal side to side boom boom boom and tune out BOOM boom BOOM. Continually readjust the overall speed on each side but checking the mixture too, to obtain a highest idle speed than always a tad richer( the width of the screwdriver slot or two).
To be sure a cold start will likely NOT idle cause the cold oil will drag down to make it stall. Jack up the throttle and tighten the friction stop until you climb on to ride.
A minor cable tweek may follow for under load i.e. dyno loaded 15 hp/ 50+mph

dynodave said:

FWIW
always set your timing first,
adjust you valves,
always adjust carbs last

This procedure follows a skilled thorough cable setup...
The whole point of the tiny balance tube is to allow a little idle vacuum cross back and forth between sides to keep the idle mixture circuit have a more consistent flow at the lowest engine pulse speeds. Without it would allow the idle circuit fuel column to drain back a little bit between pulses.
I only adjust the idle mix and speed with the balance line hooked up. My measure is to stand behind the bike and listen for equal side to side boom boom boom and tune out BOOM boom BOOM. Continually readjust the overall speed on each side but checking the mixture too, to obtain a highest idle speed than always a tad richer( the width of the screwdriver slot or two).
To be sure a cold start will likely NOT idle cause the cold oil will drag down to make it stall. Jack up the throttle and tighten the friction stop until you climb on to ride.
A minor cable tweek may follow for under load i.e. dyno loaded 15 hp/ 50+mph

Click to expand...

Great explanation. Thanks.

When I use the electronic balancer I tape it to the top of the tank and go for a ride. Try to find a hill long enough to be able to watch with a load on the engine.

milfordite said:

When I use the electronic balancer I tape it to the top of the tank and go for a ride. Try to find a hill long enough to be able to watch with a load on the engine.

Click to expand...

Sounds reasonable for what you have... On the dyno I also have thermocouples to see how symetrical the temps are besides the mercury carb stix vacuum monitoring, the temp helps with cutaway validation for partial throttle and finally full bore main jet selection.

I agree with Dynodave that syncing the carbs while the engine is at operating temp and under load is the best method; I'd advise against syncing while riding for safety considerations, primarily because it is a distraction and secondly because road surfaces vary a lot from flat, level and quality.

I have used both the mechanical and electronic (Twin Max) methods. If your engine demonstrates acceptable and equal leak down results, is equipped with carbs that are new or fresh (no slide galling, good and equal fit) have gotten compatible results. I am not a fan of using separate vacuum gauges/manometers unless you swap the pickups and split the difference. My syncs are done with the engine at OT, but not on the road or a dyno. I realize that my method is a bit lacking, but in the last 50 years has made no discernible difference in perceived smoothness or header bluing length.

Properly synchronized/jetted carbs will show, virtually, equal header bluing and unless you utilize the Madass single pull device, or equivalent, keep in mind that throttle cables can stretch differently, especially if the throttle stop is the slides hitting the carb tops. Emglo does offer a single pull twist grip assembly that has an adjustable stop which will greatly increase cable life and decrease the requirements for carb syncing once the cables break-in.

Best

The easiest way is to putt down the street at about 2500 RPM in 2nd gear and hold the throttle perfectly steady - then reach back with your foot and cover one ex pipe and then the other with your shoe - listening for the louder pipe. Lower the throttle cable adjustment on the loud side until they're even (carry your tools in your pocket).

Wow. Why is it that I can sync my carbs without a dyno or manometer? Two fingers to touch the slides and make sure they open at the same time, idle speed screws at the same depth, air screws at 1-1/2 turns out adjusted for "most stable idle" that doesn't balk when blipping the throttle. Seems to work for me.

Maybe a Colortune if I'm feeling technical.

maylar said:

Wow. Why is it that I can sync my carbs without a dyno or manometer? Two fingers to touch the slides and make sure they open at the same time, idle speed screws at the same depth, air screws at 1-1/2 turns out adjusted for "most stable idle" that doesn't balk when blipping the throttle. Seems to work for me.

Maybe a Colortune if I'm feeling technical.

Click to expand...

I do similar. I wind the throttles wide open then feel the slide cutaways. Dynos are good, but what really matters is the time it takes you to get around and between corners.

I always cable tie my cable splitter to the frame ,
And pay special attention to cable adjustment as the slides lift off of the throttle stops
Of course all of this can be done by ear or lolly sticks
But using the lolly sticks method it only tells you the slides are lifting at the same time, it doesn't account for wear in the carb bodies/slides

acotrel said:

Dynos are good, but what really matters is the time it takes you to get around and between corners.

Click to expand...

No. What really matters is throttle response between 1/4 and 1/2 throttle and unflinching rpm at steady cruise.

Most of us aren't racers.

Id say make sure your slides reach the top evenly. The make sure they hit the stops evenly. Then fast idle and adjust each air screw slowing them down and adjusting the airscrew until you reach the idle you want with the best air adjustment. Basic stuff first. Im with Dyno Dave that you do your timing first then your valves last diddle with the carbs.

baz said:

I always cable tie my cable splitter to the frame ,
And pay special attention to cable adjustment as the slides lift off of the throttle stops
Of course all of this can be done by ear or lolly sticks
But using the lolly sticks method it only tells you the slides are lifting at the same time, it doesn't account for wear in the carb bodies/slides

Click to expand...

I never use a cable splitter except on a two-stroke triple. My Seeley 850 has a twin pull quick action twist-grip off a Yamaha TZ. And if you pick a new cable up and hold it vertical, the cable inner will fall to the bottom. The cable splitter usually has too much internal friction. The throttle action needs to be light with no drag, and the slides should hit bottom with a single click when you let go of the throttle. You can usually hear when they are uneven, if your motor is not running - you get a double-click.

Eric Bauer said:

Hi guys -

So I bought a digi sync carb tuner - it was pretty cheap and frankly, I've balanced carbs the old fashioned way but I'm kind of tired of it. This was similar to biasing tube guitar amplifiers - get the process but wouldn't mind speeding it up. I find when I have the carbs on my 73' 850 perfectly balanced it vibrates way less at all rpms ranges and rides much smoother. Anyway i hooked it up in place of the balance tube and dialed everything in at around 1,000 rpms (after a 20 mile ride so warmed up). Pretty slick me thinks. Then I put the balance tube back on and fire it up and it runs beautifully but is idling at around 500 rpm?? So obviously the balance tube is effecting. So i crank the idle back up to 1,000 as evenly as possible but it's still rougher than it was. Then I connected the machine again and it's idling at 1,350 which makes sense but way too high clearly. I then resynched them at that speed, put the balance tube back on and in business at 1,000 rpm idle. Can someone explain to me what happened. Is there a better way to go about it? Thank you

http://thedigisync.com/digi-sync-top-page.html

Click to expand...

It looks a nice instrument.
I noticed a small hole the the clear panel. Perhaps it is a breather to enable the digi sync to work correctly and or to prevent damage from high vacuum pluses. Maybe this is allowing a back pulse of air into the manifolds giving the higher RPM? Probably not, but you never know.
Dave

cash said:

It looks a nice instrument.
I noticed a small hole the the clear panel. Perhaps it is a breather to enable the digi sync to work correctly and or to prevent damage from high vacuum pluses. Maybe this is allowing a back pulse of air into the manifolds giving the higher RPM? Probably not, but you never know.
Dave

Click to expand...

Thank you and good question - i'm not sure - I may check with the manufacturer. I'm just wondering that since there is no balance tube in place when checking with the instrument, could the balance tube provide that much difference when connected if there is a significant vacuum difference between the two cylinders. That was my original question, I wasn't really trying to turn this into a how to balance carbs thread. If so I wonder if there is an issue with one of the cylinders. When you rev the engine when connected, the right one certainly shows the biggest change in pressure reading. I am just trying to better understand how this all works

Eric Bauer said:

Thank you and good question - i'm not sure - I may check with the manufacturer. I'm just wondering that since there is no balance tube in place when checking with the instrument, could the balance tube provide that much difference when connected if there is a significant vacuum difference between the two cylinders. That was my original question, I wasn't really trying to turn this into a how to balance carbs thread. If so I wonder if there is an issue with one of the cylinders. When you rev the engine when connected, the right one certainly shows the biggest change in pressure reading. I am just trying to better understand how this all works

Click to expand...

One must ask the question, if the balance tube is blocked off to both carbs, then will the r h one still have the biggest change in pressure reading?