Mystery at the Carb Synch

[Guest]

Was in the process of synching the carbs when the gas line came undone and that ended the evening.

However, I was noticing an interesting trend in the gauge. Unfortunately, I had to stop before getting an actual photo, so I'm illustrating it instead.



I have a problem synching to #3. Does this indicate too large a jet or a semi-plugged jet?

I've additional questions about jetting, but I want to finish looking at what's available and cover it under a different thread.

[BassCliff]

Hi,

What rpm is this reading? As you adjust the other cylinders it will affect all the levels. Were the carbs bench synch'd first? Did you plug the vacuum port on #2?


Thank you for your indulgence,

BassCliff

[Riding Again]

Good follow-up questions Bass Cliff--as I should have included the information. I used a small, straightened paper clip as the clearance gauge for bench synching. The vacuum port was plugged (another good use for a golf tee ) I tried to keep the rpm's about 2000 by turning the idle knob after adjusting the other screw.

I was having a hard time "boosting" #3 or reducing the others.

Air/Fuel mixture at idle looks good.

[BassCliff]

Hi,

Hmmm, is the slide stuck on #3? Is there a hole in the diaphragm? I use the throttle cable adjustment at the throttle tube to raise the idle while I'm sync'ing. It seems the easiest.

Vacuum sync'ing your carbs has nothing to do with the jets. It is an adjustment for the butterfly valves so that all the vacuum levels are the same across all four carbs, all slides lift at the same time.


Thank you for your indulgence,

BassCliff

[Riding Again]

I was going to say that yes jetting does affect the synching until I thought all the way through.

#3 would react, but sluggishly at different throttle positions so yes, a sticky slide or diaphragm problem is more likely. I'll pull the top off and reinspect. I mentioned in a different thread that I had made a mistake and left a couple of the tops a little loose and that diaphragm could easily have slipped out of place.

I thought about the cable adjustment, but found that the knob was easier for me.

[Nessism]

What happens when you rev up the engine to 4000 rpm? Does #3 vacuum come up near the other cylinders? If so, the #3 cylinder may be loosing compression. Did you adjust the valves yet? I helped a member work on his bike and #3 was likewise low at idle but the vacuum came up with rpm. His bike showed evidence of valve damage on #3 as the shim thickness was about 3 sizes thinner than the other cylinders.

On a general note, the process for adjusting the carbs is to first adjust 2 and 3 together to level them, next you adjust 1 and 4 until they are level with 2-3. You establish the base line with 2-3 before touching 1 or 4.

Good luck and hope this helps.

[dr_fosg8]

Not sure about the carbs on that bike, but on mine three is not adjustable. All the other carbs have to be adjusted to match three, starting with two. What happens if you try to turn down two to match three? What does the plug in three look like? Is it even firing?

[MisterCinders]

If you bench synched them and get that low a reading on 3, look for an air leak. If the boot leaks, your vacuum will read low and adjusting the slides won't affect it.

[Nessism]

Not sure about the carbs on that bike, but on mine three is not adjustable. All the other carbs have to be adjusted to match three, starting with two. What happens if you try to turn down two to match three? What does the plug in three look like? Is it even firing?

The center adjuster is to sync 2 & 3 together. It's a matter of semantics as to whether 2 is being adjusted to 3, or the other way around. Bottom line is to execute the center adjustment first to get 2 & 3 level, then you move to the two outer cylinders. It doesn't matter what order you do 1 or 4, just as long as you do 2-3 first.

[Steve]

However, I was noticing an interesting trend in the gauge. Unfortunately, I had to stop before getting an actual photo, so I'm illustrating it instead.



I have a problem synching to #3. Does this indicate too large a jet or a semi-plugged jet?

Jetting will have no effect on carb sync.

First thing for me to clarify, though, which carb are you calling #3?
Carb #1 is under your clutch hand, carb #4 is under your throttle hand.

Because #3 is the one to which the cable is attached, you can not simply adjust it to match the others.
It is the "master carb" and all the other carbs get adjusted to it.

The vacuum port was plugged (another good use for a golf tee )

That is actually one of the very FEW uses for a golf tee.
This also sort of answers my question above, as a non-plugged vacuum port would show low on #2

I tried to keep the rpm's about 2000 by turning the idle knob after adjusting the other screw.

That is actually at the high end of the factory-recommended setting, most of us do it much closer to idle, where it really matters.
And, ... what "other screw"?

Air/Fuel mixture at idle looks good.

Mixture setting has no effect on carb sync.

Hmmm, is the slide stuck on #3? Is there a hole in the diaphragm.

Neither a stuck slide, nor a hole in the diaphragm will have any effect on carb sync.

Vacuum sync'ing your carbs has nothing to do with the jets. It is an adjustment for the butterfly valves so that all the vacuum levels are the same across all four carbs, all slides lift at the same time.

True enough, but they don't lift at all at idle speeds.


Riding Again, your apparent inability to change your vacuum settings makes me wonder, ... are you adjusting the correct screws?

The correct screws are the THREE that are BETWEEN the carbs, not ON them. Start with the screw between carbs #2 and #3. Loosen the locknut, move the screw, snug the locknut back into place. You should see your vacuum change immediately. Don't worry about what #1 and #4 are doing, just concern yourself with #2 and #3 for now. Once those two match, check the relationship between #1 and #2. The factory manual says that #1 should be just a little bit higher than #2, probably just a little more than your original illustration shows. After #2 and #1 are set, move to #4, set it just a bit higher than #3, mirroring the settings of #2 and #1.

.

[Riding Again]

I want to say thanks to all who have or may respond. I don't mean to sound arrogant or defensive with any of my responses--I'm terrible at expressing myself in written words. I draw upon the knowledge you've all gained to make it easier for myself.

What happens when you rev up the engine to 4000 rpm? Does #3 vacuum come up near the other cylinders? It comes up, but still lags If so, the #3 cylinder may be loosing compression. Did you adjust the valves yet? Valves were adjusted prior to starting the engine. All are in good tolerances I helped a member work on his bike and #3 was likewise low at idle but the vacuum came up with rpm. His bike showed evidence of valve damage on #3 as the shim thickness was about 3 sizes thinner than the other cylinders. All shim thickness are reasonably close to each other. #3 did not take a big step (+2) from the last valve change/carb synch

On a general note, the process for adjusting the carbs is to first adjust 2 and 3 together to level them, next you adjust 1 and 4 until they are level with 2-3. You establish the base line with 2-3 before touching 1 or 4.

Good luck and hope this helps.

Thank you for clarifying the order for me. I had to dig a bit as I remember it needed to start out at where the cable joins the carbs and the first available adjustment after (at) the cable junction.

The plug in #3 is firing dr. fosg8. One of the first things I checked, but I don't think it has much to do with the vacuum. I'll warm it up and do a compression check.

[Riding Again]

Jetting will have no effect on carb sync.

First thing for me to clarify, though, which carb are you calling #3?
Carb #1 is under your clutch hand, carb #4 is under your throttle hand.
The same one it should be--as you've described below
Because #3 is the one to which the cable is attached, you can not simply adjust it to match the others.
It is the "master carb" and all the other carbs get adjusted to it.

I foolishly tried to turn down the other carbs by keeping their butterflies more closed than #3's as I grew more frustrated

That is actually one of the very FEW uses for a golf tee.Yes, #2 vacuum port is plugged by the tee
This also sort of answers my question above, as a non-plugged vacuum port would show low on #2



That is actually at the high end of the factory-recommended setting, most of us do it much closer to idle, where it really matters.
2000 was an easy to identify point while making adjustments

And, ... what "other screw"?

The other screw is the carb's butterfly's adjustment. The one that is locked in place with a nut. I've circled the ones I would adjust, in green, below.

Mixture setting has no effect on carb sync.



Neither a stuck slide, nor a hole in the diaphragm will have any effect on carb sync.



True enough, but they don't lift at all at idle speeds.


Riding Again, your apparent inability to change your vacuum settings makes me wonder, ... are you adjusting the correct screws?

The correct screws are the THREE that are BETWEEN the carbs, not ON them. Start with the screw between carbs #2 and #3. Loosen the locknut, move the screw, snug the locknut back into place. You should see your vacuum change immediately. Don't worry about what #1 and #4 are doing, just concern yourself with #2 and #3 for now. Once those two match, check the relationship between #1 and #2. The factory manual says that #1 should be just a little bit higher than #2, probably just a little more than your original illustration shows. After #2 and #1 are set, move to #4, set it just a bit higher than #3, mirroring the settings of #2 and #1.

.

For some reason, I could not match carbs #2 and #3 [not adj screws indicated]. It makes me wonder if I have a valve or ring job ahead of me as Ed mentioned above. The first I'll check for is a leak in the boot/o-ring area [a nod to Mister Cinders]. I did replace all boots and o-rings this round as well to prevent this problem--but I messed up a bit before (a different thread) so a double check of my work is in order.


Dave

[Nessism]

It's hard to get at that 2-3 adjuster sometimes. Are you sure you were turning it properly?

[Riding Again]

Just to check, I would turn it both ways to see if I could make a difference. Couldn't see one made. Though logic would dictate that I should turn to the left (to close more the #2 butterfly) to align #2 and #3.

[Steve]

Just to check, I would turn it both ways to see if I could make a difference. Couldn't see one made. Though logic would dictate that I should turn to the left (to close more the #2 butterfly) to align #2 and #3.

I don't remember which way to turn the screw to move the gauge which way, but you want to OPEN #2, not close it.

The vacuum level is measuring the restriction imposed by the throttle plate (butterfly).
Since the vacuum level of #2 is higher, it is telling you that it is closed more than #3.
Opening #2 will reduce its vacuum level to match #3.

Did you do a bench sync before installing the carbs? If not, it's possible that they are just so far apart that a single turn of the adjuster is not quite enough.

.