Dead cylinder synching

[Take Warning15]

Quick question about synching my BS38s ('79). When synching the carbs using the dead cylinder method, do I adjust the right side to lowest idle with the screw between the carbs, and then the left with the idle screw on the left carb? Won't the left carb's idle screw alter both carbs? I cannot get it right and I did read the carb guide.

 

[weekendrider]

The dead cylinder method is for optimizing the fuel mixture of each carb.
The advantage is you are listening to only one cylinder running and getting the best reaction to changes in the mixture screw as opposed to listening to both cylinders firing.
If you are synching the carbs the manometer is best. You can remove them and use the paper pull, popsickle or drill bit method.
The reasoning in my mind is each carb has it's own varibles from the manufacturing process. It maybe close but I doubt if any two are exactly the same.
AND anything done to them since by people like me that can't leave it alone.
Stand by for other opinions.

 

[5twins]

No, don't touch the sync screw (or you'll throw the sync off), just lower the idle speed for either carb using the idle speed adjustment screw on the left carb. And I agree with WER, sync should be done with gauges. The dead cylinder method can and is sometimes used for syncing idle speeds on the earlier carb sets that don't have a linkage, but it takes a good (some might say "trained") ear for best success.

 

[pamcopete]

Yes, there are a number of methods and opinions. I start by setting the mixture screws to their nominal setting from the book regardless of how the engine then sounds because the object of carb syncing is to set the throttle plates so that both cylinders inhale the same amount of mixture, it has nothing to do with the idle mixture screw setting. People spend untold hours screwing with the idle mixture screws when what they should be doing is leave the mixture screws alone and sync the throttle plates. Of course, all of the above is only true if the carbs are clean. Trying to sync dirty carbs is like taking a bath with your socks on.

You start by killing the right side and adjust the left side for 1,000 RPM. Then, you kill the left side and adjust the right for 1,000 RPM using that center screw between the carbs. Then with both cylinders working, readjust the left side, which also adjusts the right side an equal amount, to 1200 RPM.

As a final touch, if you have dual exhausts, is to adjust the center screw no more than 1/8 turn one way or the other until both exhausts sound the same. I then finish by connecting the car vacuum barbs together for that really smooth idle.

If you have vacuum barbs, then the best way to kill a cylinder is to remove the hose or plug from the barb which makes the mixture so lean it will not work at idle.

Gages are fine, I suppose, but I only use them on my four cylinder bikes because I have no choice, but for the XS650, as long as you have dual exhausts, you will get best results using your ear.

Lately I have found that on the four cylinder bikes, a very careful bench sync works best, believe it or not because it allows you to precisely position the throttle plate between the two sets of idle holes. I then do a vacuum gage check after the bench check just looking for large differences. The other factor that favors this approach is that the gages themselves are not synced that well. They may be within 5% of each other, but that can produce a 10% difference if you insist on setting the throttles exactly by the gage reading.

 

[weekendrider]

Here is a post from 5twins that is alittle more detailed.
But it was "hidden" 6 threads below yours

Yes, you've pretty much got it there. I'll just clarify and expand upon a few things. First off, whenever talking about your carbs, you should state the year and type of carbs you're working with. I can pretty safely deduce from your mention of the mix screw location that you have BS34s, but many others wouldn't know this. OK, on to the critique .....

Step 1 - When I bench sync, I don't use a feeler gauge or spacer under the butterfly plate. Instead, I simply observe the cracks of light around the plate, mainly at the top and bottom. Holding the carb set up to a good light and looking in at the plates from the engine side, I back the idle speed screw on the left carb out until it doesn't touch the cable arm at all. If in sync, both plates will be completely closed and show no light (or very little). If one is open and the other closed, the carbs are out of sync. Adjust the sync screw on the linkage between the carbs until both plates are closed. Now turn the idle speed screw back in until it touches the cable arm, then give it 1 to 2 more turns in. That will crack both plates open a bit and give an initial idle speed for 1st start-up. Once you start the bike, the idle can be dialed in better, to 1200 once warm.

Step 2 - If you've done a proper bench sync, the carb sync will be close so you can do an idle speed AND mixture screw setting now.

Step 3 - Yes, now do the sync with gauges. If the idle speed changes due to your sync adjustments, re-set it.

Step 4 - Yes, re-visit mix screw settings and tweak. If this results in a higher idle speed on either carb, you may need to tweak the idle speed adjustment and the sync setting slightly.

As you can see, fine tuning isn't a "cut and dry", one step at a time procedure. You will often need to re-visit some of the earlier steps and tweak things. That's the "fine" part of "fine tuning".

And to answer your two questions, adjusting the idle speed screw will not effect sync. It will simply raise or lower the idle speed of both carbs together because they are connected through the linkage. The idle speed adjustment screw moves the whole linkage so it effects both carbs. The sync screw only acts on the right carb to allow matching it's speed to the left.

Adjusting the mix screw can effect the idle speed and the plug color. If you're off too far one way or the other from what we call the "sweet spot" (the ideal setting), the idle will be rougher and slower. The plug color can also go darker or lighter depending on if you're set too rich or too lean. You should try to achieve fastest, smoothest idle when setting the screws.

 

[Take Warning15]

I read that thread but that post wasn't there until after I posted. Didn't want to hijack his thread.

Can I install vacuum barbs if I do not have them? I think there is a plug in that spot instead of a barb.

 

[weekendrider]

If you have the 79 carbs the left carb should have a threaded plug under the mix screw opposite the choke.
And the right a thread plug on the right choke plate.
BUT sometimes when working on these people get in a hurry and put the left choke plate on the right carb or vice-versa and you end up with both access plugs on the left carb.

 

[EvenmoreXS]

Yes, there are a number of methods and opinions. I start by setting the mixture screws to their nominal setting from the book regardless of how the engine then sounds because the object of carb syncing is to set the throttle plates so that both cylinders inhale the same amount of mixture, it has nothing to do with the idle mixture screw setting. People spend untold hours screwing with the idle mixture screws when what they should be doing is leave the mixture screws alone and sync the throttle plates. Of course, all of the above is only true if the carbs are clean. Trying to sync dirty carbs is like taking a bath with your socks on.

You start by killing the right side and adjust the left side for 1,000 RPM. Then, you kill the left side and adjust the right for 1,000 RPM using that center screw between the carbs. Then with both cylinders working, readjust the left side, which also adjusts the right side an equal amount, to 1200 RPM.

As a final touch, if you have dual exhausts, is to adjust the center screw no more than 1/8 turn one way or the other until both exhausts sound the same. I then finish by connecting the car vacuum barbs together for that really smooth idle.

If you have vacuum barbs, then the best way to kill a cylinder is to remove the hose or plug from the barb which makes the mixture so lean it will not work at idle.

Gages are fine, I suppose, but I only use them on my four cylinder bikes because I have no choice, but for the XS650, as long as you have dual exhausts, you will get best results using your ear.

Lately I have found that on the four cylinder bikes, a very careful bench sync works best, believe it or not because it allows you to precisely position the throttle plate between the two sets of idle holes. I then do a vacuum gage check after the bench check just looking for large differences. The other factor that favors this approach is that the gages themselves are not synced that well. They may be within 5% of each other, but that can produce a 10% difference if you insist on setting the throttles exactly by the gage reading.

"I then finish by connecting the car vacuum barbs together for that really smooth idle."

So instead of using blind plugs, run a hose from one barb to the other. Right???

 

[pamcopete]

EvenMoreXS,

Yes, if you have the barbs. If you also have vacuum operated petcock(s) then use a tee to keep them working and select the prime position when the vacuum barbs are disconnected. Just keep in mind that you have to remove the cross connect when syncing because both carbs will be sharing the vacuum.

 

[EvenmoreXS]

EvenMoreXS,

Yes, if you have the barbs. If you also have vacuum operated petcock(s) then use a tee to keep them working and select the prime position when the vacuum barbs are disconnected. Just keep in mind that you have to remove the cross connect when syncing because both carbs will be sharing the vacuum.

I have the barbs and manual petcock. Once tuned and syncd I'll try that. Thanks. New mikuni jets should come in today as well as washers and orings for my mix screws. PK-92 Uni pods should be in early next week.

The home stretch. Or rather the long narrow straight away with another project at the end. But I should be idlin and revvn like a champ very soon.

Thanks