Carb Synch giving me fits

[BillyRok]

My morgan carbtune came yesterday so synching carbs was the goal tonight. My XJ has stock jets, exhaust, and air box. Also has YICS eliminator tool installed. Carbtune has the restrictors in the 4 tubes about 4" from the carbs.

Bike starts easily, warmed it up with fan blowing on it, and set idle at 1100 with screw below carbs. I was able to synch 1 with 2 but then their levels on the carbtune would change a bit while I was synching 3 and 4. Kept going side to side and what ever pair I synched would move while I worked on the other pair. The idle would keep changing also. And if I blipped the throttle, the rpm's would slowly come back to around 1100 but it took a few seconds to get there. My mixture screws started at 2.5 turns out (have new o-rings in there so I have some difficulty "feeling" when they are lightly seated), but turning them in or out as much as 2 full turns had little effect on the rods in the carbtune as I try to see the vacuum increase. I guess I thought I would see big changes on the carbtune as I turned screws, but the biggest thing that changes seems to be the idle speed...am I doing something wrong?

How do I keep the idle speed constant? And what is the adjustment on the throttle up near the grip used for? How much so I have to turn the mixture screws to see a vacuum increase? Is it normal for the levels on the carbtune to keep changing or should they stay pretty steady once I stop adjusting?

Thanks in advance for the tips and guidance.

[DGXSER]

Sounds like you have a vacuum leak somewhere. Try spraying around the carbs boots to the engine with some starter fluid, or carb cleaner. Or you can use propane, unlit of course.

Also, I am almost certain you want the restrictors near the carbtune. Not that it should make that big of a difference though.

[natemoen]

Also, I am almost certain you want the restrictors near the carbtune. Not that it should make that big of a difference though.

The instructions do say to have them near the carb end.

I agree on a vacum leak as well.

[mlew]

I read your post several times and it seems like you are turning the mixture screws to try to sync the carbs. They are for setting the idle mixture after the carbs are synced. The sync adjusting screws are between each carb.

Your valves must be checked and adjusted before a sucessful carb sync can be done. If the valves are out of spec it will be almost impossible to get them synced.

[skids]

I read your post several times and it seems like you are turning the mixture screws to try to sync the carbs. They are for setting the idle mixture after the carbs are synced. The sync adjusting screws are between each carb.

Good catch. I think you should synch vacuum before you finally adjust the pilot screws (for mixture) because the pilot screws don't really affect the synch much. The synch definately affects the mixtures near idle rpms! It can be a little tough holding an idle while you adjust the vacuum synch, especially if the mixture screws are not close to being right. Make sure there are no leaks at the boots and make sure that the vacuum hose for the ignition advance is hooked up. Use the main idle adjustment knob to try and keep the idle around 900 to 1100 rpms while you synch 4, 2, and 1. The adjustment for the throttle at the grip is to make sure you have just a little bit of play in the cable for the grip. After you synch, you should make sure the mixtures are OK. I like to color a set of plugs for that and check the sizzle of the headpipes. That is the way I do it...hope this helps.

[IanDMacDonald]

The instructions do say to have them near the carb end.

+1 on that

[DAVINCI]

Here's a proven good method:

#1 set mixture for each carb at the highest possible vacuum for that carb. One at a time. Then move to the next carb, adjust it's mixture for the highest, and so on.

#2 NOW, do the synch.

After the synch, go back to step one and fine tune the mixture for each carb one at a time. Again, you're looking for the highest possible vacuum. At this point, you'll probably have to lower the idle with the BIG MAIN SCREW

repeat synch.

Should be smooth as silk then.

[BillyRok]

Thanks for the replies, sorry if my first post was a little unclear. I was trying to synch by turning the 3 screws between the carb intake boots, then adjusting the 4 mixture screws inside the towers on the front, top side of the carbs. Based on reading other posts I was also thinking vacuum leak but the boots are new and clamps are tight. Then I discovered the vacuum line that is supposed to run from the vacuum sender to the #2 carb body is missing...guess that would be my vacuum leak huh?!

I'll try again later (synch, adjust mixture screws for highest vacuum, re-synch) and let you know how it goes.

[BillyRok]

Davinci pulled the trigger before I did! So it's mixture screws, synch, reset idle, mixture screws, and re-synch. Hopefully hooking up the ignition vacuum advance line will mean I'll see noticeable changes on the carbtune as I adjust the synch and mixture screws.

[IanDMacDonald]

The method DAVINCI mentioned is a little easier said than done. Using a carb tune to do this method may not show much vacuum increase on each carb adjusting your mixture screws (in my experience). I do know it works for many, and sounds good. You might be better off simply syncing them, and pulling plugs over a day of riding.

[GLoweVA]

I've been hearing about adjusting the idle mix screws to get the most vac, and while it works for some, it has not been easy for me either.

Maybe it's a result of the year of the carbs/set up.
Hypothesis:
early carbs are set up a little richer than the later carbs and thus maybe the later carbs idle mix screws (which were tamper sealed at the factory BTW) have a little different taper and such with them and so then maybe adjusting them (on the later carbs) just doesn't provide that much of a noticable change in vac.

So, test my theory out, those that have done it Davinci's way, chime in and see if you have early (78/79) carbs or later (80/81) ones.

[IanDMacDonald]

The first time I heard about this was in LoHo's thread "Tuning by ear". I believe Motoman was the first to mention it, with a few others chiming in. Not sure LoHo had much luck with it either.

[DAVINCI]

If your carbtune or carb sticks or makeshift ATF tubes or dial gauges are not showing a change in vacuum when you adjust the mixture screw, then something is wrong. EVERY carb regardless of year should react to this adjustment unless:

1. you have vacuum leaks (fix 'em)

2. You are not doing the procedure at IDLE (1100 RPM and below)

3. your idle circuits are gummed up. (you know what THAT means)

[GLoweVA]

If your carbtune or carb sticks or makeshift ATF tubes or dial gauges are not showing a change in vacuum when you adjust the mixture screw, then something is wrong. EVERY carb regardless of year should react to this adjustment unless:

1. you have vacuum leaks (fix 'em)

2. You are not doing the procedure at IDLE (1100 RPM and below)

3. your idle circuits are gummed up. (you know what THAT means)

Okay, okay...
I'll give it another try next time I break out my sync rig.

[skids]

Then I discovered the vacuum line that is supposed to run from the vacuum sender to the #2 carb body is missing...guess that would be my vacuum leak huh?!

That would be critical!