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Carb sync question

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pasteurized

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I have a Morgan Carbtune that I will get to use tomorrow for the first time. When "adjusting" the carbs to read close to one another, will I be turning the air screw on the side? Most of the instructions I read tell to adjust the "adjustment screw", and it's confusing me a bit. If I am adjusting the air screw, won't it screw up the air/fuel ratio?

BTW: Stock VM26SS carbs on my 1977 GS750
 
Yes it will. The adjustment screws (3) are on the linkages between each carb as you look down on them. They each consist of a screw and locknut. If you don't have a manual, get one. It'll pay for itself in no time.
 
I believe Guy is thinking of CV carbs.(??)
Your bike has the VM carbs. They have air screws on the side.
Yes, they should be adjusted using the "highest rpm method" before doing the actual synch. Also, be sure the ignition timing is correct and valve clearances are correct. Here's how to adjust for highest rpm:
Place the bike on the centerstand, turn the screws to 1 1/2 turns out as a starting point (if you know the screws are close to 1 1/2 you can skip this part), warm up the bike completely. Set idle to 1,000/1,100 rpm's, not higher.
Starting at any carb, SLOWLY turn the side air screw in either direction and listen for the highest rpm. The "sweet spot" will usually be between 1 1/2 and 2 turns out as a general guide. When the rpm's stop rising, stop turning. Fine tune but you don't have to be perfect. Now re-set the idle to about 1,100 rpm's by using the idle adjuster knob.
Repeat this to the other carbs, one at a time. When finished, the bike should be idling at the starting point, about 1,000/1,100 rpm.
The side air screws on VM carbs are meant to be adjusted. Such as when riding in higher elevations, you may want to turn the screws out an additional 1/4 to 1/2 turn or so to help lean the richer conditions.
 
Re: Carb sync question

pasteurized said:
Most of the instructions I read tell to adjust the "adjustment screw"
Click to expand...

Once you have adjusted the side pilot air screw, like Keith explained, then do the sync. You have to remove the gas tank (You'll need to run longer hoses to the tank, or use something else to get fuel to the engine. I just put the tank on a stand next to the bike and run longer hoses.) Then remove the top cap of each carb by removing the 3 screws that hold the cap on. The adjustment screw for the sync (and locknut) are right under the cap.
 
Thanks guys. I appreciate the input! So what happens though if the bike won't run well enough at 1100 to keep running?
 
pasteurized said:
Thanks guys. I appreciate the input! So what happens though if the bike won't run well enough at 1100 to keep running?
Click to expand...
Well, there are basics that should be done before you get to the synch part.
The factory manual and many carb vacuum tool instructions will tell you the ignition timing/points must be set correctly and valve clearances correct before synching. Also, the side air screws adjusted as mentioned.
In addition, I always make sure the filter(s) are clean, carbs clean/inspected/adjusted, no intake leaks, plugs spark quality good and gapped correctly...
Compression must be within range.
If all this is good, the bike should idle at the factory setting, which is about 1,000 rpm's. If some of this isn't checked, then you take your chances I guess.
Without knowing what's been done and hasn't been done, you may get lucky and the poor idle is because of the air screws not correct and a poor synch.
Set the air screws to 1 1/2 just for starters. These can't be accurately set at higher rpm's. They have a certain range of effectiveness. If the bike will only idle at say, 1,400 or more, you're wasting your time. Set them to 1 1/2 and do the vacuum synch. If you find the initial levels are way off, that may be the cause of the poor idle. Get them just reasonably closer, say within a couple inches of each other, not a final adjustment though. THEN, you should be able to fine tune the air screws and have the bike idling at about 1,100 rpm. Then adjust the vacuum levels closer. By the way, you should have a good manual and I believe the #3 carb is the "master" carb on your model? This means you set the levels to match the master carb initial level.
Just a side note, if the bike only idles at much higher rpm's or the idle rises quite a bit from cold to hot, you most likely have an intake leak. The most comon cause is the o-rings inside the rubber manifolds and sometimes the manifolds themselves. This has to be fixed first.
 
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