v.m carb problem

[raistian77]

whiterabbit, not knowing the history of your carbs/motor there can be things wrong that you'd never imagine (damage by a previous owner or parts mixed up/wrong parts installed). So if you get odd vacuum readings or an air screw adjustment appears to have no effect then it could be so many things.
If your pilot fuel screws (underneath) appear in good condition...
First, be sure BOTH tiny pilot circuit holes in the bottom of the carb throat are clear. There's one hole right above the pilot jet itself and then the other hole for the pilot fuel screw to regulate. Be sure both are clear by spraying cleaner or air through them. Look/feel for uniform flow so you know they are 100% clear.
Install them any order you want. Don't worry about how much of the tip sticks out in each carb bore. Simply seat each screw lightly/uniformly and back it out to 3/4 turn. 3/4 is a ballpark figure. If the bike is stock then this is a good starting point. They are set at the factory using emissions equipment and there's no way to set them accurately until you test.
Set the side air screws to 1 1/2 turns initially. On a typical bike this will be good for starting purposes. Once fully warmed up, you can adjust them by using the highest rpm method. They should respond as designed to. Most bikes with VM carbs in good condition will have their air screws between 1 1/4 and 1 3/4 turns out when tuning is finished.
Be sure you have #15 pilot jets.
Also, you should replace the manifold o-rings now. I know you said they look good and maybe they are but you must be sure. They commonly are over torqued and flattened too much and their service life shortened. Replace them and give them a coat of high temp bearing grease to help them last. Replace the stupid Phillips screws (if you still use them) with Allens and torque to about 8 or 9 ft/lb is enough. Inspect the manifolds too. If you have any vacuum leaks you'll never tune it correctly.
Just to mention, before vacuum synching you should be sure the valve clearances and ignition timing are correct.
As for the vacuum synch, I think you mentioned earlier that it was done recently? But you don't know if it was done correctly so it should be done. If for some reason you disturb the jet needles/throttle shaft then you must re-synch each time.
Since the pilot circuit controls mixture at minimal throttle openings and has effect up to 1/4 throttle, the best road testing is to ride around on level or gradual uphill roads at about 1/5 or less throttle. You want the least overlap effect from the cut-away or jet needle. It's all about throttle position and not actually speed/rpm's but just cruising around in 4th/5th gear at lower speeds (40 and under) will be fine for testing. After full warm up, I'd go somewhere that you can hold a steady speed for several minutes and then chop off and check the plugs. The pilot fuel screws are sensitive to adjustment and just a 1/8 turn can matter. Do what the plugs/performance say to. Don't worry too much if the final tuning results in the screws not being perfectly uniform in their adjustment. They are for fine tuning EACH CYLINDER and because of small differences in each cylinder this is normal. Besides plug color, look for a good/steady idle with reasonable warm up time and no obvious signs such as a puff of dark exhaust when you lightly blip the throttle off of idling several seconds.

Got a question Keith. In my Mikuni carb manual they state not to do a vacumn sync. It states that with the tops off, it creates a vacumn leak that throws off the sync. I do vac syncs because I think it is much more accurate than mechanical. Have you came accross this?

 

[whiterabbitt]

WOW! thanks keith that is the kind of info I was looking for in the beginning, very helpful info.

as for the morgan carbtune, I wish I could afford to spend that kind of money; $100+ and then spend another $50-$60 on the adapter, plus s+h

thats just my real problem atm not enough money, gf is still ****ed at the money I have already spent. otherwise yeah I'd buy a carbtune and a shim kit and a dyna s and a 4-1 and gsxr frontend and an 850 kit and.......
.....and pay someone who knows what the hell they are doing to put it all together!!

as it is I am contemplating using A vacuum gauge and homemade adapter to try and sync it, probly take forever, but thats where I am at.

 

[KEITH KRAUSE]

Got a question Keith. In my Mikuni carb manual they state not to do a vacumn sync. It states that with the tops off, it creates a vacumn leak that throws off the sync. I do vac syncs because I think it is much more accurate than mechanical. Have you came accross this?

Your Mikuni manual isn't talking about VM carbs. You can only access the slide adjusters on VM's by removing the top cap. The factory manual shows the synch being done the only way it can be done...with the caps off.
As for accuracy, synching with a vacuum tool is the most accurate. To me, the bench synch serves only to get the slides fairly close and that makes the final vacuum tool synch much quicker/easier. If careful, you can get them pretty close manually but there's no substitute for the vacuum tool synch.

 

[Big T]

as for the morgan carbtune, I wish I could afford to spend that kind of money; $100+ and then spend another $50-$60 on the adapter, plus s+hQUOTE]

I don't know where you got this info, but if you look on the carbtune site, it's $95 shipped airmail with all the adapters.

I do recommend the case, however

Using a single guage is hopeless, you need to compare all 4 at once, as changing one often changes the other 3.

 

[raistian77]

Your Mikuni manual isn't talking about VM carbs. You can only access the slide adjusters on VM's by removing the top cap. The factory manual shows the synch being done the only way it can be done...with the caps off.
As for accuracy, synching with a vacuum tool is the most accurate. To me, the bench synch serves only to get the slides fairly close and that makes the final vacuum tool synch much quicker/easier. If careful, you can get them pretty close manually but there's no substitute for the vacuum tool synch.

Actually it is talking about VM carbs, just the smoothbore carbs.

Quoted from manual
Mikuni recommends that all smoothbore type carburetors be synchronized mechanically rather than with vacuum gauges or mercury manometer.

Sync adjustments on smoothbore type carburetors are underneath the top cover of the carburetor, which when removed exposes a large vacuum leak to the cylinder being adjusted, resulting in inconsistent vacuum levels from one carb to the next.

Carburetor circuitry is throttle position sensitive. If the slides are not in exact mechanical sync, then one carburetor may be operating on a different circuit than the others.
Quote end

For the record my shop manual states to vacuum sync my 79 GS850 so thats what I do. Just curious why the bike manufacture states one thing and the carb manufacture states another.

 

[BassCliff]

Basscliff, there's an error in the VM carb rebuild section. I can't remember Paul Musser's username if he even still comes around here. Maybe he could make the change if he was contacted. He asked me to check the whole section out before he printed it and I don't know how it was missed.
In step 37, it mentions vacuum synching. It says to use the IDLE ADJUSTER knob to raise the rpm's to 2,500 to 3,500 rpm's or approximately. That is of course incorrect and would cause major headaches if someone follows that procedure.
You simply begin the synch with the bike idling at 1,000 rpm's or what the factory calls for. Then, I suggest synching at approx' 3,000 rpm's by using the throttle.

Thank you Mr. KEITH KRAUS. I'll make a note of this on my website.


Thank you for your indulgence,

BassCliff

 

[KEITH KRAUSE]

Actually it is talking about VM carbs, just the smoothbore carbs.

Quoted from manual
Mikuni recommends that all smoothbore type carburetors be synchronized mechanically rather than with vacuum gauges or mercury manometer.

Sync adjustments on smoothbore type carburetors are underneath the top cover of the carburetor, which when removed exposes a large vacuum leak to the cylinder being adjusted, resulting in inconsistent vacuum levels from one carb to the next.

Carburetor circuitry is throttle position sensitive. If the slides are not in exact mechanical sync, then one carburetor may be operating on a different circuit than the others.
Quote end

For the record my shop manual states to vacuum sync my 79 GS850 so thats what I do. Just curious why the bike manufacture states one thing and the carb manufacture states another.

Well, if Mikuni themselves say the smoothbores should be synched mechanically then so be it. I don't know why significant leaking would occur in the smoothies and not the VM's but they're certainly not the same exact design so I wouldn't argue.
I was giving advice based on what our GS models are equipped with...standard VM's or CV's. And the Suzuki factory manual does show removing the tops when synching VM's.
It has me scratching my head though. I can't see how the smoothies would leak any more than the standard VM's. When the standard VM's are properly synched, you can SEE all 4 levels at the SAME time. So I don't know how a proper vacuum tool synch on the smoothies would result in the inconsistent levels the Mikuni manual is concerned about.

 

[tejasmud]

After doing the vacuum sync on my VM29s with the "tops off," and then checking the sync with the "tops on" I observed no difference. The thought had entered my mind at the time as well if there would be a difference in levels with the "tops off" vs. "tops on," which is the reason I checked.

Lucky? Possibly, but just check them in both conditions to eliminate any concern.

When they are out of sync they will not run very well as with any carbs. Anytime the needle position is changed and the levers are removed from the rod it will more than likely change the sync of the carbs.

E