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'79 550 carb settings

Steve

Steve

GS Whisperer
I am working on a friend's '79 550E (with the VM carbs). Have ended up dipping the carbs, so they are entirely stripped. Somebody else had worked on the bike before I got the chance to work on it, so I don't know what starting points are for all the jets.

The bike is all stock, airbox and pipes. Besides the sync adjustment at the top, the only two adjustments I can see are the air bleed screw (?) near the intake throat and the idle mixture (?) on the bottom. (I am assuming that's what these adjustments are.) The air bleed screws were all about 1 1/2 turns out, but the idle screws were all about 4 turns out. That does not sound right to me, so I thought I would ask for opinions here.

The last carb is in the dip tonight, so I would like to start putting them together tomorrow night, and maybe get it running by the next night.


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If the carbs were worked on before, I suggest checking the float levels for correct adjustment. If the o-rings were hardening, I'd replace them too with a kit from Robert Barr, a member here. I always suggest replacing the manifold o-rings whenever the carbs are out too. Inspect the manifolds too.
Be sure the sharp tipped pilot fuel screws (underneath) are never tightened down, just seated lightly when counting/adjusting them. Stock bikes generally have them set from 3/4 to 1 1/4 turns, so I'd try 1 full turn out for starters. The side air screws are always adjusted using the highest rpm method which you can find with a quick search. 1 3/4 turns out is a good initial setting. Use a good fitting tool because the screws can be stiff and the heads stripped easily.
Be sure to bench synch the slides. Again, search under VM bench synch and I'm sure you'll find detailed help if needed. The bench synch needs to be followed by a vacuum tool synch though.
Your stock main jets are #80. The stock pilot jets are #15. I'm not sure what the factory position for your jet needle e-clips are...either position two or three from the top. This is VERY important if there's a chance they've been moved.
Sounds like you're in a hurry to get the carbs back together but I still think the above should be done.
 
Oh, and just to add: the pilot fuel screws (underneath) regulate a second small passage. These screws assist the pilot jet and fine tune the amount of fuel in the mixture.
The side air screws regulate air flow.
There should be an idle adjuster knob, either under or above the carbs for adjusting idle speed. Correct idle speed for your model is about 1,000 rpm's. This is also the rpm that you use as a base setting when doing the highest rpm check.
 
Thanks, Keith.

I have done the CV carbs on my two '82 bikes (650 and 850), so have a decent feel for them, but this is the first set of VM carbs across my bench.

All the screws look good, have been removed from the body for dipping, and screw in and out easily. \\:D/
I do know that a previous mechanic replaced the intake o-rings about a year or so ago, so they should not be a problem. I will take a chance with them at this time. 8-[
My biggest question about his work is how far he went into the carbs. Considering the previous screw settings, I'm not sure he did anything but 'tweak'.

Got them re-assembled tonight, will tweak the screws to your suggested settings and do the bench sync tomorrow night. Might be Wednesday before getting them on the bike.


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OK, in a nutshell, I rode the bike over to the friend's house.

Set the pilot screws (underneath) to 1 1/2 turns out. Never did change them. Set the air bleed screws (upper side) to 2 turns. Played with them after bike warmed up, left them at 2.

Did the bench sync using some repair wire (baling wire?) for my gauge. One of the carbs was a bit open, compared to the rest, so I closed it down a bit. After getting the carbs attached to the bike and filled with fuel, bike started on the second push of the starter button. \\:D/ The owner was right there and said it had NEVER started so easily. :cool: I reminded him that it was only going to get better after the vacuum sync. Checing the mercury columns, they were amazingly close. I have never done a bench sync before, I think I will be doing them routinely if the carbs ever come off again. After full warm-up, we adjusted sync and played with jetting again. Left jets as they were. Again, have not played with the mixture screws under the carbs.

Test drive went well. Before I started working on the carbs, I had trouble coaxing the bike to 45mph. Tonight, I had it over 65 before I realized it was still accelerating well, and I had better cut it out and go home. 8-[

Another successful project. :cool:

Now, if he stores it properly, he will actually be able to register it and RIDE IT next spring!

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Glad the bike is running better. Makes you wonder why someone adjusted the pilot fuel screws to 4 turns out on a stock bike, but funnier things are done.
 
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