Adjusting for highest vacuum???

[79XS11F]

Carb #1,#2 & #4 are to be matched to #3 because #3 has no sync adjustment screw. It goes without saying that all other engine adjustments need to be correct if the sync process is to be correct. I always sync when the engine is HOT so I do it after a 15 to 20 min ride. Never had an issue doing it this way as per the factory manual. It is normal for idle to increase and need to be adjusted down as the carbs are synced closer and closer to one another.
Rob

[3Phase]

The part that got me was that quote #2 seemed to be talking about synching, starting with adjusting #3 which has no adjustment, but that's been explained, what was really being said is they adjusted the #3 idle mix using the vacuum method, then adjusted the synch between carbs as normal.

You have to set carb #3 for highest vacuum first with the main idle adjustment screw when you start to synch the carbs. That will ensure that its throttle butterfly is almost closed and pulling fuel only from the idle port. The other carbs are adjusted after #3 has been set.

There is another hole further down the carburetor throat on the engine side of the carburetor after the idle port and the throttle butterfly. It's a transition port, just like in a regular carburetor, and it provides the fuel for off-idle and low speed running.

If the throttle butterfly is open too far at idle then the idle port is uncovered and so is the leading edge of the transition port. You can only adjust idle mixture with the idle port.

The transition port is a fixed size and will flow fuel as it's uncovered. The only adjustment for the transition port is the size of the pilot jet in the float bowl and the pilot air jet in the carburetor inlet bell. It is otherwise unmetered.

You can get the bike to run with some of the butterflies too far open but it won't be right. Set #3 carb for highest vacuum first, then #4, #1, #2, then synch the #1/#2 pair with the #3/#4 pair.

After you've got the mechanical part of the synch done, drop the idle down to ~900 RPM and try adjusting the idle mixture screws by ear, again, beginning with #3 carb. Only adjust one mixture screw at a time. Set the mixture screws at 3 turns out to start and turn them in 1/4 turn at a time. Wait for the idle to settle and listen for the RPM to drop. Back the mixture screw out 1/4 to 1/2 turn from that point, check the idle speed, then move on to the next carb.

You will probably have to go through the mechanical synch and idle mixture setting procedure a couple of times to get it right but it'll start, idle and run nicely after your finished.

If you can't get the bike to synch and idle at ~900 RPM to set the mixture screws there are other issues that need to be corrected.

Good luck, and make sure you have a fan blowing air over the engine while you work!


Regards,

Scott

[Yahman]

You have to set carb #3 for highest vacuum first with the main idle adjustment screw when you start to synch the carbs. That will ensure that its throttle butterfly is almost closed and pulling fuel only from the idle port. The other carbs are adjusted after #3 has been set.

I think what you are saying is, you have to set carb #3 for highest vacuum first with the fuel mixture adjustment screw, and the rpm's at the proper setting by having the throttle butterfly almost closed with the idle adjustment screw.

Other than that it sounded about perfect to me.

Larry

[XJOK2PLAY]

Thanks for a very good job of clearifying exactly what I was trying to say....
LOL.... guess it was a classic case of disconnect between brain and typing fingers...

That's on the money, Scott. I concur, and that's what we did... right down to the fan blowing on the front of the bike part.

Thanks for spelling it out clearer!

[3Phase]

I think what you are saying is, you have to set carb #3 for highest vacuum first with the fuel mixture adjustment screw, and the rpm's at the proper setting by having the throttle butterfly almost closed with the idle adjustment screw.

I haven't tried setting the mixture on my bike with a vacuum gauge, just the mechanical balance. I'll have to try it next time I synch the bike. Setting for peak vacuum on regular carburetors made the idle too rich or too lean, depending on the type of idle circuit, so I'll see how it goes on my bike.

Other than that it sounded about perfect to me.

Thanks! I have to buy or make another set of gauges before I try vacuum setting the mixture screws. The last ones got crushed in my buddy's garage and spilled mercury all over the floor. That was fun to clean up... I used GarthXS' new MotionPro last fall, I think, to set the mechanical synch but it's been over a year since I adjusted the mixture screws. It's time to dig in and do it again so I'll try using vacuum this time and see what happens.

Thanks for spelling it out clearer!

You're welcome!

I didn't use a fan the first time I set the carbs on my bike. Of course it got too hot so it was adjusted wrong for normal idle/operating temperature and I had to do it over again with fan cooling -- no one to blame but myself for that one!


Regards,

Scott

[Yahman]

I haven't tried setting the mixture on my bike with a vacuum gauge, just the mechanical balance. I'll have to try it next time I synch the bike. Setting for peak vacuum on regular carburetors made the idle too rich or too lean, depending on the type of idle circuit, so I'll see how it goes on my bike.

Regards,

Scott

You still need to confirm if it's set rich or lean with a plug check, then tweak the mixture screws as necessary.

Larry

[3Phase]

You still need to confirm if it's set rich or lean with a plug check, then tweak the mixture screws as necessary.

I know, that's why I've been putting it off!

I'll try to set it before I go off adventuring again at the end of the month and to the rally in June but the mix is pretty close right now. I might be able to eke another MPG or two out of it but its getting ~35 MPG now. It definitely needs to be done before I even think about heading up north for the Oregon/Washington rally. Last year the mileage was atrocious!


Regards,

Scott