Carb tune and sync

[ddebonis]

Hello.

My '84 owners manual doesn't say anything about tuning the carbs and clymer says take it to the dealer.

I want to believe that it's safe for me to do this job at home. my friend has a TwinMax (and a R75/5), but we weren't sure where to plug it into the carbs.

I'm guessing it goes where the tube marked "A" in the picture below goes. Is that right? (And what is that tube and what does it do?)

Bonus question: What is the metal pipe marked "B"? It comes from the cylinder to the airbox. CO emissions control?

Thanks! Danny

[Bob_Roller]

A, is the vacuum line that goes to the valves in the airbox, with higher vacuum the valves open and allow air through the tubes B into the exhaust valve port of the cylinder head .

To use a manometer, or Twin Max, you pull the rubber hosesoff of the carbs and put your tester hoses on there .

Just remember to put the hoses back on, or the engine will run poorly .

[ddebonis]

Thanks.

Owners manual says close idle mixture screw and open 3/4 turn.
Adjust throttle butterfly screw til it touches lever. Turn clockwise 1/2 turn.
Start engine.
Adjust idle speed 950+/-150 by turning both butterfly screws.
Adjust both idle mixture screws simultaneously to maximize idle speed. Then turn clockwise not more than 1/4 turn.

I guess I'm confused about when/why I adjust the mixture versus the throttle butterfly screws.

I read NOT to do one side at a time because it's bad for ignition system. I assume it's fine to have both sides firing.

Any tips on this procedure is greatly appreciated!

-Danny

[montmil]

Before attempting any carb adjustments, run the valves and confirm or set valve lash.

Then take the bike out for a long ride to get the engine up to full operating temperatures. Five minutes around the neighborhood won't do it and you'll probably soon be displeased with the carb settings.

I prefer to not have the throttle screws touching the levers until after I'm satisfied with the idle mixture and idle revs. Owners that complain of a high idle and/or their engine idling too fast when at a stop... usually have an idle mixture issue created by; 1) tuning a not properly warmed engine, or 2) throttle screws interfering with the mixture setting.

Set up a big fan to blow on the engine while tuning. Avoid spending too much tweakin' time thus overheating that air-cooled Boxer.

[nhmaf]

+1 what Monte said.   You can check your air mixture screws first by marking with a sharpie pen where the screw slot is and gently turning it in - don't overtighten after it stops as you'll damage the seal.    IF it seemed to be somewhere between 1/2 turn and 3/4 turn of the screw, then set it back to where it was and verify the same on the other side.   Just make sure that they are both the same amount to start with, and somewhere in that range.   Then, just leave them along until you get the idle screws adjusted and the engine seems to idle relatively consistently at the ~1000-1100 RPM.    When you are happy with that - then you can go back and try to "fine tune" the air mixture screws, counting any adjustments from your baseline by the 'clicks' or width of a screwdriver blade of rotation +/- from where you started from.   You may find that the engine will idle a little smoother, or rougher, as you go in one particular direction or another - and again, just make small adjustments, and on each side - eventually you will develop an 'ear' for it.

After this, then it comes down to synchronizing the cables so you get even pulling/vacuum from both sides and check with your twinmax that the vacuum levels are close to balanced out - I generally check at just above idle and at 4000 RPM - they won't be perfectly matched at both points, but you want them to be close as you can manage, and I generally err for allowing a bit more deviation at idle for smoother running at road speed.

[Luca]

I read NOT to do one side at a time because it's bad for ignition system. I assume it's fine to have both sides firing.

Yes, most of us tune our carburetors with the engine running on both cylinders.  Just go from side to side making fine adjustments while checking your manometer (such as a twinmax), and be sure to blip the throttle between adjustments.

It's bad for the ignition system if a plug lead is not grounded.  The juice needs a place to go.  With some adapters/extensions plugs you can short out the plugs via a screwdriver between adapter and cylinder, thus safely running the bike on one cylinder at a time (but you don't need to worry about this with the twinmax)

[tvrla]

The instructions they gave you would confuse me too.

There are essentially three steps in adjusting the carbs:
1. Set idle mixture
2. Set idle speed - synchronize
3. Adjust cable pull so both open the same amount

1. Ensure there's slack in the cables so they can't hang the idle high.

Adjust the mixture screw slowly out and in and leave it where you've got the highest idle. If the idle goes very high you won't get a proper setting so drop it back down a bit and adjust the mix some more. You want the idle to stay in the normal range they gave you - roughly 800-1000. I don't like my idle any higher than that.

I don't know how you could adjust both sides at the same time - I've never done it that way, much less heard of that method before.

2. Adjust the idle speed screws to balance both sides according to the twinmax.

3. Adjust the cables to remove most of the slack, and so they show they're balanced on the twinmax at off idle or about 2K.

That's about all there is to it.

[Barry]

The only things I would add to what's already been said is that I have never felt the need to use a cooling fan as it only takes a few minutes (I guess it depends on the local climate though) and my personal preference for the idle mixture setting is to start from the base setting and turn in until the idle slows and then back out again only far enough to restore the smooth idle. I believe this give a slightly weaker setting than peak idle speed. Actually "weaker" gives the wrong impression. Idle mixtures are always relatively rich so it's better to say it gives a less rich setting. This mixture setting impacts significantly above idle at small throttle openings so it affects low speed running and fuel efficiency as well.

The advice about doing valve clearances first is important as making changes will affect the idle speed and mixture settings more than you might think.

[montmil]

The only things I would add to what's already been said is that I have never felt the need to use a cooling fan as it only takes a few minutes (I guess it depends on the local climate though)...

...and the experience, knowledge base and skill level of the owner.

[jusgus]

Per the Bing instructions, when I finish the cable play, idle speed balance and idle mixture, I go for a test ride and readjust the idle mixture for best transition from idle to mid speed. I do this by removing the plug wires in turn and pulling off from a standing start on one cylinder (I still have points so I don't think it hurts the electrics). Then I recheck the idle speed when I get back home. I only have a homemade manometer so that probably explains the extra steps.  

[Ed Miller]

If you're going to run on one cylinder, you need to ground the opposite spark plug.  Even with points ignition it's bad for your coil if you don't.  The pull-the-spark plug off method originated in magneto times, and apparently it doesn't hurt them.  

Even my owner's manual says to do it, and I have electronic ignition.  Oh well.

[jusgus]

Thanks for that info, I will have to make a ground shunt. I am most appreciative.

[Ed Miller]

I made a grounding tool:  one alligator clip big enough to hold the threaded part of a spark plug, and the other one about the right size to clamp to a cylinder head fin.  They are bolted together.  I just use a spare spark plug to ground the wire, leaving the current one still in the spark plug hole.