CARBY TUNING - by ear and with gauges

Tune by Ear | Tune with Mercury Sticks

Subject: Carby Syncronising by Ear

DATE: 17/10/00
FROM: Theo Bekkers

Zebee Johnstone wrote;

Same here.

But I dunno if I'm using them right. I can't no how get the two columns the same level, one (the RHS) is getting on for twice the length of the other.

This is what I did...

With engine off, I opened and closed the throttle, trying to get to the point where I heard only one click. OK, got to the point of hearing two *really* close, another bit of adjust, can only hear one. I asssume you were adjusting the cable lenght, it is not necessary to do any adjustment to anything before connecting the sticks.

Then I screwed the idle screw all the way in both sides, then 4 turns out. That was to get them out the same amount, It doesn't matter much where the idle screws are when you start.

Next the mixture screws. To full turns out. The mixture screws are important but not relevant to synching the carbs. I note that as the engine note rises, the mercury for the side I'm working on does too. I find the visual cue helps.

But they are no way near each other, should they be? IF so, how do I get them there?

First set the mixture screws to the 'book' setting, then ignore them. Make sure you have some slack in the cables. Adjust the idle screws untill the engine will idle. Next it is important to understand what the mercury is telling you. The higher the mercury, the more vacuum you have. More vacuum means that that carby is open _less_ than the other. Unscrew the idle screw on the side that is low untill the columns match. If idle speed gets too low for the engine to run, screw in the high one. Note that with most carbs you should lift the slides slightly when screwing in the screws to take the load off them. The screws are meant to be a stop, not a lifting mechanism. When they are level adjust both a bit at a time untill you get the idle speed you want. Then open the throttle a bit and adjust the cable (shorten the cable) on the high column untill they are level. This will mean that your cables match and are pulling the slides up at the same time. Recheck idle level. You are done. This whole proceedure takes me about one minute. The mixture screws are a separate adjustment and have no relevance whatsoever to carby synching.

Oh - they both have bubbles, and there seems to be a bit of water or something in there, is that normal?

They shouldn't have any bubbles. You may not have enough mercury in the sticks. I made my own sticks with a bit of tubing and a piece of wood to mount them on. Cost me $15, including the mercury, about 20 years ago. Bring the bike to Perth and I'll show you how it's done. :-)

Cheers Theo

Subject: Carby Syncronising with Mercury Sticks

Date: Mon, 16 Oct 2000
From: Robert Johnson

Zebee wrote;

Can someone recommend a good tutorial on carb syncing or explain same? I have been trying it by ear, and I have a set of mercury carb sticks. BIke is 850T with 32mm pumpers. How do I set the idle screw at the start? Should the columns of mercury be the same length, if so how do I get them there?

This is what I have done:
- opened and closed throttle with ignition off, listening for the clicks. Adjusted cables till I heard them closer and closer together, then till I heard only one. - screwed idle screws all the way in then back out each 4 turns.

- started motor, adjusted idle same amount each side till it idles.
- turned off motor
- screwed out mixture screws 2 turns.
- hooked up carbstix.
- started motor

At this point, one column is much much longer than the other.

- fiddled with short column (LHS) mixture screw, listening for revs and watching column. highest revs and longest column occur when screw is almost all the way in. Less than one turn out I think. Column is closer to other but still they are quite different in length.

- fiddled with long column mixture screw (RHS). it is less sensitive, eventually liked about 1.5 turns out. Column is now much longer than the other one. - adjusted idle evenly both sides.

The bike seemed to be running a bit rough, and eventually I scrwed the RHS idle out a bit more than the LHS and that made it a bit better.

After a chat with a friend, I tried it all again, except instead of setting idle all the way in then out evenly, I screwed them both all > the way out, then screwed the RHS in till the bike idled. Then screwed the LHS in till the cylinder seemed to cut in, then screwed > the right out till they seemed to vaguely match. Very vague though!

But the columns are still quite different lengths, I thought the idea was to geet them the same or close to? What affects that? Seems only the mixture screw does, but if I adjust both to highest revs (which leads to longest column) they are very differnt lengths.

Zebee

Zebee, you probably have a Haynes manual. If you do go to the section on carburettor idle speed and synchronisation. (page 90 in mine). It tells you to set your mixture screws to the manufacturers specs then set the idle speed at 1000 to 1200 rpm.

This is so the engine will run on one cylinder. You then disconnect one spark plug and adjust the mixture screw on the opposite side carburettor until you get the fastest tick over possible on one cylinder. Then reverse the process and do the opposite sides.

After this, hook up your carb sticks and adjust the idle screws so you get both tubes reading the same. After this don't touch the carburettors again! Do all the rest of your adjustments with the throttle cable adjusters, or, if you have flat top carbs, the cable adjusters above the flat tops. Adjust these so the carb sticks both read almost the same at your most used engine speed,say, 4000rpm.

If your engines a bit worn you probably wont be able to get the sticks reading the same right through the rev range so its best to get the carbs balanced at your most frequently used engine speed.

Be careful of revving the engine above 5,000 rpm and shutting the throttle as you can suck mercury into your engine. If you do this take it for a ride to get rid of it. Under no circumstances let it get on your skin. Also remember to take the little stopper out of the top of the mercury container or you may get a false reading. I've also found it a good idea to swap the carb sticks to the opposite carbs upon completion of the balancing job to make sure they still read the same.If they read different this will let you know if there are air leaks in the sticks or tubes.

Rob Johnson