I've bought myself a nice 'carbtune' kit and set out to balance the carbs using the technical forum description.
I balanced them nicely at tickover, then adjusted the mixture. All seems well. Took the bike for a ride... seems to go OK. Then tried to balance them through the throttle opening stages and I'm really confused!
Where the instructions say the metal tube should rise at the same rate, when I open the throttle the tubes go in opposite directions! one rise whilst the other one drops! I changes over the plastic pipes to the carbtune to see if it was the pipes.. it wasnt.
I am really confused, how can the vacuum increase in one carb whilst decreasing in the other?
Thanks, stuart.
Carburettor balancing challenges
Hi Stuart,
I've recently done the same job - I think if you refer to the carb balancing section (point 11, maybe) under tech tips on this website you'll get a better description of what you're trying to achieve.
Before adjusting the nuts at the top of the carbs the columns will most likely go in opposite directions but it should be possible to alter this by turning the nut at the top of the carb just a little bit.
Have a read at it and see if you manage to sort it out.
I found it helpful - though I have to say I didn't really manage to hear the engine getting faster and slower and 'stumbling', as described in the earlier steps. (Mixture screw adjustment).
Nick
I've recently done the same job - I think if you refer to the carb balancing section (point 11, maybe) under tech tips on this website you'll get a better description of what you're trying to achieve.
Before adjusting the nuts at the top of the carbs the columns will most likely go in opposite directions but it should be possible to alter this by turning the nut at the top of the carb just a little bit.
Have a read at it and see if you manage to sort it out.
I found it helpful - though I have to say I didn't really manage to hear the engine getting faster and slower and 'stumbling', as described in the earlier steps. (Mixture screw adjustment).
Nick
The vacuum is based on two things. How far the slide is open and how fast the engine is turning over.I am really confused, how can the vacuum increase in one carb whilst decreasing in the other?
If your carbs are out of balance, then one slide is lifted first. The vaccum drops in that carb because the opening is now bigger. It then rises again because the engine is running faster. On the other carb the slide hasn't moved, but the engine is running faster and so the vacuum increases.
I find the best procudure is to adjust the throttle stops until the engine is idling reliably and then balance the throttle cables first (at about 3000-4000rpm). This is actually the more important adjustment as it affects the way the engine picks up. Then adjust the throttle stops, then the mixture. You may well have to do the last two stages a couple of times as they interact. If you have a balance tube between the carbs, then the idle speed rises when you put it back on, so take that into account when setting the idle speed, or turn both idle screws counter-clockwise through the same angle to reduce the idle speed.
The Morgan 'Carbtune' is a good quick instrument, but the collumns tend to vary a bit, so it's worth checking by using a 'T' piece and connecting them to the same vacuum source.
A 'U' tube manometer connected between the carbs is far more sensitive and accurate, but ideally needs to be mounted on a wall. Checking for gross imbalance with a carbtune first will stop you sucking the fluid into the engine!
I don't like the throttle cables with inline adjusters because they introduce extra joints that give inconsistent cable tension. The Dart with it's splitter box is prone to this and I used to find it difficult to keep them as well synchronised as I would have liked. You can reach the carb top adjusters under the tank without too much difficulty. An ideal way of doing the job is a 'Drip stand' providing the fuel so you can take the tank off.
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
Paul has pretty well described the process, but I think we need to clarify a couple of points.
I think that we all understand that by turning the throttle on the handlebar, we raise some cylinders in the carburettors, and that then increases the flow of air/fuel mixture.
When the throttle is closed, the cylinder sits near the bottom of the carb. The idle screw has a tapered end, and the bottom of the cylinder sits on this taper. By adjusting the idle screw in or out, we can therefore adjust exactly how close to the bottom of the carb the cylinder sits. We are effectively changing the bottoming out position of the cylinder. By adjusting both carb's idle screws, we obtain a balance WHEN THE THROTTLE IS CLOSED. The idea here is to obtain the exact same gap between cylinder and carb bottom in both carbs. The actual magnitude of that gap affects the idle speed, but we are initially only after getting the balance right. Once the balance has been obtained, the actual idle speed can then be adjusted by turning both idle screws by the same amount and in the same direction.
As the throttle is opened, the cylinders are raised because they are connected by the throttle cables. There are two separate cables, one for each cylinder, and each cable will have a different amount of slackness. This slackness is affected by the length of the cables, and by their routing through the bikes frame and around other components. We make adjustments to this slackness using the adjusters where the cables enter the top of the carburettor. The idea here is to balance the slackness in the cables so that the cylinders are lifted in a synchronised manner.
The difference here is that when the throttle is closed, the cables are left slightly slack, so the idle screw provides a mechanically adjusted height (bottom) position. If there was no slackness left, we could not guarantee that the cylinders will sit on the idle screw. When the throttle is opened, the synchronisation of both cylinders lifting is dependant on the variation in slackness between the cables. So by balancing the slackness in the cables, we ensure that the cylinders move at the same time.
This got a bit wordier than I originally thought! Hopefully it is clear enough and will help explain why you can have a good balance at idle, but then a poor balance when the throttle is opened.
I think that we all understand that by turning the throttle on the handlebar, we raise some cylinders in the carburettors, and that then increases the flow of air/fuel mixture.
When the throttle is closed, the cylinder sits near the bottom of the carb. The idle screw has a tapered end, and the bottom of the cylinder sits on this taper. By adjusting the idle screw in or out, we can therefore adjust exactly how close to the bottom of the carb the cylinder sits. We are effectively changing the bottoming out position of the cylinder. By adjusting both carb's idle screws, we obtain a balance WHEN THE THROTTLE IS CLOSED. The idea here is to obtain the exact same gap between cylinder and carb bottom in both carbs. The actual magnitude of that gap affects the idle speed, but we are initially only after getting the balance right. Once the balance has been obtained, the actual idle speed can then be adjusted by turning both idle screws by the same amount and in the same direction.
As the throttle is opened, the cylinders are raised because they are connected by the throttle cables. There are two separate cables, one for each cylinder, and each cable will have a different amount of slackness. This slackness is affected by the length of the cables, and by their routing through the bikes frame and around other components. We make adjustments to this slackness using the adjusters where the cables enter the top of the carburettor. The idea here is to balance the slackness in the cables so that the cylinders are lifted in a synchronised manner.
The difference here is that when the throttle is closed, the cables are left slightly slack, so the idle screw provides a mechanically adjusted height (bottom) position. If there was no slackness left, we could not guarantee that the cylinders will sit on the idle screw. When the throttle is opened, the synchronisation of both cylinders lifting is dependant on the variation in slackness between the cables. So by balancing the slackness in the cables, we ensure that the cylinders move at the same time.
This got a bit wordier than I originally thought! Hopefully it is clear enough and will help explain why you can have a good balance at idle, but then a poor balance when the throttle is opened.
Nick - 1979 500 Strada