Carburettor idle adjustment advice needed

[Foggy]

I am seeking advice on idle speed adjustment for my carburettors on an M600. The bike was refusing to idle even when warm so, in an attempt to address the issue, I adjusted the screws located at the front and behind the carbs to approximately 2.5 turns out from their fully closed positions (after several attempts at other settings). However, this has led to the engine racing at several thousand rpm upon ignition.

In the past on other bikes I have started the engine and done the usual turn the screw clockwise until it gets lumpy then turn the screw anti clockwise until it gets lumpy then turn it back to the midway position of the two. But because of the high revs I am reluctant to leave it running so I switch it off meaning I need to do the adjustment “blind” without the engine running.

Could anybody please guide me on which adjustment screws need to be modified and by how much to obtain a normal tickover speed?

As an aside, is the front adjustment screw (by the air filter) for adjustment of air intake so clockwise makes it richer and is the rear adjustment screw for fuel intake so clockwise makes it leaner ? Or is the front screw just for air/fuel mixture and the rear for idle speed ?

[slob]

Are you sure you haven’t got any air leaks on the engine side of the carbs?

What year is your 600?

[dunf]

It sounds like you've adjusted the slide screws rather than the mixture screws? These push up the slides as they are tightened and if overdone have the same effect as pulling on the throttle cable.

[Zimbo]

Could you be a bit more specific about which screws exactly you have adjusted?

[Foggy]

Thanks guys for your replies and apologies for the delay in replying. I was expecting a notification email but nothing arrived so I viewed the post by chance and noticed your messages.

THe bike is a 1995 M600 with a Mikuni 38 BDST twin carburettor.

Looking at the photo kindly posted by Zimbo I think I can now correctly identify the screws and conclude that I haven't touched the idle mixture screws, but it seems I have been adjusting the idle speed screw and the synchronisation screw.

Is the carb in the photo a Mikuni too ? The reason I ask is because of the 2 screws I have been adjusting ..... one of them I have accessed from the front of the carb (idle speed adjustment I assume) and the other from the back of the carb (synchronisation adjustment ?) .... however in the photo it looks like the synchronisation can be adjusted from the front as well as the idle adjustment.

It sounds like I need to turn the idle speed screw out anti clockwise which hopefully should reduce the high revs upon ignition. Not sure how much though but I guess it's trial and error.

My concern now is whether the carbs are out of sync as a result of me messing about with the syncronisation screw.

Do I need to synchronise these carbs and, if so, what equipment do I need ? I assume a vacuum gauge, any recommendation for an affordable but reliable piece of kit and tips on how to use it ?

[Zimbo]

The carb synchronisation is a fairly easy job once you have the equipment. The cheapest and also most accurate way is to remove the two M5 screws from the inlet manifolds and temporarily replace with M5 hose barbs, some plastic pipe from those hose barbs to a manifold you can make up from cheap parts with a tap on each inlet so they can be independently shut off, and a vacuum gauge that can read the vacuum from either manifold depending on which tap is open and which is closed. It's useful to damp the vacuum gauge with a restriction to prevent the needle from fluctuating. Once set up open the taps to read the vacuum from one carb and then the other, adjust the synchronisation until you get the same vacuum reading from both. Once set, remove the gauges and pipes and replace the M5 blanking screws in the inlet manifolds. Inexpensive carb vacuum gauge kits are available on eBay etc if desired!

[Foggy]

Originally Posted by Zimbo

Could you be a bit more specific about which screws exactly you have adjusted?

Originally Posted by Zimbo

The carb synchronisation is a fairly easy job once you have the equipment. The cheapest and also most accurate way is to remove the two M5 screws from the inlet manifolds and temporarily replace with M5 hose barbs, some plastic pipe from those hose barbs to a manifold you can make up from cheap parts with a tap on each inlet so they can be independently shut off, and a vacuum gauge that can read the vacuum from either manifold depending on which tap is open and which is closed. It's useful to damp the vacuum gauge with a restriction to prevent the needle from fluctuating. Once set up open the taps to read the vacuum from one carb and then the other, adjust the synchronisation until you get the same vacuum reading from both. Once set, remove the gauges and pipes and replace the M5 blanking screws in the inlet manifolds. Inexpensive carb vacuum gauge kits are available on eBay etc if desired!

Thanks for the info, much appreciated. I'll take a look at that when I have corrected the idle speed. Fortunately I took a note of the settings before I started so I'll set the synchronisation back before I check the vacuum settings.

[Nickj]

I use the high science technique, a ball bearing that has a rolling fit into a tube.
Works fine on my S2R even though it's very pulsey, worked on a Yam 600/4 and my little VFR just more involved on a four as you set one carb as a reference then match to that.

My father used to manage balancing twins with a bit of tube or a straw, did look a bit daft with it stuck in his ear though. He was dead chuffed when straws with a bendy section came out!