Carburettor idle adjustment advice needed

Foggy · 19-09-2024, 05:48 PM

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 · 19-09-2024, 11:43 PM

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

What year is your 600?

dunf · 20-09-2024, 08:58 AM

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 · 21-09-2024, 04:10 PM

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

Foggy · 28-09-2024, 04:59 PM

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 · 28-09-2024, 05:48 PM

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 · 28-09-2024, 06:01 PM

Quote:

Originally Posted by Zimbo

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

Quote:

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 · 28-09-2024, 06:07 PM

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!

Darren69 · 28-09-2024, 10:40 PM

Something like these

https://www.ebay.co.uk/itm/353959718...mis&media=COPY

https://www.ebay.co.uk/itm/387270918...mis&media=COPY

The gauge kit doesn't appear to have enough of the right size conectors from what I can see.

Foggy · 29-09-2024, 12:19 PM

Quote:

Originally Posted by 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!

Ha, that's incredible but makes sense from a scientific point of view.

Foggy · 29-09-2024, 12:24 PM

Quote:

Originally Posted by Darren69

Something like these

https://www.ebay.co.uk/itm/353959718...mis&media=COPY

https://www.ebay.co.uk/itm/387270918...mis&media=COPY

The gauge kit doesn't appear to have enough of the right size conectors from what I can see.

Thanks, I was thinking of buying a gauge kit like this and great tip about the connectors. I assume I need the M5 connector (x2) ? Any particular length, does it matter ?

Dukedesmo · 29-09-2024, 02:58 PM

I like the Morgan Carbtune, works well on all my bikes.

https://www.carbtune.co.uk/carbdtls.html

Also comes with the correct size adapters.

Foggy · 29-09-2024, 03:53 PM

Quote:

Originally Posted by Dukedesmo

I like the Morgan Carbtune, works well on all my bikes.

https://www.carbtune.co.uk/carbdtls.html

Also comes with the correct size adapters.

OK thanks I'll take a look. Morgan was also recommended by some carb expert on YouTube so it's good to have it confirmed. Cheers.

jerry · 29-09-2024, 04:36 PM

I use Morgan carbtune for many years

was chatting with chaps at louigimoto ,, on race bikes they prefer modified Mikuni BDST over keihan FCR unless customer insists

Foggy · 29-09-2024, 09:43 PM

Quote:

Originally Posted by jerry

I use Morgan carbtune for many years

was chatting with chaps at louigimoto ,, on race bikes they prefer modified Mikuni BDST over keihan FCR unless customer insists

Yeah I think I'll go for the Morgan carbtune, not a bad price either. Good to know about the Mikunis, just need to get mine tuned ... can't wait.

19-09-2024, 05:48 PM	#1
Foggy Registered User Join Date: May 2024 Location: Diss Bike: M600 Posts: 32	Carburettor idle adjustment advice needed 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 ?

28-09-2024, 06:07 PM	#8
Nickj Too much time on my hands member Join Date: Jun 2007 Location: Forest Of Dean Bike: S2r Posts: 3,208	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! __________________ "The final measure of any rider's skill is the inverse ratio of his preferred Traveling Speed to the number of bad scars on his body." Song of the sausage creature

28-09-2024, 10:40 PM	#9
Darren69 Transmaniacon MOC Join Date: Sep 2013 Location: Sutton In Ashfield Bike: Multiple Monsters Posts: 6,098	Something like these https://www.ebay.co.uk/itm/353959718...mis&media=COPY https://www.ebay.co.uk/itm/387270918...mis&media=COPY The gauge kit doesn't appear to have enough of the right size conectors from what I can see. __________________ Roast Beef Monster! Termignoni and Bucci - Italian for pipe and slippers! S4 Fogarty, S4R 07T, 748, Series 1 Mirage

29-09-2024, 02:58 PM	#12
Dukedesmo Registered User Join Date: Sep 2010 Location: Leics Bike: M900 Posts: 2,902	I like the Morgan Carbtune, works well on all my bikes. https://www.carbtune.co.uk/carbdtls.html Also comes with the correct size adapters. __________________ M900, 916, LeMans II.

29-09-2024, 04:36 PM	#14
jerry Old Git Join Date: May 2006 Location: Cricklade Bike: Multiple Monsters Posts: 2,913	I use Morgan carbtune for many years was chatting with chaps at louigimoto ,, on race bikes they prefer modified Mikuni BDST over keihan FCR unless customer insists __________________ MONSTERMAN

19-09-2024, 11:43 PM	#2
slob . Join Date: Jul 2003 Location: East London Bike: Multiple Monsters Posts: 9,738	Are you sure you haven’t got any air leaks on the engine side of the carbs? What year is your 600?

20-09-2024, 08:58 AM	#3
dunf Registered User Join Date: Jan 2022 Location: Dunfermline Bike: M1200s Posts: 33	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.

21-09-2024, 04:10 PM	#4
Zimbo Too much time on my hands member Join Date: Apr 2003 Location: Stonehouse, Glos Bike: Multiple Monsters Posts: 2,085	Could you be a bit more specific about which screws exactly you have adjusted?

28-09-2024, 04:59 PM	#5
Foggy Registered User Join Date: May 2024 Location: Diss Bike: M600 Posts: 32	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 ?

28-09-2024, 05:48 PM	#6
Zimbo Too much time on my hands member Join Date: Apr 2003 Location: Stonehouse, Glos Bike: Multiple Monsters Posts: 2,085	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!