Carburation problem - mikuni 30mm

Frog · 03-27-2012, 10:40 AM

Hi,

I have still not quite sorted the carburation of the mikuni 30mm fitted to my 200cc GY5 (pushrod).

The problem now is an intermittent misfire during prolonged high speed running. The bike runs along fine and then will 'stumble' for half a second and then pick-up again. Feels like fuel starvation...

Exactly the same with two different CDIs and plug cap is a new NGK. Plug is only 2000km old and is an iridium one.

Jets are 20 slow running (tried a 25 but too rich) and 110 main. Plug looks a little rich (checked after riding for a while rather than a high speed plug chop)- so dropped the needle a couple of groves (now on middle slot)

I'm thinking float height could be the problem - the fuel level is about 4-5mm below the carb body to float bowl joint.

Anyone any thoughts? :?:

Other than this the bike is running really well. :?

Cheers

Frog

gloud · 03-27-2012, 01:20 PM

Check fuel flow (petcock) and tank vent . you can adjust the floats too.

Frog · 03-27-2012, 01:50 PM

Thanks for the thoughts Gloud.

No problem with the vent or tap. I think it may be the float but I do not know the correct float height or fuel level... :(

Frog

FastDoc · 03-27-2012, 01:52 PM

http://www.dansmc.com/carbs.htm

There's a section on setting the float in this article.

Weldangrind · 03-27-2012, 02:33 PM

I've installed a few of the 30mm Mikunis with good success. I simply remove the float bowl, invert the carb on the workbench and sight the parting line of the plastic floats. I bend the tang until the parting line is parallel with the bowl mounting surface.

When you say you dropped the needle a couple of grooves, does that mean that you moved the clip from the bottom position to the middle? If so, perhaps the main jet isn't being completely uncovered at nearly WOT. Try moving one groove down.

Frog · 03-29-2012, 05:38 AM

Many thanks Fastdoc and Weldandgrind.

I followed Weld's advice - raised the floats until the seam on the plastic casting was level with the bottom of the carb with the carb fully inverted. In the event this amounted to raising the fuel level the couple of mm's I thought was needed and produced a result that matched the advice in the link Fastdoc pointed me to - i.e. a fuel level just below the carb to float bowl joint....

This has cured the problem! It may have been noted before perhaps but Weld does indeed know stuff

His method is a good one for these carbs.

Of course I now need to clean up the bottom end of the range again but I think that will be just slow running screw adjustment.

So thanks both and to gloud for confirming my thoughts.

Frog

Weldangrind · 03-29-2012, 03:01 PM

Great news!

Do you plan to move the clip again?

SpudRider · 03-30-2012, 01:18 AM

I'm glad you got your carburetor fixed, Frog!

Indeed, Weldangrind does know his stuff.

Spud

Frog · 03-31-2012, 06:35 AM

Quote:

Originally Posted by Weldangrind

Great news!

Do you plan to move the clip again?

Hi

I think I will leave it where it is - the carburation is good and the plug is a good colour - pale chocolate brown. When I refit my less restrictive exhaust I will prob raise it again and do a plug chop to see where I am.

Thanks again for the advice.

Frog

Weldangrind · 03-31-2012, 01:30 PM

You're welcome.

BTW, if you fit a less restrictive exhaust, you'll want to lower the clip, not raise it.

03-27-2012, 10:40 AM	#1
Frog Join Date: May 2008 Location: Hampshire England Posts: 191	Carburation problem - mikuni 30mm Hi, I have still not quite sorted the carburation of the mikuni 30mm fitted to my 200cc GY5 (pushrod). The problem now is an intermittent misfire during prolonged high speed running. The bike runs along fine and then will 'stumble' for half a second and then pick-up again. Feels like fuel starvation... Exactly the same with two different CDIs and plug cap is a new NGK. Plug is only 2000km old and is an iridium one. Jets are 20 slow running (tried a 25 but too rich) and 110 main. Plug looks a little rich (checked after riding for a while rather than a high speed plug chop)- so dropped the needle a couple of groves (now on middle slot) I'm thinking float height could be the problem - the fuel level is about 4-5mm below the carb body to float bowl joint. Anyone any thoughts? :?: Other than this the bike is running really well. :? Cheers Frog
	0

03-27-2012, 01:52 PM	#4
FastDoc Join Date: Mar 2009 Location: Southeastern Washington desert Posts: 14,761	http://www.dansmc.com/carbs.htm There's a section on setting the float in this article. __________________ Happy to serve.
	0

03-27-2012, 02:33 PM	#5
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	I've installed a few of the 30mm Mikunis with good success. I simply remove the float bowl, invert the carb on the workbench and sight the parting line of the plastic floats. I bend the tang until the parting line is parallel with the bowl mounting surface. When you say you dropped the needle a couple of grooves, does that mean that you moved the clip from the bottom position to the middle? If so, perhaps the main jet isn't being completely uncovered at nearly WOT. Try moving one groove down. __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
	0

03-29-2012, 03:01 PM	#7
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	Great news! Do you plan to move the clip again? __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
	0

03-30-2012, 01:18 AM	#8
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	I'm glad you got your carburetor fixed, Frog! Indeed, Weldangrind does know his stuff. Spud __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894
	0

03-27-2012, 01:20 PM	#2
gloud Join Date: Sep 2011 Location: Westhampton Mass USA Posts: 155	Check fuel flow (petcock) and tank vent . you can adjust the floats too.
	0

03-27-2012, 01:50 PM	#3
Frog Join Date: May 2008 Location: Hampshire England Posts: 191	Thanks for the thoughts Gloud. No problem with the vent or tap. I think it may be the float but I do not know the correct float height or fuel level... :( Frog
	0

03-29-2012, 05:38 AM	#6
Frog Join Date: May 2008 Location: Hampshire England Posts: 191	Many thanks Fastdoc and Weldandgrind. I followed Weld's advice - raised the floats until the seam on the plastic casting was level with the bottom of the carb with the carb fully inverted. In the event this amounted to raising the fuel level the couple of mm's I thought was needed and produced a result that matched the advice in the link Fastdoc pointed me to - i.e. a fuel level just below the carb to float bowl joint.... This has cured the problem! It may have been noted before perhaps but Weld does indeed know stuff His method is a good one for these carbs. Of course I now need to clean up the bottom end of the range again but I think that will be just slow running screw adjustment. So thanks both and to gloud for confirming my thoughts. Frog
	0

03-31-2012, 01:30 PM	#10
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	You're welcome. BTW, if you fit a less restrictive exhaust, you'll want to lower the clip, not raise it. __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
	0