Thoughts on Mikuni carburetors.

Ariel Red Hunter · 07-14-2016, 03:08 PM

I do not want to oversell the idea of changing a Hawk to a Mikuni carb. The Keihan is a very good carb in it's own right. The problem with Keihans has always been the difficulty of getting jets, needles, and slides. And the advantage of the Mikuni carb is how easy it is to get almost anything you need for the little darlings. A 26VM Mikuni has a 26mm choke(venturi), and 30mm opening where it bolts on to the cylinder head. You should not have to change the needle, but be forewarned that the needle jet is a wear item. This is not hard to visualise when you realize this engine is inhaling up to 2000 times a minute. When the engine inhales, the needle is dragging on one side (engine side) of the needle jet. Eventually, this wears the needle jet, and it will over fuel in the midrange. The needle it self almost never wears. If you ride a lot, maybe you should think about changing to a new needle jet after a year or two. Once you have settled on the main jet, the needle normally has the clip in the middle slot. Raising the needle richens the midrange, dropping the needle leans it. The pilot jet controls from idle to 1/4 throttle. If you ride much off road, especially in the eastern United States, you will want the engine to idle as slow as possible and still run. Coming down a steep trail with the engine idling too fast is not fun. If neccessary, switch the engine off with the shut-off on the right handlebar, and come down in second gear. Personally, I always set my carburetor so that the engine will not run, or even think about running, when I release the twist-grip. But most of you will want the engine to idle because you are used to cars that always idle. We are all comfortable with what we are used to. Now when you have jetted the carburetor correctly, the first movement of the twist grip will be applied as you roll out near the bottom of the hill, and the engine will fire immediatly. This is why you want to have the right pilot jet in there.

2LZ · 07-14-2016, 03:19 PM

In the olden days of CB's, all I did to both Hensims (db150's) and the X-moto (GY200) was raise the needle a slot on the PZ carb. It was still lean but definitely an improvement. I did eventually find a couple jets (from a neighbors shoebox) for the X-moto and it really helped a ton.

Now? With the lack of easily accessible jetting for the PZ and most importantly, everything coming to CA sealed up now like on my Rhino, the Mikuni was a no-brainer. Even at "full price" (35.00?), it's super cheap and jets are plentiful. Even comparing the build quality to the stock PZ, the Mikuni (or fake Mikuni?) was obviously machined better in the throat and slide bores. Just a better overall carb, IMHO.....and I can blame Weldangrind for pointing me that direction.

pcspecialist · 07-14-2016, 05:24 PM

Mikuni Large Round main jets (N100.604) work great in the Keihan, idle jets are another story, though, the stock one works fine all the way down to sea level (idle mixture turned out just shy of 2 turns).

This is the main I used: http://www.ebay.com/itm/152156398660

I shimmed up the needle with three brass #4 washer, though, you can get an adjustable needle for it here: http://www.ebay.com/itm/291807976434

Weldangrind · 07-15-2016, 11:36 AM

Quote:

Originally Posted by 2LZ

....and I can blame Weldangrind for pointing me that direction.

I accept that. I first learned about them from Gio, because they had Mikuni carbs installed on their CG bikes from the factory. As well, that's all they stocked for spares back then.

Ariel Red Hunter · 07-16-2016, 01:53 PM

Hopefully, by now, everybody knows that generally speaking, these carburetors operate on the pilot jet circuit from idle to 1/4 throttle, the needle and needle jet from 1/4 to 3/4 throttle, and the main jet from 3/4 to WFO. The little phrase "generally speaking" is because it is not quite that simple. The pilot jet contibutes from idle on up all the way. At one quarter throttle, the needle controls the fuel mixture from the main jet, which is added to what is still being contributed by the pilot. At 3/4's throttle the main jet, now un-encumbered by the needle, controls fuel feed the rest of the way. The Amac carburetor, a two lever device came out about 1912. About 10-12 years later they hooked up with Binks Carburetor Co. By the late 1920's the AMAL (Amalgalated Carburettors, Ltd.) combined the two, and bingo, we have the predecesser of the Mikuni.

07-14-2016, 03:08 PM	#1
Ariel Red Hunter Join Date: Jun 2016 Location: akwesasne, NY-13655 Posts: 2,220	Thoughts on Mikuni carburetors. I do not want to oversell the idea of changing a Hawk to a Mikuni carb. The Keihan is a very good carb in it's own right. The problem with Keihans has always been the difficulty of getting jets, needles, and slides. And the advantage of the Mikuni carb is how easy it is to get almost anything you need for the little darlings. A 26VM Mikuni has a 26mm choke(venturi), and 30mm opening where it bolts on to the cylinder head. You should not have to change the needle, but be forewarned that the needle jet is a wear item. This is not hard to visualise when you realize this engine is inhaling up to 2000 times a minute. When the engine inhales, the needle is dragging on one side (engine side) of the needle jet. Eventually, this wears the needle jet, and it will over fuel in the midrange. The needle it self almost never wears. If you ride a lot, maybe you should think about changing to a new needle jet after a year or two. Once you have settled on the main jet, the needle normally has the clip in the middle slot. Raising the needle richens the midrange, dropping the needle leans it. The pilot jet controls from idle to 1/4 throttle. If you ride much off road, especially in the eastern United States, you will want the engine to idle as slow as possible and still run. Coming down a steep trail with the engine idling too fast is not fun. If neccessary, switch the engine off with the shut-off on the right handlebar, and come down in second gear. Personally, I always set my carburetor so that the engine will not run, or even think about running, when I release the twist-grip. But most of you will want the engine to idle because you are used to cars that always idle. We are all comfortable with what we are used to. Now when you have jetted the carburetor correctly, the first movement of the twist grip will be applied as you roll out near the bottom of the hill, and the engine will fire immediatly. This is why you want to have the right pilot jet in there.
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07-14-2016, 03:19 PM	#2
2LZ Join Date: Mar 2010 Location: Volcano, Ca Posts: 7,086	In the olden days of CB's, all I did to both Hensims (db150's) and the X-moto (GY200) was raise the needle a slot on the PZ carb. It was still lean but definitely an improvement. I did eventually find a couple jets (from a neighbors shoebox) for the X-moto and it really helped a ton. Now? With the lack of easily accessible jetting for the PZ and most importantly, everything coming to CA sealed up now like on my Rhino, the Mikuni was a no-brainer. Even at "full price" (35.00?), it's super cheap and jets are plentiful. Even comparing the build quality to the stock PZ, the Mikuni (or fake Mikuni?) was obviously machined better in the throat and slide bores. Just a better overall carb, IMHO.....and I can blame Weldangrind for pointing me that direction. __________________ "Light a fire for a man, and you heat him for a day. Light a man on fire, and you heat him for the rest of his life." 2007 Suzuki DRZ400S (SM convert) 2009 Q Link XP 200 1967 BSA B25 250cc Starfire 2022 Royal Enfield Interceptor 650 2023 Royal Enfield Scram 411 1948 Royal Enfield Model G 350
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07-16-2016, 01:53 PM	#5
Ariel Red Hunter Join Date: Jun 2016 Location: akwesasne, NY-13655 Posts: 2,220	The Mikuni Hopefully, by now, everybody knows that generally speaking, these carburetors operate on the pilot jet circuit from idle to 1/4 throttle, the needle and needle jet from 1/4 to 3/4 throttle, and the main jet from 3/4 to WFO. The little phrase "generally speaking" is because it is not quite that simple. The pilot jet contibutes from idle on up all the way. At one quarter throttle, the needle controls the fuel mixture from the main jet, which is added to what is still being contributed by the pilot. At 3/4's throttle the main jet, now un-encumbered by the needle, controls fuel feed the rest of the way. The Amac carburetor, a two lever device came out about 1912. About 10-12 years later they hooked up with Binks Carburetor Co. By the late 1920's the AMAL (Amalgalated Carburettors, Ltd.) combined the two, and bingo, we have the predecesser of the Mikuni.
	0

07-14-2016, 05:24 PM	#3
pcspecialist Join Date: May 2016 Posts: 583	Mikuni Large Round main jets (N100.604) work great in the Keihan, idle jets are another story, though, the stock one works fine all the way down to sea level (idle mixture turned out just shy of 2 turns). This is the main I used: http://www.ebay.com/itm/152156398660 I shimmed up the needle with three brass #4 washer, though, you can get an adjustable needle for it here: http://www.ebay.com/itm/291807976434
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