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ET: Engine (Twin) Mikuni carbs

Glenliman

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VOC Member
Two front head & 34 mm Mikuni VMs. There are two tees, one just barely visible in the photoIMG_20200527_144140.jpg
 
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brian gains

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VOC Member
vibrac: it would be interesting to know if you find any appreciable muck in the carb'. In conversation this week I was told that the floats 'stick' on their vertical guide pins after being left dry due to vibration causing notches on the pins which stop the float rising with the level of the fuel.
Anything is worth considering but have yet to pull mine apart to inspect. At the moment I'm always at the ready to give the float chamber a tap with the handle of a screwdriver.
 

Robert Watson

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John McDougall took every 34 mm Mikuni he put on a bike, which was more than a few, and took a drill to the guide holes as he experienced several that would have a float stick on the guide pins.
 

MSVH Y3

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VOC Member
I did the same using numbered drills. Then I carried the next size of drill with me. One carb float had to be resized 3 times.
 
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vibrac

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vibrac: it would be interesting to know if you find any appreciable muck in the carb'. In conversation this week I was told that the floats 'stick' on their vertical guide pins after being left dry due to vibration causing notches on the pins which stop the float rising with the level of the fuel.
Anything is worth considering but have yet to pull mine apart to inspect. At the moment I'm always at the ready to give the float chamber a tap with the handle of a screwdriver.
Thanks Brian I will look today the carb (not old) has been empty all winter had one run before a strip for primary leak and then on next start up the float or needle stuck. Tank has always been clean and I empty it in the autumn and squirt some WD 40 in over winter. Stand by for photos I have a new float needle somewhere.
As an aside I have a pre monoblock carb on my KSS velo on its third float chamber and miriad float needles and reseating attempts it still floods at random times on tickeling I blame the so called petrol.
 
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erik

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VOC Member
Hello Vincent ! On your bike the two carbs are connected with a bypass! Is there any advantage to do this? Regards Erik
 

vibrac

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Thanks Brian I will look today the cartb (not old) has been empty all winter had one run before a strip for primary leak and then on next start up the float or needle stuck. Tank has always been clean and I empty it in the autumn and squirt some WD 40 in over winter. Stand by for photos I have a new float needle somewhere.
As an aside I have a pre monoblock carb on my KSS velo on its third float chamber and miriad float needles and reseating attempts it still floods at random times on tickeling I blame the so called petrol.
Ok after breakfast I whipped off the Mikuni and opened up the float bowl, first problem is that you have to upend the carb to easily undo the 4 cross heads so there is no way you can leave the float & needle undisturbed
the carb was the cleanest thing in my workshop not a spot of any dirt. I removed the float tab and inspected the needle under a glass no ridges or blemishes so the reason for flooding remains a mystery.
I will add a filter however, who knows I may be forced to go into a supermarket filling station one day...
 

vibrac

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VOC Member
Back together with filter added.
Now its sat for 10 mins taps on, no leaks , I give up!
NB see Where the starter sits next to the Alton and F106?
1590661333111.jpeg
 

vibrac

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VOC Member
No from the short brass outlet in the 'middle' (I think)
 

greg brillus

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Had a customers Comet with a 600 top end kit on it.........we installed a 36 mm Mikuni and after there seemed to be fuel dripping from the clear overflow drain lines......I think the owner ended up lowering the float level to fix it. But I think if the carby hangs slightly lower at the rear, this may cause the problem, as I don't think they like that angle too much. Didn't they originally come factory on Yamaha XT/TT 500's.
 

Nigel Spaxman

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VOC Member
Heres two pictures of my bike with two front heads and VM34 mm carbs. I put the battery under the swing arm. There is a tee fitting on the right hand petcock and a tee behind the front carburettor air cleaner. The only filters are the screens in the tank. I have had no problems in 12,000 miles. My floats don't stick for some reason. I have two other bikes with Mikunis and they sometimes stick after being taken out of hibernation. A tap on the side of the bowl usually solves that. The front carb has a special top with a brass pulley to turn the cable 135 degrees quickly with no friction. There isn't much room under the tank.
 

vibrac

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VOC Member
I did note my float needle tip was solid brass I thought they were viton or some sort of rubber tip nowadays?

added at 17.55 Oh Dear its flooding again. So what is meant by 'drilling out the guide holes' in post #23 does it mean the floats or the lever holes? its not clear
 
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Robert Watson

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The floats ride up and down on two small brass pins with holes through the floats, these can have burrs inside them and that is where one needs to clean up the hole.
 

Comet Rider

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The floats ride up and down on two small brass pins with holes through the floats, these can have burrs inside them and that is where one needs to clean up the hole.
Hi Robert,
On my 34mm on the Comet the holes in the floats had a small brass tubes/sleeves in them, but on the 28's on the Rap just as moulded

Neil
 

Chris Launders

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VOC Member
You've got me wondering now, I'm sure my 34s floats pivot on a bar as I've had one that was nipped by the posts after a winter lay up, I shall have to dig the bike out and have a look.
 

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