Single Mikuni VM 36 top cap leak ?

motohubsf said:

Hello everyone
Just acquired my first Norton which has been sitting a few year and it has the single Mikuni Carb setup. I got the bike started and it is generally in very good shape so no need for intake gaskets etc as its a recent build. MY question is about the top cap the Mikuni VM36 as the cap is stripped where the throttle goes in via the 45 degree cable guide.

I would imagine this would create a small air leak even with a good slide to carb bore and the cap does have a rubber seal underneath to seal to carb body, photos attached . I can see this being an issue with CV carbs but can't wrap my head around this round slide CV on whether I should change the cap as I am having some intermittent problems with coughing and running correctly ( but am also dealing with a 30 year old Boyer I am replacing soon as it has intermittent fault ).
Any input on specifics of the cap seal appreciated in terms of air leaks. I did my best searching online and didn't come up with much , suppose I could Mikuni , I will eventually chance the cap but had to ask out of curiosity.

Hopefully the google photos links work ... thanks in advance

https://photos.app.goo.gl/MhTWYBvmumYUSRbT6

New item by Dmitry Yev

photos.app.goo.gl

New item by Dmitry Yev

photos.app.goo.gl

Click to expand...

motohubsf said:

thanks for reply fellas. Ill update when I get this carb soaked and cleaned and replace the rotor which has week magnets .

Click to expand...

baz said:

With a Boyer make sure your battery is tip top charged up especially if a MK3 Boyer
Make sure every connection is perfect
Also the ignition switch and cut out if you have one
As a test you can run a power feed directly from the battery to the igniter to find problems

Click to expand...

DennisMo said:

I have a VM34 and it has a rubber seal as in link below see part number 1.
I would get a new cap is it were my carb

https://mikunioz.com/vm-round-slide-carbs/vm34-carb-parts/?v=5435c69ed3bc

Click to expand...

motohubsf said:

helpful . thanks

Click to expand...

SteveA said:

If you want to eliminate the cable fitting as the source of your problem, put some sealer around the threads. Sure, if the sealer you have is silicon based, fuel might eventually degrade the sealant, but there isn't much fuel at the top of the carb, and it won't degrade before you have understood if it is your problem.

Of course, if you have some fuel resistance sealer to hand like Permatex, use that.

I would replace the top, but in slower time, just to get a more stable cable adjustment.

But, I don't think it will help the poor running much, because I would suspect your Boyer is the source of your problems, in particular low battery voltage and the pick up plate connections are a regular source of (intermittent) ignition issues

Click to expand...