Mikuni VM 33 Carbs

Injected wrote:

I just did my initial start up on #25 pilots but its way too rich, going to drop them down to #17.5's. That air box I am running must be way more restrictive than I thought.

I am running stock air screws with the hole thru them so the setting on those would be a maximum of 3/4 turns out.

I will set my needles and mains with my portable O2 sensor and AFR gauge. Plugs chops are fun but there is always some jackass in the way!

You may be right about the air box. Was thinking it might be. I bought new air screws from jetsRus along with other things. Did you notice the jet holes in the bottoms? Wonder why they did that? I'll be watching what you come up with. Out here there's not much traffic in the mornings so I should be able to do them. 

SWest wrote:

Injected wrote:

I just did my initial start up on #25 pilots but its way too rich, going to drop them down to #17.5's. That air box I am running must be way more restrictive than I thought.

I am running stock air screws with the hole thru them so the setting on those would be a maximum of 3/4 turns out.

I will set my needles and mains with my portable O2 sensor and AFR gauge. Plugs chops are fun but there is always some jackass in the way!

You may be right about the air box. Was thinking it might be. I bought new air screws from jetsRus along with other things. Did you notice the jet holes in the bottoms? Wonder why they did that? I'll be watching what you come up with. Out here there's not much traffic in the mornings so I should be able to do them. 

 

Do you mean a hole in the air screw tip? I only saw the cross hole thru the body.
 

Yes there's one in each tip.

SWest wrote:

Yes there's one in each tip.
 

No I guess I missed that, will take one out and give it a closer look.

Took out the VM33 air screws today and had a closer look at the tip under a 4X loop, saw the tiny thru hole on the end coming out into the larger cross hole.

Since I missed this feature I put a .015" guitar string thru the tip hole and it just about fit perfect, maybe about .001 to .002" clearance.

Took the rest of the screws out to make sure the string would pass thru them all just to be sure. I did clean them in a small ultrasonic tank, but never passed a string thru them as far as I can remember.

My question is why did they use the holes in the first place. Miller soldered my old ones up and reshaped them to be like other air screws. 
Steve

SWest post=914351 userid=43940My question is why did they use the holes in the first place. Miller soldered my old ones up and reshaped them to be like other air screws. 

Steve

It may be a example of Bernoulli's principle, speeding up air flow by restricting a orifice. They are connecting two larger orifices with a smaller one, the cross hole controls how much air gets choked off (throttle) 

It is possible that these race carbs needed some help in order to let the engine idle smoothly. The tiny tip hole would help suck some more air thru as race motors usually don't create a ton of vacuum at idle. It is basically a extra fixed air jet.

This is just a theory on my part.

I also suspect idle RPM will need to be higher than a regular VM carb set.

I just picked up an estate sale A5 with 33mm smoothbores. It is an old canyon racer with a 1135 kit. 

Cleaned the carbs and now I need to figure out float height setting.

Is that covered on one of the 33 pages in this thread or online somewhere?

I use the clear tube method but here's the static method. 

www.jetsrus.com/carburetor/carb_mikuni_V...oded_view_parts.html

With the wet method you want to be 3-4mm below the gasket surface. You can buy a fancy threaded adapter but I used a beer bottle rubber stopper you can buy at homebrew store for $1 and a 1/4" plastic elbow.