Walbro carb question

Does it matter whether the pulse port is up or down?

I have a 25cc Homelite blower motor with a Walbro carb. It has a primer bulb "on top" and the fuel pump cover is facing down. So the pulse port in the carb body lines up with the port in the heat dam.
That part between the carb and cylinder - I've heard it referred to as "isolator", "insulator", "heat dam", "manifold", "block thingy", etc... My Homelite manual call it heat dam.
Everything works fine, and in this position the choke lever and mixture screws face out (away from the motor shaft), away from the mounting plate and tire.

Now I'm doing another build with a 28cc McCulloch from weed wacker. It has a different Walbro carb. It has the fuel pump cover facing up (primer bulb is external; not attached to carb). The choke lever faces out, but the mixture screws are in, towards the drive shaft. When this is bolted to a mounting plate and put on a bike, the carb screws will be very hard to get at.
So I would like to flip the carb over (rotate 180). The heat dam can't be flipped because the port in the heat dam has to line up with the port in the cylinder. But the heat dam has a semi-circular groove running from the pulse port. I'm assuming this allows the carb to be installed with the pump or diaphram either up or down. Is this correct?

I know that a diaphram-type carb can operate "upside-down", like in an RC airplane flying upside-down. That's all I can find when trying to google my question.

The attached image of the heat dam is blurry, but you can see the port on the bottom and the circular groove. Can the carb be put on with the diaphram side up or down?

Shouldn´t be any trouble. A Walbro carb is used a lot in chainsaws, and their internal "pumping action" does that the carb works also upside down. main thing with this carb, is to be sure its airtight,without any airleaks, if it could suck just a tiny bit of air from other places than trough the carb, you got failure! And remember to use a gasoline filter before it, they easily get clogged without!
In fact at my Jonsered saw engine, it sits with the pulse port down, but seen on a Husquarna saw, its up! Good Luck!

As Leaded said the carb could care less how its mounted as long as it gets the pulse.

I found this on YouTube on Walbro Diaphragm Carb

http://www.youtube.com/watch?v=zUuVQfvWSnI

Another video better explanation below a more detailed one with actual carb taken apart explanation:

Actually a two part video I found on Youtube I posted on your thread

http://motorbicycling.com/showthread.php?t=37460&page=2

The two part video..... seems that the primer bulb on these carb really don't prime but rather take bubbles out of the fuel in the carb and lines and so there is a route of tube putting the air bubble (froth) back into fuel tank. Also quite unique the crankcase power and compression stroke make pressure on the bottom of piston and also vacuum to run two things in carb. 1st the fuel pump with pressure and 2nd the diaphragm with vacuum for additional draw of gas to chamber to be available for draw of fuel in venturi of carb when you start and run engine.

I think this video is really quite good but if a professional could use more light and better camera it would be astounding.... check it out if this stuff interests you

http://www.youtube.com/watch?v=5_K4Hx518AA


http://www.youtube.com/watch?v=CC3DnuQD00o&feature=related

MT

As others have said......yes the carb can be installed either way as long as the pulse from the crankcase pumps the carb (which it should using that heat dam).

Thanks for the response guys.

I assumed that the groove in that heat dam was specifically for allowing the carb to be mounted up or down. But you know what they say about assuming.

Should be getting a new ring in the mail from Frank Bowman soon. I'll let you guys know how that ring and the new build comes along.

You will notice a big difference with the Bowman ring. Have run them in the Ryobi R/C conversions. Compression increase is dramatic. Some how the makes them to run with virtually no gap.