Does R* can run without fuel pump ? I not, why ?

Yes, and No... There is a great article on the delphi forum about this right now... here is the text, but I'd recommend you join that group too... it's free for a basic membership.
*text snipped from another news group*
http://forums.delphiforums.com/1602/
Hey dude,

Update for all .... I will be finalizing the write up on this ASAP. First off, much thanks to Joel Odo Parker for agreeing to run the "final" set up on his end, I wanted a second tester to make sure of the results.

There's been many questions about "why we can't run pumpless", or "why does the bike seem to cut at WOT" (some, must be the REV limiter)?"

Well it all came down to physics. The basic flow formula for a liquid is flow = square root of (pressure IN minus pressure OUT). The pressure OUT is the bowl pressure, that's pretty constant, and the fuel pumps have a regulated discharge pressure, this would be pressure IN. So the fuel flow rate is limited by the needle valve set and NOT by anything else. The fuel pump can flow way enough, but with a regulated fix discharge pressure(pressure IN to the needle valve), it's the orifice size the the stock needle valve that becomes the restriction at the entrance of the bowl. This where the problem lies.

It does not matter how more flow capacity of a fuel pump you put between the tank and the carburetor bowl, unless you increase the pressure going to the carb (entrance of the needle valve), it won't pass anymore flow. At or near WOT, all the of engines fuel needs comes from the carb bowl. The float tries to maintain the level at a reletavly set height in order for all the passages sucking fuel are well supplied. The stock needle valve set seems to have a MAX flow capacity slightly under the MAX sucking need from the bowl. In other words, the bowl has more being drawn from it than can come in it!

By replacing the needle valve set with a higher flow rated one, with a geniuine Mikuni part, that we finaly found, eliminates this issue. Also, as a BONUS, there is more than enough flow capacity directly through the fuel line, by gravity, to feed the carb. All of you that have ever drain there tanks must have asked themselves the same question; "at the rate this tank is draining, if the engine was swallowing it that fast, I'd run out of fuel in 5~10 minutes!" Right?

Well you were right questioning it. Where the problem lied was the needle valve set would not allow the fuel to enter the bowl fast enough. We have a flow size of 2.0, if you've read the previous posts on this thread, all this was explained.

ANYBODY that has had to "INCREASE" the jetting of there carburetors in order to assure proper fuel ratio mix in the engine, will reach this bowl starving issue earlier/faster. Therefore, it would be a good idea to concider replacing this.

The write up for the Road Star Clinic is almost done, it's in the final stages of review.

Em ...

You go Dave

No problem! Check out my write up on the tech tip pages!

Em ....

PS: I just joined this forum, Hi all!

Welcome to the Clinic Em!

Its a great article, and we are really glad to have it here.

GRAM

First of all thanks go out to Em44, Odo, and Gram for a great write up on going pumpless with the needle valve change out.

The question I have is assuming we are still using a fuel filter (in-line) on this setup, does the stock fuel filter provide enough gravity flow or should we be looking into "higher flow" fuel filters. I will be doing this mod aswell as venting my gas cap in the very near future.

Thanks in advance,

Mike

Mike,

Here is a suggestion/answer to a similar question that was posted on the Delphi forum;

"For the filter, I'm running with the screen mesh from the pingle petcock .... I now, I don't have the stock one, but I changed it prior discovering the needle sizing problem.

I would suggest to you to do a simple test, drain the tank from the stock petcock through you filter. I did that, the flow through the stock filter was at a good rate, leave the cap on the tank. If the engine swallowed the gas that fast, you'd run out of gas about 7 minutes down the line! You try it and judge for yourself. This will also tell you if you have venting issues on your tank."

Em ...

I have another question?

If fuel is essentially "vacuumed" from our float bowls through the "main jet" (Venturi affect), is this same "vacuum" also "sucking" fuel from our gas tanks through the needle valve assembly when the engine is running and no electric fuel pump is being used?.

I've also posted this question on Dephi

Comments please!

Thanks,
Mike

Post edited by: bongo, at: 2005/12/11 09:12<br><br>Post edited by: bongo, at: 2005/12/12 14:10