My bike had that GS tendency to starve if run hard for extended periods.  Lately it's gotten very bad though so tonight I did some measuring.  I timed the fuel flow from the Tank Valve RUN, RES ports, and then from the petcock in RUN, RES, and PRI positions.  I then did the same test after the filter. 

Filter bad.... 

Here are the #'s
Tank Ports-
RUN     -     19.6GPH
RES     -     19.6GPH

Petcock (10" vacuum applied using a Mityvac)
RUN     -     13.64GPH
RES     -     13.64GPH

After the Filter
1.67GPH

As I said - Filter Bad.  I made some interesting observations though.....

1.  The vacuum signal coming to the Petcock from my motor is WACK.  It bounces around so bad I can't see it.  I'd guess it averages 10" but who knows...it's seriously bouncing around - I guess this indicates the state of my motor - 28,000 hard miles and infrequent tuneups.
2.  The flow rate through the system is enough to support a whole lot of HP!!!  Using some generic formula I googled, I calculated the flow will support over 150HP (assumes 25% engine efficiency).  Just a googled formula, I make no claims for the accuracy.
3.  Continuous WFO operation would limit the engine to whatever fuel was in the bowls.  This explains why the engine would nose over after a pass down the strip with the clogged fuel filter.

4.  If you're doing any serious work, run with the Petcock in PRI and you should have no fuel starvation.

Has anybody measured the vacuum signal on their bike?  I'd be interested to know what it looks like.

I noticed the same thing about the vacuum signal.  I was worried about it for a few minutes until I thought about it.  I realized that it's the vacuum from a single cylinder.  Of course it's going to bounce around, it's only going to have vacuum 1/4 or the time, on the intake stroke.  It's not like a car where the engine vacuum comes from all 4+  cylinders at once. 

How does your tank look?  Is it rusted inside?  What kind of fuel fliter are you using?  If you're using one of those tiny motorcycle filters, you might consider a larger automotive filter.  They have quite a bit more surface area, so it shouldn't restrict flow as much for a given amount of crap in the filter. 

There are quite a few posts about the fuel starvation issue.  It's not solely due to your fuel filter.  The stock petcocks also restrict the fuel flow enough to cause the problem with extended periods of full throttle.  I personally haven't decided what to do about it yet.  I need a new petcock, but most of the Pingel petcocks apparently attach to the tank and some don't have a RES, but I want RES and I want a petcock that I can switch to RES on the fly like the stock setup. 

Edit: Okay, it's bedtime.  Here I am telling a veteran about petcock issues....

I was browsing the Pingel (Pingle?) site the other day and they have a very thorough selection of high-flow petcocks.  The closest they had to a direct replcement with on/reserve/prime that's vacuum activated was like $150.00 though.  Those guys are making a killing, do they have any reasonably priced competitors?

-M

The problem is not necessarily finding an alternative petcock, but finding one that is going to fit and going to give you the fuel you need.

Quote from: V8Pinto on February 26, 2006, 07:10:38 PM
Has anybody measured the vacuum signal on their bike?  I'd be interested to know what it looks like.

I just did my carb synch yesterday, and it started out with No.1 Cyl drawing 13 inches of Mercury, and No.2 drawing 15 inches.  I then synched them both to 15 inches @ 1300rpms.

I experience the fuel starvation at constant full throttle on Sunday.  I was lucky it was just before an extermely dangerous downhill twisty part of a major freeway under constriction (i.e. no shoulder ).  Major pucker factor!

I had the bike out at a track day the previous weekend and I saw something that seems to indicate one additional issue on the fuel starvation problem.  When I stopped the bike at the track I saw the fuel in the clear plastic filter was bubbling.  I am assuming that I am shedding enough heat off the engine to cause the filter to get hot enough to boil the fuel (like vapor lock).  I did not think to look on Sunday (FYI it was 90 degrees).  The engine would not restart on the shoulder.  Just turning to Pri for a couple of secs got the engine to start and then it ran fine.

1) what temp does gas boil at?
2) could the gas have been boiling, or are the bubbles a characteristic of the plastic filter with a brass screen?
3) based on the posts above, how would I confirm that a filter is designed for sufficient flow rate to keep up with the upstream flow?

Thanks!

according to the web, gasoline boils somewhere between 104 and 167 F.  Thanks to Scratch's engine temps, I could have a situation where the filter (which is close to the engine case) has stagnant fuel in it and starts to boil when riding.  I need to move it and get one with minimal fuel flow reduction.