"When you explore the unknown, by definition, you do not know what you will find."

I ended up having more time this afternoon than I expected, so was able to run some tests. My basic setup was an old fuel tank from a Toro SnoPup. It is about 3 1/2" diameter, about 8" long, and holds about a quart. I didn't measure precisely, because I use the same tank for all testing. I don't know if would make a difference, but ambient temperature was about 58 degrees F.

Initial test: hold my finger on the outlet nipple, fill the tank, remove my finger and start my old analog stopwatch. When fuel flow stops, stop the stopwatch. Time, 97 seconds.

Second test. Attach the filter (TW FASTEX VISU FILTER) which gave trouble on my CH80 scooter with about 4" of plastic line. Hold the filter upright to allow fuel to fill it. Put finger over outlet of filter, fill tank, remove finger and start stopwatch. Time to drain 88 seconds - less than the unrestricted tank? Repeat and get 86 seconds. Total distance from bottom of tank to bottom of filter, about six inches.

Third test, a Briggs and Stratton filter, PN 691035, which on its packaging says it is for engines with fuel pumps. Time to drain the tank was 74 seconds.

Fourth test, a filter with a UPC of 37049 92399. I cannot remember whether I got it at Walmart or Ace Hardware. Time to drain tank, 70 seconds.

Fifth test, the tank with the 4" of line, but no filter. Time 81 seconds.

The gas was on hand from three or four months ago, and I think it was what Bill would have called "100% pure gasoline." The gas I had used in my scooter was E-10. When the filters dry out I'll get a gallon of E-10 and try again, just in case it makes a difference.

All filters are clear plastic, pleated paper. I am not sure how to go about attaching more than one picture to a post, or I'd do so. But as I have said before, if I have to use a third-party site it ain't gonna happen.

Checked at AutoZone and the filter package states it is a Duralast FF7144DL universal filter that is made in China.  It has ¼ - 5/16 inlet and outlet on it, you just cut off the ¼ to have the 5/16 I am trying to use.  I have a picture of the filter, but cannot find the instruction on how to get a picture into a post.  

I also checked my fuel tank and I am at about 1/3 full, so that could have some bearing on the issue of flow.  I guess it does not really matter because I need the filter to flow all the fuel in tank to be useful.  Wondering about the Chinese filter media now.  Still cannot get over the flow just pouring gas into the filter verses from the petcock.

The outlet of my tank was (unmeasured) about 1/4" ID. A 5/16 hose fits nicely on the nipple, but I used a 1/4" ID and forced it on. One of the filters was a 1/4 and 5/16 combined size; the others were nominally 1/4".

I used 4" of tubing to give me room to bend the filter "up" so as to allow it to fill with gasoline before I pointed it down for the test. As I said, the total drop or head was about 6" from the bottom of the tank to the bottom of the filter, with the filter and hose more or less in a straight line pointing down. Total drop from the top of the tank to the bottom of the filter would have been no more than 10". There was no line attached to the outlet of the filter - I just stuck it into the collection tank. The S40 head varies from about 12" with a full tank to about 4" with an empty tank.

I admit to being somewhat surprised when the second test (the first filter test) resulted in a shorter time than the unrestricted tank. I attribute it to the additional "suction" of the longer drain line, and that is why I ran the final test with the 4" line but no filter.  My testing is by no means rigorous, and I welcome constructive criticism.  

This should be the filter which gave trouble on the scooter.

And this is the third filter.

All three filters use pleated paper, but none of them gave specs as to their filtering abilities (xx microns). When I put the filter on the scooter, the filter didn't seem to ever really fill up. It is hard to judge, because the body panels on the scooter hide the filter from view. I guessed that was because there really wasn't any way for the air in it to pass back up the fuel line into the tank. That is also why I pointed the filter "up" for my testing. When I removed the filter from the scooter, I blew through it and noticed a distinct resistance. Today, when I got through testing it, I blew through it and didn't notice the resistance. I plan to try it again with known E-10 and see if it makes a difference. I am not completely sure what the fuel was that I used today, since I cannot even remember for sure when I bought it.

The one I have looks amazing like the third one Charon tested and worked great.  Here is the one I was trying to use without much luck.

Thanks Charon for the testing you did and both Oldfellow and your input on this. I am losing my mind on why it will not flow out of the petcock in prime, which is a totally the stock vacuum petcock at this point.  From what I have seen the prime position is total free flowing depending on the float valve to keep it from running into the engine or do I have my facts wrong ?  The diaphragm does not come into play except on the ON and RES positions right.

Prime = sittin on the bike, grabbing the lever & swinging it to the rear of the bike. If you arent gettin gas like that, sumpins RongGG