Vacuum Lock/Flow Rates

[gareth]

I had a hell of a time keeping up with the gang at WOT on Sunday out on Highway 90, ( although you mightn't think it if you watch the video..) It felt like I was running out of fuel after a few minutes. The bike started bogging down, made a roaring noise through the pods and slowed to a crawl. It also got really hot. I initially thought the bike was going to seize because I just KNEW I had gas. We were 11 miles from home at that point and with a break at a gas station at the next exit I got home fine. When you are the guy that everyone has to stop for, it can get a bit embarrassing. This was the second incident for me, ( I lost a side-bag lid that morning...). It didn't faze anyone. They are a very patient, cool bunch of co-riders.

Riding back home with Randy on Monday, the same thing happened on the Taconic. Three times. Gas was OK so we spent a while dismantling and cleaning the gas-cap at a Park'n'Ride on the side of the road. Still problems. I was having trouble calibrating the vacuum advance on the Keihin carbs that I use and I figured that the bike timing was over-advanced at WOT. After falling back AGAIN, I plugged the vacuum advance, left the gas-cap open, and made it home.

I just did some flow-tests on the fuel system. I checked to see how long it took for 2.5 litres of fuel to drain from the tank under various conditions.

Gas Cap on, Petcocks on Reserve, to drain 2.5 litres into a bucket....5.48mins

Gas Cap off, Petcocks on Reserve, "..............................1.55mins

Yikes!!!!

I replaced the gas cap internals with one from my old Special tank..

Special Cap, Petcocks on Reserve, to drain 2.5 litres.....................2.27mins

Then I checked the fuel hoses and filters.

I have a couple of the small brass porous filters on the petcocks and a small paper/ plastic filter at the carb.

With the higher flow from the new cap, the paper filter slowed the time to drain the 2.5 litres to 5.26 minutes!

So with a bad gas cap, and a restrictive filter it's a miracle the bike ran at all above idle..

And that is why I was stopped on the side of the road.

Five times.

So I'm dumping the paper filter, and asking if anyone can suggest a high efficiency, compact replaceable fuel filter that does a better job than the goofy one that I picked up at AutoZone. I also wondered what kind of damage I might have done running a bike lean at WOT with an over-advanced ignition.

[fredintoon]

- - - So I'm dumping the paper filter, and asking if anyone can suggest a high efficiency, compact replaceable fuel filter that does a better job than the goofy one that I picked up at AutoZone. I also wondered what kind of damage I might have done running a bike lean at WOT with an over-advanced ignition.

Hi Gareth,
check the plugs, if they are bright white do a compression test.
Any old fuel filter will stop great big flakes of rust but it needs a really small pore size filter medium to get the small particles.
Alas that means lots of screen area to get the required fuel flow.
I'd guess that a filter that'd stop 5 micron particles and flow a gallon of gas in 5 minutes would be as big as a beans can.
You gotta compromise and perhaps run both taps to double up on the filtration capacity.

[Ken Talbot]

So, 2.5 litres is approx .66 US gallons. If you're getting a (very poor) 30 mpg, this amount of gas should take you pretty close to 20 miles. At 120 miles per hour, that will take you 10 minutes. If your 2.5 litres will flow through in 5 minutes, wouldn't that be fast enough?

[gareth]

If your 2.5 litres will flow through in 5 minutes, wouldn't that be fast enough?

Probably...... but let the head of pressure drop as the fuel level drops in the tank and try to get that anemic pressure to overcome the vacuum blockage and the restricted filter and then the trouble starts.

My test was done with a full tank of gas and I just added back the stuff I drained out for sucessive tests. On the road, the bike would keep up while it was near full. As soon as it dropped to below reserve the pressure dropped and the trouble started...

For the scientists..

Flow rates/ time measured for the gas cap with the blocked vent...

.5 litres......... 00:35
1.0 litre.........01.58
1.5 litres........03.18
2.0 litres.........04.34
2.5 litres.........05.48

Now maybe someone could plot the flow-rates against time and remaining head of gasoline in the 20 litre tank, and figure out exactly when the head of pressure would no longer over-come the vacuum caused by the blocked gas-cap and so stop the highway fun.
But the data is complicated by the original problem, which COMBINED the vacuum restricted flow-rate of 2.5L in 5.48 and the restricted flow caused by the in-line filter and measured with the UNRESTICTED gas cap...
The filter more than halved the measured flows..( 2.5L in 2:29mins vs. 5:26mins)

So rather than go through all that...

I'm going to take a guess...

At full throttle, with about 1.8 gallons of gas in the tank, the bike would bog down, go lean, overheat. This is empirical evidence gathered from observation of the bike doing just that 5 times this weekend, and once earlier today...


The moderators should feel free to move this posting to someplace more appropriate..Like maybe to the bottom of Queechee Gorge..


G.

[DiverRay]

The moderators should feel free to move this posting to someplace more appropriate..Like maybe to the bottom of Queechee Gorge..

MOVE IT?!?!
We LIKE it here This gives us a thread to point someone to when they say, what can a gas cap do?
Good work finding this. And for the record, I use the small bronze element filters om my bike. No flow restrictions.