Reserve does not work

[clcorbin]

Well, I finally pulled the tank off and set it up on my work bench for full drain testing. The result? I appear to be a bold faced lier!!!! The bloody things worked exactly as they should work!

Here is what I did: I set the half full tank on my work bench with the petcocks hanging over the end. I attached a length of clear hose to the fuel nipple and set it down in a 5 gallon bucket so I could see fuel flow rates. I then connected a length of 5/16" hose to my vacuum pump and then to vacuum nipple on the petcock (tested one at a time). The hose was loose by design as this pump is an industrial vacuum pump and it can pull a LOT more vacuum than the engine can, so I didn't want to hit the diaphram too hard and damage it.

As expected, when I hit the switch, the pump cam on and the petcock started flowing in "On". The second petcock responded exactly the same. So, I drained the tank down until I just had a tiny trickle coming out of the fuel nipple, then switched it to reserve, expecting nothing to change.

But, low and behold, the fuel flow jumped right back up where it should be! The other petcock behaved the same bloody way!!! I then tested both petcocks in prime and that too worked exactly as it should. I then left both petcocks on prime and let them run until they stopped flowing. There was only a few ounces of gas in the tank (barely enough to hear sloshing when I shook the tank). Again, exactly as it SHOULD work!

One thing I did notice is that I had to push the vacuum line up against the nipple and petcock to get a bit better seal on the vacuum line before the fuel would flow as fast in reserve as it did in prime, so that has me thinking.

So, here is where things stand:

1) The fuel filters looked a bit worse for wear. They have less than 15000 miles on them (1 1/2 years or so) and they did have a bit of sentiment in the bottom of each one. New filters have been installed.

2) The vacuum made me think. I am going to try and clean out the vacuum nipples on the carb boots to see if I have some blockage in there that is causing insufficient vacuum to be seen at the petcock. I'm also going to install new vacuum line, just to be sure.

3) I also picked up new fuel line, so I am going to replace all the fuel line before buttoning things up.

Then, I guess it is time to test reserve again and see if things actually work "under fire". If reserve doesn't work in use, then I have NO idea what is going on!

I would like to apologize for leading everyone astray. I did test those petcocks a couple of years ago and they did not work like they did today. Perhaps my testing methodology was incorrect.

[3Phase]

Clint, I'm lost so bear with me, please!

It shouldn't make a bit of difference if the taps are in the On or Reserve position so the missing/reduced flow Reserve problem's got me baffled but did you by any chance work on or rebuild the vacuum diaphragms in the fuel taps and stretch the springs a 32nd of an to make them work?

The springs in the vacuum valves are a little finicky. Too loose and the fuel won't stop flowing; a fine frog hair too tight and the vacuum valve won't stay open during acceleration. The engine will bog out and die and switching to Reserve doesn't do anything particularly useful. Sometimes it'll happen during low speed/low RPM street riding and/or at highway cruising speed too if there's not enough manifold vacuum to keep the valve open.

When that happens (DAMHIKIK) the engine has to run on whatever is in the float bowls until there's enough vacuum to open the valve and refill the float bowls.

[clcorbin]

I'm wondering if the problem wasn't a partially clogged fuel filter. It was always two cylinders that cut out first and I always headed to the nearest gas station when that happened, so I never got to see how long the other two cylinders would run... My theory goes like this: partial clog, so the filter needs a certain height of fuel above the filter to push enough fuel through the filter at a rate equal to the rate the engine is consuming it. Once the fuel level drops down below that, two cylinders (assuming only one partially clogged filter) start to miss as they slowly start to starve for fuel. It wouldn't matter what position the petcocks where in, as it was a function of fuel height only.

Hopefully, I'll be able to find out in the next week or so. If things don't start working with the changes I have made, then I will have to look at the tees and see if one of them is partially clogged. I remember cleaning them out the last time I had the carbs out and apart (2 years ago), but who knows?

[3Phase]

A bad inline fuel filter might do it; not enough pressure from the weight of the fuel left in the tank to move the fuel through the filter when it gets down to Reserve.

If the bike sat for a while with a partial tank of bad fuel and left varnish/gunk on the lower half of the plastic screens on the filter towers that would do it. You'd have fuel flow until the level in the tank got down to the plugged part of the towers and then it would stop.

You'd have to take the fuel taps out to check the screens for something like that. You can't just blow air up through the fittings because it would go through the upper half of the filter that isn't plugged.

[DGXSER]

Physics being what it is, adding the height of the reserve fuel capcity would/should add some pressure to the system. The lowest height of the fuel that can flwo in reserve is lower than that of the ON setting, and with the same upper level of fuel, an increase in pressure would occur. Might not be enough to matter though, kind of like the change in fuel pressure due to changes in outside air temps, not enough to matter to anyone except a physics major.

I would bet your on to it with the fuel filter though, if your fuel filters were still in play when you were running your experiments on draining the tank.

[clcorbin]

Well, I FINALLY have a complete answer to the whole "reserve doesn't work" question. Unfortunately, due to a tank of bad gas, it took a month to get to the bottom of it (and my carbs are STILL not 100% back to normal!)

So, the short answer is this: after replacing all the vacuum lines, fuel lines and inline fuel filters, reserve DOES work exactly like it should. This was not the case before replacing everything, so I am ASSUMING I had partially plugged filters, but that is only an assumption.

It is kind of nice being able to put it on reserve when the bike starts to miss (usually the right bank first, but that is probably due to the crown in the roads and the very slight left lean they cause) and then things clean up and runs perfectly normal again. Yesterday, I filled it up after going on reserve right when I got home the night before and put in 4.5 gallons (I try to fill it up exactly the same way every time as I measure the fuel economy every time). That is about 0.3 gallons more than I have been able to put in it since at least the last three years. And prior to that, just to get to 4.2 gallons had the bike running on two cylinders when I got to the nearest gas station!

So, things look like they are back to normal with the fuel supply. Now I just have to pull the bloody carbs AGAIN to clean out the crap left there by that tank of bad gas...

[3Phase]

Nice work and thanks for the follow-up!

Changing filters is a lot easier than taking out the fuel taps to clean or rebuild them!

[DGXSER]

The pressure is off.....or on depending on if your a cup half ull or cup half empty person. So it all boiled down to the pressure required to overcome the filter plugage. On had it, reserve not enough. Great work figuring it out! Congrats and many miles of smiles to follow.

On the carbs, you could try adding seafoam to your tank of gas and seeing if that clears it out.