Fuel pump issue or just carb service?

[hiero]

So I was just reading through the '1999 M900 - No Fuel Pump Installed' thread and didn't want to hijack. 

In any case, reading through that thread got me thinking, what's the symptom of a fuel starved bike with the rectangular vacuum pump?  My bike has recently started to die on me after a period of time going freeway speeds and then stopping at a light or something.  After dying, leaving it alone for a few minutes then hitting the starter several times (keeping the button down for a while, stopping, then doing it again) seems to get the bike started again with no issues until some time later when the same thing happens.  It happens after sustained rides it seems.

I was thinking it had been a while since I replaced the needle jets and maybe I was getting more fuel than I needed and was flooding the engine out, but I'm not so sure (not sure if I'm even thinking correctly).  Haven't taken the plugs out to inspect yet either.

In any case, could it be the rectangular pump I installed failing?  or something else I'm not thinking about?  Maybe the vacuum operated petcock is closing out or something (still original with original lines), but I was under the impression those usually failed open. 

In any case, it's been pretty busy for me lately and I haven't been able to put a lot of brainpower or time to it since I've got the 748 running, so a little help from the DMF would be great.  Any thoughts?

[ducpainter]

Vacuum petcocks can fail open, closed, or the diaphragm can rupture and flood the cylinder the vacuum line is connected to.

It could be a failing pump...

or it could be a blocked/kinked vent line/tube to the tank.

It also can be that little black rubber nub under the fuel cap is blocking the vent in the cap, or the vent in the cap itself.

I'd start by checking the vent lines and that black fitting.

If you open your cap the next time it happens and you hear a whoosh as the tank sucks in air you know you have a blocked vent and not something wrong with the pump.

[Dellikose]

I have heard, under similar circumstances, that it could be your fuel hose heating up and collapsing due to the heat of the engine.

Check the routing of the lines when the tank is down and secured.

Also, it seems like the pentagon pump flows the best. You should be able to find one of eBay if you wish.

The vacuum petcock fails open, flooding the engine.

Edit: Beaten!

[hiero]

Vacuum petcocks can fail open, closed, or the diaphragm can rupture and flood the cylinder the vacuum line is connected to.

It could be a failing pump...

or it could be a blocked/kinked vent line/tube to the tank.

It also can be that little black rubber nub under the fuel cap is blocking the vent in the cap, or the vent in the cap itself.

I'd start by checking the vent lines and that black fitting.

If you open your cap the next time it happens and you hear a whoosh as the tank sucks in air you know you have a blocked vent and not something wrong with the pump.

yeah, the vent block was my first guess when it happened to me on the road, I didn't hear the whoosh though when I tried opening the cap or anything and it took quite a bit to get the bike restarted again for what that's worth.  The vent lines are pretty old at this point, they could use a change up anyways.

[ducpainter]

yeah, the vent block was my first guess when it happened to me on the road, I didn't hear the whoosh though when I tried opening the cap or anything and it took quite a bit to get the bike restarted again for what that's worth.  The vent lines are pretty old at this point, they could use a change up anyways.

probably not the issue.

Fuel filter on a carby is less likely to be a problem than on an injected bike, but is worth considering.

There is also a small filter in the carbs where the fuel line from the pump connects, and it is worth a look.

As Dellikose said it could be a collapsing fuel line.

I don't think it's your needle jets causing this issue, but they could be worn. What do the plugs tell you?

It seems there is a lot of talk about pump problems, but a well respected tech I know claims he rarely sees a bad hexagonal pump.

Maybe the ethanol is starting to take its' toll.

[hiero]

thanks! The filter's pretty new and the filter at the carb was cleaned not too long ago either...
I just got a chance to look though and it looks like the vacuum hose that goes to the pump is slightly oblong, looks like it has an awkward bend that I didn't realize when I installed it. I'll put a new longer line in so the bend radius isn't so extreme and see if that's the culprit!

[Howie]

Just in case the vacuum hose doesn't do it, the easiest way to confirm fuel starvation is drain to open the carb float bowl drains and see how much fuel comes out after the bike stalls. 

After going through the same thing I am now running a manual petcock and am back to the pentagon shaped pump.  The rectangular pump does put out
enough volume for your bike when in good shape, but not much more.  A little deterioration and you have starvation.  My guess is once in a while one that is not ethanol resistant slips through.

Replacing the vacuum shut off is not a bad idea since failure is common and failure to shut off can go unnoticed.  Another option is replace both with an electric pump.

[hiero]

So I finally have gotten a chance to sit down with the monster.  I replaced the vacuum hose to the square pump (damn if it's difficult to do when you don't want to take anything off the bike) and I haven't had a fuel starvation issue yet!  I need to take the bike out for a long run and see for sure, but just looking at the hose routing, you could tell the middle of the hose was already oblong and susceptible to collapsing. 

Incidentally, I cursed myself for throwing my old pentagon pump away and recently had to purchase a used one on ebay to have as a backup when the square fails...