1999 M900 Carb Fuel? Issues

[koko64]

A full tank can flow thru but once past half a tank or so. You fitted a manual tap and deleted the vacuum tap iirc?

[Dellikose]

Correct, and I had the petcock open, so that would explain that.

Also, I am going to change the vacuum line from the intake manifold to the pump just to be safe. Do you know the ID off the top of your head?

After some searching, it looks like it is a 5mm vacuum hose.

[Howie]

But the lack of fuel at idle leads to a bad pump since you now have a manual petcock.  The fuel pump can pump less fuel when hot.  You are also correct that a collapsing vacuum line can be a problem, as can be the fuel hose that runs from the petcock to the pump, particularly if it is the original hose, running close to the engine.

This is what I did with my 750, using heavy walled fuel hose.



You can buy a new pump or rebuild kit here.
http://sudco.com/Catalog37Digital/index.html#/222/

Their pump comes with two outlets, but it is a lot cheaper than OEM.  Just Y them together.

[Dellikose]

Thanks howie.

I rebuilt the pump this past fall with a Mikuni MK-DF62 kit. I rebuilt it because I originally diagnosed the pump when this problem started.

Along with replacing the vacuum feed to the pump, I will replace the fuel line from the petcock (my petcock is before the fuel pump) to the bottom feed of the pump. The original fuel line makes some pretty crazy bends to get to the bottom feed, and it also sits in the corner made by the engine case and vertical cylinder - it must get pretty warm there.

[koko64]

I wonder if e fuels eat pump components up your part of the world?

[Howie]

I wonder if e fuels eat pump components up your part of the world?

Definitely shortens the life, but not that fast.  Still could be a pump problem.  I don't know what the proper volume at idle would be for that pump installed on a Ducati, but that would be the true test.

I always forget, there is a screen in the fuel inlet to the carbs that can clog.  Have you checked that?

[Dellikose]

Well there's yer problem...



The vacuum hose split where the clamp was on the intake manifold nipple. The pump couldn't draw enough vacuum to keep up with the fuel demand. Seems to make sense to me.

Confirmation that the vacuum line is indeed 5mm...



Once I get this replaced hopefully the soap opera is over.

[koko64]

 

At least that's a cheap and easy fix.

[Dellikose]

So turns out that I still have an issue. I will let the bike sit and idle for 5 to 10 minutes and it will eventually bog down and die.

The vertical cylinder spark plug is looking good, but the horizontal is reading way too lean. I need to check for a vacuum leak at either the carb/manifold boot, the vacuum taps, or the manifold base gasket. If nothing checks out there, I will dig into the carbs again.

I am thinking issues in the carb could be a plugged jet, torn diaphragm, or cracked float.

After that I will go to electrical and look for heat soak issues, but the plug is not soaked with fuel when I pull it - so I am assuming fuel delivery issue and not electrical.

Any other advice?

[SpikeC]

 Don't let it idle so long.

[Dellikose]

One of the first things I noticed is that the vacuum line to the horizontal cylinder has an issue. I can rattle the canister on the line to the vertical cylinder and hear the ball inside, but did not hear it on the line to the horizontal. Half a can of carb cleaner later and now the ball rattles freely. I would like to replace these lines, as they are pretty brittle. Can I omit these canisters? They seem like a one way deal - draw fresh air into the carbs when needed, and shut off by the ball when not drawing air. I imagine to prevent gas vapors from escaping.

I would reroute up near the battery to get them in to still air, since I no longer have the frame pods that they previously connected to.

[Howie]

There are filters in there.  Forgot about them   If they clog or the check ball jams you will have a hard time off idle.

[Dellikose]

I am wondering if the vacuum filter gave up the ghost. None of this crud was in there when I had them apart last time. It looks like the same grit that I flushed out of the vacuum line.

Horizontal cylinder carb:

[Howie]

Yep.

[Dellikose]

For those who run into the same issue - I had a look inside the filter that is connected to the vacuum lines. As it turns out, there is not a ball check inside, but 2 or more screens to keep out debris. The rattling is from the screens having some free play within the barrel section. I will try to get a picture later.

As you can imagine, mine were seriously gunked up after 30,000 miles. It even looks like there are remnants of a bee caked on one of the screens - poor guy.

After a good cleaning I am going to reuse them, since I would rather have the protection so nothing gets sucked up into the carb diaphragms.

Edit: Here's a picture of the filter after cleaning.