Yet another M750 bogging down with throttle

[pneumadeux]

Hey everyone,

2000 M750.

For several months, I ignored the fact that the exhaust smelled ridiculously rich. (Famous last words...)  Then, last month, I packed up for a multihour ride, only to have the bike sputter black whisps of smoke and bog down on the highway merely 25 miles into the ride. (Small blessing, I guess.) Got her towed home.

- Pulled the carbs. Cleaned them surgically.
- Cleaned the repeatedly wet n' fouled plugs... Probably need new ones, but these were just installed prior to the attempted trip, darn it...
- Float needles nice n' springy. Bowls currently set to 13mm (per DucatiSuite method of measurement)  I have read 12 is considered "stock" by some, and that FactoryPro suggests 14. We're splitting the diff. Open to thoughts on that.
- Replaced main jets
- Replaced all gaskets and o-rings in the carbs
- Idle mixture screws set to 3 turns out
- MotionPro carb balancer to arrive any day now
- New belts installed prior to the fateful trip

When I last tried to start her up, she fired up, only backfired out the exhaust like the 4th of July for about 20 interminable seconds. Discovered the hoses that vent to those little gray airboxes each had 1 significant crack. I've trimmed and reattached the now crackless hoses at the little black air filters with vacuum connectors and flexible plastic cement. They aren't re-installed yet. But I digress.

After the backfiring ceased, (this was prior to discovering the cracks) I restarted, adjusted the idle, gave her some healthy revs, and rode her around the block.

She took off like a champ, until I shifted into second, and gave her throttle.

That's where she lugged and bogged; I was practically wide open and she barely got to 3300 RPMs.

How much â€" if any of that â€" was a set of cracked vent hoses?

Before I get her all back together again, I wanted to toss this story into the ring. And with that, I await the brilliance of the assembled throng.

[Howie]

Check the needle jets (emulsion tubes).  If they seem at all oval change them.

[pneumadeux]

 Thanks for the quick response, Howie.

Needle Jets are good. Nice and round. Not egg shaped.

[koko64]

How many miles on those needle jets? Are they the bronze coloured type? The oem needle jets can wear in as little 5 to10,000 miles and you may not see it. I would replace them with hardened Factory Pro parts if I was overhauling the carbs. I would set the floats at 14mm while I was in there.

Hopefully you may just have a carb flooding issue and a use of the search function will provide the reasons for it and a number of solutions. There are a few threads on this issue which should help. Just enter "carb flooding" and a great deal of info appears.

[Howie]

Koko is correct that visual inspection may not pick up worn needle jets.  If you are going to take them apart anyway this would be a good upgrade.  DO recheck float height.  I had Factory Pro jets with DynoJet needles in my old 750.  The new owner is still riding with them trouble free, oh maybe 30+K miles.

[Blackout]

Take a look at your fuel filter.

[pneumadeux]

Thanks, Blackout.

I failed to list in my original catalog of maintenance that a brand-spankin' new fuel filter has been fitted.

That did make a world of difference.

She also has HICAP starter cables from MotoElectric. So starting, when she doesn't have fouled plugs â€" is absolutely instantaneous.

[pneumadeux]

If I remember correctly, changing out needle jets is a matter of pushing that white, circular plastic piece through from the guts of the carb...

Can someone walk me through it so I don't screw something up?

Perhaps an order from Factory Pro is a good idea.

[pneumadeux]

And another question â€" 

If I install new Factory Pro needle jets, does it follow that I need to replace the jet needles, too?

[tbyte]

Check the vacuum line to the fuel pump for holes.  Also, +1 on replacing carburetor vent lines.  My winter hard starting problems disappeared after that.  I ignorantly always thought it was battery.

[koko64]

What type of fuel tap do you have there? Vacuum fuel taps cause trouble and people replace them with a manual tap.. Was it both plugs fouling? If so then the vacuum tap hasn't sprung a leak into one of the cylinders via the vacuum hose, but it is worth replacing with a manual tap which can help deal with carb flooding.

The domed bolt in the float bowl secures the needle jet, jet holder and white slide frame to the carb body. The notch on the needle jet has to line up with the pin in the jet holder upon reassembly. The parts are delicate although it requires some hand force to push the needle jet out/in. The dome bolt is thin gauge, so be careful both ways.

1mm on the floats makes a difference, but shouldn't foul plugs like you are describing.
What needles you running?

[pneumadeux]

If by "fuel tap," you're referring to the fuel cutoff... it has the stock vacuum cutoff as pictured here: http://www.ducatitech.com/bikes/m900_fuel.jpg

Forgive the noob question here, but, there are three lines running to that vacuum tap. I've only ever seen a straight on/off valve with a pair of in / out ports. Can you point me to what I should be installing?

Both plugs were fouling; horizontal was worse.

Needles are currently stock. However, thanks to you guys, I did place an order for a "Fresh Start" carb kit from Factory Pro, today 

[koko64]

 

That fuel cut off valves often give trouble. One line is the vacuum line to the manifold and needs to be plugged when fitting a manual tap.

Have a good read of the threads on carb flooding via the search function. A manual tap helps with that occasional issue. Try our sponsors first, but Motion Pro and mower shops have in line taps that you can fit in a spot that lets you get to it easily even while riding. The early oem tap bolted to the frame but that bracket was later removed. It was a good set up with a big tap knob.

[koko64]

Just remembered, bogging can occur when the slide diaphragms are not located and seated correctly. Check that the slides lift when you rev it by looking in the airbox.

[pneumadeux]

Slides were lifting beautifully during that previous ill-fated run around the block. I remember checking before fitting the K and N cans.

I will check back in after the stage III Fresh Start kit from Factory Pro arrives.