Help please... '00 M750 fuel issues

[Raziel]

This is my carbed '00 M750 that lay dormant for some years. I've been trying to get it back on the street for a while and to date have put in new properly-tensioned cam belts, cleaned/rebuilt the carbs thoroughly and set float height, gently cleaned the vacuum fuel pump (diaphragms seemed OK), replaced the fuel lines, rinsed the tank with a bit of clean fuel and Sea Foam and changed the oil. New fuel filter.

I have a new battery arriving soon since the old Yuasa is toast. In the meantime, I've jumped it from a (non-running) car battery. It finally started and ran with the choke on, and idled for a few minutes. Eventually I closed the choke though it was idling a bit rough (below 1K). I can twist the throttle and get some RPM's for a minute or so but soon it sputters and when I hit the throttle I can hear some "chuffing" through the airbox. Checked the tank vent and lines for kinks, those seem OK. Opened fuel cap while running too so I don't think it's a vapor lock issue. Plugs are a bit black and sooty but dry with not too much fuel smell, but I'm not a good judge of that.

I need to give the K&N filter a good cleaning, but I tried running it with the lid off as well with no change that I could detect. The carbs were rejetted some time ago and ran pretty well with the cored cans that I installed. They are open cans, no back pressure to speak of.

Also I've got fresh 87 octane non-ethanol "Stabil-ed" fuel in there, not as high octane as it should be I now realize after some reading.

Any and all advice appreciated, I figure I've got a while left to ride before the snow hits.  

[the_Journeyman]

Well, I run 87 normally in both my carbed '99 M750 and my FI 900SS, so I don't think your 87 octane is a problem.

It sounds to me as a fuel delivery problem.  How carefully did you check the passages is the carbs & jets?

JM

[Raziel]

The carbs were torn down completely and well-cleaned...they were heavily varnished. The butterflies wouldn't open or close initially, they are still a bit stiff and even after adjusting the opening and closing throttle cables they don't "snap" closed, but they do close fully when the throttle is twisted back. Was hoping that some run time would alleviate that. I inspected all passages and jets and blew compressed air through the whole works.

[Raziel]

Bumping! and clarifying my long-winded post...

Bike starts with choke, and idles without choke after 1-2 minutes. Takes throttles and revs fine briefly, but then stumbles and chuffs and dies unless throttle is rolled back. It will start again but won't take throttle.

Where do I start?

[ducpainter]

Bumping! and clarifying my long-winded post...

Bike starts with choke, and idles without choke after 1-2 minutes. Takes throttles and revs fine briefly, but then stumbles and chuffs and dies unless throttle is rolled back. It will start again but won't take throttle.

Where do I start?

I would ascertain that the slides were actually lifting when you roll the throttle open.

[alibaba]

Just a shot in the dark from a Brit - Iron dinosaur.  I assume the float levels are correct.  I am not at all familiar with the carbs on the Ducatis (Dellortos?) but with the Amals I am very familiar with this is indicative of a problem in the pilot circuit which controls starting to just off of idle.  Amals are notorious for both the fuel pilot jet and the pilot discharge holes to varnish and clog up.  I can't tell you how many owners with similar problems swore their carbs were clean only to have me find them clogged.  I like to spray WD40 thru all the passages and check the output.

[Raziel]

alibaba, they are Mikuni 38s. Good thinking on the pilots.

Still battling this problem, but I think I've narrowed it down...

New fully-charged battery installed, tank ~1/4 full and propped up so I can watch the slides with the filter out. No kinks in the fuel lines anywhere. Fuel cap loose to eliminate vapor lock as a cause. Starts up with a little difficulty but is soon idling OK without choke at about 800-900. After about five minutes I adjust the idle up to around 1,100. It's running great at this point and takes throttle. After a few more minutes I start to open it up (the slides open fine) and eventually am hitting WOT with no stumble or cough, very smooth. 

Back to idle. About a minute later it chuffs and won't take throttle without wanting to die. And then of course it dies. Can't get it started again. I'm pretty sure it's fuel starvation at this point. The fuel filter is new, but that's an easy swap.

So I think I'm down to troubleshooting fuel delivery.

What's the best arrangement of fuel lines to do this? Must be a tried and true method. Fuel filter out straight to the pump, bypassing the vacuum petcock altogether? Or bypass both and go straight to the carbs?

I figure its best to eliminate the pump and petcock before pulling the carbs apart to check the float bowls?

[citizin]

I just had a similar problem, water-related, and humorless dp suggested I squirt some fuel into the carbs while the bike is running and keep squirting to keep it running. Maybe you've got a hidden clog or something and you just gotta force it out.

I figure its best to eliminate the pump and petcock before pulling the carbs apart to check the float bowls?

Good thinking.

[Raziel]

I was just reading your thread! Glad you got it sorted, I know how annoying it is. . The fact that it starts and runs strong for a while and then suddenly bites it leads me to think it's fuel delivery... but I wouldn't rule anything out at this point.

[citizin]

Yeah I finally got it to a point where it would run a bit, but it would sputter and die. If you get past the tap and pump and it still misbehaves - running then stopping, then try what i did. I got a small spray bottle full of gas, got it idling, and spray the makes the beast with two backs out of it straight into the carbs. If there is something like water or gunk in the lines maybe it'll loosen/force it out.

Just a thought - humorless dp suggested it and it worked. I've got a 2000 with the same carbs as you.
Indeed 

[Howie]

For test purposes you can bypass the fuel shut off and fuel pump.  For the bike to run the fuel level in the tank needs to be higher than the carbs or you could run an external set up.  A way to shut off fuel in case of carb flooding would be a good idea.

[koko64]

For test purposes you can bypass the fuel shut off and fuel pump.  For the bike to run the fuel level in the tank needs to be higher than the carbs or you could run an external set up.  A way to shut off fuel in case of carb flooding would be a good idea.

+1

[MongoReturns]

Yeah I finally got it to a point where it would run a bit, but it would sputter and die. If you get past the tap and pump and it still misbehaves - running then stopping, then try what i did. I got a small spray bottle full of gas, got it idling, and spray the makes the beast with two backs out of it straight into the carbs. If there is something like water or gunk in the lines maybe it'll loosen/force it out.

Just a thought - humorless dp suggested it and it worked. I've got a 2000 with the same carbs as you.
Indeed  

Mine did exactly this - start for a bit, then sputter & die.  I was pretty sure it was the fuel pump, but replaced the filter anyway.  Yep, it was the pump.  Have you guys ever rebuilt or replaced it?  If no, you're way overdue.  Fuel pump issues can be misleading depending on how much gas you have in the tank - full tank, will run longer if pump buggered (gravity).

Pump rebuilt kits are $20, or I know lots of people like CA-Cycleworks' replacements.  Edit:  I see Razkiel cleaned the pump.  I don't know if that would fix it or not - when I replaced my membranes, they actually didn't look too bad,  though one was was stretched into a lense shape.  Pretty sure that was the problem.

[citizin]

Mine did exactly this - start for a bit, then sputter & die.  I was pretty sure it was the fuel pump, but replaced the filter anyway.  Yep, it was the pump.  Have you guys ever rebuilt or replaced it?  If no, you're way overdue.  Fuel pump issues can be misleading depending on how much gas you have in the tank - full tank, will run longer if pump buggered (gravity).

Pump rebuilt kits are $20, or I know lots of people like CA-Cycleworks' replacements.  Edit:  I see Razkiel cleaned the pump.  I don't know if that would fix it or not - when I replaced my membranes, they actually didn't look too bad,  though one was was stretched into a lense shape.  Pretty sure that was the problem.

I've got the pentagon pump, and I'm unsure if the cycleworks pump would fit onto the frame where the old one currently is. A rebuild is definitely in order - found a rebuild kit on BikeBandit for the pentagon Mikuni pump.

[MongoReturns]

I've got the pentagon pump, and I'm unsure if the cycleworks pump would fit onto the frame where the old one currently is. A rebuild is definitely in order - found a rebuild kit on BikeBandit for the pentagon Mikuni pump.

Yep I got the Bikebandit kit - I really like the pentagon for no particular reason.  Real pain to get out of the frame though!  And make sure you re-assemble it right, not half upside down like I did, then wondered why it wouldn't fit back in the frame. 

Also something else to thing about - there's a vaccuum petcock somewhere that may cause an issue (Ducpainter discussed it elsewhere).