carburated Ducati 900ss starving for fuel at 5000 rpm -FIXED- !!!

[randy yocum]

I recently bought a good used 1997 Ducati 900ss here's what I have done to it so far ,installed a (New) clutch,fuel pump,fuel filter,starter solenoid,cleaned the carburetors twice,including correctly setting pilot screws and balanced both carbs to specs.Both cylinders show gobbs compression 175lbs on both cylinders.Timing is spot on, and both sparks plugs have nice hot blue flames. I've ridden the bike 400 miles in the last couple days ,it starts beautifully,has a nice lopping idle, underway it pulls very strong in any gear till I hit 5000 rpms and the the power drops off.I experimented on the highway in 5th gear at 5000 rpm as the power was dropping off I pushed the choke lever on full and it was like an after burner kicked on.The bike accelerated hard right up to 100mph and I backed off. So its got to be fuel,like I said the carbs are squeaky clean,could the main jet size be wrong? I'm not sure that at 5000 rpm the main jet is even involved.Could the needle in the slide need to be moved up.These are CV's carbs with rubber diaphragms and they are in great shape.The needle is aluminum and is not adjustable I suppose a guy could maybe shim the needle up a little with a washer or two.I just don't know where the problem lies,any Ideas out there?   Does anyone recognize this scenario? any help would be appreciated

[Tom]

Electronic ignition with rev. limiter??

[RinkRat II]

  Hi Randy, couple of things.... did the bike come from the same or similar elevation where you are now?? At that rpm your on the main jet and may need to go up a few sizes along with the tweaking the slide opening etc. Stock airbox? Join a Ducati forum specific to the model you have and learn, learn learn. Congrats on the  SS. Enjoy!


    Paul B

[oldbike54]

 These are the stock Mikuni CV carbs ? I think you gave the clue Randy by mentioning that closing the choke helped . You might also make sure the diaphragms are still pliable and have no holes in them .

 Dusty

[randy yocum]

Yes,it has the stock air box with a K+N filter ,and factory stock Mikuni carburetors.The bike had been sitting unused for 12 years here in Missouri where I live,so same elevation.

[RinkRat II]

      Double check all vacuum lines and the connectors, internal ports to the diaphragms as thats what controls the slides.  Any specs in the factory internals will help too.

        Paul B

[LowRyter]

http://www.factorypro.com/Prod_Pages/prodd09.html

[Cal3]

If its been sitting awhile.....dont ignore the simple stuff.....is the petcock gummed up or not flowing enough, what about the tank vent, make sure its clear....disconnect that anti-spill/check valve gismo on the vent tube.  Good luck....nice bike!!

[SmithSwede]

Hello Randy.   Wow.   What a pretty bike.    I must admit I’m falling more and more in love with my red carby 1996 Ducati 900 SS. 

Not sure what to tell you, on account of my Ducati ignorance. Mine pulls stupid strong up to about 7500 or 8000 rpm, then runs out of steam.   Like you said about yours, mine starts *instantly* when the button is pushed. 

I’m thinking since the problem is resolved with the choke, then it’s not electrical or spark or mechanical or bad gas.  Gotta be something with intake manifold, carbs, fuel supply, etc.

I’m pretty sure that at full throttle and 5,000 rpms, you are mostly or entirely on the main jet.   

Since it starts and runs well in other conditions, I’m thinking there is not a gross air leak.   Is the float bowl height adjusted right?  Sounds like it’s fuel starved upon high demand.   

I’d also wonder about the CV diaphragm.   Does it move freely?  And sticking, holes, etc.?

Do you still have this problem if you crack open the fuel cap to ensure there is no vacuum problem?

I’m also wondering about the “choke” or fuel enrichener.  Is there maybe a leak or fault in that system such that when you move the handlebar adjuster, it corrects the problem and runs better?

[motoguzzibill]

Randy,
  The "choke" is really an "enricher", it puts more fuel into the carbs. If it runs better with the enricher on then for some reason you are starving for fuel. At 5K your probably at  1/2 throttle or so. Fuel lines are pretty tight under the tank between the frame be sure you haven't pinched one. My 900SS had Mikuni flatslides so I'm not real familiar with the diaphram carbs. On mine the fuel pump ran continuously with the overflow fuel returning to the tank. Different system but it worked. Do you have accelerator pumps on your carbs?
Bill

[SmithSwede]

It’s too bad that there is not a Ducati website that is comparable to WildGuzzi.    I’ve looked but haven’t found one. 

Which is a backhanded way of saying how wonderful this forum is. 

[fotoguzzi]

It’s too bad that there is not a Ducati website that is comparable to WildGuzzi.    I’ve looked but haven’t found one. 

Which is a backhanded way of saying how wonderful this forum is.

totally agree, I found that out pretty quick with my new Ducati. And the forum is not nearly as active.

VIVA wild Guzzi!

[RinkRat II]

     Might just be some fun reading here....https://www.ducatiforum.co.uk/search/34907513/?q=900ss+carbs&t=post&o=relevance
   
       These guys seem to be as close as your going to get to WG. They have some good info in general and I visit once a week or so even tho I have a 907ie  and some of the stuff crosses over.

         Paul B

[randy yocum]

 Motoguzzibill,you and I have the same carbs, flat slide Mikuni's, the top of the slide has the rubber diaphragm. Here's pictures I took of mine.you can see the rubber diaphragm attached to the bottom of the slide. I assuming these carbs are the constant velocity type ,i'm not an expert but when you open the throttle,your not pulling on the slide like a normal carburetor,your opening a throttle plate like a valve. Here I took this off the internet."The Constant Velocity carburetor has a variable throttle closure in the intake air stream before the accelerator pedal operated throttle plate. This variable closure is controlled by intake manifold pressure/vacuum. This pressure controlled throttle provides relatively even intake pressure throughout the engine's speed and load ranges." 

[Chuck in Indiana]

I'm not sure that at 5000 rpm the main jet is even involved.

I would imagine it is very involved.  Even the idle jet is a little involved. You are in the area of needle/orifice, though.

It's probably ignition.  A weak spark needs a very rich mixture.. <shrug> Don't ask me how this has been pounded into my thick skull.. more than once.

[Rough Edge racing]

 Does the SS have a vacuum fuel pump like my 96 Monster ?  Check the two CV chamber  vent hoses from the sides of the carbs for kinks or improper routing.. There's also a common fuel bowl vent hose between the carbs, check that also....

[LowRyter]

It’s too bad that there is not a Ducati website that is comparable to WildGuzzi.    I’ve looked but haven’t found one. 

Which is a backhanded way of saying how wonderful this forum is.

concur on both counts

[LowRyter]

totally agree, I found that out pretty quick with my new Ducati. And the forum is not nearly as active.

VIVA wild Guzzi!

yeah, I'd like a little bit of crossover regarding 937 Testaretta engines (SS, HM, MS) and software.  Notchy neutral, cold clutch, oil consumption, breather reed valve, muffler valve, TSU setting, etc.  Lots of guys just spitballing it.

[oldbike54]

 Kinda though WG is the Ducati forum .

 Dusty

[Rough Edge racing]

 I bought my 96 900 Monster about 4 years ago..I ran ok but had the often mentioned problems with the stock CV Mikunis..I got some info from the various sites but many said the stock carbs are junk and put on a set of very expensive Keihins.. I keep messing with the Mikunis, changing float levels and jetting. I removed the stock airbox to make pulling carbs easier...I put on large K&N pods instead and was told the engine will never run correctly...I was told by some that messing with float levels was wrong....
   After some frustration but persistence I got the engine running perfectly with the pods... It does not run lopey at low speeds like many Ducatis, I can cruise along smootly at a steady 2500 rpm in town..The engine responds cleanly to the throttle and pulls to over 8000 rpm on an accurate tach...
  In the long run you often have to tune the engine the way it wants to be tuned and not what others may do...

[jpv7]

I recently bought a good used 1997 Ducati 900ss here's what I have done to it so far ,installed a (New) clutch,fuel pump,fuel filter,starter solenoid,cleaned the carburetors twice,including correctly setting pilot screws and balanced both carbs to specs.Both cylinders show gobbs compression 175lbs on both cylinders.Timing is spot on, and both sparks plugs have nice hot blue flames. I've ridden the bike 400 miles in the last couple days ,it starts beautifully,has a nice lopping idle, underway it pulls very strong in any gear till I hit 5000 rpms and the the power drops off.I experimented on the highway in 5th gear at 5000 rpm as the power was dropping off I pushed the choke lever on full and it was like an after burner kicked on.The bike accelerated hard right up to 100mph and I backed off. So its got to be fuel,like I said the carbs are squeaky clean,could the main jet size be wrong? I'm not sure that at 5000 rpm the main jet is even involved.Could the needle in the slide need to be moved up.These are CV's carbs with rubber diaphragms and they are in great shape.The needle is aluminum and is not adjustable I suppose a guy could maybe shim the needle up a little with a washer or two.I just don't know where the problem lies,any Ideas out there?   Does anyone recognize this scenario? any help would be appreciated

My buddy and I both had carbed '97s.  We installed Factory Pro jet kits (with more tapered needles than stock) after a few thousand miles and the bikes ran great.  Over time, the bikes seemed to run richer and richer and we couldn't understand why.  I went to flat slides, but my buddy didn't.  After a while, he discovered that the Factory Pro needles were wearing out the needle jets, causing the bike to run richer.  Factory Pro denied they were the cause, but began providing plated or harder needle jets.  The weird thing is that the other years didn't seem to be affected.  In any case, the issue was resolved after he installed the new jets, and returned the carb settings to the factory pro specs. 

Does your bike have the original needles, and what do the needle jets look like?  How many miles are on it?

[jpv7]

To check or confirm that you are indeed running lean and not rich, have someone follow you to see what happens at 5000 rpm.  If rich you will see some black smoke.  If lean you will get popping.  The choke circuit is obviously fixing it, but that doesn't mean that it's lean.  Hope you find the issue.

[randy yocum]

Thanks everyone for your insight and suggestions,as to jpv7's questions,the bike has only 13,000 miles and the slide needles look brand new,I can't detect any wear,which surprises me because the are made of aluminum.I still think the problem is not enough fuel.My friend an auto mechanic suggested I try shimming the slide needle up, a couple thin washers should do the trick ( these needles don't have the graduated grooves like many have).Then test it and see if it behaves better.It won't cost anything but my time,I will report back soon with my results .

[jpv7]

Thanks everyone for your insight and suggestions,as to jpv7's questions,the bike has only 13,000 miles and the slide needles look brand new,I can't detect any wear,which surprises me because the are made of aluminum.I still think the problem is not enough fuel.My friend an auto mechanic suggested I try shimming the slide needle up, a couple thin washers should do the trick ( these needles don't have the graduated grooves like many have).Then test it and see if it behaves better.It won't cost anything but my time,I will report back soon with my results .

The slide needles won't wear.  While you are in there, take a close look at what they are going into - the needle jet.  That was our problem.

[Tom]

Outside thought.  When you had the carbs open....did you check to see if  rubber diaphragms had any holes or wear.  They are old.

[SmithSwede]

Did you have this problem above 5,000 rpm before you did anything to the carbs?   That's a potentially important clue.  If these same carbs weren't doing this before you got into the carbs, but are doing so now, you probably induced a fault somehow when you were working on the carbs. 

I've pondered this, and I strongly suspect that there is something wrong with the CV slide.  At lower speeds, the other jets, and maybe a partially opened slide, flow enough gas mixture for the engine to run ok.  But when you want lots of power above 5,000 rpm, you can't get enough gas mixture because the slide won't rise up far enough.

So I'm thinking there is probably a vacuum leak of some sort.  Are the gaskets properly in place?  Everything snug?   Or maybe there is some crud in the mechanism that prevents free motion. 

Sounds like you can spray propane from an un-lit propane torch all around the outside of the carbs while the engine is running.  If there is a vacuum leak, the propone will get sucked in and the engine speed will increase, confirming the existence and location of the leak.

If you take the carbs off again, I'd look to see if there is a way to apply a vacuum to the correct area to see if the CV slide moves freely, and fully, to the top.  I've never seen these carbs up close and personal, so I'm not sure if that's feasible. 

[LowRyter]

I posted the link to Factory Pro jet kits and carb rebuilds above.  (if that turns out to be the issue)

[randy yocum]

When I got the bike it had been sitting in dry storage unused for several years.I discovered the fuel pump and starter solenoid didn't work,the tank was pretty clean with no rust.The bike wouldn't start so I pulled the carbs they were dirty but easily cleaned .The rubber diaphragms attached to the flat slides looked really good and pliable.So to answer SmithSweed  I don't know if this is an old problem or new,because I have no history on the bike.I like your idea of using a porpane gas around the carb area while the engine is idling to find out if there is a vacuum leak.I like your idea of attaching some form of vacuum to the carb to watch the motion of the slide. I've never heard of anyone doing this ,I would like to see a video of someone doing it to one of these mikuni carbs.I wouldn't know where to  attach a vacuum line to induce  the slide to rise,cool idea though. You know these bikes are unique in that you can easily raise the tank up exposing the air cleaner,battery,electrics.great idea.Anyway remove the air filter  and your looking straight down at the carbs in the airbox,with the slides right in your face.Start the engine and you can see the slides and the throttle plate move as you turn on the throttle.The thing is my mechanic friend told me the vacuum slides won't behave the same with the bike sitting in neutral in my garage as they would  under load going down the highway,too bad it would be an easy way to test if they are working correctly.   
   I've got company coming in this weekend,I spent the day getting ready for them  so I haven't had a chance to do anymore with the bike .It will have to wait till next week.Thanks again to everyone for your help.

[SmithSwede]

Yep!   I love the pivoting gas tank and how easy it is to get access to everything.   Brilliant design. 

In my opinion, the diagnostics totally change since you don’t know the history of the bike before messing with the carbs.  Could be wrongly sized jets, improperly adjusted screws, who knows what.  I’d go back to stock everything.   

Here’s another idea.  If you take the carbs off again, why not drain the float bowls to get rid of all the gas, then experiment with putting the hose of a powerful shop vac on the engine side of the carbs.  Not sure how much vacuum a shop vac can induce, but I’ve got one that seems pretty sucky.   Then play with the throttle plate and see if you get good smooth motion of the slide.  You should be able to see this action from the intake side. 

Alternatively, maybe run a leaf blower into the intake side and inspect slide action from the engine side.  I’m less sure that would work since the engine side may not see enough vacuum in the open.

There should be a passageway on the engine side that “sees”
the engine vacuum, and it communicates that vacuum up into the slide diaphragm.   Is that passageway blocked?  Meanwhile look for any potential leaks in this system. 

As to your mechanic friend’s suggestion of raising the jet.  Seems like that might work, certainly if the prior owner had it too low.  But I’d be somewhat worried that it might merely mask the problem by raising the needle, when the real problem could be that the entire slide isn’t raising up completely. 

[SmithSwede]

Randy:

The stock carbs are Mikuni BDST 38mm CV.   These were used on many other bikes, such as the Yamaha XTZ 750 Super Tenere and the TDM 850.  So maybe other bike forums would have diagnostic information, and a Yamaha dealer could supply parts. 

Looks like you can buy a pair of new Mikuni carbs for about $600.  I suppose that is the guaranteed fix. 

I have confirmed that the “choke” on this bike is a separate enrichment circuit that draws fuel from the float chamber through its own jet.   So it adds more fuel (as opposed to restricting air).  Seems to me the most important clue is that the bike runs fine if you give it “choke,” i.e. more fuel above 5000 rpm.  Which I think means spark is fine, there is enough fuel reaching float chamber, etc. 

Here are the OEM specs according to my Haynes manual.  Since you don’t know what the prior owner may have done, I’d suggest confirming that you have all stock parts and if not, replace with the stock unit. 

Main Jet is 140

Main air jet is 70

Needle jet is Y-2

Jet needle is 5C19, with clip set at 4th notch from bottom

Pilot jet is 42.5

Pilot air jet is 60/1.4

LT Synder manual suggests that you stick your finger into the carb body and push the slides up.  You should hear the vacuum suck, and both slides should feel exactly the same. 

The Synder manual also shows a small O-ring underneath the diaphragm cover.  I bet that O-ring is easily lost or overlooked, and a likely source of vacuum leaks. 

Finally, I’d make double darn sure the carbs are fully seated into the rubber intake manifold boots, and there aren’t any splits or cracks there, and everything is snug and tight.  Synder says to “push them home,” which suggests to me it’s possible to put them only part way in.