ducatipaso.org

Hey gang -

Last year my uncle & I bought a '90 750 Sport as a fixer-upper and it's finally running again.

Sparing you all the gorey details, here's my dilemma. The bike has had the stock Weber and airbox tossed in exchange for a set of 38mm carbs from a later model 900ss. It has long intake runners, and the carbs are connected. The carb mouths are just a few mm from the battery. It has Cobra F1 slip-ons. Anyhow, I had the bike worked on recently - new belts, valves checked, new plugs, carbs torn down and cleaned throroughly, jetting adjusted, and installed foam pod filters on the carbs.

My mechanic's first comment is these carbs are way too big. He said they'll be perfect if we do a high compression piston kit down the road, but right now they are overkill and were extremely hard to tune (4 hours on the dyno getting it right). He still isn't happy with the performance. But the bike runs really good compared to before. He measuerd 61 hp and 46 lb-ft torque at the rear wheel.

The shop is about an hour's ride from my home, and I rode it home with no problems - ran flawlessly. But the more I ride it, the more I feel a hesitation from time to time. Kind of feels like a fuel starvation situation - not a misfire or lack of spark situation. It happens at steady throttle, usually above 45 or so MPH, and usually when it's in traffic. Not sure if it's temperature related or not yet (it's been pretty hot here, around 85 to 90 degrees F). Definitely more noticable in traffic than running alone on the road.

My question is... is this normal for an open-pod setup? Every other bike I've ever owned has had an airbox/pipe/jetkit so I don't have anything to compare it to.

Here's a pic. Still needs to get the Harley grips taken off, install the new tires, install new brakes pads, get some stainless steel braided front lines, and install bar-end mirrors. Then we're going to ride the wheels off it.

If it happens from time to time, this looks more likely to be a fuel supply trouble.
The fuel filter is brand new?
how does the pipeline run?
the carbs look to be placed in quite an high position, this might need a fuel pump (like the weber has)
Anyway, the 38 mikuni properly jetted fit very well on 750 engines

I have heard, that Mikunis NEED GOOD AIRBOX to function well, so i am wondering now, that U have fitted foam air filters to Your bike...

Good questions, thanks for you responses.

No we didn't replace the fuel filter, but as luck would have it there's a whole post on just that subject on this very forum. I'll do that first.

As for the airbox question, you are correct we only have pod filters, which is something I was concerned about. There's nothing to keep engine heat out of the motor. Finnpaso is that something you heard related to the Mikunis as well?

Hang on, I just remembered something... when I replaced the fuel lines I did add an in-line fuel filter to the line between the tank and the carbs. It's one of the clear filters that you can watch fuel flow through.

Is there one inside the tank as well? There is a fuel pump in there, I can hear it whirring when I turn the key to the On position.

There's a pre-filter which is kind of like a big long tea bag. The fuel pump and a filter if it's like a typical SS setup.

It does sound a lot like fuel starvation. But... the fuel pump and plumbing must be made right first to eliminate it from the equation.

Awesome! Thanks Jon, I'll take it apart and see what I find...

don_zielke wrote: Finnpaso is that something you heard related to the Mikunis as well?

I have bought just set of Mikunis from my one Ducati friend(888&900 Elephant). He make modification to his Elephant: Put M16 Ecu and hole injection system from ST2 and then quite new Mikunis came useless=>I bough them with manifolds and Elephant airbox. He have played alot time with injection and eprom systems and he told to me, that Mikunis NEED very good airbox to function well. I trust him, cause he knows more, than me from injection and carburators(He have had also KTM supermotard with carbs and he have played also with them quite alot. I know, that Dellortos dont need such airbox, but Mikunis need. He explaned something, that Mikunis working needs some underpressure to air intake, so airbox needeed. I havent had time yet to read more about Mikuni principales, but as they use those in Monsters, ss, etc in 90s, those were fitted with normal airbox, NOT any pod filters. I think, those Monsters, ss, etc function quite well with normal settings(airbox)....

Guys,
You do not need the air box for the Miknuis. I have foam pods with the Mikunis and it works wonderfully.

Fraser.

Thanks, Fraser Btw, have YOU dynoed Your bike before and after Mikuni conversion ??? It would be nice, if someone have, that know, is there any reason for conversion... Surely behaving is better, but does that conversion give anything "more", like more hps, torque, etc.... I was thinking to adopt Elephant airbox to Paso and use Elephant normal air filter, what is easy to find from many shops... Elephant box looks such, that it takes less space from under tank, than Pasos airbox. Surely it needs also modification with fibreglass, but thats easy job to me.

Fraser, Hows Your jetting, cause it seems work wonderfully??? I need to find surely right jetting, but i think i am already close with normal Elephant jettings, cause that Elephant modell have smaler valves and quite same cams, as in 750 Paso. Anyway all notes about Mikuni jettings are welcome...

He measuerd 61 hp and 46 lb-ft torque at the rear wheel.

Pretty good IMO. FYI 600cc carbed Ducs have 38mm Mikunis, too.



I have ridden my Monster w Mikuni`s without any filters w/o problems.
Pods worked well f this guy: http://www.ducatisuite.com/jetkit.html

BUT,
the hoses for vacuum operating of the slides needs still air,
f e behind battery on a Monster or w the original grey sidepods.
Very important!

A bit of air box, or just simply some hose length between the carb and the aircleaner will always help the symptom you mention. I used some 2 inch/50mm rubber intake hoses that are 90degree elbows. It helps contain the fuel fog that comes back out of the carb when the intake valve closes. This has much more affect at lower RPMs. It also helps to align the airflow a bit more, which should help across the entire range.

It also helps to put the aircleaners in in a place where you can put some nice big ones on.

So in short, any amount of intake track helps all conditions, just make sure it is big enough so it doesn't restrict flow. A 50mm intake hose for a 38mm carb is a pretty good set up. A couple of inches is plenty. do a search for intake hoses and you will come up with plenty of options.

Cheers,
Phil
90 Sport
More info on the Sports at http://www.sport-classic.net/ in the forums.

+1 on Phil.