Paso 906 - Spluttering at high revs

Pez · Post by **Pez** » Fri Aug 25, 2006 4:07 am

Hi All. I've just recently bought my first Ducati - a 1990 Paso 906 and absolutely love it! I've always had sport Jap bikes and this is my first V-Twin. The sound, the power low to mid range and the feel is fantastic, however I have noticed at high revs (say above 6k), she tends to die off a little and splutter - almost like a rev limited has kicked in. The original owner had removed the Webers and air box and replaced them with 38mm Mikuni's. He told me this is a pretty standard mod that most Paso's have done because of the 'problems' the Webers give - whatever that means. I'm wondering if anyone else out there has this problem and also had the carbs changed like this. Is this normal and should I really be looking at putting the Webers back on? As I mentioned, I've always had Jap bikes and really used to the high rev acceleration that seems to be lacking here. I'm keen to get her going at her best and would love any advice you guy's could give me.

Thanks.....

Duc750 · Post by **Duc750** » Fri Aug 25, 2006 10:09 am

Do you mean high revs and high speed.

Paso780 has an interesting theory about the Mikuni's being run without an airbox which I'm inclined to agree with. Basically at high speeds the air flow past the carbs is too much for them to fuel correctly, when I get my 750 back in one piece then I'm going to play with a blanking plate up there to create so still air for the carbs to sit in.

ducapaso · Post by **ducapaso** » Sat Aug 26, 2006 9:39 am

You can try just to change the spark plugs... they're a very sensble part of the paso's engine: bad starting, excessive choking, additives in the fuel can make plugs source of various bad symtoms.
test also voltage in the orange wire upon the coils while starting the engine: it must be the same of the battery, otherwise, go there
http://it.msnusers.com/pasochisti/Docum ... 0plant.doc
for more suggestions, or search "electrical" in paso forums.

Pez · Post by **Pez** » Sat Aug 26, 2006 1:38 pm

Thanks guy's for your replys.

The problem occurs at high revs and high speeds. It just loses power and starts to flatten out. I have given the bike a full service - plugs replaced and electricals checked. Thanks anyway ducapaso for your advice.

I think you are on to something Duc750. The airbox is probably the key. The air flow past the carbs at high speeds would play havoc with the air/fuel mixture and cause the symptoms I am experiencing. I will play around with some sort of blanking plate as you have suggested and see how it goes. I'll let you know the outcome and maybe save you some time with your 750 when you put it back together.

Thanks again.

Post by **Skins** » Sun Aug 27, 2006 6:45 am

I think you're more likely running out of fuel than running out of air with this problem at high speed, because at full power you need a rich mixture, and if you were running out of air in the mixture, that's what you'd get. I think this especially if she runs OK for a while at high speed, then cuts out.

I had this problem with a motor running out of fuel once, running in speed events. She would run fine at or close to full throttle for several seconds before cutting out, then pick up again once the revs had dropped.

Turned out not enough fuel was getting into the float bowls for running at full throttle, and she was draining them.

ducapaso · Post by **ducapaso** » Mon Aug 28, 2006 7:04 am

Skins, you're right. I happened the same thing many years ago when my car started splittering at high revs! I was forgetting that!
the trouble was fuel pump gone, but the same can happen also because of wrong/gone fuel filter... Pez did you cecked the fuel pipeline?

Post by **paso750** » Tue Aug 29, 2006 1:55 pm

as written above, for me it`s just a theory. If the engine runs lean at high rpm you will have sudden power loss. I experienced that on my Gilera when trying aftermarket exhausts without rejetting.
If it runs too rich problems I think are similar but less abrupt and combined with more exhaust smoke.
reading about the problem Harry has would indeed indicate on a too lean mixture.
Still I wonder if the engine gets enough air when driven w/o airbox as the carb is standing so upright. I`d love to know about any effects using a backplate on the filters or to different jettings. So guys keep posting your results.
Although jetting is probably best adjusted on a dyno I think using open filters will definetely require some driving tests as I don`t think the effects of the airstream at high speeds can be properly simulated in a shop.
I personally always drove with an airbox and will stick to that also if it`s a lot of work building one.

G.

Pez · Post by **Pez** » Thu Aug 31, 2006 7:25 am

Hi Guys, Thanks for all your suggestions and advice. I have been running premium unleaded in the bike (98RON) and then I read somewhere the other day that you should be running the lowest RON grade you can without the bike pre-igniting. So last weekend I filled her with standard unleaded (90RON), took her for a big blast making sure I reved her nice and high and all of a sudden I noticed the flat spot and spluttering started to disappear. By the end of the ride she was running beautifully. I think the combination of fuel and a nice hard ride really cleared her out. I would however still like to fit an airbox to see what happens. Does anyone know where I can find a link to someone that has done this before to a 906 with Mikuni carb mods? I'm keen to experiment...

Post by **paso750** » Thu Aug 31, 2006 9:38 am

http://forums.ducatipaso.org/viewtopic. ... cle&sid=29

this is on a 750 showing the most simple solution modifying the original airbox. The negative part is that it loses volume so I`d say a K&N filter becomes mandatory.
The original carb jettings from a SS or also a Dynojet kit may be used as starting point for rejetting.
Recovering the lost box volume is difficult unless you create your own construction.

drop me a PM

jomo · Post by **jomo** » Sun Oct 01, 2006 12:03 am

Pez,

Are you running air filters or just open ram tubes?

I've got 40mm Dllortos on my 906 and it had ram tubes to begin with, but I wanted clean air, so I tried all sorts of filters. The problems were the same as you experienced. I changed to large K&N s and all is perfect. Now I have that part sorted, I may have it dynoed just to get it perfect.

My suggestion is to just remove the filters and take it for a blast.

Good luck,

jomo