Dellorto Carbs....Again

Post by **jcslocum** » Fri Dec 30, 2005 2:48 pm

I have yet to completely figure out what the issue is with the carbs on my Paso and I'm looking for any ideas to try. I had a choke cable break a few weeks back and finally got around to replacing it yesterday. I also took the time to try to setup the carbs once again since the bodywork was off and they are better but not good.

The problem is that the idle will stay up if you roll off the throttle nice and easy. The idle is set around 1200 RPM but it will stay at 2500-3000 RPM when this occurs. If the throttle is closed fast, the idle returns to normal. If the idles stays up at the 3000 RPM level, all I have to do is "blip" the throttle and it settles right down to 1200 RPM.

I have checked all of the mechanical settings; Cables are not sticking, air slides are free and fully returning, choke valves are closed, etc...

I have adjusted the mixture properly and get a full range of adjustment from them. If I turn the screw in too far it will beging to miss and then stall. If they are turned out to far the engine begins to stumble/blubber from being to rich.

I'm stumped by this condition as it usually occurs when the mixture is set up too lean but I don't think this is the problem. I get a normal range of adjustment from the mixture screws and they carbs have been synced.

Any ideas???

Post by **Skins** » Fri Dec 30, 2005 6:32 pm

Jon, it sounds to me very much like you have little wear dimples in your slides where the throttle-stop screws strike them. The end of the screw is hard and sharp, and the slide is very soft, and after a year or two ...

The little dimple hole plays havoc with the idle. The vibration of the motor causes the slide to 'ride' the end of the screw, jiggle about, and the result is chaos. Believe me - sort this one out, balance your carbs according to my 'Ducati cuddle' technique, and you'll see what the lovely smooth torquey Paso 750 is all about.

Take your slides out and you'll see the little wear dimples as plain as day, especially under a magnifying glass. You will clearly understand how the bike cannot idle properly with those wear dimples there. Follow the instructions in what I believe was my first post on this site, and enjoy your Paso 750.

ansim · Post by **ansim** » Fri Dec 30, 2005 6:49 pm

Exactly Skins.

I had the same problem as Jon, idle at 3000 and not going down.

When I serviced the carbs, I noticed the "dimples" on the slides.
I carefully grinded them and when spring was setting and the first ride was performed, the 3000 idling was gone.

No problem after that.

//Anders

Post by **jcslocum** » Sat Dec 31, 2005 3:34 am

Pull the slides it is then. I'm riding Sunday (New Years tradition) so I might try to get it fixed tomorrow, otherwise it will be yet another project for later in the winter.

I can get parts for the Dell's here so maybe I'll just order 2 new slides and throw them in.

Post by **Skins** » Sat Dec 31, 2005 4:56 am

Good stuff, Jon. And I strongly recommend that your last check is my 'Ducati cuddle' slide synchronization to within 4 thou of slide movement or better, if possible. Four thou movement of a slide probably equals at least 200 rpm.

Probably the cables will need very careful attention to ensure things are going to stay right once you get them right.

Paul · Post by **Paul** » Tue Jan 03, 2006 4:32 pm

Jon,

I had exactly the same problem on my PHM40s. I followed Skins' advice and I even posted pictures of the dimples. I used a needle file to file them down and set the carbs according to Skins' advice. It was much better but it definitely didn't cure it. It still does this occaissionally (more often when it is hot).

If you find the complete solution I would be interested to find out what it is.

Thanks

Paul

Post by **Skins** » Wed Jan 04, 2006 12:36 pm

I didn't realise you had 40's on your bike, Paul. Carbs that size would make things even trickier. If you have everything else perfect and it still not right, you could try slides with a smaller cutaway.

Paul · Post by **Paul** » Wed Jan 04, 2006 12:49 pm

Skins,

I am not quite sure why anyone would want to put 40's on a 750, but that's what it has. I have learnt to live with the problem so I am not too worried about it. It was just that Jon made a post regarding this so I thought "if he finds a cure I can try whatever he did".

Are you still "migraine free" ?

On a completely different subject, I saw a "Supersport 900 manual" for sale in the local "Bike SA" classifieds yesterday for R100 (about US$16) so I phoned the guy this morning. Aparently it is a CD version. I explained that I had a 750 Paso and the engines were basically the same. He said that he thought he had a manual for one of those too. He phoned back later to say that he did. I am not sure what it is that I will be getting but I am going to collect it tomorrow morning. I'll let you know if it is worth having, and if it is appears to be worth uploading to the Downloads section.

Paul

DesmoDog · Post by **DesmoDog** » Wed Jan 04, 2006 1:56 pm

Paul wrote:I am not quite sure why anyone would want to put 40's on a 750, but that's what it has.

Took them off that beat up old '74 750SS that was sitting in the back of his uncle's garage maybe???

Post by **Skins** » Wed Jan 04, 2006 5:29 pm

Yeah, mate, still migraine-free ... it's wonderful!

I hope the manual turns out OK. I have the factory Paso 750 Manual, which seems pretty good in most respects, although I have noticed one or two inconsistencies.