LCGB Forums

I thought I would share my findings!
Just built a mugello 198.
Over the years I have built loads of reliable motors, but was stumped as I couldn't get this one to idle....
Plug black despite trying pilot jets as small as 45. Sooty black plug.
So I got to the point of stripping it slowly to recheck everything.
Turned out to be the slide was slightly out of round and thus sticking. Had a rubbing point on lower edge.
This was a second hand carb and at some point the slide had been slightly squished.
Just another thing to check if your bike won't idle!
I hope this post helps save others loads of grief and time for what is a simple thing.

I could be missing something but I don't understand how a sticky slide would cause the symptoms you describe. It would be more likely to cause the idle speed to be too high. The sooty black plug and reducing the idle jet with no effect sounds more like the choke sticking on

Hi Coaster, yeh I thought choke piston was not closing fully, however I disconnected it as well as making sure it was visually.
The plug was sooty and the bike won't idle.
The reason I think it is the slide is that it is sticky and has a ware area at skirt. I think this is causing slide to not go to bottom of carb. The slide is also visually out of round and sticky. This I think means that the needle is not going into atomiser as low as it should on idle.
I really welcome your shared knowledge as a second inside is always good.
I will let you know later how I get on as I have to swap a slide from another carb to test.
I'm living in France so can't just run down to Camb Lam!

For now, you could fit a stronger spring (from anything that will do the job) or even ‘double up’ to prove the theory, because Coaster is almost certainly correct.

it would be helpful to know which carb and list the jets fitted .
If the slide is sticking then you will need to dress the slide lightly with some 1200+ wetndry until it slides smoothly.
If you rub some engineers blue on the slide and operate the slide over its full travel you'll see exactly where it is rubbing and needs easing.

Try disconnecting the slide from the throttle cable and operating it manually and see if it still binds

My earlier suggestion was only as a stop-gap measure.

Unfortunately, the chances are as with all Dell'Ortos, the carburettor body is a soft alloy & very easily distorts as well as stripping threads. I think it's doubtful that the slide has distorted, although it would suffer the effect from any distortion of the body. The chances are either one or the other has suffered a bit of a bang.

Good condition slides in a good condition carburettor will not stick for any other reason than something causing a problem, & that won't be due to wear......

Bit puzzled with this one ?

If the slide was sticking open wouldn't that cause it to rev' ?

When you're saying you can't get it to idle do you mean it's dying or revving too much ?

Float valve stuck open?

well whatever his problem his post suggests that it is now fixed
Sometimes in removing cleaning and refitting carbs and setting them up... you can fix an unknown 'problem' without ever knowing what it was

Ok put in replacement slide number 50.
Now able to set idle with mixture screw, pilot 48 now.
Idles great, but wouldn't rev without backfiring.
Rechecked timing17 degrees- Indian lightweight electronic.
Replaced plug as it was fouled.
Runs great now clean all though rev range.
Spec:
28mm phbh
266 atomiser
X3
118 main jet
Second notch from top.
No dynos where I live!
So tune by ear, plug colour, reving cleanly and returning to idle cleanly.
I believe if it is really loud then it is generally rich.
I always say though if you can use dyno then do, small investment to get it spot on.
Thanks for all the comments, their is a lot of genuine wisdom shared on this forum.

x3 needle ? or was it x13?

Simon_T wrote:x3 needle ? or was it x13?

Think you must be right mate,

Rich'