Carburation issue?

[FenlandExile]

I have an issue at midrange engine speeds on my 1977 350 Sport. Up to 4000rpm the bike runs perfectly, and above 5000rpm it again runs cleanly, pulling like a good un should. But between 4 & 5K it coughs and splitters quite badly. I'm thinking this is a main jet/needle/atomiser issue. Anyone out there come across this and know the solution? At the moment I'm on 115 main jet, 260BD atomiser, and E17 needle. Any advice to sort this much appreciated.

[MickeyMoto]

1. Where in Northumberland are you?

2. I think the main jet should be 112

[FenlandExile]

Hi Mickymoto,

1. I'm in Choppington at the moment, but will shortly be moving to Longframlington.

2. Yeah, I'm thinking of buying a range of jets, needles etc and trying different combinations. Before then I was hoping someone on here had come across this issue already and have found the sol to save me some time and expense.

[morini_tom]

Are you running a standard exhaust and airbox? These things have a big effect on jetting and even a change in silencers to something similar looking but not fundamentally the same as original will have an influence.

However, assuming your bike is as standard, I would be considering the following:

1) Sports are often a bit rich in the 4-5000 rpm range and you may find depending on gearing and tyre size that I top gear at ~60mph uou might find yourself needing to downshift when on an incline for example.

2) 115 sounds a bit big. As suggested, try a 112

3) 260BD and E17 sounds ok, but how old are they? They do wear which will richen things up which will only make rhe 4-5000rpm rich hole worse

4) how are your choke rubbers? The pads deform which can lead to poor sealing and could lead to a change in fuel delivery in normal unchoked running

5) rich running can make a weak ignition system more prone to breakdown, so don’t discount bad plugs, leads, caps and transducers as a factor


If it were me would do 2 and 4 straight away as they are quite cheap and easy to do, and if that doesn’t vastly improve it I would try to get new needles and atomisers (no point in replacing individually) although 260BD are hard to find so you may end up having to modify something else

[MickeyMoto]

Hi Mickymoto,

1. I'm in Choppington at the moment, but will shortly be moving to Longframlington.

2. Yeah, I'm thinking of buying a range of jets, needles etc and trying different combinations. Before then I was hoping someone on here had come across this issue already and have found the sol to save me some time and expense.

Longframlington is close to me, Choppington not far. I may have some 112s, i'll look tomorrow.

The Running Fox will be a good place for cake. There is a new Moto cafe in Amble.

[FatherB]

I had a similar issue on my Strada which is now rebuilt with Sport pistons and an L5 cam. I dropped the needles (E17s) to the top notch which more or less cured it. Needles and atomisers were new and correct for a Sport. I think modern fuel caused my issue and weakening the mid range cured it.

[FenlandExile]

Hello all and thanks for the replies.
Very interesting to hear about the rich running in the mid range, it does feel like that is happening on my bike.
I'm not overly familiar with Dell Orto carbs, but experience with Amals makes what fatherb is saying make a lot of sense. Main jet size, on Amals at least, just affects running on full throttle, and I don't seem to have an issue with that.
So when I get a chance the first thing I'm going to check is where the needles are set in my carbs, if the clips are in the middle of bottom grooves then I might have a quick solution. If the needles are already in the lowest position then I'll have to consider replacement parts. On that front, if 260 BD atomisers are hard to find, can someone recommend a nearest equivalent please?
I'll post how I get on with this as and when.


Hi Mickeymoto,

Are you in Amble? I think we met up at the national rally near Norwich a couple of years ago. I was on my CB750 then. It would be good to meet up again. I'm not familiar with the biker cafe in Amble. What's it called and where is it?

[MickeyMoto]

I have found some jets, of which 2 may be 112s. They are a bit chewed up as were stuck in the carbs but welcome to try them.

Cafe is called Ethos on the Industrial Estate just past Morrisons.

[George 350]

Hi,
Unlike the others above, I still run 115 main jets, as these are standard for a Sport. A good thing to do (IMO) is to fit #42 or #43 pilot jets. That and as already suggested, new needles and needle jets plus choke plunger rubbers.
Pay close attention to carb balance too as this can also make a bad situation worse.
Perhaps it is why Morini fitted a 6 speed gearbox in the first place!
Good luck,
George

[3potjohn]

I think the main jet size did go up to 115 for a year or two then reverted to 112 prior to the K2 era. My K1 has been fine on 112. Everything standard—ish.
What atomisers are you running? Are the passageways clean?
The choke pistons as mentioned do degrade with an increasingly over rich condition.

[FatherB]

Your issue sounds very like atomiser and needle adjustment as I mentioned above. If your bike is running well below 4,000 rpm as you say it's unlikely to be connected to the idle/pilot jet circuit and if it's picking up cleanly from idle then it sounds like the idle/pilot to slide stage is OK otherwise you'd have a stumble or hesitation there. You said you were familiar with Amals: remember that the mixture screw on a Dellorto VHB controls the fuel going in to the idle circuit and not the air as on Amals. Therefore screwing it out will richen the idle mixture not like an Amal where screwing the mixture air screw out weakens the mixture. If your bike is idling well and reliably and picking up cleanly from idle then I'd leave that alone. It's unlikely that the choke slider rubbers are leaking as that would definitely affect early throttle positions but they're always worth checking and cheap to replace if in doubt. I think dropping your needles a notch will clear up the mid range problem. Please let us know how you get on.

[norbert]

This hole about 4500 revs is well known.
A friend of mine told me once that he cured it putting a small washer unter the atomizer . Could be worth a try and don't cost Nottingham.

[MickeyMoto]

Sounds a bit Mrs. Schilling's orifice!

[norbert]

I know that it sounds strange. It' just a cheap way trying change the posicion of the holes of the atomizer. Nothing to Do with magic

[norbert]

At this midrange nearly everything of the carbsytem has some influence.
I don´t have that problem using since years 501 motors with PHB28 carbs with uncomon 268T atomizers and 50 slides (not the usual 264BC with 40 slides).

I wondered as well when he told me about this washer. But this friend works since decades as a professor at the Hambug university department vehicle tecnology ... Maybe it´s woth a try and as said above does not cost nothing. (of couse I don´t know Mrs. Schilling )