Carburettors

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Steve Brown
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Re: Carburettors

Post by Steve Brown »

Sorry but the figure I was suggesting was just that-a suggestion! I can't see where it is now either and that's the trouble with the internet. Too much well intentioned help that is not to be relied on! I might even be remembering a figure I used for Amals or Mikunis!
If you get yours both set the same at say 5mm for instance that should be a 'safe' starting point on the rich side. It wouldn't hurt to then try 6mm and so on. You'll soon know if you've gone too far as it will be weak everywhere and obviously don't run it too long in a weak state. A short test should be fine.

And it's probably the ignition at fault anyway. :wink:
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mbmm350s
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Re: Carburettors

Post by mbmm350s »

I should also add that the two carbs have the connecting fuel line between them (that is how it came).
This is normal as there is only one electric fuel tap as standard, the second tap is a reserve tap,
do you mean the vacuum balance pipe on the vacuum side of the carbs?
In any case the vacuum balance pipe, if present, should be removed and the vacuum ports blanked or used to connect to the vacuum gauges/manometer during carb balance.
I am now stumped, how could 50 idle jets and 43 idle jets need the same adjustment at the mixture screw to obtain a blue burn? Also why do I have to adjust the mixture screw to almost full in before I get a blue flame?
Because as designed the diameter of the idle jet is not a controlling factor in the idle mixture circuit,
selection of the idle jet is based on opening (and closing) the throttle, the idle jet is important in progression and overrun.
So what does control the idle mixture when the carburettor is in good conditions, i.e no wear or blockages and all seals etc good.
1. The size of the progression hole
2. The vacuum, or rather lack of, this needs to be weak
3. The type of taper on the mixture screw, various models with HS carbs had a longer taper.
BS carbs usually have the short taper.
4. The position of the screw.

This is important to understand as its not obvious.
During idling the progression hole draws air in to weaken the mixture passing through the idle circuit (from the idle jet)
This is intended to weaken the idle mixture, actually I would say it provides a correction to the fuel air ratio but I will not argue with the Dell Orto manual.
How much of this weakened mixture is fed to the cylinder is controlled by the mixture screw,
The mixture will mix with a small amount of additional air coming under the tiny gap below the slide.
Often I have found that worn carburetors will never satisfactorily idle on the idle circuit.
simply too much air is leaking, the weak vacuum required by the idle circuit is destroyed, the circuit needs the air through the progression hole
so its too rich, we try to wind in the screw but it bottoms out, our only recourse is to weaken down the idle jet, drastically,
but this is a bodge.
What we should do is to figure out if the slide is leaking too much or is far to open. I can tell if a cylinder is idling on the slide as I call it.
The commonest thing is the mixture screw has no effect at all.

Typically as Paul says the cylinder is actually idling on the slide and
On the other hand if retarded, the slides may end up too far open to
compensate and you'll be on the progression circuit, not idle.
So you must check that the ignition is right. Not only the 6000 rpm full advance but also that it backs off at idle. There are
so many possible combinations of pickup and transducers that could affect the advance. The standard system
generally goes from about 22 degrees to 34 degrees, but often there is less advance range
If happy with the ignition timing.
The selection of a smaller idle jet is beneficial in some cases to improve the progression particularly
on worn carburettors but it wont fix the idle properly. A new slide would be a superior fix.
The conclusion I have come to (maybe wrongly) is that the carb is getting the additional fuel in some other way, possibly the float bowl is too full and the other jets are picking it up irrespective of the idle jet size?
To establish fuel level is extremely common place in other manufacturers carbs
and the special modified bowl connection and sight tube is readily available.
Excessive staining at the vent hole would indicate badly awry float level, but since this is not a two stroke
I don't think the fuel level to be super critical. The fuel level is the same for all type of floats suitable for VHBZ
carbs i.e round solid, more squarish solid, black round and hollow and the beastly white hollow ones which were
standard fit on Darts.
Another source could be via a poorly seated choke valve, the small rubber seal of which there are two types, can be perished
I think new seals can be obtained, but certainly new choke plungers can be.

Often overlooked is this:
The atomiser is 2.60mm and the standard needle is 2.45mm, therefore only (2.60-2.45)/2 = 0.075mm of wear on the
slide face or the carb body itself is enough to cause the needle to come in contact with the atomiser wall in this
case there is a similar effect to capilliary action causing unatomised fuel to draw up the needle. Though this tends to affect
progression very badly due to poor fuel atomisation at least until we are on the taper of the needle after 1/4 throttle.
Now to compensate the slide needs to be a little more open to get more air.
Then you get that typical coughing off idle of Morinis with shagged carbs these days.

There are some small compensations to jetting for modern fuel and many report that progression is improved with a smaller idle jet,
with 45 being a standard.
The question arises why the 260BD for A sports, with the additional higher holes in the atomiser,
which should be above the fuel level were placed to weaken the mixture in the midrange.
My understanding is the atomiser solution was developed for the Sport which was running too rich in the mid range.
Too rich? Yes indeed! Someone can ask Ing. Lambertini if we ever get to Morano!
Whilst the 260BD is no longer available it is easy to drill the holes needed by modification of a 260K
full details available to club members via ATG a few issues back.

Tuning guide is available from Eurocarb for very reasonable price.
Mark
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Ming
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Re: Carburettors

Post by Ming »

Reading through this topic, despite the interest and detail alone, gives two thoughts. 1) Carburettor slides sound to be a good project for 3D printed replacements. 2) Has any thought been given to a FI system?
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Paz2112
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Re: Carburettors

Post by Paz2112 »

Mark,

Firstly thank you for the detailed answer, there is a lot to take in there but also a lot that makes sense to me. The possible issue of the slide does make complete sense and this might be going down the correct path. My Slides can be moved both from side to side and back to front approx 1mm. The needle does seem to be a little off dead centre of the jet as well. If the vacuum was strong due to the worn slide then this could explain the additional richness of the idle which is difficult to change with the mixture screw until it is almost all the way in.

Last night I started to make myself up a banjo joint and 8mm nut for the bottom of the float bown, to allow me to easily measure the exact fuel height in the bowl. Once I have checked that I will also check the spark, timing and ignition coil resistance again. I did all of that two years ago and it was good, but I will complete a full check list now to be sure.

If all of that checks out I might invest in two new slides, they are expensive but it might just make the difference.

I would appreciate if anyone had the instructions/diagram for modifying the atomiser to make it a 260K.

Again Mark, thank you for all your suggestions and help, this is greatly appreciated.
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Steve Brown
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Re: Carburettors

Post by Steve Brown »

Ahh, 1mm? that is a lot.
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mbmm350s
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Re: Carburettors

Post by mbmm350s »

Reading through this topic, despite the interest and detail alone, gives two thoughts. 1) Carburettor slides sound to be a good project for 3D printed replacements.
Eurocarb have had made at considerable expense new slides for the VHBZ. I think they are good value really.
The important thing is that they are anodised, and that the anodising is complete,
a simple visual inspection of loss of anodising on the vacuum side, starting at the edges, is the precursor to a worn out slide.
In extreme cases we can detect a noticeable ridge. I even saw a pair that had be refinished flat by removal of the anodising completely.
These must be scrapped. I read that the Italians have had some success in resurfacing.
My Slides can be moved both from side to side and back to front approx 1mm
That's not neccesarily the issue as the spring holds the slide against the vacuum side,
the wear limit is much smaller in comparison. Its about the wear on the face of the slide thats important.
It is very difficult to determine the concentricity of the needle in the atomiser.
Do check that the needle is not loose in the holder in the slide, meaning the tip shouldn't move very much.

Cheers mark
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Paz2112
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Re: Carburettors

Post by Paz2112 »

Please forgive the quality of the image but these are the current state of the slides in my carbs.

Front Cylinder slide
Image


Rear Cylinder slide
Image

The needles do feel firm enough in their position.
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mbmm350s
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Re: Carburettors

Post by mbmm350s »

Hi,

Thank you for the pictures.
Not quite the worst I have seen but very bad.
These are most likely worn out. Even the venturi area anodising is missing.
Place the slides on a flat surface and see how much feeler gauge
you can get under the edges towards the top of the slide which will be more worn,
you can see where the anodising remains at the top, if you can get 0.07mm or more then they are worn out.
If you can feel a step then they are worn out.

I would also say that the needle is worn by contact with the atomiser, see how it is shiny on the engine side but clearly stops
at the point where the needle is above the atomiser. There is also a pitted ring of corrosion at the top of the
worn area. I would replace the needles and atomisers too.

There is no guarantee that new slides will fix the issue
if there is wear in the carb body or other problems.

This kind of wear is caused by unsupported carbs with non standard air filters,
or after very high mileage.

Paul asked about whether the carbs were properly clamped in a previous post.

Mark
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Paz2112
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Re: Carburettors

Post by Paz2112 »

Mark,

They are held in place by replacement Start Gomma rubber inlets for the inlet side and the original rubber air funnels on the airbox side. They are, I guess you would say, standard and secure although the replacement inlet rubbers do not seem as stiff as the old ones they replaced but I put that down to the age of them making them more solid than the new replacement ones.

I will replace the atomisers and the needles. A forum member has kindly offered to send me two 260K atomisers to try and another friend is lending me two less worn slides to see what effect that will have. I will update again once I have tried all of this. I do feel like I am getting somewhere now and I appreciate the detailed advice on the carbs.
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Paz2112
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Re: Carburettors

Post by Paz2112 »

With alternative slides on load from a friend along with a handful of other parts I have been busy resetting/testing/reviewing the carbs and bike. I am not finished yet but so far, replacing my worn slides with less worn slides has helped the bike to not stumble and stop while idling. The bike also seems to have better mid range power. I am now going to order new slides, needles and mixture screws and set about rebuilding my existing carbs to the best of their ability. Hopefully this will cure the issue enough for the future.

While working at this I noticed that the rubber inlet manafolds that I purchased around 5 years ago from startgomma.com have already started to perish which is disappointing to say the least but is also an issue with my quest to get the Morini running as good as possible. I wanted to ask where other member have bought their manafolds from and or if anyone would know where I could buy good quality ones or indeed if anyone would have two good quality manafolds that they would sell me? In an ideal world these would also have balancer nipples fitted?
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3potjohn
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Re: Carburettors

Post by 3potjohn »

I think you will find with the new parts fitted it should run better. The intake manifolds on my 350, which I got from Startgomma several years ago look OK. However the ones on my 507 Morini are badly cracked on the external surfaces. Whilst this seems not to have penetrated to the interior I have for now covered them in clear silicone sealant.
The Startgomma ones were the best at the time but the price and increased cost of importation has put me off .......for now.Maybe a bigger order would be more cost effective.
I keep wondering about a suitably curved radiator hose.......
John
mbmm350s
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Re: Carburettors

Post by mbmm350s »

With alternative slides on load from a friend along with a handful of other parts I have been busy resetting/testing/reviewing the carbs and bike. I am not finished yet but so far, replacing my worn slides with less worn slides has helped the bike to not stumble and stop while idling. The bike also seems to have better mid range power. I am now going to order new slides, needles and mixture screws and set about rebuilding my existing carbs to the best of their ability. Hopefully this will cure the issue enough for the future.
That's good news indeed.
Check the mixture screws for the VHB and VHBZ 25 BS carbs (350s) these should have the short profile not the long profile. Please do check carefully down the hole that there is no remains of any previous screw embedded in the carb body. Also please make sure that the correct washers and o-ring are fitted.
I think you should also fit new atomisers - even new 260K will be better than worn out ones. These are difficult to measure for wear.

Please confirm also that the ignition timing is correct - as an experiment I moved the pickup on an easy to start bike to nearly its most advanced position and changing nothing else the idle revs rose by 500 rpm (and it was difficult to start).

Morini Riders Club : Keeping Morinis on the road.

Cheers
Mark
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Re: Carburettors

Post by Stubaker58 »

Just taken my Sport out for the first time this year. I changed the pilot jets from 50s to 45s. What a difference! It now pulls cleanly from low revs and picks up really nicely, I’m also approaching a reliable tick over. Best ten quid I’ve spent in a long time. A really enjoyable run.
Now it just pops and bangs on the over-run so I suppose I’ll have to look at the exhaust sealing rings?
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Paz2112
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Re: Carburettors

Post by Paz2112 »

I wanted to update this thread for the benefit of other owners with similar problems. Knowledge and all that.

After borrowing and using some less worn slides and mixture screws from a fellow owner (thank you Brian), I found that with them the idle was more reliable so I decided that I would spend the money and replace these components within my VHBZ carbs. I ordered the following from eurocarb,

2 x 7447.50 Slides
2 x E17 Needles
2x Mixture Screw + Spring + Washer + O-Ring

with these I built up the carbs as follows

Carb : VHBZ 25BS
Choke : 50
Idle : 45
Atomiser : 260k
MainJet : 115
Slide 7447.50 (NEW)
Needle Type : E17 (NEW)

Once I had balanced the carbs and set the idle the 1st big difference was the bike would idle without tumbling into a stall after 30 seconds. This has always been an issue with my bike and was one of the biggest barriers to enjoying riding it. Having the bike simply drop revs and stop at a set of traffic lights gets very old very quickly.

I took the bike for a decent 1st run, 30 minutes of hard riding and back home to waiting balancer. I then again balanced the carbs, adjusted the cables, set the mixture and set the idle in the normal roundabout method. After 15 minutes of this I was able to get the bike to idle at 1400rpm(ish) which again was never possible. That ended Saturdays work. On Sunday I decided to go for a good run to the coast and stretch the Morini's legs. Idle stayed reliable and I even stopped worrying about blipping the throttle at every junction. As I got closer to the coast the air got a lot colder and I noticed that I was down on power or more a feeling of having no real torque. I also noticed that if I cracked the throttle open hard while idling, the bike would stall. My conclusion is that the idle jet of 45 is now too small and I should go up to 46 or maybe 48 again, most likely the slides have had the biggest impact on reliability and actually I don't need such a small idle jet anymore.

Could I ask the more knowledgeable owners (I am looking at you Mark and Paul) for their thoughts on my findings?

Lastly I am making a plea for someone to please sell me a pair of 260BD Atomisers if at all possible. I am not able to find them anywhere and I think they would also be a good addition to my rebuild.

Thank you.
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Re: Carburettors

Post by EVguru »

Image
Paul Compton
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