Carburettors

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Paz2112
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Re: Carburettors

Post by Paz2112 »

Hello again fellow 350 owners. While servicing my bike today and getting ready for some spring rides, I decided to go over the carbs again to try and get them perfect and also to make the bike less likely to stall. I have always seemed to suffer from the dreaded stumble from low revs. I replaced the seals on the enrichment lever to make sure were good. I changed the idle jets to 48 with a view to helping the smooth idle. Now, the problem arises. On the rear carb, I was not able to adjust the mixture screw in the manner I would expect. I started by setting it fully in (screwed in) and then setting it one and half turns out. This resulted in a yellow burn at both idle and when revved. I then turned the mixture in a half turn but got the same colour of burn. It was only when I got to about a quarter turn from full in that I achieved a blue flame. So I changed the idle jet to a 46 and started again. Same result, only getting a blue flame at about a quarter turn from full in. Seemed strange. So I went again and used the 43 idle jet and to my utter confusion the same result remained.

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?

When I rebuild the carbs a few years ago, they were fully cleaned, all seals were replaced. They are currently balanced in terms of slides opening at the same time. I should also add that the two carbs have the connecting fuel line between them (that is how it came).

It has got me wondering if my timing is actually correct and if I don't have some other problem with the burn, like ignition?
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Re: Carburettors

Post by EVguru »

Make sure the throttle cable has slack.
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Paz2112
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Re: Carburettors

Post by Paz2112 »

I will check this again, I am fairly confident that it has a few mm of travel before opening. That said, should the slide close completely down, visibly in the carb, or is it OK to see a gap below the slide when throttle is closed?
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Re: Carburettors

Post by EVguru »

When I'm rebuilding carbs and presetting them off the bike, I'll set the stop screw to barely create a gap under the slide. If the ignition is too advanced, the idle speed can be right, or even too fast with the slides fully shut. 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.
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Paz2112
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Re: Carburettors

Post by Paz2112 »

Understood, but with that said at the most basic level, when built up, should the slides in the carbs fully close down? Mine are visibly still open at the bottom.

Also the manuals always stated that the 350 S needed the 260 BD atomiser but my carbs have the 260K. Would this be an issue?

Currently This is Their Setup :

Front Cylinder Carb : VHBZ 25BS
Choke : 50
Idle : 45
Atomiser : 260k
MainJet : 115
Slide 7447.50
Needle Type : E17

Rear Cylinder Carb : VHBZ 25BS
Choke : 50
Idle : 45
Atomiser : 260k
MainJet : 115
Slide 7447.50
Needle Type : E17

They are mounted to rubber intake manifolds with jubilee clips. The carb body as cuts in it but no plastic insert as others show. Not sure if this is an issue.
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Re: Carburettors

Post by EVguru »

Paz2112 wrote: 28 Mar 2021 14:42 Understood, but with that said at the most basic level, when built up, should the slides in the carbs fully close down? Mine are visibly still open at the bottom.
Yes, there should be a slight gap under the slide.
Also the manuals always stated that the 350 S needed the 260 BD atomiser but my carbs have the 260K. Would this be an issue?
The atomisers affect mixture under acceleration.
They are mounted to rubber intake manifolds with jubilee clips. The carb body as cuts in it but no plastic insert as others show. Not sure if this is an issue.
Both the white sleeve and the black insert missing?
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Re: Carburettors

Post by Paz2112 »

Thanks Paul. Yes my carb has neither sleeve inserted in it. Do you think this could be a part of the issue?
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Re: Carburettors

Post by EVguru »

It's not going to help with air leaks. The black adapter isn't available from Eurocarb, so I'd make a one piece insert that does both jobs.
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Re: Carburettors

Post by Paz2112 »

What jobs did the two different pieces perform? I did not realise that cabs had both.
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Re: Carburettors

Post by EVguru »

White sleeve is for push on applications (like the early metal inlet manifolds).
Black inner smooths transition from carb to rubber inlet manifold.
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Re: Carburettors

Post by Paz2112 »

OK so a small update and another question. Current detailed carb info, please read to see if you can add/help

Front Cylinder Carb : VHBZ 25BS
Choke : 50 (rubber seal replaced)
Idle Jet : 45 (lowered from original 50)
Atomiser : 260k
MainJet : 115
Slide 7447.50
Needle Type : E17 (set to middle ring)
Float : 7450.2 14gr Old Style Black round with brass arm
Float Height : 25mm (from carb body to outer edge of float)
Needle Valve : 200

Rear Cylinder Carb : VHBZ 25BS
Choke : 50 (rubber seal replaced)
Idle Jet : 45 (lowered from original 50)
Atomiser : 260k
MainJet : 115
Slide 7447.50
Needle Type : E17 (set to middle ring)
Float : 7450.2 14gr Old Style Brown round with flat bottom and stainless steel arm
Float Height : 25mm (from carb body to outer edge of float)
Needle Valve : 200

My issue is that trying to make the carbs leaner to achieve bunson blue flame at idle is proving to be nearly impossible. I only manage to get blue when just a quarter turn out from full in on the mixture screw. Changing the idle jet from 50 down to 43 at the lowest point did not seem to lean out the mixture and I was only able to get the bunson blue at the quarter turn out from full in, with any of the jets I used. I used a 50, 48, 45 and 43. This does not make sense to me.

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?

Any experience, help or pointing the general right direction is appreciated.
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Re: Carburettors

Post by EVguru »

I don't use a Colortune on a motorcycle. The volume of them is too big compared to the cylinder and leads to misleading readings. There are motorcycle versions with reduced volume. The idle mixture atomisation of the Delorto is also quite crude and give a misleading colour.

Very slowly turn in the mixture screw on one cylinder until the revs start to drop (the manifold walls will be wet with excess fuel). If you can't get the revs to drop, or even actually get the cylinder to cut out, then something is preventing you from running lean enough. That might be too high a fuel level, or it could be the slide is too far open and you're on the progression circuit (at least partially) rather than on idle. Back the screw out until you get maximum revs. Repeat on second cylinder. Reset idle speed and balance and then start all over again.
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Paz2112
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Re: Carburettors

Post by Paz2112 »

I use a colortune that I have had since the 1990's which is for motorcycles. I am intrigued as to why do you not think it is accurate, knowing how much you have worked at bikes and engines.

http://www.carbtune.co.uk/colortune.html

I will start again today and go by the sound and feel of the engine to see if I can get it set. One other question in relation to the floats and the measurement of them. My carbs both have different versions of Dellorto floats, which are different shapes. Would this not mean that the measurement float height would be slightly different for each?
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Re: Carburettors

Post by Steve Brown »

Paz2112 wrote: 03 Apr 2021 07:56

http://www.carbtune.co.uk/colortune.html

My carbs both have different versions of Dellorto floats, which are different shapes. Would this not mean that the measurement float height would be slightly different for each?
It may well make a difference. In that case check the fuel level as opposed to the float height. There is an indication of the fuel level to aim for in the dellorto manuals and 'I think' it is 5mm + or - 0.5mm from the float bowl joint. Check the books though, obviously. This way you know that the correct amount of fuel is available to the jets.
Also the floats come in different weights for different applications, but if you can adjust the float to get the fuel level right you'll have sorted it.
I made and adaptor to attach a clear plastic pipe to a float bowl nut/main jet holder.
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Paz2112
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Re: Carburettors

Post by Paz2112 »

Steve, I had came to the same conclusion late last night. The only way I will get it right is to visibly make sure it is right. When you say float bowl joint, are you referring to the very top of the bowl and therefore 5mm +/- 0.5mm from the top of the bowl?

Can I also ask for a link to the manual which is most specific to my carbs, VHBZ 25BS?

I have found a few links to manuals/guides that look possibly correct but not specific, as follows:-

This tuning guide does not seem to discuss the VHBZ range
https://www.vortex-rok.com/gestione/all ... /88_40.pdf

This manual which is very similar to the above one but also does not seem to relate directly to the VHBZ.
http://www.iwt.com.au/private/Dellorto%20manual.pdf

Dellorto's own page for expanded diagrams and float levels which is useful but they dont have the tuning guide.
https://www.dellortoshop.com/contents/e ... loads.html


If anyone could link me to the definitive guide/manual for the VHBZ 25BS I would really appreciate that.
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