The K2 carburetter saga goes on

[davel182]

Well I appreciated your atomisers table very much. As an owner of a 250T, I had already established that the pattern of holes in an 'R' is the same as an 'AH' and wondered why the two existed, but Paul seems to suggest that the size of the holes may also vary, which would explain the R/AH but introduces another property to be considered if you were going to modify one pattern into another. I have an atomiser labelled 'K' with a 'BD' pattern - a 'conversion' which I believe NLM do, but they presumably have access to most of the range so can measure them.

Sorry I don't have anything positive to suggest, but I'm following your saga with considerable interest.

Dave

[penman]

Well I appreciated your atomisers table very much. As an owner of a 250T, I had already established that the pattern of holes in an 'R' is the same as an 'AH' and wondered why the two existed, but Paul seems to suggest that the size of the holes may also vary, which would explain the R/AH but introduces another property to be considered if you were going to modify one pattern into another. I have an atomiser labelled 'K' with a 'BD' pattern - a 'conversion' which I believe NLM do, but they presumably have access to most of the range so can measure them.

Sorry I don't have anything positive to suggest, but I'm following your saga with considerable interest.

Dave

Thank you! I have just noticed another thing, which I should have spotted sooner. Looking at the carb spec table on the Dutch website, all the models with the VHB(Z) 25 BS carbs seem to have the richer "K" or "BD" atomisers, while those with the "HS" carbs seem to have either "D" or "CM", the two leanest atomisers (there are a couple of exceptions, late model Kanguro and Excalibur which have an intermediate atomiser). Maybe there IS a difference between the two carb variants, other than the provision of vacuum ports and breather stubs on the HS. By the way, I'm using the term "lean" a bit loosely here, I'm talking about lean in the mid-range.

I'm still trying to find a source of "CM" atomisers, but I am seriously considering modifying the 260BS to 260CM. This will involve blocking four holes as well as drilling two new ones, so it's not dead easy! Still, I've nothing to lose, if I end up with no useable atomisers I'm no worse off! The hole size is a slight worry, but I'll have to do what I can based on the existing hole sizes and available drills. At the worst it should tell me if I'm moving in the right direction.

I abandoned the Morini project yesterday and took the Bonneville out for a "spirited" ride round the lanes, which has restored a bit of enthusiasm, much needed! Mind you, it was flipping cold, though I don't think I can blame the Dellorto atomisers for that!

Regards,
Joe.

[penman]

An update, and a pretty disappointing one.

In the end, modifying the "BD" atomisers to "CM" was pretty easy. I used a mini-burr in the Dremel to slightly countersink the unwanted holes, then applied solder and finally smoothed it off with a fine file. The nearest drill I had to the existing hole size was 0.8mm, so I marked the position of the new holes and drilled them. I think 0.8mm might be slightly small, but 1mm was significantly too big, so I think the original holes were something like 0.85mm - hard to be certain. I must say, I was expecting this to be quite an improvement, even if it wasn't perfect.

The result is not an improvement. It isn't worse, it's different. The big change is that it now won't let you 'blip' the throttle, making down-changes a bit of a pain. I guess this is telling me that the reserve of fuel in the well isn't being replenished quickly enough, due to the lack of a hole on Level A. On the other hand, the flat-spot effect is slightly improved, though not enough to make the bike easy to ride. I suppose the next step should be to try a "Q" atomiser, just a case of drilling one hole, but my enthusiasm is now on the floor! This is a bog-standard bike, so why is it so hard to get the thing performing properly? Since George's carbs worked so well, I am assuming that the rest of the engine is blameless. Certainly my compressions are good (Front - 175, Rear - 170 psi) and the ignition is fine. So now I've fitted the correct carbs with the correct jetting etc and it runs like a pig. Not sure what I'll do next!

[EVguru]

Basic carburettor setup tends to get leaner over the years to meet increasing fuel emissions standards.

That usually means that more enrichment is needed for clean acceleration, so less slide cutaway for more vacuum and/or richer atomisters.

The ultimate case on Morinis are the notorious VHBH30's fitted to US spec Camels. Really too big for the engine size/state of tune, but the smallest Dellorto then fitted with an accelerator pump. Very tricky to get set up right.

[72degrees]

"There has to be a reason why George's BS works, but my HS doesn't."

Try another pair of BS carbs and see if they also work with your innards?

So the VHB(Z) has a vacuum balance connection? I'm sure you have mentioned it before but presumably the link tube is in absolutely perfect condition. No tiny split that could bleed in air under high load conditions?

I did find one site suggesting that the VHBZ uses a removable float valve seat (like a PHBH).

"Thanks to Matt Cooper from Eurocarb for providing the following information describing the differences between the various VHB carbs.
VHB was the original carburetor for all 20 mm to 30 mm sizes. Then Dellorto changed to VHBZ for 20 mm to 26 mm carburetors and VHBT for 27 mm to 30 mm carburetors. VHBZ and VHBT have a separate needle and seat."


Confusing they just say needle valve when they mean *float needle* valve.
http://www.dellorto.co.uk/shop/dellorto ... 150-225-2/

These come in different sizes. Could the 'fixed' VHB one be significantly different to the size used in the VHBZs on your K2 ? I can't find any mention of what size the float valve seats should be as standard on a K2. Is it possible someone has treated it to new seats and float needles at some point and not matched the original spec - are yours marked with a size?

I don't know exactly what effect an incorrect float valve seat size might have , but perhaps a too small one might not help with these symptoms?

[penman]

"There has to be a reason why George's BS works, but my HS doesn't."

Try another pair of BS carbs and see if they also work with your innards?

So the VHB(Z) has a vacuum balance connection? I'm sure you have mentioned it before but presumably the link tube is in absolutely perfect condition. No tiny split that could bleed in air under high load conditions?

I did find one site suggesting that the VHBZ uses a removable float valve seat (like a PHBH).

"Thanks to Matt Cooper from Eurocarb for providing the following information describing the differences between the various VHB carbs.
VHB was the original carburetor for all 20 mm to 30 mm sizes. Then Dellorto changed to VHBZ for 20 mm to 26 mm carburetors and VHBT for 27 mm to 30 mm carburetors. VHBZ and VHBT have a separate needle and seat."


Confusing they just say needle valve when they mean *float needle* valve.
http://www.dellorto.co.uk/shop/dellorto ... 150-225-2/

These come in different sizes. Could the 'fixed' VHB one be significantly different to the size used in the VHBZs on your K2 ? I can't find any mention of what size the float valve seats should be as standard on a K2. Is it possible someone has treated it to new seats and float needles at some point and not matched the original spec - are yours marked with a size?

I don't know exactly what effect an incorrect float valve seat size might have , but perhaps a too small one might not help with these symptoms?

Yes, the "HS" carbs have a vacuum port which normally provides a connection for a balance pipe. It's also handy for vacuum gauges, though I find that if you put care and attention into balancing the carbs mechanically, i.e. both slides rising exactly together, you'll be 95% of the way there without bothering with vacuum gauges. On the advice of EVguru I now have this blanked off, though I did try it with and without the balance pipe. It made no difference, though putting the balance pipe on causes a small decrease in idle speed - I'm told in normally speeds up by a few hundred rpm. This is odd, and maybe there's a clue here but I don't know what it means. Yes also to your second question. The VHBZ does have a removable needle valve seat and they come in various sizes. 200 is specified for all the 350 models and that's what I have - it's a new needle and seat by the way. Both the BS and HS variants use the same needle and seat. By the way, my bike originally had the idle mixture screws with the long tip (see a photo I posted a week or so ago) - maybe that has an effect on the behaviour of the balance pipe.

I did try a transparent balance pipe at one stage and was a bit surprised to see a kind of mist streaming back and forth in the tube, presumably air/fuel mixture, so I guess it's logical that it should idle faster with the pipe on, whichever cylinder is on its intake stroke gets the chance to "steal" a bit of mixture from the other carb

Paul mentioned jetting becoming leaner over the years to meet changing emissions rules. I guess changes in the composition of the fuel also tend to mean that the ideal jetting of 30 years ago may not be ideal today. Certainly on the Mk II Amals which I am more familiar with, moving to a slightly smaller pilot jet and starter jet can be a minor improvement these days. Having said that, unless you've changed the state of tune of the engine, the original jetting, specified back in the '70s will work perfectly well, without any of these major performance issues I'm seeing here.

If I can summon up the energy to do it, I'll open up another hole on the bottom level of the atomisers, making them into "Q" spec. as used on the '86 Excalibur. I'm running out of ideas now though!

Joe.

[72degrees]

Ah, should have read the start of the original thread properly. Presumably all early VHBs have a fixed 200 size float valve seat?
I missed exactly what type the replacement pair provided by NLM are BS or HS?

Very frustrating. Get George to sell you his carbs

[penman]

Ah, should have read the start of the original thread properly. Presumably all early VHBs have a fixed 200 size float valve seat?
I missed exactly what type the replacement pair provided by NLM are BS or HS?

Very frustrating. Get George to sell you his carbs

As far as I know, the VHBZ carbs have always had a replaceable (screw-in) needle valve seat. The Dellorto spec gives size 200 for the Morini 350 models and 250 for the 500s. Eurocarb still stock the 200 but not the 250.

My replacement carbs are HS model, I did that deliberately because that was the type originally fitted. Not so sure it was a good choice now!

I'll probably give this a rest for a day or two, it's not doing a lot for my sense of humour, but I will have another go at the atomisers before the week's out. I've got my sights on the "Q" spec now. I do have this feeling that I'm missing something though! I wish I really understood the difference between the BS and the HS, but I've examined them side by side with a magnifying glass and I can't find any difference - of course it may not be externally visible. These are just lumps of metal though, if one works and the other doesn't, they MUST be different!

[72degrees]

I was wondering what size the very early VHB25BS (not Z) ones, with fixed float valve seats, sitting on my desk are. I'm guessing 200.

Sounds like there may be a hidden difference between HS and BS but as HS is standard on a K2 it's still a mystery why correctly jetted ones don't seem to suit yours. By the way my early VHBs have 7447.50 slides like George's VHBZ pair. I can't find reference to 8896 dellorto slides anywhere on the internet so that's a bit of a mystery.

I expect such tribulations with engines that have non standard cams, but with a mileage of only 3000 that surely can't be the case with this one.

[MickeyMoto]

Was it a US model? Maybe there were other EPA enforced differences? The EPA did contrive to make the engines gutless......

Glad all that effort has worked. Whilst we fret about our 1.7% of Global CO2, the US, Russia, China and India produce about 75%. Good job, EPA!

[EVguru]

There were small and large body VHBs, the small version being replaced bt the VHBZ and the large by the VHBT.

The fixed float valve seat orifice on VHB carbs probably varied with choke size. Too large an orifice can make the carb prone to flooding, although there is a sprung tip needle available, which helps a lot. They're usually used in high vibration applications, like off-road bikes.

What height are you setting the floats too? Float weight obviously makes a difference to the fuel level. When I was chasing a problem on my 250 2C, I modified a float bowl to take an external site tube so I could see the fuel level.

[penman]

I was wondering what size the very early VHB25BS (not Z) ones, with fixed float valve seats, sitting on my desk are. I'm guessing 200.

Sounds like there may be a hidden difference between HS and BS but as HS is standard on a K2 it's still a mystery why correctly jetted ones don't seem to suit yours. By the way my early VHBs have 7447.50 slides like George's VHBZ pair. I can't find reference to 8896 dellorto slides anywhere on the internet so that's a bit of a mystery.

I expect such tribulations with engines that have non standard cams, but with a mileage of only 3000 that surely can't be the case with this one.

I have tried both types of slide and they both work fine in George's carbs, so I conclude that that's a "red herring" one of many!

I am not really sure what the "correct" jetting is for this bike, a 1985 US model because what it came with is quite different to the Dellorto spec sheet. I think if someone had gone to the trouble of, for example, changing the cam, they would have done other things like fitting rear sets and lower bars - of course I can't be sure of anything. However, fitting the "BS" carbs with standard 3 ½ jetting, it runs well so my guess is that it's standard.

I think I'll send an email to Eurocarb/dellorto and ask what the difference is between BS and HS - if for example the air feed to the atomiser is different, that could explain all. However it still doesn't explain why it runs so badly on the specified standard jetting. I do wonder if this is why the bike had so little use, someone got fed up fiddling with it and shoved it up the back of the garage! I have some sympathy with that approach! One thing I can do is take my HS carbs back to NLM and ask to swap them for BS versions. I'm sure they'll do it if only to get rid of me! But that feels like a defeat! I'll keep plugging away, I have a couple more atomiser setups to try - and I must try the Imperial mini-drills, one of those may be closer to the atomiser hole size - or it may not!

MickeyMoto - yes there are emissions-driven differences, the most obvious being all the breathers taken into a collector box and then into the airbox. That doesn't include the carb breathers however, those are just piped along the frame tube and end by the steering head, open to the air. I've tried it with them disconnected, no difference as expected. The carbs also had smaller pilot jets (44 instead of 50) and smaller main jets (108 instead of 112). There is a rubber seal between the two halves of the airbox, which has the effect of reducing the intake area. The blue book talks about different main jet sizes according to whether this seal is fitted or not. At the moment I have the seal out and the larger (112) main jet fitted. That seal is very perished, so I can't sensibly re-fit it. I suppose I could seal it with tape though as a test. Hmm, that's another thing I suppose I could try.

Regards,
Joe.

[penman]

There were small and large body VHBs, the small version being replaced bt the VHBZ and the large by the VHBT.

The fixed float valve seat orifice on VHB carbs probably varied with choke size. Too large an orifice can make the carb prone to flooding, although there is a sprung tip needle available, which helps a lot. They're usually used in high vibration applications, like off-road bikes.

What height are you setting the floats too? Float weight obviously makes a difference to the fuel level. When I was chasing a problem on my 250 2C, I modified a float bowl to take an external site tube so I could see the fuel level.

Paul, your posting crossed with mine.
I am setting the floats as per the diagram in the blue book and elsewhere. That gave perfect results with George's carbs, using my floats, needles and seats. George has lent me some modified float chamber bolts with a "sight glass" tube attached for checking the fuel level. I haven't actually made that measurement, maybe I will, but it means swapping the carbs over yet again! I've also bought a set of the newer white floats, which are lighter (12.5g vs 14g) - fitting those makes no difference at all to my symptoms.

I've said it before, I do feel I'm missing something, surely it shouldn't be this difficult! If you fit the standard carbs with the standard jets, surely it should run pretty well, even if with modern fuel there may be room for a bit of "fine tuning". What's going on???

Regards,
Joe.

[72degrees]

I've said it before, I do feel I'm missing something, surely it shouldn't be this difficult! If you fit the standard carbs with the standard jets, surely it should run pretty well, even if with modern fuel there may be room for a bit of "fine tuning". What's going on???

Quite. Particularly when it runs OK on carbs where the only variable now seems to be the body type. Exhaust system type/condition can be a factor but this bike presumably still has the standard system in reasonable condition? Even a silencer change can have a significant effect on mid throttle roll on (particularly if 'whacked open').

My tuned 604cc Gilera Nordwest single was treated to an FPM (Frigerio) silencer. I'd got the twin KeihinCR33s jetted pretty well, but if you whacked it open under heavy load it would still 'bog'. I remembered Luigi mentioning when I collected it that removing the last 'dB killer' type baffle for 'racing' was a good plan. It was only held with a self tapping screw so between runs at Gurston Down I removed it - what a transformation!

Which still doesn't explain why George's carbs work

[penman]

To answer the question, yes the exhaust system is perfectly standard and in excellent condition.

I put the 258BD atomisers in this morning and it was as bad as with the 260CM. I am wondering if a new issue has crept in, because this business of refusing to "blip" the throttle is a new symptom. Whatever it is though, it is affecting both carbs. I must admit, I replaced the atomisers with the carbs in situ on the bike, not at all difficult and it saves a lot of dismantling and reassembly. Maybe I've disturbed something, can't think what though.

A quick question. If I set the idling to idle well when cold or let's say, half warm, when it is hot it then idles at 2000rpm or even more. If I then adjust the mixture and throttle stop to give good idling when hot, it constantly dies at the lights when cold or just warm. Is that normal? For a given temperature I can get excellent idling, it's just that it seems to be very temperature sensitive. I try not to make unreasonable comparisons, but my Bonneville idles pretty well the same hot or cold, about 900rpm and reliable. OK it's a completely different engine with completely different carbs, but should I expect the same standard? The Morini's a bit easier to cold start, particularly with electric start, so it's not all bad! Maybe this hot/cold idle issue will disappear if I ever get the carbs sorted, I'm more interested in the bigger problem just now!

And an even bigger problem - I now have to go out and try to start the leaf blower, with which I don't enjoy a good relationship!

Regards,

Joe.