The K2 carburetter saga goes on

[penman]

Hi all,

Well I got 2 new (second hand) carb bodies from NLM, gave them a thorough clean in the ultrasonic tank, fitted all the jets, etc which had been working perfectly in the carbs I had on loan from George, new gaskets, O-rings and so on all round, balanced the carbs, adjusted the idle mixture (excellent idling) and took off for a test ride. It's awful. To be fair, it's not quite as bad as it was before, but I seem to have the biggest flat spot I've ever experienced. It doesn't actually die as it did previously, but at around 60mph in top gear, the engine is hunting and opening the throttle a bit actually makes it slow down. Opening the throttle a lot makes it hesitate for a few seconds and then take off like a good 'un. As before, up to about quarter throttle it's absolutely fine, impressive, and beyond half or three quarter throttle it goes really well.

I'm running out of ideas here! I have three sets of carb bodies and I am transferring ALL the removable parts from one to another. The carbs I have on loan work perfectly, I have no problems with their performance. My original carbs are unrideable - in the mid range the engine actually dies and is hard to re-start. The third set, just bought at great expense give me a huge flat spot, but the bike is just about rideable, albeit very unpleasant. I thought I had this cracked, though I must admit I didn't understand what was wrong with the original carbs. I was just about to start stripping off all the shiny bits for a bit of cosmetic work, but it seems I am back to square one.

Atomisers - rightly or wrongly I am focusing on this area. The Dellorto spec sheet gives 260CM for this bike. This is sadly difficult to get or I would just order it. The bike originally had 258BD and NLM recommended 260BD, which is what I have. George's carbs have 260K, but those caused a minor flat spot in his carbs and made my original carbs a lot worse, though they were bad enough already! I could try the 258BD, but they are a bit corroded so I'm not sure it would prove anything. I have one 260D in good condition and that one has holes higher up, so may be leaner, but it's just one and they aren't available from Dellorto.

Pilot jet - the spec says 50, I have 44 and George has 42. My idling is best with the screw about 3/4 turn out, so I could probably use a smaller pilot jet - but would it make any useful difference?

Main jet - the spec says 112 and that's what I currently have. Originally the bike had 108.

Raising or lowering the throttle needle just moves the trouble spot around a bit, it doesn't fix anything.

This is a US spec bike, I don't know if there were any differences, there easily could have been but I have no information on that. I suppose I should put back the original jetting:
108 Main
258BD atomiser
50 pilot
E24 needle

- I suppose I'll do that if I can find the enthusiasm, but I can't really see it making much difference. Any ideas??

I've had to deal with plenty of dodgy old worn out carburetters in the past, but my experience is that if you clean them, fix any actual faults and make sure you have the correct jets, they just work. So what's different here? Why are these such hard work?

Many thanks for reading this frustrating saga, suggestions gratefully received. My short term plan is a curry and a couple of beers!

Regards,
Joe.

[72degrees]

Truly baffling.

If you want to try the old VHB25BS pair sitting on my desk PM me an address and I'll send them (the bodies at least). I'm making progress with setting up the new PHBH28s on the project so won't be needing these for a while, if at all. They can't actually be a pair as I find one has a 260K and the other a 260S atomiser. One must be from the original Hillclimber (76) and one from the 77 350 that became Forgotten era hill climber Mk1.

That 1/4 to 1/2 throttle range is the one that can be problematical on Morinis in my experience, but not so much on ones with standard cam and exhaust system. You haven't mentioned the slides but I assume they will the standard '50'. "Opening the throttle a lot makes it hesitate for a few seconds", sounds a lot like my tribulations over the years with 375/L5/PHBH28s/pod filters/2:1 exhaust. Will it go better if you open the throttle progressively from 1/4 to 3/4 giving time for the revs to build?

When I had a 350 Sport I found that if you opened the throttle wide in 6th cruising revs when it was two up it didn't have a flat spot but it just didn't make impressive progress unless you stirred the box down a couple of cogs and got the revs up. The K's have a much milder cam though and as I remember my K2 was smooth throughout the range, if a little lacklustre for me. I like a bike that noticeably comes 'on the cam' - then I'm also a fan of power valve two-strokes

The obvious answer is that George's carbs must be Magick! Or must be somehow free from some odd quirk that the other pair don't have. That doesn't help much, sorry, but if you try mine that would at least widen the data set.

[Haboola72]

Choke plungers. Winkle out the rubbers and reverse them.
It's worked for me in the past...

[72degrees]

Choke plungers. Winkle out the rubbers and reverse them.
It's worked for me in the past...

It could, I think penman said all the removable parts have been swapped (presumably including the complete starter plunger mechanisms). I've found perished (to the point of disintegration) O-rings on the starter jets on old carbs, but once again they have been swapped in to 'George's Bodies' (sounds like an episode of Waking the Dead).

[EVguru]

The important seal on the starter plungers is the one in the end that covers the fuel port. The very thin O-ring at the top seems to make little difference.

The O-ring on the starter jets holds the float bowl on whilst you screw in the main jet holder. I've found it makes no difference to the running.

Going too lean on the idle jet will make you too lean through progression. You can nearly always get idle mixture right because you have an adjustable screw. The off idle stumble on my 125H was bad enough to cause stalls and a leaner idle jet did improve it. Going leaner stll was worse and just before I sold it, I went back to a stock idle jet and a larger slide cutaway.

[huub]

i have had a set of aprilia pegaso carbs that absolutely refused to behave , despite ultrasonic cleaning a dozen times.
they must have spent at least a day in the ultrasonic tank.
I eventually dunked them in a tin of aceton fr a full day, wich solved the problem. ( literally)
just because they look clean doesnt mean they are.

if you swap all the removable parts over , you swap the atomisers over too?
if they work on one set of carb bodies they are unlikely going the be the problem?

if everything else fails, just get a set of VM mikuni's.
they are cheap and easy to set up....

[park3164]

Hi Joe,
Old atomiser from my bike, showing the holes. Sorry re photo quality. Same 2 lines of holes on the reverse side.
Fiona, Sydney. '86 K2

[penman]

Hi all,

and many thanks for all the replies, very much appreciated. Fiona, thanks for the photo, that confirms that the CM atomiser is leaner than any I have here, so once again, I will try to get some.

The choke plunger is an interesting idea. I confess, I didn't swap those over! I did notice that there is a pretty deep indentation in the end of the plunger - I think someone said the rubber seal can be got out and turned over - I'll try that. I didn't see that seal on the Eurocarb website and new choke assemblies were a pretty scary price! I did invest £8.38 +VAT per carb in a set of gaskets and O-rings. I guess that seal is just a small rubber disc, so I should be able to make one if necessary.

At the moment I have 44 pilot jets and idling is excellent. Also off-idle response is smooth and instant, so I'm assuming the pilot jet is OK(?)

These carbs are actually quite easy to clean, and easy to verify as clean. All the passageways are straight, so it's possible to poke a wire through them and with a bit of ingenuity, you can see both ends of all passages. The progression orifice is in line with the pilot jet, so you can see daylight through that if you remove the jet.

By the way, I don't like the suggestion that George's carbs are magic, though it's by far the most compelling argument I've heard so far!!!

I'm not giving up yet!

Joe.

[penman]

Atomisers. I hope this will be interesting to some!

I have been trying to get my head around the differences between the various types of atomiser. The atomiser sits in a tube extending down from the venturi into the fuel in the float chamber. Fuel is admitted to the bottom of the tube via the main jet. (I'm just stating the obvious here, by the way, I'm no expert!). I'll call this outer tube the "atomiser tube" as distinct from the atomiser itself. In addition, there is an air feed to the top of the atomiser tube via the little brass tube projecting in the intake. The atomiser, with the throttle needle, has the function of metering the atomised fuel into the venturi as well as mixing fuel and air. Fuel enters the atomiser via the end with the screwdriver slot, which has a calibrated hole, the "260" part of, say "260K". There are holes drilled in the sides of the atomiser which admit air from the atomiser tube to mix with the fuel. I am not absolutely clear as to how this works, but holes low down on the atomiser (Level A in my picture below) can allow fuel from the main jet to enter the atomiser tube, those holes are never quite blocked by the needle. As the needle rises, the atomiser will see suction from the venturi, fuel will be drawn up via the main jet and air will be drawn in via the holes in the atomiser - and maybe some extra fuel will flow into the atomiser tube from the lower holes and back into the atomiser, mixed with air via the upper holes.

Like I say, I am not really clear as to how this works, however I am pretty sure that holes low down on the atomiser will make the mid-range mixture richer, and holes higher up will make it leaner. So, I have drawn up a table of all the atomiser variants I can find for the VHBZ carburetter. Each atomiser has two rows of holes cross-drilled through it, making a total of four lots of holes as you turn it around. These holes seem to be on 5 "levels" which I have called ABCD and E, A being closest to the main jet end. In addition to those shown below, there is an "S" type, which has no holes at all and could therefore be a starting point for making an atomiser of any other design. My table shows the two cross-drilled layouts, there are two of each row shown.

Looking at my table, I reckon "CM" is the leanest, with "D" being pretty close. Any views on all this heavy-duty carburation malarky? I do hope this table proves useful to someone!

[72degrees]

I can see how a dodgy starter jet plunger seal might allow excess fuel to be sucked in to the works under heavy vacuum conditions - like wide throttle under heavy load at low rpm. Well worth a try with George's starter mechanisms in the other bodies.

However, I've just come back from another running in/carburation test on the project engine installed in the road 2C (more details in my 'blog'). That is still behaving fussily under those conditions - but the carbs, and hence the starter plunger seals are brand new!

I suspect I could improve things by an atomiser change or cunning drilling but I think I will take evguru's advice and save that for a dyno when the engine is housed in the ready to race machine with the exact exhaust system and ignition system to be used. Having a trailer makes that possible.

You will get there in the end!

[EVguru]

Figure 24 shows the full-throttle system used on four-stroke
engines which utilises air to change the amount of fuel delivered
by atomiser following sudden throttle openings.
There are several side holes (6) in the atomiser (5),
communicating with the air intake (2). On opening the throttle
fuel metered by the main jet (3) flows into the atomiser where it
mixes with air drawn through the side holes of the atomiser and
the resulting fuel-air emulsion flows into the barrel (4) where it
further mixes with air coming from the main intake (1).
A larger internal diameter of the needlejet atomiser produces an
increase in fuel delivery at all throttle valve positions while a
smaller size results in a decrease in fuel delivery at all throttle
valve openings.

The atomisers fitted to Carburetors intended for four-stroke engines are manufactured with different
types of side drillings because the positions of these holes affect acceleration response.
Atomiser holes positioned high up cause a weakening in the mixture since they are above the float
chamber fuel level and only let air in; conversely, holes lower down cause mixture enrichment because
they are below the chamber fuel level and draw fuel from the well to the barrel.
The result is that, to weaken the mixture under acceleration, atomisers with holes drilled higher up are
required, while to enrich the mixture, atomisers with holes lower down are needed. The holes' diameter
determines how long the well takes to empty and it is therefore also necessary to select a suitable size.

[mad muller]

for what its worth , as i can make out the loaned pair of carbs run fine , the rebuilt ones are identical body, jets , slide etc, but bike runs poor , i would look at the floats its not fueling right set them so they stop just before it floods out into the air box then you can be sure its got enough fuel to burn . muller.

[penman]

Paul, many thanks for that excerpt. It bears out what I thought, but the bit I don't understand is:

"conversely, holes lower down cause mixture enrichment because
they are below the chamber fuel level and draw fuel from the well to the barrel."

What I don't really understand is how the fuel gets into the outer atomiser tube. The only way in is via the main jet and the hole at the bottom of the atomiser, but that takes the fuel inside the atomiser - or does it then come out through the lower positioned holes and get drawn back in via the higher ones? Anyway, the bottom line is that I'd like to try a leaner atomiser, so that's the current quest. I will also pursue the choke plunger issue, that would be a nice easy fix. I must admit I'm doubtful though because if it is leaking enough to destroy mid-range running, why doesn't it affect idling?

for what its worth , as i can make out the loaned pair of carbs run fine , the rebuilt ones are identical body, jets , slide etc, but bike runs poor , i would look at the floats its not fueling right set them so they stop just before it floods out into the air box then you can be sure its got enough fuel to burn . muller.

I've put quite a lot of effort into getting the fuel level right, including using the same floats and needle valves in both sets of carbs and checking that the measurements come out the same. I don't think this is the cause, but I'll keep it in mind - like everything else!

Joe.

I'll keep plugging away, I'll crack this eventually!

[EVguru]

The bottom of the atomiser is below the level of fuel in the float bowl, so fuel will flow up through the main jet into the atomiser, out through the holes at the bottom and into the atomiser 'well'. When you accelerate, the main jet will limit the rate of flow, but you have a reserve of fuel in the well to provide enrichment.

[penman]

The bottom of the atomiser is below the level of fuel in the float bowl, so fuel will flow up through the main jet into the atomiser, out through the holes at the bottom and into the atomiser 'well'. When you accelerate, the main jet will limit the rate of flow, but you have a reserve of fuel in the well to provide enrichment.

OK, thank you. I'm sorry to be labouring this, I wouldn't be so interested in the atomiser if I didn't think I had a problem in this area!

From your note Paul, it sounds as if the fuel in the "well" provides a kind of "accelerator pump" effect on opening the throttle (correct me if I've misunderstood). It also sounds as if on say half throttle, the well would quickly be emptied and fuel from the main jet would flow straight up the atomiser, drawing in some air through the holes to help with atomising the fuel. What I'm asking is, on a half open throttle, is it drawing fuel from the holes in the atomiser, or only air. If fuel is flowing out of the holes on level A (my earlier picture) and back in through the holes on say level C, that would seem to richen the mid range mixture. Hmm, I do wish I had a box of atomisers to try!

I would like to measure the diameter of the atomiser tube "well" to see if there is a difference between the "HS" carbs and the "BS" variety, but I have no suitable tool. I'll figure something out. There has to be a reason why George's BS works, but my HS doesn't. There must be a difference - or a fault which I can't see.

By the way, I contacted Herdan Corp in the US and they no longer do international shipping, not even Canada, so that is unfortunately a no go. I'm tempted to order a couple of "S" atomisers from Eurocarb and drill the hole pattern for "CM", but at Dellorto prices these are expensive experiments! I'll try the 258BD first, though they're a bit corroded.

I WILL crack this.