V50 Mk II carburetter jetting

I have been playing about with the jetting on the VHBZ carbs on my V50 Mk II and I wondered if anyone else has been down this path.

The original problem was the fairly familiar issue of the engine hesitating coming off idle, it can be very irritating when pulling away from the lights or blipping the throttle on a down-change. There are a few possible causes, one of which is an over-rich pilot jet, causing the engine to bog down a bit as the throttle slide starts to rise and the progression orifice comes into play. On the Dell Ortos, the idle mixture is adjustable with the screw, but the progression mixture is not, it is supplied direct from the pilot jet. So, having been through the exercise of making sure the carbs were balanced and clean, I have changed the pilot jet from 40 to 36. This has made a huge difference, much more than I expected and it’s a big improvement. Idling is much better and the off-idle response is much snappier. So far so good.

But there’s another problem. It always wants the choke on a cold start (fair enough) but it won’t run on choke, you have to nurse it along with the throttle. My Moto Morini with similar carbs will run happily on choke for 30 seconds or so at a fast idle until it starts to warm up, when it starts to get lumpy, then you can close the choke, give it a couple of blips and it’ll settle down to a normal idle. I’m wondering if the choke jets are also a bit too rich, but I don’t have any smaller ones in my collection to try and at about £6 a pop, experimenting would be a pricey business! Has anyone else played around in this area? Any views?



Hi Joe,
Has you got standard jets? I started with new standard jets and worked from there. Have a look at this site, lots of useful info.

Hi Steve,
Thank you for the link to this old tractor - I have been there before, but your message prompted me to re-read the tuning guide.

I did start from standard jetting and in fact the only thing I’ve changed so far is the pilot jets, reduced from 40 to 36. I just did about 50 miles yesterday, being a bit fussy about how the bike is going. To be honest, it’s not too bad, though I think it could be better

Originally, I had poor idling as well as severe hesitation as I came off idle and on to progression. In fact if I opened the throttle suddenly, the engine tended to stall. Starting from cold, the engine started easily on choke, but wouldn’t run on choke. I had to nurse it along with the throttle and close the choke asap. It then idled unreliably until thoroughly warm, when it idled reliably but too fast and not particularly evenly. I noticed that the idle mixture screw was only about a quarter turn out on both carbs, and the idle speed screw (throttle stop) was almost fully in.

My first thought was that the idle orifice might be blocked, so I gave the carbs a thorough session in the ultrasonic tank, followed by squirting carb cleaner and air through all the passages. I am confident that everything is clean. I also balance the carbs every time I fiddle with them. This made no difference at all.

My next move was to change the pilot jet from 40 to 36. This seemed to make quite a lot of difference and the idling is much better and sits at around 1000rpm, but the idle speed screws are still fully in and although the mixture screws can now be further out, they don’t really make much difference. Hesitation coming off idle is better, but still not perfect. It still won’t run on choke, though it needs it briefly to start.

I’ve checked that both choke plungers are sealing OK when the choke is off and the air feeds to the choke and pilot circuits are perfectly clear. The air cleaner is pretty new and besides, it runs well on an open throttle. I’m sure the air filter isn’t choking it.

I’m running out of ideas! Any suggestions?

By the way, this V50 II uses the square slide VHBZ type carbs. At 24mm they seem very small for a 500cc engine!


There is quite a bit f info on tuning Dellorto carbs onthis page put together by a guy that built a Guzzi powered morgan replica.

A guy from the NE Branch successful changed from VH to PB due to slide wear and unobtainable parts. A few calls to Eurocarb and jets sorted. They are very helpful.

My mk II is still running original carbs and original spec jets. I did replace them all a few years back was all – I figured that the passage of time, fuel and impurities had likely opened them out. Been through any number of choke (enricher) plunger, they’re cheap enough that when in doubt I swap them out. I’m still running the std air box, and this set up has worked fine with various different silencer types over the years, currently on some from Keihin (sp?).

Bike is a little rich at the bottom end and always has been. Will often start without choke as long as you catch it on the throttle as it fires. So if I do start it on the choke I can switch it out almost immediately. I have the tick over set at about 1500 rpm, maybe a little less. I think that is what is recommended, I certainly think that 1000 is too low. Bike shows no bogging on pick up off the stops at all. My pal’s similar bike is set up the same and behaves the same.

Your set up doesn’t sound quite right somewhere.

I’ve got my Camel set up with a remote lever on the handle bars for the chokes and that allows some gradation on the setting. But then that is a pig to start on the kicker anyway, and often as not just plain refuses to tick over at all.

Like cyclobutch has said, standard jets and airbox work great for me. I have the same start procedure, may be the choke for 5 to 10 secs. I know mine is a Mk3 but when it was rebuilt from an eBay non runner with a box of bits thrown in, I fitted new original size jets and no prob since. The only concession I made was to fit a K&N filter in the air box (mg020).


I removed the airbox to have a look at the filter element, which was as good as new - it was new about 6 months ago. I needed to check it though. I then started the engine with no airbox and took the trouble to set up the carbs properly. It runs fine like that and idles well - so this places the fault in the airbox area. I cleaned and reassembled everything and once again, it ran very badly, idling was too slow even with the throttle stop screws right in. On a whim, I then disconnected the two breather tubes from the cylinder heads and that completely cures the problem. The breathers connect to the bottom of the airbox, which has a simple ball and washer valve which I take to be a PCV valve. What I didn’t try is blocking the entry to the valve, so I don’t know if the improvement was really due to disconnecting the breather, or just to opening a hole on the carburetter side of the air filter. I will have to put it all together again and try that.

Anyone else had a problem with this breather system?


The breather system is not great, but I’ve not noticed it impairing the carburation. It’s more the case that on mine every so often it loses the plot and decants a whole load of oil onto the top of the gearbox. I just presumed it was because the ghastly cheap ball valve thingy would occasionally hang up. Leaves a good tide line on the air filter also. I’m pretty lazy on replacing these – the UK is not a massively dusty place anyway I suppose. I’m probably/maybe only on my 5th or 6th air filter ever.

As I noted before, I’ve not been aware of running issues that might have been down to this.

cyclobutch - thank you for that, you are confirming that the standard airbox/breather system does work, albeit with a tendency to leak oil. Mine was oily inside, but then it’s supposed to be, and there was no liquid oil. all the pipes are clear and in good condition. The ball valve is a bit crude, but I can see nothing actually wrong with it such as a rusty ball or a bent seat. I don’t suppose we’re looking for a perfect gas-tight seal here anyway, just something to block it from sucking air into the crankcase too readily. I must say though, I would have expected something to stop the ball clattering about quite so much. I was thinking of putting a light spring behind it, but I’ve gone off that idea now, on the grounds that it shouldn’t be necessary and may actually interfere with the breathing.

Not sure what to do next, I’ll think about it for a day or so.


Hi Steve,
Do you have any more info on this? My 500 Morini has the PHBH carbs and I find them generally “better”. Easier to set up and better performing. I can see that the PHBH carbs should work OK, why not? But I can imagine some hassle with the connections to the airbox and to the cylinder head inlet stubs, which could easily make it more trouble than it’s worth. I think the smallest PHBH is 26mm, while the VHBZs on the V50 are 24mm.

I spent a lot of time cleaning and reassembling the airbox/breather assembly again and the result is that the bike is generally performing satisfactorily again. If you think “satisfactorily” sounds like a weak word, you’d be right. Idling is a bit on the slow side, though reliable. It also soots the plugs on idle, so it is still rich on idle though OK on a more open throttle. After a lot of faffing about, I’ve settled on the 36 pilot jets. I’m not sure why it is so rich on idle or low revs and it usually starts fine without the choke. If I put the choke on while it’s running, it dies immediately unless I give it quite a lot of throttle. If I do that on the Morini, the revs rise and it runs rough, but it does keep going. That bike requires the choke to start, even on a warm day. It needs to be properly warmed up to start without choke.

I was hoping to take the V50 to Spain at the end of September for the Moto Piston Rally, but that’s quite a high mileage event, so the bike has to be right! I have started to prepare the Bonneville as well, so I have a plan B!


An update, but first a quick re-cap:

  1. To begin with, the bake was idling very unevenly and was hesitating coming off idle and onto the throttle. Idling was too slow when cold or half warm and too fast when warmed up. It felt rich on idle.

  2. I put both carbs through the ultrasonic tank, checked all drillings etc. Replaced floats, float needles, throttle needles (one was bent!) and choke plungers.

  3. Reassembled and was surprised that with the existing throttle cable adjustment, the slides were about 5 or 6 mm up, way beyond the adjustment range of the throttle stop screws.

  4. As before, it idled too fast and the idle mixture screw had no effect at all. If I lowered the slides onto the throttle stop screws, even screwed fully in, idling was too slow, about 800rpm. However, the mixture screws now have an effect, so it is idling on the idle orifice, albeit too slowly. Idle seems too rich and it tends to die after a short time. Plugs very sooty.

  5. I found that disconnecting the breathers from the airbox improves things a bit, but I now think this is a bit of a red herring. It’s still too slow and still tends to die after a short while.

  6. I have reduced the pilot jet from 40 to 36 - a 19% decrease in fuel flow. It is marginally better but still not very good.

  7. The update is that today I took it for a longer run and once fully warmed up, it was awful. Idling very slow and feeling too rich and the engine usually dies as soon as I stop. I cut the ride short and ran for home. I should mention that on quarter throttle and above, it runs really well.

So that’s where we are. I would say that it isn’t getting enough air on idle, but everything is clear and clean and I’ve paid particular attention to the air feed to the pilot circuit - it’s clinically clean! The air filter is as good as new and everything is correctly assembled.

Compression is measuring 160psi on both cylinders - I don’t know how good that is, but it’s well out of the danger zone. I’ve checked ignition timing.

So, at the moment I’ve given up and started to give the Bonneville a full service ready for the Piston Rally. I also have my Morini 500 as another option, but I used that last year and I really wanted to take the Guzzi.

Any ideas? I’ve had plenty of trouble with carburettors over the years, but never this much! Usually if they are clean and in basically reasonable condition, they just work in my experience…



Carbs can be fickle things to get set up correctly when they start playing up. Slightly concerning that you say when you reassembled the carbs that they were sitting 4or 5mm high, Are you sure you haven’t put the slides in the wrong way round? The flat face goes towards the engine.
Also assembling the spring and needle assembly can be a bit of a fiddle.
Where abouts are you, could someone local take a look, often a fresh pair of eyes spots something you may have overlooked.
From a fellow Bonneville owner.

I don’t have any contact details, but I will ask him at the next NE meeting. It sounds as if you have a pair of serviceable carbs at the moment so you’re not far away from sorting. As Don-Spada says carbs can be time consuming. Mine took two weeks, after sorting the jets, to get to a stage I was happy with.

Hi “fellow Bonneville owner”!

The slides are the right way round. I’ve never tried putting them in the other way but I would guess it would make the engine race. These carbs have been on and off so many times in the last couple of weeks I could easily assemble them blindfold now! With the slides raised by mal-adjusting the throttle cables, the bike was rideable, but the idling was too fast and somehow weak. The off-idle response was very hesitant. That was clearly no good and it wasn’t using the idle orifice at all, I think the major air-flow was through the carb venturi, not through the idle orifice.

Since I need to have a bike in good shape for the Piston Rally on 21st September, I have reluctantly pushed the V50 back into the “naughty corner” so I can service the Bonnie and the Morini, both of which should be good prospects. The Morini had a routine oil change a couple of weeks ago, so that’s one less job!

I need some new ideas on the V50, I’ve reached a stage where I’m poking around with no rationale!

I would welcome another pair of eyes on this - I’m in Milton Keynes.

Harris Bonneville eh? Very nice, mine’s a 1978 Meriden job re-imported from California.


Well I used to have a Co-op T140 - real hateful carbs on that one. And I currently have a Morini (Camel) on the fleet too.

Happy to try swapping my carbs onto yours for a test but I’m mid-Essex so that that’s not quite so convenient.

Hi Guys,
This is a rather late reply, but I wanted to be sure of what I’m saying. The good news is that I have completely cured this problem, by replacing the throttle slides with ones with a deeper cut-away. The original V50 spec is for a #40 cut-away and I have replaced it with #50. 40 by the way is 4mm and 50 is 5mm. I just happened to have these in my Dell Orto box of bits. I am still running the #36 pilot jets. Idling is now strong and even at a shade over 1000rpm and it no longer sounds lumpy. It now needs the choke to start from cold and will run unattended on choke for maybe 30 seconds before bogging down. The off-idle response is excellent, strong and instant. I’ve put about 250 miles on it since making this change, so I’m pretty confident. I guess this is another example of modern fuel needing a different set-up.

Unfortunately I didn’t get this done in time for the Moto Piston Rally, and in the end I used the Morini 500 again. I had a brilliant time, the Morini is just about perfect for those mountain roads and I think the V50 would be every bit as good. Oh well, maybe next year?


I’m shocked. I really expected this to be something weird and unusual - like a big hole in the carb bodies, or the choke plungers having been replaced with M&Ms or suchlike.

So are you saying that you changed the std spec slides for different ones, not that you went back to standard?

Just to be clear, I’ve had my bike from new and run it std on the intake side all those years. It ran best on 5* fuel out in France of a cool summer’s evening, but still runs OK now as is.

My Mk 3 has standard jets and carbs, and a standard air box. It also has a decent electronic ignition system. I run it on 97 or 99 fuel (depending where I buy from. Runs like a dream.
Bring back 5 star!!

My Monza was all standard including the air filter. I found that it needed “choke” to start but would need to flick the lever off almost immediately. It was sensitive to carb and ignition adjustment but when set up right it was a lovely crisp bike to ride.
I’m quite interested in reading about carb experiments as I’m about to buy an off-the-shelf PHBH24 for my little Ducati 175 project, and will then need to play around with getting it to run right. The MB22 it came with is beyond saving.