Jetting issue?

[Matt Hale]

Hi All- hope track day goers had a great day+...I have sooty plugs possibly causing stuttering /one cylinder running when engine warm after a mile...starts and runs ok before this...I’m running 43 pilot jet but with 115 main with standard (new) air filters fitted. No splits in rubbers anywhere. Should I use a smaller main? Would oversize main cause sooty plugs with this set up? I’d like to use the air filter...thanks in advance for any help!- Matt

[Steve Brown]

Fuel level in float chamber? Throttle needle set high or needle jet worn-quite common? A quick way to test the air filter contribution is to disconnect the rubber hoses from carb to air box and refit them pointing away/down so picking up unfiltered air. You can't ride far or fast like this as the rubbers will flap about and possibly even close up causing another decoy fault. A mile or so up the road should be ok though. If not then a lot of carb faults are ignition faults and vice versa. If they soot up from clean in a mile there is something way off!

[Matt Hale]

Many thanks Steve- I will check float heights, lower needle height (needles not worn I don't think) and re strobe timing as I have moved it without re checking. I will then try no air filters if these adjustments don't improve things. best Matt

[Steve Brown]

Many thanks Steve- I will check float heights, lower needle height (needles not worn I don't think) and re strobe timing as I have moved it without re checking. I will then try no air filters if these adjustments don't improve things. best Matt

It's usually the needle JETS that wear rather than the needles, the jet gets oval. Hard to detect the wear by eye too!

[3potjohn]

112’s would be worth trying.

[Matt Hale]

Hi 3potjohn and Steve- thanks for your thoughts. I update you....I re strobed the timing as I had moved it about on the road in desperation. I now have OK advance at 5,000 ish revs ( no rev counter to check that). It needs to be set in retarded position in relation to standard as I am using blue Ducati transducer boxes. Starts OK but not ridden yet. Awaiting 98 + 112 main jets to try from Eurocarb in Pangbourne. In the workshop manual it says use 98 mj with air filters/box. I have 115's in now so maybe too rich?. I will see what they do when I fit and ride. I may also lower needles- currently in the middle position. I'm keen to keep using standard air filters...
I have not adjusted the floats as I don't want to mess there unless all else fails. I have checked and they seem correct but I do find that a bit hard to be sure on. I don't see why they would have moved. No leaks in them.
I'll get back with further update soonish- many thanks for your support.
Best wishes to you and all.
Matt

[mbmm350s]

Hello Matt,

I don't know where you get the information that a 79 sport has 98 main jet. That might be the case for USA Emissions spec (California) but these have a sealed airbox and in later versions a different airbox, please don't try it even if it fixes something its just wrong.

79 onwards A and K, K1 Sport if fitted with VHBZ25HS carburettors were as John says using a 112 main jet
The emulsifier tube should be 260D

If yours is an A/3 sport fitted with VHZS22BS then its 115 and 260BD
Both models (A or K) will run on the Stradas 260K reasonably well, all use E17 needle, middle notch
It must be born in mind that the minor variations of the emulsifier tube is "the cherry on the icing on the cake" of tuning

Unless you ride everywhere at 3/4 or more throttle the main jet 112 or 115 will make no difference, the metering is controlled by the size difference between the taper of the needle and the emulsifier tube. If you put in a 98 you might be using the main jet for middle throttle metering which is just wrong.

As Steve points out wear in the needle and especially in the emulsifier tube are very common and must be eliminated first, serious wear in the slide or a loose needle causes the needle to make contact with the emulsifier tube wall, this upsets correct atomisation. Wear in the slide can be detected by laying the slide on a flat surface and attempting to slide feeler gauges under the edge, if the anodising is non existent or only partly there then sorry the slide is worn out.

Its very common to find the choke is not sealing off correctly.

If you haven't already done so I suggest you get a copy of the Dell'Orto tuning manual before changing anything, this also gives detailed drawings of all the common needles, though its very difficult to measure them for wear and people have an annoying habit of polishing them

All of the above pre-supposes that the ignition is running correctly. If you don't have a tach, just wind the engine up until the ignition stops advancing and make sure that aligns with ANT1, there is very little if any advance after 6000 rpm.

Cheers
Mark

[mbmm350s]

Hello Matt,

I don't know where you get the information that a 79 sport has 98 main jet. That might be the case for USA Emissions spec (California) but these have a sealed airbox and in later versions a different airbox, please don't try it even if it fixes something its just wrong.

The "star" in the manual is for the American models setting (Emissions controlled states eg California) - I don't think this is applicable to all US states, may be an American contributor could confirm.
To re-iterate please don't use this setting unless you have an emissions controlled USA spec model


Mark

[Matt Hale]

Many thanks Mark, 3potjohn and Steve. I will stick to 115 main jet- got mixed up with American ties as you say. I think my issue maybe the other carb potential problems you kindly listed and I’ll check them. Was trying a 45 pilot but now 50 at present. Starting difficult from cold or impossible. Using choke makes it harder/does nothing. Seems v easy to get wet plugs…timing is correct according to my strobe check and advances correctly now.
I'm bit surprised the bike is so sensitive to carb wear as older carbs on other bikes always started OK but were sometimes a bit rich. I've added an image of one of my carb slides here for your consideration! I do wonder about just buying new alternative carbs but don't know which I could use....Thanks once more!!

IMG_3683.jpeg (47.17 KiB) Viewed 3861 times

[3potjohn]

You are absolutely certain that the chokes are properly sealing when off.

[Matt Hale]

Hi 3potjohn-not certain chokes seal OK...but... I did buy new chokes last year and when I put them on/use the chokes I can hear a change in sound as if it is sucking through different area of carbs so I think they seal OK when not in use as sounds different. Is there a way to check this?

[MickeyMoto]

Chokes on, testicles revved off, chokes off when cold stall, when warm ticks over.

[Matt Hale]

Thanks MickyM...currently bike not start on choke...

[Matt Hale]

Hello Matt,

I don't know where you get the information that a 79 sport has 98 main jet. That might be the case for USA Emissions spec (California) but these have a sealed airbox and in later versions a different airbox, please don't try it even if it fixes something its just wrong.

79 onwards A and K, K1 Sport if fitted with VHBZ25HS carburettors were as John says using a 112 main jet
The emulsifier tube should be 260D

If yours is an A/3 sport fitted with VHZS22BS then its 115 and 260BD
Both models (A or K) will run on the Stradas 260K reasonably well, all use E17 needle, middle notch
It must be born in mind that the minor variations of the emulsifier tube is "the cherry on the icing on the cake" of tuning

Unless you ride everywhere at 3/4 or more throttle the main jet 112 or 115 will make no difference, the metering is controlled by the size difference between the taper of the needle and the emulsifier tube. If you put in a 98 you might be using the main jet for middle throttle metering which is just wrong.

As Steve points out wear in the needle and especially in the emulsifier tube are very common and must be eliminated first, serious wear in the slide or a loose needle causes the needle to make contact with the emulsifier tube wall, this upsets correct atomisation. Wear in the slide can be detected by laying the slide on a flat surface and attempting to slide feeler gauges under the edge, if the anodising is non existent or only partly there then sorry the slide is worn out.

Its very common to find the choke is not sealing off correctly.

If you haven't already done so I suggest you get a copy of the Dell'Orto tuning manual before changing anything, this also gives detailed drawings of all the common needles, though its very difficult to measure them for wear and people have an annoying habit of polishing them

All of the above pre-supposes that the ignition is running correctly. If you don't have a tach, just wind the engine up until the ignition stops advancing and make sure that aligns with ANT1, there is very little if any advance after 6000 rpm.

Cheers
Mark

Hi Mark+ Many thanks Mark, 3potjohn and Steve. I will stick to 115 main jet- got mixed up with American ties as you say. I think my issue maybe the other carb potential problems you kindly listed and I’ll check them. Was trying a 45 pilot but now 50 at present. Starting difficult from cold or impossible. Using choke makes it harder/does nothing. Seems v easy to get wet plugs…timing is correct according to my strobe check and advances correctly now.
I'm bit surprised the bike is so sensitive to carb wear as older carbs on other bikes always started OK but were sometimes a bit rich. I've added an image of one of my carb slides here for your consideration! I do wonder about just buying new alternative carbs but don't know which I could use....Thanks once more!!

Following your advice I'm thinking of buying this lot below from Eurocarb- can anyone confirm correct? Ie Standard 350 Spor VHBZ 25 BS carb ...Not sure which slide-50 or 60...?

x2 9526 AP Type atomiser (VHB(Z) 20-26mm)
x2 2343 E type needle (VHB(Z) 20-26mm) - E17 needle
x2 7447 VHB VHBZ 20 – 26 Slide- 50 OR 60

[Ming]

FWIW I never used a strobe on the ignition on any of my Morinis, just set static timing to the mark. Likewise carbs were tuned (ahem) by ear or wires under the slides. I did change a couple of choke rubber discs. I think your carb problem might turn out to be ignition related. Even a badly set up carb should allow the engine to run badly at tick over.