Main Jet

Hi,

I pulled the carbs on my 850 Mk 3 as it was running on only one cylinder and the carb adjustment screw o-ring were turning to dust as I tried to adjust them. The carbs were very dirty and I wanted to remove them to check their internal condition. I had the bike 4 months and only ridden it round the block once so I am trying to find out as musch as possible about the general conditiond of all systems.
The inside of the bowls were covered in a red dust and the airway seemed dirty. They cleaned up well in an ultrasound washer.



The one on left is uncleaned. You can just see the float bowl drain plug is almost disintegrating on the right.

Here is the dirty stack.

I was expecting to see a 260 main jet but it is a 220.

The manual and the tech info on this site suggest that it should be a 260.

Should I leave the old jet or replace it with a new 260 and needle. ???

I have standard downpipes and stright peashooters with no baffels.

What is your experience with jets and recommendations. Anything else I should look for while I have a the carbs in a pile of bits ?

Regards,
Sean

My MK2A came with 230's but with peashooters instead of the factory fitted Black caps but it would not take full throttle, I went to 260's and the problem was cured. The MK2A setup is virtually the same as the MK3 so 260's would be best for you too as a starting point.

Your carbs should have have a stepped spray tube, can't see from the pics if these are fitted.

Even with the cleaning you have done I would still clean out the pilot jets with a #78 drill (16 thou) glued into the end of a WD40 tube.

Hi,

Thanks for that comment. I need to check the spary tube too. I think I might have the standard one and not the Norton one.



I was planning to just change out all the jets and not try to clean them. They are relatively inexpensive but soooo important.

Regards,
Sean

The pilot jet is not removable on these carbs so cleaning by using the #78 drill is best, alternative is to drill it out oversize and fit a removeable pilot jet in the 2 stroke position which will possibly cause other problems.

You need the steeped Norton spray tube to remove a flat spot, whether this was due to the silencers being black cap make make this moot of course but if you get a flat spot then that is the first item to look at.

Stock airbox/Filter?

Yes - stock airbox and filter. New filter elements fitted.

The stepped spray tube was introduced with the black air box and the black cap silencers, as you now have peashooters you may not need them but I have them on my similar setup eg peashooters no baffles and black plastic air-filer with foam element and they do no harm but never tried without either.

Maybe theyre very old mainjets , and theyve worn to 260 . :shock: They do have a service life - if to match spec . Guess the filters help here . The old fuels abrasive over the miles . 20.000 & youd see wear .

I had problems with the red residue inside the Amal bowls too.

The mild rusting/oxidation in the steel fuel tank passed through the fuel tap filters, the fuel line banjo filter and an inline filter.
Probably this mix would act as an abrasive and accelerate wear on the fuel metering parts especially as the needle moves up and down in the jet.

The fix was to apply the Caswell epoxy liner in the tank. No more residue!

Didn't the 930 Amals for the 750 come with 220 main jets and the 932 had 260 main jets and the red dust in the bowls would be old fuel left in the carbies when the bike has be sitting for some time, should drain the carbies before any time left in storage, as for main jets and needles, they should be replaced every 2 to 3 years of running, over time the needles vibrate which causes the jets to wear oval, they are cheap to replace.

Ashley

Hi,

That is what I was thinking. I have 932´s but the jet is wrong.



I will change it and see how it goes.

Regards,
Sean

My MK3 came with 230 main jets. Factory WSM said this was correct. It was totally stock but as I lived in Colorado at the time, went down to 220 to get the mix correct. I seem to remember the rule of thumb was to reduce jet size 3% for every 3000ft gain in altitude so at 5000ft or so 220 was about right. Now I'm pretty much at sea level and with straight thru peashooters with one of those K&N's that hobot likes so much and I've found that 240 mains work pretty well. I may see about an increase to 250 though as fuel these days is oxygenated and may cause an engine may run leaner with previously perfect settings. That's my take on it anyway.

Biscuit said:

My MK3 came with 230 main jets. Factory WSM said this was correct. It was totally stock but as I lived in Colorado at the time, went down to 220 to get the mix correct. I seem to remember the rule of thumb was to reduce jet size 3% for every 3000ft gain in altitude so at 5000ft or so 220 was about right. Now I'm pretty much at sea level and with straight thru peashooters with one of those K&N's that hobot likes so much and I've found that 240 mains work pretty well. I may see about an increase to 250 though as fuel these days is oxygenated and may cause an engine may run leaner with previously perfect settings. That's my take on it anyway.

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I'd agree with this. Manual/Stock jetting should only be used as a starting point/guideline. What with all the different fuels these days....