Jota Pilot Jets

tom3c75

tom3c75

Hero member
Location
Cape Town South Africa
When I rebuilt the Dellortos (new brass, slides, needles, gaskets, seals, etc) on my 1980 Jota, I fitted 55 pilot jets.  This was the same size as previously fitted (the bike came from a high altitude location) and the size spec?ed in the literature for this model.

When I started the bike, it spat and popped in the exhaust, so I fitted 58 pilots, which overcame the problem. 

After running the bike for a few hundred kms to get the engine settled, I thought I?d try the 55?s again.    However, the bike definitely did not like them and suffered from the same symptoms as above, only worse, and running performance from stand still was poor.  I tried screwing out the idle screws, but this did not really help.  So, I refitted the 58?s, and things are OK again.  I was also tempted to fit 60?s, but didn?t.   

The engine has the 1980 big valve head and is fitted with high comps and 4C?s.

Does anyone have any experience with this situation?  I seem to recall that the original Slater Jotas had slightly bigger pilots.  Can anyone confirm this and do you know what size they were?   

Thanks
Tom

 
Depends on state of tune of the engine, shape of combustion chamber etc.
I fitted a Corsa spec head to my previously standard 1200 (running 65 pilots initially) and this simple change skewed the pilot settings.
I was lucky to get away with screwing the mixture screws out further, but a size higher on pilot jet would likely have got it back in it's sweet spot.
 
Hi Tom, there are many variables, but one which caught me out big-time a while ago was valve timing - it can can have a massive impact on pilot jet need. At the end of the day, the jet which runs best for your bike is the right one. IMHO 58s dont really indicate you have big problems. Paul H.
 
Haggis said:
Hi Tom, there are many variables, but one which caught me out big-time a while ago was valve timing - it can can have a massive impact on pilot jet need. At the end of the day, the jet which runs best for your bike is the right one. IMHO 58s dont really indicate you have big problems. Paul H.
Click to expand...

Totally agree.

To add to this though, you have only given us information on half of 2 circuits. The pilot jet directly affects the idle circuit and progression. Its partners in these 2 circuits are the mixture screw for idle and the slide cutaway for progression.

The best way to look at a Dellorto is holistically. There are no on and off valves in there and all circuits work at all times though they do have areas where they are more influential and the proportional influence alters with throttle opening. The Dellorto tuning guide is simplistic at best.

If you are in any doubt then put 200ish mains in and see if you can get it to idle smoothly with no other changes. It won?t but the tuning guide states that the main jet only really affects wfo.

There was a thread just recently about spitting carbs and the remedy was to raise the needle a notch. It worked but that action would significantly alter the AFR at all throttle openings rather than just at idle. A small increase of pilot jet, as you have done, will affect the closed throttle position mainly and its affect will be quickly diminished as the slide rises.

A 55 to 58 idle jet increase allows an increase in fuel of 10% but due to the relatively small size of the jet this will be a minimal increase at WFO but there will be one.

Rod
 
Thanks all for your replies.

Interesting point about the cam timing.  At this stage I haven?t gone beyond checking that the timing marks line up, but perhaps I should.  Time to study Phil?s notes!

Rod, the slides are 60/2 and the needles are K1 middle notch, both of which are standard.  Mains are 130.  The air screws, with the 58?s, are about 2 turns out.   

The bike feels pretty much in the zone with 58?s.    Running between 3 and 4K revs results in plugs that have a dark brown colour, with the porcelain insulators a clean, light brown.    I haven?t checked the plugs at higher revs, as the pistons and rings are new.    However, I have given it a brief squirt to 5K revs in top, that?s about 160kph, and the bike did that effortlessly and on a wisp of throttle. 


 
I found 130 mains too big on my Jota. The PO had fitted 135s and it was way too rich. After a bit of dyno time, I ended up with 120s.The bike ran much better and made a few more horsepower. It's well worth booking a dyno session to get the fuel mixtures right.
 
what you ned is a one and one eight Amal feeding all three
and I didnt know you could get the fueling correct just by booking a dyno.
CLEM
 
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