Hi , rode in a trial last week and after a while of carb experimenting got the bike to be realy smooth at the bottom end and carburates realy well everywere else . problem is now even when not very hot I have pinking or pre ignition just over tickover ie under load , not terrible but its there , im on middle position needle , with air screw 1.5 turns out .When the needle is raised , the pinking goes and the air screw needs to be screwed all the way in to run ok , but only ok not smooth at the bottom end like the settings I have now. My thoughts are to go up on the pilot jet first , what do any of you guys think ? Carb is mk1 concentric and not worn on the slide , float level and main settings are as factory. Its doing me in as apart from the pinking its never run as nice , fuel is esso super unleaded as well

Crank seals ?

Assuming you are in the UK, try Shell V-power petrol. With ordinary unleaded you are trying to get an old system to run perfectly on a mix of chemicals never even dreamed of when it was designed. The V-power at least has some better properties - sorted my Beta out anyway so I use it on the SWM too.

Sorry, just noticed your reference to Esso SU. I find Shell the best so might still be worth a try.

Edited December 4, 201411 yr by 2stroke4stroke

Sounds like a little lean on the air screw, that said Buls pull better when just a tad

fat on the bottom, Amals do tend to be very touchy about atmospheric changes.IMHO

Suggest soldering a bit of metal on the air screw for easy tuning. (in rich)

Try the air screw & leave the needle alone because next will be mid range unrest.

Larry

Hi , don't suspect crank seals as it runs perfect apart from the pinking then doesn't pink when needle raised , so to me its pinking because its too weak that's why I thought of increasing the size of the pilot jet , will try this first I think

Air screw and or pilot jet whatever, Amals can be strange animals.

Have a nice day.

At least with the Amal there is infomation available for the original jetting / settings, so you have a starting point. From memory I think pilot is #20, needle jet #106, main #160 and slide #3.0. There is only one needle for the 2-stroke concentric and it should be centre groove.

I've never had to raise an Amal needle, they've always run fine with the clip on the middle groove. What's confusing is when you raised the needle which richens the mixture, it weakened the pilot as you had to turn the air screw in which richens it more... That isn't right as raising the needle will richen the mixture at the bottom of the rev range, so you'd expect to have to wind the airscrew out a little if the increased richness caused it to run lumpy at idle or coming off idle.

As for individual jets and the part of the rev range they are supposed to affect, I've found to the contrary of general thinking. A weak main jet can make a bike pink at very low throttle opening when the engine is warm as I've just had this very problem with my Ossa. I had a 130 main fitted (Mikuni) instead of a 160 and the bike nearly blew itself apart on the road, even with the choke on, and this was at around 25mph in top on a bike that will touch 70mph, so that gives you an idea of how little the throttle was open - and the main jet is only supposed to affect 3/4 and above throttle opening.

It's possible you have a partial blockage in the pilot system, so it would be worth soaking it in carb cleaner and blowing the pilot circuit through with an airline, or if you can, get it sonic cleaned.

Also, what spark plug does it have. Too soft and it will pink when warm. Generally, an NGK BP6ES or 5ES works fine. Avoid race plugs as they are no use in a trials bike, they're designed to run constant high revs, they can easily foul up in trials engines.

Check the jetting against spec first, also make sure you haven't got a 4-stroke needle which is weaker than the 2-stroke, is longer and has much less of a taper. I'm not sure if it woould even run with the 4-stroke needle but worth a check I guess. I forget what the 2-stroke needle is stamped. but it has a U in it I think.

You didn't say whether the bike has always done this or whether it only began after you tweaked it at that last trial.

Will try main jet first then , to be honest never thought of the plug . Its never realy run what i would class as perfect , but sunday came pretty close apart from the pinking , absolutley no hesitancy from the bottom at all , thanks for the reply

Hi bult360,I have had some Amal parts in the post today to re-build a MK 2 for my Alpina. The needle is a 2B1 & needle jet a 106 . Very quick service from Burlen fuel systems. I have had enough nonsense with MK 1's over the years,& I think the best use of them is a new career as fishing weights!!! However,I do know that they can work as the designer intended,sometimes. The new needle has 5 grooves,hope this helps.John.

E

Just some thoughts since 1973 with Bultacos .( Mod 92 came with 25 Amal)

As John so aptly puts it; nonsense & fishing weights good plan!!!

Mikuni & copies way to go!

Side main originals will work without a main jet for most sections, certainly not steep long climbs

where we would be needing a proper main.

Before doubting please experiment.

Larry

Hi bult360,I have had some Amal parts in the post today to re-build a MK 2 for my Alpina. The needle is a 2B1 & needle jet a 106 . Very quick service from Burlen fuel systems. I have had enough nonsense with MK 1's over the years,& I think the best use of them is a new career as fishing weights!!! However,I do know that they can work as the designer intended,sometimes. The new needle has 5 grooves,hope this helps.John.

E

John, MK2 needle is different from the MK1 needle

Well???

Please don't leave us in the dark.

We are never to old and set in our ways to learn.

Larry

Not had time to experiment as yet , have got together various pilots a new needle and needle jet of the correct size and mains and will go from there , changing one thing at a time ,this weekend if i get chance ,and report back

Thanks for the reply, as I am old and understand time constraints.

Wish you good luck, please don't rule out electrics, as they can

cause mysterious happenings.

Larry

Side main originals will work without a main jet for most sections

Larry

I assume you are referring to this type :-

of Spanish-made Monobloc and main jet is #30 in the illustration ?

(I have the drawing, but not the corresponding parts list)

I acquired a number of these jets "n.o.s" in various sizes along with some other Bultaco parts I bought many years ago, and at the time was unable to find anyone able or willing to identify them. All appeared to have been supplied originally as official Bultaco parts and were packaged and labelled as such; more irritatingly, they are shown in Clymer manuals in similar pics. of carb. and photo stripdown sequence, etc. - and not mentioned in the text ! .

Bult 360 -

(with apologies for going off-topic a bit)

You don't say which Mk. I Concentric - size ? UK replacement, or Spanish original ?

Edited December 10, 201411 yr by lorenzo