Hitchcocks Forum

Hi Guys
I’m trying to tune my new 32mm Concentric Mk2 for my 1996 535cc and having great difficulty starting the bike reliably – haven’t yet worked out the settings for such minimal throttle so sometimes it starts immediately and sometimes only after 20 kicks (starting "by guess and by god"). Bike has ~8.0:1 compression (our hosts 8.5:1 535cc piston but compression plates allied with head flattening give me an uncertain compression).
Currently I have a 220Main (tried 240 and 260 to no avail and they both gave sooty plugchops), 106 needle jet (tried 107, no better, similarly sooty), needle in position 2 (tried them all, even those in the Kama Sutra – I think the bike’s messing with me now), 35 pilot jet (tried 25 and 45), choke jet 40 and air jet 3.5mm (have a 2.5mm).
One thing I am unsure of is the last item, the air jet. The tuning details from Amal say it “controls the amount of air which pre-atomises the fuel before it enters the mixing chamber body” and that it “affects the main jet depression”. Well I’m getting pretty depressed even without touching this jet. Some writers here have said “use the 2.5mm air jet” as it makes it better. But what does it actually do? Does a smaller air jet affect the starting, does it make the whole mixture richer or what?
Hope you can help, and thanks for the input.
Cheers, ChrisD.

Chris, I believe the choke is similar to the Micarb and other Jap carbs, ie a plunger type? I found that with the Micarb, if the choke is left on for more than about 3 kicks or so, the engine would flood and not start until it is cleared. If the bike doesn't start after about 2-3 kicks with choke, turn choke off then try. The best way to clear a flooded endine is to turn off the ignition and choke, turn throttle fully open, open decompressor, then kick it over a few times. The Micarb choke is very rich, though some seem to think the opposite and keep kicking with choke on!

Air jet – this allows fuel to pre-atomize before entering the mixing chamber – the jet size may be changed to fine tune performance, a smaller jet will increase depression and a larger jet will decrease depression, therefore a smaller jet will result in a richer mixture and a larger jet a weaker mixture.

The cold starting jet can be changed to give a richer or weaker starting mixture. Also it’s always worth checking with the Mk2 whether or not the cold starter plunger is allowing the cold start jet to be fully opened for starting. (The operating lever on this carb is very crude and often does not allow the start jet to be fully closed or fully opened.) If you’re constantly getting a sooty plug it may be that the starter jet is not closing off fully.

Hi Chris , you ve got good advice there from Nettshubby and Presto ( thanks for explaing the air jet to me too !! )
I also run a 32mm MK2 on my 535 , comp valves and an unknown American piston , but its got a lump on it ! I have no idea what compression I m running at , however I am suffering from the same rich running at low traffic speeds , under normal and fast riding conditions my settings are fine .
You may want to compare .. so its a Mk2 with a 3.5 slide a 2A1 needle ( 2nd from top groove )240 main jet 106 needle a 20 pilot and a 2.5 air jet .
I must say mine will always start 1st kick or so , unless ive been running slow and the plugs sooted up .
I have tried various plugs from the std B8ES , an Iridium BR8E IX and a hotter B7ES, no real noticeable help with the soot at slow speeds !
It has been suggested to me recently that maybe at slow speed the larger carb has not got enough mixture velocity to fully atomise the fuel ..any thoughts on that ?
I may try going upto a 3.5 air jet now and see if it helps mine , let us know how you get on Chris
Cheers

Chris , just a quick thought have you found a way of mounting your carb so it sits upright ? mine is at an angle and have , although Amal say I m ok for about 22 degrees , wondered whether that contributes to bad fuelling , I cant imagine the float liking it much ..
Phil

The air jet doesn't have any effect at very small throttle openings, so I don't believe it's got anything to do with the starting problem.



Question: by how much are you "cracking" the throttle when starting it?

Phil535: I did 2 things to make the Amal fit. I raised the back of the petrol tank by 1.5cms (welded on an extension plate) and I made an offset manifold. I cut almost through the stub from the left side exactly where it joins the flange. Bent the stub sideways from the flange by ~8mm so that it was in the correct position but still held in place by the uncut 3-4mm of stub. Then I drilled through the flange into the cut stub face to fit 3mm bolts and glued them in place so the stub and flange were fixed permanently at that angle. Then I filled the gap with J&B weld and smoothed up the inner surface of the tract. Then I made sure the flange was flat and smooth. All OK after a few hundred kms. Should it fail and start leaking, I’ll get a proper one made at that angle offset.
Also Phil thanks for the comment about the air jet. Amazing that Amal doesn’t mention where it has an affect.

Thack: I don’t open the throttle but have always, since getting the Bullet, wound in the throttle stop a turn or so to make for better starting – then turn it back once the bike starts. Worked every time with the Mikcarb.
Had to drill a 2mm hole through the head of the extended Amal throttle stop and insert a thin copper rod to make it easier to turn with gloves on. J&B weld there too (great stuff).

Presto, that’s a good point about the cold start jet, and I’ll go check out the amount of movement.

All: starting is a bit easier now that I’ve moved the inlet cam to “on the dots” but again I’m getting reversion so much of the fuel above half throttle gets blown backwards out of the carb now. Not sure what to do. I’ve the Hitchcocks Performance cams and the overlap is considerably larger (~100degrees) than that of the Redditch cams (documented as ~75degrees). Tried the cams in so many different positions now, advanced, retarded, also the adjustable Pinion. The bike doesn’t run properly “on the dots” (i.e. with the cams set on the proper dots) so I ring the changes to try to get the best combination of easy start and effective throttle working past ½ position without misfiring from reversion. Since there have been issues with both the carb and the advance/retard mechanism, there have been (and still are)many things to test and one-at-a-time takes forever. I have never managed to get the bike to run at much more than half throttle position, despite all the money spent on mods and tuning, and so the maximum speed is still ~115kph although it will do that easily on half throttle. So, no success there yet!
ChrisD

Chris , Yeh that JB Weld is useful stuff !! I am going to try and make another manifold for mine and as we speak I have had my timing cover off with a plan to change my cams , still std at the moment, as mine starts to run out of puff after about 70mph ..as is bottom end ..but on finding a few of those little shiny flakes of metal in my oil this afternoon its looking like my time and £ is going to be spent on a crank rebuild
There is a Very interesting and informative write up on cams and cam timing on the U.S Enfield forum , if you Google " Lets talk about cams " it should come up , worth a read !
Hope you get it sorted and would love to hear any progress
Cheers Phil

Yes, Phil535, I have followed Ace Café’s excellent work and reviews on the USA forum for the past few years – indeed from his site I decided to adjust the cam phasing both with the OEM cams and now with our hostss Performance cams, 8.5:1 piston and adjustable pinion (actually this gives 6 degrees of advance or retard not 4.5 degrees). However, neither gave me access to full throttle because it still misfires. It does run better with the inlet cam retarded ~12degrees, but it won’t start reliably so is not useful. This was the reason I went sequentially for twin-plugging, new distributor and Concentric 32mm+Ace Café air cleaner to try to solve this issue – also to no avail. The bike goes best with the shorty megaphone (PN 91009) or the English shorty (PN 91007), occasionally revving to almost 2/3 throttle but both are unacceptably loud.
Taking into account the extra cost to import parts here in South Africa (adding some 80% to the listed cost) I have so far spent almost the price of a new bike on this one, but “hope springs eternal….”.
I shall try the jet changes to see if that helps.
ChrisD