Author Topic: B40 running lean  (Read 277 times)

0 Members and 1 Guest are viewing this topic.

malj

  • Star
  • *
  • Posts: 11
    • View Profile
B40 running lean
« on: 10 August, 2019, 21:33:00 »
Hi, Iíve fitted a new Amal premium concentric carb on my b40, 130 main jet, .106 needle jet. The bike has a circular pan type air filter and a straight through trials type exhaust. Itís also fitted with electronic ignition.
 To get the bike to start I had to turn the pilot air screw almost all the way in itís only open around a quarter turn. The bike then ticks over nicely but alas wonít rev up off idle backfiring through the exhaust the exhaust also seems to be getting very hot, Iím pretty new to Amal carbs and British bikes and am running out of ideas, but I think all these signs point to the bike running very lean.
Iím going to double check the ignition timing and recheck that thereís not any air leaks at the carb/engine joint. If they check out ok would I be right in think that if I change the main jet for a larger one that would richer it up, itís running a 130 at the moment how big should I go?. Any help would be gratefully accepted, cheers

hoogerbooger

  • Royal Star
  • ***
  • Posts: 175
    • View Profile
Re: B40 running lean
« Reply #1 on: 11 August, 2019, 08:52:26 »
I presume you have a Concentric 626. I don't as yet know the best set up for 626 on a B40, but did query this with Dave Smith (Rupert Ratio)  recently as I am thinking of getting a new one for my B40F Star ( so different exhaust). His advice is only as a starting point for tuning and he suggested:
Main jet 140; pilot jet 20; Slide cut-away 3; needle position; needle jet 0.106 ( But note I had said I have  gauze filter, whereas you have a paper one which will reduce the air flow/lean the mix a bit )

I'm guessing you may have the set up for the 1967 Australian Army B40 which was as above, but with a 130 main jet. It had a paper air filter. I had previously presumed this would be ok on mine, but perhaps needed to increase the main jet to 140 in account of the extra air flow through the gauze filter . i.e as Dave Smith Suggested. However Julian S on this forum suggested this may leave the bike lean at higher revs. I queried this with Dave Smith who said 140 should be ok as a starting point, but suggested going up to a 160 main jet if lean.

(The original monobloc had a 190 main jet and Dave Smith said as a general rule of thumb, if you switch to a concentric of equal size ( a 626 is a tad smaller) the main jet should be reduced by 20)

So no clear answer I'm afraid, but the above is hopefully of some use for further tinkering. Good luck ....& please post if you work out a set-up for a 626 that seems good on your bike.








« Last Edit: 11 August, 2019, 08:57:15 by hoogerbooger »

Bees

  • Blue Star
  • **
  • Posts: 61
    • View Profile
Re: B40 running lean
« Reply #2 on: 11 August, 2019, 16:47:16 »
I would start with the timing. Some of those symptoms sound like too far retarded to me.

Csx355

  • Blue Star
  • **
  • Posts: 67
    • View Profile
Re: B40 running lean
« Reply #3 on: 12 August, 2019, 07:35:28 »
Hi Mal - another useful check while it is running is to check for air leaks between the carb and head. Spray some GT85 / WD40 or similar around the head/carb joint. If the tickover picks up that could be the issue. Also check the head bolts are tight.
Bantam - Needing Rescuing
B40 - Work in progress
B44 Trail - sort of finished
B44 Road - In Progress
A65 Thunderbolt - Always the next one to do!

malj

  • Star
  • *
  • Posts: 11
    • View Profile
Re: B40 running lean
« Reply #4 on: 13 August, 2019, 21:39:43 »
Thanks for the replyís, Iím beginning to think it is timing related. Iíve checked and sealed up leaks on both the exhaust and carb Iíve also checked the timing and it seemed to be pretty much on, however Iíve advanced the timing a touch (I think) and itís running slightly better so I think Iíll advance it a touch more and see what happens.
I said before that I think Iíve advanced the timing, itís fitted with a hpi ignition which is very similar in appearance to the electrix unit with the electronic gubins encapsulated in plastic and mounted on the stator plate Iíve found some set up instructions but there pretty basic, looking from the primary casing side the engine turns anti clockwise so Iíve turned The stator plate against the engine direction clockwise, can anyone confirm this for me that this is advancing the timing. Cheers

AdrianS

  • Royal Star
  • ***
  • Posts: 234
    • View Profile
Re: B40 running lean
« Reply #5 on: 14 August, 2019, 09:35:52 »
If you turn the stator plate in the direction of engine rotation you will retard the ignition timing. Rotating against the direction of rotation will advance the timing.

malj

  • Star
  • *
  • Posts: 11
    • View Profile
Re: B40 running lean
« Reply #6 on: 14 August, 2019, 17:09:22 »
Cheers Adrian thatís what I thought.