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Monobloc question

Discussion in 'Other Norton Motorcycles' started by qside, Jun 19, 2019.

  1. qside

    qside

    Joined:
    Sep 8, 2016
    Here is the History: Working on a 1966 N15 with 389 monobloc carbs. One carb is chopped model. I am having trouble getting one cylinder to idle. I am pretty sure its the idle circuit because the bike runs well when throttle is opened, but will not fire on one side on idle.

    This all started because I had to swap out the chopped carb because the original was warped and the slide was sticking. Now the replacement chopped carb wont idle. I have flushed and probed all the idle airways that I could find. Still wont Idle.

    I then compared the 389 chopped carbs. The only difference between my 2 carbs is that the original carb had a hole (port) drilled from under the carb side cover through the inner wall into the area were the main jet would seat. This would in essence form a gas reservoir. On the replacement carb (the one that wont idle), this hole is not there and therefor there is no gas reservoir forming.

    My question: Is this hole necessary? And could this be why the idle circuit is not working properly?
     
  2. Paul W.

    Paul W.

    Joined:
    May 12, 2018
    What’s the number on your replacement chopped carb? Your original should be a 389/211. My 389/211 does not have a hole into the reservoir. In essence, it’s an empty chamber.
     
  3. qside

    qside

    Joined:
    Sep 8, 2016
    I will check and report later... but is the hole necessary? Could it be affecting my idle circuit on that side?
     
  4. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    My chopped Amal does not have a hole. From your description, it seems it was meant to be an extra fuel reservoir, but IMO, serves no purpose.

    Monoblocs were noted for fuel starving the 750 engines, and likely hopped up 650's as well. This condition is due to feeding two carbs off one needle and seat. The needle and seat is the choke point, what one does downstream becomes irrelevent.

    The hole falls into the category of the extended float chambers that attempted to address the fuel starvation problem. These only provide a very short term "buffer".

    As far as idling ..... Check your pilot jet. Excessively rich fuel mixture will not ignite and burn.

    I once had a leaky jet block gasket that flooded my right cylinder so badly, I was running on the left only. That nearly drove me crazy, until I noticed the poor condition of the gasket.

    Slick
     
  5. qside

    qside

    Joined:
    Sep 8, 2016
    its number 389/219
     
  6. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    It is the suffix number that matters ..... a 389/219 was fitted to 1965 Atlas, the Bore,Pilot Jet, Cutaway, Main Jet, Needle Jet, Needle Position are: 1 1/8", 20, 3, 350, 0.11, 3.

    The N15 used other numbers, including /211. Check here ....<http://porklips.org/~mason/moto/amal_specs.html>

    Slick
     
  7. qside

    qside

    Joined:
    Sep 8, 2016
    Ya I realize that.That is a great reference. i have used it before. But this is what came on the bike. It was running good. I never had a reason to check the jets before this. Now It looks like I will get some larger main jets just to get it back to specs... But this shouldnt affect the idle circuit, right?
     
  8. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    I am a big fan of returning everything to spec when trouble shooting. I agree, it should not affect idle, assuming your pilot jet is correct (#20).

    Slick
     
  9. qside

    qside

    Joined:
    Sep 8, 2016
    yep pilots are correct. Larger jets on order. I also checked my jet block gasket as you recommended. Can you explain to me how a faulty gasket affects the mixture?
     
  10. texasSlick

    texasSlick VIP MEMBER

    Joined:
    Jan 2, 2013
    As I explained, in my case, it was flooding the right cylinder, and I was running only on the left. Not really sure how it causes this ... I think fuel was aspirated directly into the spray nozzle, bypassing the main and needle jets. I replaced the jet block gasket mostly because I was at my wits end. Fired up the Atlas and was running on both cylinders.

    Since you changed out a carb that was running fine, except for sticky slide, why not try swapping the slide from the carb that will not idle, into the one with the sticky slide? Maybe the slide will not stick.

    Slick
     

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