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Warm Start Troubles

Discussion in 'Norton Commando Motorcycles (Classic)' started by Tornado, Jul 1, 2018.

  1. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    I try to get into habit of closing fuel taps before reaching my garage and just run the carb out of fuel while idling once parked...to help clear pilot circuit of fuel. This will leave some fuel in the bowl, just lower team idle pick up passage. Don't want residue building up in idle circuit if bike isn't used for long periods.
     
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  2. franko

    franko VIP MEMBER

    Joined:
    Dec 29, 2013
    In reading several post a day in this thread, I lost track of that it was an Amal flanged set up.
    Is the boot to Ham Can tight, as in pushing against the carb when they heat up and expanded?
    Could this be just enough pressure to tilt the carb and get a small leak at the O ring?
    Maybe a small smear of high temp RTV could be placed around the flanges and used to test this theory.
     
  3. RobinKent

    RobinKent

    Joined:
    Jan 31, 2010
  4. Craig

    Craig VIP MEMBER

    Joined:
    Dec 20, 2005
    Interesting reading on Amal ,RK ... glad the single Mikuni was already installed on my bike when I got it ... I think maybe 5-6 years ago I gave the Mikuni a quick clean ... since then , drain the carb at end of season and that’s it .... while I like original guess I love simplicity more
    Craig
     
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  5. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    Yes, seems doing more mixture screw tweaking might have cured much of the hot start issue. I think I'm now around 1 1/4 turns out on the screw....richer than before. Running #78 drill into pilot didn't seem to improve stuff. She's idling quite nicely at around 900 rpm...lower than I was comfortable with before. Plugs looking good so far.
     
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  6. maylar

    maylar

    Joined:
    May 13, 2007
    Gonna hijack this thread, since I'm having similar issues.

    Is the idle circuit the only thing in play when starting the engine? Mine will start from cold with one kick after tickling and with choke, idles nicely at 900. But hot starts are very difficult unless I "half tickle", and it shouldn't be that way. Just wondering what I should tweak.
     
  7. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    After a good ride,shut down engine and immediate touch the carb bowl(s) to check temp...mine was cold, lower than ambient. Then with fuel taps closed, left it 20-30 minutes and checked temp again...too hot to touch. So heat soak to carbs is a real issue that may effect starting. Turning on fuel taps coiled down the carb quickly. Of course leaving fuel taps open during your stop should keep temp down...but will use up fuel as it evaporates from bowl doing the cooling.

    Ultimately, what seemed to resolve the poor hot starting on my bike was tweaking idle mix...I think it ended up richer...some 1/4-1/2 turn towards closed.
     
  8. RobinKent

    RobinKent

    Joined:
    Jan 31, 2010
    Hi Maylar, this is how I see it. Tickling floods the idle circuit but also the needle jet and may also put fuel directly into the inlet manifold, giving you a rich mixture for starting. If we don't do this then we would need to create enough vacuum in the inlet manifold to suck the air out of the idle circuit and then draw the fuel up it, mix it with air and inject it into the manifold. All through a hole just 0.016 inches wide. We just can't get enough vacuum with a kick to do that.

    When running, the circuit is kept full by the vacuum but when we stop so does the vacuum and fuel drains back down, replaced by air. We are back to where we started.

    The time taken to get there varies from bike to bike, altitude, rate of evaporation, fuel volatility, leaking gaskets, sync of carbs etc etc.

    The choke extended into the airflow helps a cold engine by increasing the vacuum over the two tiny inlet ports on the engine side of the carb but perhaps more by blocking some air and making the mixture richer until you get to operating temperature. It doesn't really help the bike fire up though. I don't have a choke fitted at all and usually start first time after tickling. Doesn't matter if its hot or cold. I just look after the idle till its warmed up.

    The pilot screw controls the idle mixture but is set in conjunction with the air slide that controls RPM, ie vacuum. As you pull away it hands over to needle jet pretty much instantly. It does have a range and is ideally set to make idle and transition optimum.

    In my view both the idle and needle jet contribute to starting with success dependent on availability of fuel in the jets. I always start with some throttle.

    So in normal circumstances full or half tickling is normal. If you are stalling due to problems in those circuits and then struggling to start then there is some scope to investigate why. Otherwise just get the timing spot on, valve clearance right and then sync and set per the Bushman instructions (Adjusted for slide size) and learn roughly how long you get before you need to put fuel in those circuits by hand...
     
    Last edited: Jul 9, 2018
  9. RobinKent

    RobinKent

    Joined:
    Jan 31, 2010
    As Tornado says, 1 1/4 to 1 1/2 out is pretty normal but you can get the same idle with richer or leaner setting by altering slide height. Bushman is good.
     
  10. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    Update: after still getting some harder starting following short 5-20 minute shutdowns, today I tried not tickling, not choking...and got 3-5 one kick starts straight. So now I'm thinking my issue was over fueling/flooding by tickling a hot carb where fuel might have already been expanding due to heat up above the float level and perhaps into the carb throat/intake. Plenty there for good starting until a tickle putting even more fuel into the intake.

    Thoughts?
     
  11. Mr. Rick

    Mr. Rick VIP MEMBER

    Joined:
    Feb 6, 2009
    My thought is that whether a tickle is useful or not wd be a function of how long the bike has been stopped and how long it takes for fuel to evaporate below the idle circuit level. If you tickle the carb and fuel spills immediately, you prob didn't need to. The only "flooding" problems I've heard of had raw gas dribbling out constantly, usually a float (fuel) level incorrectly set.
    But why mess with success? If you are getting one-kick starts, you're doing something right for sure.
    As for choking, I went years without one, and now use it (very briefly) only for dead cold starts.
     
  12. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    Been running and starting quite well past few weeks. I placed a fresh Phenolic gasket between carb and manifold, eliminating the rubber Oring entirely, using Permatex grey gasket maker (low viscous type apparently similar to Yamabond) on the carb side of the phenolic and just Hylomar on the manifold side to ease future removals. Also placed fiber washers under each carb to manifold stud bolt to help reduce heat conduction. Happy to report carb bowl no longer too hot to touch after riding. Bike is starting reliably with no tickling or choke 20-30 mins after hot shutdowns. Tickle seems to flood it and take 5-10 kicks before firing again.
     
  13. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    Oh, I also swapped out the carb throttle slide with the one for my other unused 932 Amal...seemed to slide more smoothly and not stick in bore at fully wot position...no more stuck open throttle when carb hot or not.
     
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