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The correct use of choke

Discussion in 'Technical and Troubleshooting Torque' started by typhoon, May 31, 2008.

  1. Well, it's definitely cooled down enough in the southern states, and there's lots of new riders experiencing their first winter, so thought a post on the choke would be useful. If you have a fuel injected bike disregard, as your bike does it all for you (ya bastards!).

    O.K, the theory behind a choke. When starting a cold engine, the incoming air is cold and travelling very slowly, compared to when the engine is running. This makes it more difficult for teh carburettor/s to atomize(mix up in air) the fuel. The choke serves to add extra fuel to the incoming air, ensuring combustion in the engine.

    A cold engine needs significantly more fuel, even when first started and running, to make up for fuel that is not atomised (usually pools in the intake, passes right through the combustion chanber unburnt etc).

    There are two type of choke mechanisms on motorcycle engines, one is an enrichener, and the other is a choke plate.
    The enrichener works by adding extra fuel through a small port in the carburettor wall. It is a simple plunger type setup, more choke you pull on, more the plunger travels, and more fuel can be admitted.
    The choke plate type works by restricting the amount of air entering the carburettor. It creates a higher vacuum on teh engine side of the choke valve, and this higher vacuum draws extra fuel out of the idle and transition circuits of teh carburettor (adds more fuel).

    Correct use of choke is to FULLY engage the choke when cold cranking and use no throttle.
    As soon as the engine catches and starts to run, trickle on some throttle. You will need some judgement with your particular bike with regards how much throttle to add. As soon as the engine is up and running, reduce choke as far as you can without the engine starting to stumble. When the engine is running smoothly on the choke, you should be able to let the throttle return to idle. It is normal for the engine to rev higher on choke, even with no throtle.
    You can safely leave the engine do it's own thing with some choke on, put your gloves on etc as it warms up. You should slowly keep reducing choke as the engine needs it, some bikes will take a few seconds of needing choke, some more. Again, experience and judgement will work.
    It is even normal to need a little choke to ride off with on cold mornings.

    A good test to see if you need choke is to blip the throttle lightly. If the engine stumbles just off idle, you need some more choke. Remember of course, to return the choke to off as soon as reasonable, again, use throttle blip to see if you need it. Smooth blip, no need for choke anymore.

    Sometimes, rarely, you may flood an engine (flooding is when you allow the engine to draw too much fuel in and the fuel/air mix is either too rich for combustion, or liquid fuel has coated the spark plug, allowing the electric current to run straight to ground with no spark) using too much choke. It is pretty hard to do on a bike with a healthy ignition system, but if you do suspect flooding, you need to know how to identify it and correct it.
    If you have flooded the bike, you will notice either a strong fuel smell, or liquid fuel running out of the exhaust somewhere. If the engine is turning over, and occasionally firing on one cylinder, it is NOT flooded. Use more choke.
    If you do flood an engine, the best thing to do is walk away from it for a few minutes, the restart with NO CHOKE and full throttle till the engine catches, and if it doesn't start after catching, reapply choke, but less this time.
    Probably the very best way to start a badly flooded engine is roll starting down a long hill.
    A side not to cold starting an engine is engines with vacuum fuel taps. Sometimes, the carburettors may have the fuel evaporate otut of them if left for a couple of weeks, leaving very little fuel in the carburettors. Normal cold crankng of the engine does really not provide enough vacuum for the fuel tap to flow fuel, so you should switch to prime and crank on prime. This allows fuel to run to the carburettors regardless of engine vacuum. An engine not starting is fairly frequently caused by this, and can be misdiagnosed as flooded. Try prime position if the engine has sat for a week or so.
    So, there is absolutely nothing wrong with using choke, it is not a sign of a badly tuned engine that you need choke for cold starts, nor is it bad to ride off with SOME choke. Remember, the choke is there for a reason, use it, but use it properly.
    Consult your owner's handbook for any pecularities for your bike.
    And remember, a well maintained ignition system is your friend on a cold, winter morning, as are well maintained carburettors. Spend the money to aviod much frustration!

    Regards, Andrew.
    • Like Like x 1
  2. Top stuff Andrew!!
  3. [​IMG]


    [​IMG] Typhoon

    Young noobs today would probably never have been in a vehicle with a choke. Well timed topic.

    Speaking of chokes, I remember reading a readers digest story once about a lady who dropped her car off to a mechanic, complaining about poor fuel economy. The mechanic did a service and said he couldn't find anything wrong and that the car was in good condition and operating well. The lady paid and as he watched the lady prepare to drive off, he noticed her do something odd. He went up to the car and knocked on the window. "Excuse me ma'am, but I couldn't help notice your hand bag, do you often leave it there?", "Oh yes, it's very convenient, I pull this out and hang the bag on it... why is there something wrong?", "Ah, yes ma'am, you're hanging your bag on the choke lever... that would explain your fuel consumption problem." :grin:
  4. Well Done Andrew!

    :LOL: :LOL: @ Wallace
  5. Top post Andrew. I've taken the liberty of forwarding it (with appropriate reference) to to a UK forum where this is very topical.
  6. hah Rob, I remember that story at the time. The car was a Mini Minor, and the choke button protruded horizontally from the lower dash; a perfect spot to hang a hand-bag!
  7. My bike stalls if I start it with the choke :?
  8. Thanks for this, good stuff.

    One question - I note you say to engage the throttle fully when first starting. After trial and error, my current practice is to switch the throttle on halfway, believing it best to give the engine only just as much as it needs to fire. I'm obviously mistaken - not difficult given that I'm a mechanical moron. Would you plese explain then, why a full throttle is the way to go as opposed to partial? (Dumbed down answers appreciated)
  9. Maybe you misread the post, or perhaps teh part on starting a flodded engine:
    Regards, Andrew.
  10. Probably needs a carb tune up/ rebuild.

    Regards, Andrew.
  11. No my mistake, sorry for the confusion. Where I said throttle, I meant to say choke. D'oh, told you I was mechanically challenged. Does the question make sense now?
  12. Yeah. Generally full choke till the engine catches is the way to go, then feather it till it runs smoothly. As I hinted, it's a pretty good general system, and fine tuning to your particular bike will make cold starting even better.
    But the process I posted will start pretty much any internal combustion engine with a choke, it's a good place to start :roll:

    Regards, Andrew.
  13. I always use a similar method to Donna, choke on about 2/3rds...no throttle, just wait till the engine starts to rev up while I gear up, then drop the choke to about 1/3rd, while I reverse out. Then choke off. (Except for the other day when I got 5 k's down the road, pulled up at an intersection and realised I still had it on :oops: oops)

    So probaby more the scenario below, but from the word go without the first stage you mentioned using the throttle --

    So, what is the benefit of trickling on the throttle? :)

    Thanks for the post Andrew.
  14. Purely to get idle speed up a little bit, nothing more.

    Regards, Andrew.
  15. Well, whaddaya know.....
    seems I've been doing it wrong too, only using half choke.
    I went out to the garage and tried your method, using full choke, worked a treat. Thanks Typhoon, I'm a happy camper again :)
  16. There ya go, Andrew, in the age of fuel injection, you've posted a truly valuable thread, well done.

    Funny that at the servo at Wilberforce a couple of Saturdays ago, a young lady driver with a young male passenger couldn't get their little tin-box started after fuelling up. I knocked on the window and said, "You've flooded it; leave your foot off the accellerator and don't touch it till it fires." I think the boy thought I was trying to pick up his lady because he gave me a filthy look, but the young lady did as I suggested and the car started, and spat out a gout of unburnt fuel as she drove away, smiling sweetly and waving while his nibs hunkered down in the passenger seat and scowled even more :rofl:.

  17. My work here is done. :p
    I am glad it helped, I knew my many years of pain with manual chokes would come in handy some day!

    Regards, Andrew.
  18. Yup thats the one :grin: and I can tell you the fix as well.

    They left the manual choke just where it was as a 'hand bag' hanger, and fitted one of those new fan dangled automatic chokes. :cool:

    I happen to know the workshop and the mechanic that did the job.
    The workshop was the main BMC workshop in at Zetland Syd in the very early 70's. As for the mechanic involved.... lets just say my dad loved telling that story to anyone that would listen :LOL:
    But I wont mention the involvement he had in the P76 development :oops:

    Ps: The choke/handbag was a common problem back in the dark ages of the automobile. But the Mini Minor one... I know well :rofl:
  19. Now don't you be nasty about the P76, Bob. I had a '74 Executive V8 and it was a better car than either Ford or Holden was DREAMING of building in THEIR shops.
  20. As my dad was very fond of saying "it was ahead of its time! it took another 10years before other company started to drop the wipers below the bonnet line" Not to mention the boot size :)

    I got photo's of a very youngish VTRBob sitting in the driver seat of the very 1st Targa Florio still in the experimental stage :)

    As well as in a 2 door model that never made it into production.