when jetting is lean, why does the engine surge, w

Discussion in 'Bike Mods, Performance, Maintenance & Repair' started by georgieboy, Nov 26, 2001.



  1. georgieboy

    georgieboy Rookie DRN Member

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    I am just wondering why an engines starts surging when it is acting lean. Why is it not just going flat, dies or blows to the moon and then quits.Why the up and down motion of the engine?
    And then an other thing.
    I am at WOT. I realise that my engine is flat at top. I expect it to be too lean. I am closing the throttle at it starts to surge. The surge occurs at closed throttle. Why, my lean condition is at WOT, not at idle.
    And then the last question.
    I start my bike with choke on, after it starts i rev it two times and then close the choke. Warm it up for a few more minutes and then start riding slowly. After that it is warmed up, say after ten minutes from the start it is acting lean at the pilot circuit(1/8 throttle).
    I dial in the airscrew to fix the problem. So far no problem.
    But the thing is that i do not really understand why the engine starts acting lean after 10 minutes, why not right away when it is still cold and so needs more fuel to compensate the condensation of the fuel against the cilinder etc. I would say that after it is warming up it needs lesser fuel in stead of more as it occurs.
    I know i am putting salt on the issue but maybe you guys can straighting things out for me.


  2. David Trustrum

    David Trustrum Subscriber

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    Would help to know what the bike is. Maybe you have an airleak that gets worse when the bike is Hot. Maybe the float jet is adjusted so the bowl is starving after being on WFO for a while.

    When the mixture gets too rich the fuel can form droplets & fall out of suspension of the gas which can give lean symptoms due to incomplete combustion.

    We assume you are turning the airscrew the correct way to richen the mixture at idle ie in? (unless your bike has some weird carb).
    Are the jets clean? Replace them if it is a Dellorto.
  3. georgieboy

    georgieboy Rookie DRN Member

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    David, sorry for not being clear about it. It is my handicap that i think dutch and then try to write it down in english.
    There is no problem with my bike, at least not that i know of:)
    Temperatures have dropped here since winter is closing in. So my lean condition is due to lean jetting. I will fix that this week to ride coming weekend.
    But what me interrested is why does a engine surge when it is acting lean.
    I understand the "bog", it just starves there from fuel.
    I understand the "going flat"
    But i don't understand the surging. It just seems the engine is looking for fuel but there ain't no. I just don't understand why it is surging.
    That is the same with going WOT. Ok it is flat, so probably lean. But then when you close trottle is starts to surge.
    So it is not why is it surge but more what is a surge.
    Just a stupid question.
  4. David Trustrum

    David Trustrum Subscriber

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    Well your english is a damn sight better than my dutch. In fact your english is better than half the 15yr olds on this forum.

    If it goes lean it often stutters, again I believe due to incomplete combustion. There is supposed to be a theoretical 14:1 air-fuel mix which ends up needing to be higher in a real engine. If the mixture is not favourable to combustion the sporadic running may be the result.

    Well that’s my take, perhaps someone has a better explanation.

    The only stupid question is the one that remains unasked. Stupid answers however are a dime a dozen. . . The only stupid question is the one that remains unasked. Stupid answers however are a dime a dozen. . . & I think I’ve used my ten cents for the day
    Last edited: Nov 27, 2001
  5. georgieboy

    georgieboy Rookie DRN Member

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    I was thinking that maybe a surge is the result of the up and down motion of the carb. The carb is jetted too lean, but since the bike hops around sometimes it will receive more fuel as the fuellevel increases. At that moment the bikes act as being happy with the mixture but again the fuellevel drops to normal and the lean jetting is kicking in, making the engine revving again. And there we have the surge.
    On the other hand, if it wasn't for dirt riding, but for example outboard motor, the carb is fairly still of motion, so it won't surge but just goes lean and dies, or screams to hell. How sounds this?
  6. dirt bike dave

    dirt bike dave Sponsoring Member

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    It seems to me that an rpm surge while lean is caused by the excess heat resulting in more complete burning of the fuel and oil. The lean rpm surge is common in other two strokes, such as chain saws, and I don't think they even have float bowls.

    The example of how the warm bike behaves differently than the cold bike also points toward engine temperature as a factor.

    My theory is that when properly jetted and at normal operating temp, a portion of the fuel/oil remains unburned by the ignition of the spark plug. Some of the unburned fuel then evaporates, which helps cool the combustion chamber before the next intake charge enters. The unburned oil improves lubrication and ring seal.

    I've got no proof of the above and would love to hear other theories, but it makes sense to me.
    Last edited: Nov 27, 2001
  7. David Trustrum

    David Trustrum Subscriber

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    It sounds like BA-b-b-b-baa-be-baa-bb-b

    Generally the float bowl should have a constant enough level that vibration etc should not cause a problem (like it used to in the bad old days of solid mounted carbs).
  8. georgieboy

    georgieboy Rookie DRN Member

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    Yes, i must be wrong because if you think of it my way, it is fairly impossible to get a carb working nicely anyway with all the hopping around it does.
    Doing wheelies, climbing hills, jumping, cornering, (falling down) etc. It is a miracle that a carb works like it does. Nice piece of invention.
    Dirt bike dave, i did not know about the chain saw. So it is operating without a "steady" fuel level?
    To close things down, is a idle surge caused by too lean a mainjet, just a preignition because of too warm an engine? After a few minutes the idle surge will be gone, as the engine cools.
  9. Chief

    Chief Subscriber Damn Yankees

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    Georgieboy,

    As far as which circuit is lean, all I can tell you is I recently experienced the same problem. After running wide open, then going to an idle, the bike would surge up and down for about 15 seconds.

    This occurred after I went leaner on the pilot. from 52 to 48. After I bumped the pilot richer to a 50, the problem of surging was eliminated.

    My suggestion is to try richening the pilot until the surging stops.

    As far as why an engine will surge when lean, I've been wondering about that myself. What you guys are saying about the more comlete burn makes sense to me.

    Also could it be that having too lean and therefore ,a less dense vapor at the carb reduces the vacuum at that point and allows the engine to rev higher than it normally would?

    Joe Chief
  10. dirt bike dave

    dirt bike dave Sponsoring Member

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    Hey, there, georgieboy,
    Here is a link regarding chainsaw carb tuning.
    http://www.madsens1.com/sawtune.htm

    The chain saw page might have some insight about lean conditions and revving, as well as how different circuits effect different parts of the range. The page also notes that overly rich jetting will prohibit the bike from reaching max rpm.

    Regarding the lack of float bowl, the chain saw's fuel tank itself appears to serve as the 'bowl', and jetting should be set with the tank full. According to the page, setting jetting with the fuel tank nearly empty will result in the saw running rich when the tank is full.

    I agree with Joe that a lean idle surge would be influenced by the pilot jet and airscrew settings, and not the main.

    Turning in the airscrew will richen the low speed circuit, and some bikes are very sensitive to this. For example, my bike starts easier and runs better when cold with the air screw about 5/8 turn out. When the bike warms up, turning the airscrew out 1/2 turn gives better performance and throttle response.
  11. smb_racing

    smb_racing Master of None

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    Perhaps the problem of surging after it's warm is a product of your lean pilot circuit. The choke richens the pilot circuit to allow the engine to start better when cold. This richening could keep your bike running smoothly until the engine warms up completely. To see if your pilot circuit is lean, when the bike starts surging pull the choke and see if it settles down. If it does then go one richer on your pilot jet until your problem is solved. Maybe this information will help you out somewhat.
  12. georgieboy

    georgieboy Rookie DRN Member

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    SMB-racing, i always forget to use the choke as a guide whenever i am riding. Next time, coming weekend, i will keep it in mind to use when nessesary. In the meantime i go bigger on the pilot.
    Joe, the less dense vapor makes the surge. That would explain the situtation with the chainsaw. Since the chainsaw doesn't have a real floatlevel the surge is caused by the venturi-area.
    Dave, thnks for the site, i ve read it.
    Smb it think you also got it on the surging after 10 min when i ve started the bike. Could be caused by first loading it up due to the choke, and after it gets warmed up it starts out becoming lean as all the fuel it burned.
    A last thing-banging in the pipe. It occurs whenever i was full on the pipe. Is this caused by the mainjet?
  13. smb_racing

    smb_racing Master of None

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    banging on the pipe? I'm not familiar with that term but it sounds like maybe an engine knock? What octane of petrol do you run? If your engine knocks then you might need a higher grade fuel, but as for surging or sputtering at WOT then that is controlled by the mainjet and needle.
  14. G. Gearloose

    G. Gearloose Pigment of ur imagination

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    In regards to the original question..
    I have experienced peculiar surging when I had an elusive coolant leak. A little air trapped in the head caused the temp to locally run away, and the engine surged at idle-to-1/8 throttle. The bike was fine when first started, of course, like you said. I suspect carbon build-up could cause similar effects.
    I hadn't seen it mentioned yet, so it may be a good thing to verify..
  15. georgieboy

    georgieboy Rookie DRN Member

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    thnks you all guys for yr input.
    i will keep everything in mind whenever i facing some of above problems again, meanwhile thnks a lot. happy riding.



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