Runs best with choke partially on?

Discussion in 'General' started by Andy Gouge, Jul 10, 2018.

  1. Andy Gouge

    Andy Gouge New Member

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    Had a local shop tune my 82 CT70 up a couple of years ago and it’s ran and started great up until this year. Guess where it has sat all winter. It fires right up with choke on and then it runs good for a couple of minutes after choke is off but then starts to sputter a bit under full acceleration and won’t idle. I can put the choke back on about a small bit (1/4) and it runs and idles fine. What do I need to do to get it to run and idle without a little choke?

    Thanks,

    Andy
     
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  3. racerx

    racerx Administrator
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    Before getting into the carb itself, check the fuel flow to the carb, as well as the air filter element. This could also be something as simple as a restricted tank vent, which is in the cap.

    The carb probably needs a good cleaning, possibly a rebuild. 21st century pump gas has a very short lifespan, after which it turns into TBLA.:censored:
     
  4. cjpayne

    cjpayne Well-Known Member

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    My first thought was to up the size of the main jet and/or move the throttle needle up. Is the stock aircleaner assembly all still there? Are you running ethanol?
     
  5. racerx

    racerx Administrator
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    If it was calibrated (jetted & tweaked) lean, then it'd be weakest before coming up-to-temp. The colder the motor, the weaker it would run due to being functionally leaner. Fuel doesn't atomize or vaporize well until the motor is hot. But, it seems to be acting leaner as the runtime accumulates...when it should be getting functionally richer. That's what has me thinking "restricted fuel delivery". Ever forget to switch the petcock, or the tank vent, "on"...only to be reminded as the bowl runs dry? The symptoms would be similar to those described.
     
  6. Etown

    Etown New Member

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    Fuel flow is a good one but I am 98% sure your carb jets just need to be cleaned. The choke set at 1/2 to1/4 means that you have to restrict the airflow to get the bike to run right because you are not getting enough fuel to the engine. Pump gas these days is horrible for little engines. Only takes a few rides for your jets to reclog. The solution for that is to clean your gas tank, put inline filters on your fuel lines, clean your carb’s jets, and refill it with non ethanol fuel. Otherwise you will clean your jets and in a couple rides you will have the same problem. If you put a less restrictive air cleaner on your bike it is the same principle, you have way to much air coming in and not enough fuel. I would start with a 70 main jet and go from there. (If you changed your filter) Good luck
     
  7. wanrep

    wanrep New Member

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    If it ran fine up until it sat all winter, it's a classic case of ethanol gummed something up in the carb.
    It doesn't take much to partially clog up one of those tiny jets.
    Like these guys have said, check the easy stuff first.
    Fuel flow and air filter. Where I am (in the sticks), I see a lot of problems with mice getting in the airbox and chewing up the filter over the winter.
    If fuel and air is ok, clean the carb jets and it should be back to normal.
     
  8. racerx

    racerx Administrator
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    I've run nothing but pump gas since day one. Only problems I've encountered have been related to the polymer parts of the system (petcock packing & fuel lines), dirt in the fuel (there's lots) and, more & more commonly, the occasional tank of crap fuel (power goes down, oil temp & fuel consumption go up).

    I went to Tygon fuel hose. It gets brittle over time but I've gotten up to 7 years from a set; It's when they have to be R&Rd after a long time that replacement is needed. The petcock packing gets greased, prior to offseason hibernation (storage); that limits the drying & shrinkage, it swells back to size when exposed to gasoline, in the springtime. I do run an inline fuel filter, along with a screen in the petcock. The bad fuel issue...well...that goes away with the next fill-up. I've not so much as pulled the float bowl on the carb since 2007. With 23,800 miles on the engine & carb, no problems. The key, IMO, is draining the entire fuel system dry...tank, lines, carb & petcock, prior to storage along with never leaving the same fuel in the system for more than 2-3 weeks. The sole concession I've made is sealing the tank. Where there's ethanol, there's water, and steel gets rusty with water exposure.

    Ethanol-free gasoline is a better way to go. However, even E0 (ethanol-free) gasoline has been reformulated and is not a whole lot more stable...unless you source aviation fuel, or specifically-formulated storage fuel (VP hydrocarbons has it). That's gasoline as is was 40 years ago; it's pricey and not widely available. For those of us who rely pump fuel, those are impractical solutions. E10 is survivable, it's actually an okay fuel. It does run cleaner and ethanol has a cooling effect. It does, however, affect jetting...making the mixture leaner. And, with these little bike, you can expect a 5-10% drop in MPG...which, at the rate these machines sip fuel, is not a big deal.
     
  9. wanrep

    wanrep New Member

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    Sounds like you've figured out how to live comfortably with E-10 as have I.
    The other 98% of bike owners have not or are too lazy to take the steps you do.
    That's why half of my business is cleaning carbs.

    I remember the good old days when I was a kid and our minitrail would sit for 6 months.
    The gas would still smell like gas and it'd fire right up.
    We did buy the good stuff....Ethyl but it was pricey...50 cents a gallon. lol
     
  10. cjpayne

    cjpayne Well-Known Member

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    That ethanol can also cause high speed leanout. It did my k1.
     
  11. racerx

    racerx Administrator
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    Oh yeah, back when Gulf "No Nox" and the original Sunoco 260 were available...containing ~5gm TEL per gallon. In the Detroit area, those juices were 36 cents per gallon. It's a wonder any of us survived, if you believe the hype...on either side. "nostalgia ain't what it used to was";)
     
  12. racerx

    racerx Administrator
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    The carb can adjusted, or/and rejetted, to compensate for this and stoichiometry (ideal balance, chemically) restored. In 2006, I noticed the sudden change when "reformulated" pump gas hit, right at the beginning of summer. Went from #160 main, to #170, with the jet needle 2 grooves higher, no problems since then...other than the occasional tankful of lame fuel. The bike usually gets ~96mpg +/-4. When that drops into the high 70 - low 80s and oil temp is 5-10C higher than normal, the reason is clear... with the next tankful.
     

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