79cc predator is backfiring out carb and exhaust won't start

Discussion in '4 Stroke Bicycle Engines & Kits' started by Tsears0627, Sep 10, 2016.

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  1. Tsears0627

    Tsears0627 New Member

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    Okay so I bought this 79cc predator to replace my Chinese 4stroke that the rod snapped in, and I am now having issues with this motor not starting.
    First thing I did even before firing the motor was remove the governor. I followed afg's instructions to the letter. Put it all back together making sure cam was lined up correctly at TDC. Checked the rockers to make sure that the push rods were still in their home.

    The issue now t won't flipping start, I check the plug it's got plenty of spark, and it's getting fuel. I pulled the shroud and pull start off and am using a drill to crank the engine as pull start was killing my arm. The motor is backfiring out of the carb and exhaust is spitting flame :-|| the carb is spitting gas out of it, and I'm at a complete loss please help as this is my only transportation to and from work! zpt:-||laff
     
  2. Tony01

    Tony01 Active Member

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    Check your valve lash as well! Sounds like the engine may be at full throttle or something with the choke full on.
     
  3. Tsears0627

    Tsears0627 New Member

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    Is there a way to get set valve lash with out feeler gages? I checked them by hand and they move slightly but are under pressure, and as for the throttle going wide open with the choke, that's a yes, but with throttle closed and choke on its still has air pulsing out the carb, is that normal? I'm thinking of just getting a new carb, I've been messing with this for days, and I'm getting very tired of it.
     
  4. Tsears0627

    Tsears0627 New Member

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    Update : with throttle closed and choke on, it's spitting flames our of the exhaust, which is just a straight pipe, from eBay, could it be that there is not any back pressure in the exhaust?
     
  5. FFV8

    FFV8 New Member

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    Is the flywheel key installed correctly? Not sheared?

    You will have to pull the flywheel to check it.
     
  6. Tsears0627

    Tsears0627 New Member

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    Yes the flywheel is installed correctly
     
  7. Venice Motor Bikes

    Venice Motor Bikes Custom Builder / Dealer/Los Angeles

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    Double check that the cam gear is lined up with the crank gear properly.

    Also... make sure the valve rockers aren't too tight (you need a feeler gauge)

    Make sure the two dots on the gears are lined up like this...

    image-3232836367.jpg
     
    #7 Venice Motor Bikes, Sep 10, 2016
    Last edited: Sep 10, 2016
  8. Tony01

    Tony01 Active Member

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    I did not think it was the flywheel as you don't need to pull it to remove the governor. Try starting with the choke off and the theottle closed. Make sure the gas you're using isn't more than a week or two old. Make sure there's oil. So there are flames, and it's firing? Still sounds like a flywheel issue but you say it is good. So I dunno what to tell ya. I have no experience with the stock carb. I also did not run the engine before putting it in the bike. I took everything off it, removed gov, fabbed a new intake and exhaust, took the fins off the flywheel and converted to rope start, and oh yes!! I had the same issues. I had forgotten to tighten the jets in my 19mm carb and it was doing what yours is doing. Now I dropped the bike on the intake side which has caused an air leak... LOL
     
  9. Kletus

    Kletus Member

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    I'll give you 10:1 odds that the cam gear is not indexed correctly. Or the intake valve is stuck open because of a reassembly error.
     
    #9 Kletus, Sep 10, 2016
    Last edited: Sep 10, 2016
  10. Tsears0627

    Tsears0627 New Member

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    I will double check all of this and update you all if and or when I figure out what the problem is, I appreciate all the help and if any one else has any ideas or has had the same thing happened please keep posting the help is much appreciated
     
  11. Slogger

    Slogger Member

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    I had a stuck float needle on a 1600cc Roadstar once. There was dirt holding the needle open.
    It shot fire out of the intake about 2 feet, backfired in the pipes, and made quite a spectacle of us.
    Worth a look.
    Good luck, hope you get it cleared up.
     
  12. Greg58

    Greg58 Well-Known Member

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    Simple question, did the engine run before you did anything? Its always best to start a HF engine before doing anything, getting it replaced may be a hassle if they see you have taken it apart.
     
  13. WECSOG

    WECSOG Member

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    Sure sounds like it's 180* out of time.
     
  14. bluck

    bluck Member

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    Did you ever fix the issue I'm having the exact same problem with mines
     
  15. bluck

    bluck Member

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    If anyone wants to know I had the exact same issue and I was able to turn on my engine by adjusting the valves using a feeler gauge at .004.
     
  16. motormike777

    motormike777 New Member

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    I may have my timing 180 degrees off. I'm having this same issue...anyone have any updates?
     
  17. bluck

    bluck Member

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    Check your timing, if it's correct then you need to adjust your valves. Go to autozone or harbor freight ask where they sell "feeler gauges". Buy them and go on YouTube and it will explain how to use the feeler gauges on the 79cc predator. I adjusted mines to .004 and after that a couple pulls later the engined turned on for the first time.
     
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  18. motormike777

    motormike777 New Member

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    Thanks bluck. I don't know how to post on here. I have no choice but to: I'll be homeless in a few days: It's still not starting. I set valve lash to .004, put a #85 jet in my nt carb. Took everything apart, checked the timing again which is completely fine, tried hooking up a spare kill switch still nothing. Although now I can hear some of the exhaust like a it want's to start since I swapped to my 2nd NT carb. And after switching from #78 jet to #85 in the 2nd nt carb I can hear less exhaust now when trying to start. Really trying to get this running tonight. I need it to.

    As for the stripped out drive shaft I used a longer bolt with wider spaced and deeper threads; if you looked down the length of the bolt it's maybe just a hair wider than the original because of the threads. Shoved about 9 strands of copper in the hole, tapped the bolt with a hammer to get it started and it worked very very well. When it started to feel like it was getting slightly looser I tapped with the hammer a few more times and tightened up again no problem.

    I doused it in red thread-locker.
    Just trying to get on the road, even though my hilliard clutch will be arriving in a few days... I'll burn this one out first.

    Anyone know the proper air gap for the magneto?? I can't find any info whatsoever!

    Not giving up this fight; lost too damn much to go out like this.
    Even if I have to sit in my neighbors shed and eat dry ramen and vienna sausages for a week, i'd appreciate any help you guys can give me.
     
  19. curtisfox

    curtisfox Well-Known Member

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    Not sure what the gap is on these new ones, but old small engines like Briggs, used to take a match book cover or 3 x 5 recipe card set the cap to it and should be good about 16 thow........Curt
     
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  20. motormike777

    motormike777 New Member

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    I appreciate your input I went with .30 air gap and she fired up- with NT carb at half choke with a #85 jet. She very shaky reddd Going with a 56T and hilliard 10T extreme duty clutch, and an intake manifold from AGK.
     

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