Help a Veteran with New engine crackiling and 4-stroking

Discussion in 'Motorized Bicycle General Discussion' started by salorr33cc, Jun 11, 2016.

  1. salorr33cc

    salorr33cc Member

    Apr 30, 2016
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    I purchased a Silver Flying Horse Kit from Gas Bikes .. I believe it is a GT5 38mm stroke motor. I know there are a lot of different kits out there,, like PK80, 40mm stroke, 2 different pistons and 5-6 connecting rods out there,,, I seem to have the 38mm smaller piston and no marking copper color rod,, I would like to say that my cylinder was in good quality,, barely any casting flaws.

    I have the slant head with spark plug towards rear of engine.
    stock NT carb with needle clip in 2nd down from top out of 5 positions, the first carb they sent had all chinese markings on it and a 4 position needle, they sent me a new one cause plastic float leaked, new one has slight differences like 5 postion needle and different looking main jet.. I know that these carbs have a way to big.079 main jets and that can cause 4-stroking
    I have a banana type expansion pipe,
    stock cdi and magneto in 1 oclock postion and ohms out at 320
    spark plug boot real crappy and has like this loose spring clip inside it,,, dont like it but get spark
    tried all types of different spark plugs ,, no difference
    running 32 to 1 synthetic mineral 2 stroke Bel Ray oil..
    tried adjusting needle clip posistions,,, a little better a low throttle and rpms
    i port matched the exhaust pipe flange to be more oval then small round
    trimmed base gasket where there was overlapping but have not case matched to jug or done anything with intake except add o-ring to prevent leaking
    bike has real good compression and lots of power,,, i never have to pedal,,, and starts right up even with out choke,,, never used choke yet
    i live in Stroudsburg PA, and probably a little high here in elevation
    **** Like I said,, when starting off under load and throttle it purrs good and smooth, as soon as it starts to rev up like when the pipe should be kicking in some powerband it just starts to break up and crackle and loose power and at 15-20mph half throttle no load cruising it does the same crackiling and popping.

    and also what should i do with my dremel and light porting,, like i can see a good bit of piston in exhaust port a BDC which also means my transfer ports arent getting un covered all the way either,, and the intake is all overlapping and can see piston skirt at TDC.. and bottom case to cylinder jug transfers is also all overlapping not matched metal... please help as i read and hear all types of things to do and everyone says he or he is wrong ... confusing... just want it to run smooth from low to high and feel a powerband kick at like 5000 rpms... I think if i port matched everything and smooth the flow out thru engine should run better,,, dont know what to do about piston skirt on intake side and raise exhaust port a little,,, or use thick base gasket and sand down head ???? also should i just adapt a Walboro 15mm carb and adjust hi and low speed settings like on my 50cc scooter... please help

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  2. Nashville Kat

    Nashville Kat Active Member

    Apr 20, 2009
    Likes Received:
    Looks to me like a standard 66cc China Girl.

    If you have an older stock carb- those were built originally for the 50cc mortors-

    when I did my first build in 09- that was the setup- 66 motor original stock NT carb-

    EVERYBODY on here was complaining of bogging down at full throttle- CNS and other cabs were just coming out-

    My own solution was found when I put a straight billet intake on- that cleared the four stroking

    The NT SPEED carb came out then- Marked with the word "Speed" and generally with the carb shut off valve also- that also helps for the larger 66 motors- an upgrade built specifically for the larger version.
    That's all I've ever run. The Speed carb runs well on a 50cc too.

    also BEWARE of brushing CHOKE lever on- I've done this a couple of times - not terribly close together, and then wondered why the bike was running sour- a choke lever on perpetually will big it down too.

    A smaller back sprocket would give you better speed, but more importantly not so much rev and vibration at a cruising speed- some 1.75 street tires or even 1.5 would give you a better roll, if you're not on rough roads, but they do wear just a bit faster.

    A 415 Industrial/Trike chain is the same size but with smaller profile- half the weight, and much les rolling resistance.

    Can't tell you much about porting- I think the easiest and most powerful purchased item would be a cylinder jug with a wide 40mm intake and then a billet intake to match, and Delloto style or CNS carb. But haven't tried one- my cruiser with 700c wheels and 34 sprock will cruise at 30 and can get to 35 not quite fully opened. And that's fast enough here.
    #2 Nashville Kat, Jun 11, 2016
    Last edited: Jun 11, 2016
  3. Agreen

    Agreen Member

    Feb 10, 2013
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    This is easily fixed. Your main jet is too large. Solder and drill it smaller, or buy a $15 kit of jets. I prefer soldering and drilling. One drill kit will cover every carb, every jet, and can do the same size multiple times. A jet kit can only be used for one type of carb, and if you use one size, it can't be used on another engine.

    Anyway, thin out the main jet, but understand that you might need to readjust the needle once it's rejetted. The needle effectively modifies the size of the main jet, making it smaller for when you're at partial throttle. So with a smaller jet, you may need to go down on the clip (this raising the height of the needle) to compensate.

    Don't do any porting until you have read Gordon Jennings book about 2 stroke tuning. If you cut the skirt of the piston to where you can't see any of it at tdc, you will actually lose power. I did it, and immediately regretted it. The engine is port timed, so if you change the height of the port, you change the opening and closing times. Bad things can happen if you don't know exactly what to do.

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