No Load, Higher RPM break up crackiling

Discussion in 'General Questions' started by Diamond Back, Jun 11, 2016.

  1. Diamond Back

    Diamond Back Active Member

    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


  2. Diamond Back

    Diamond Back Active Member

  3. HeadSmess

    HeadSmess Well-Known Member

    sounds too rich, especially if you arent needing the choke. i had one particular engine, would take 50 metres to even pop, a few hundred metres to warm up...then off it went like a scalded cat. engines tend to get "richer" when warm.

    the piston never completely clears the exhaust port, thats a 66cc design flaw... they always cleared on the 48's.

    dont play around with the port timings unless you know what you are doing or have replacement parts for the steep learning curve that follows.

    go wider on ports rather than higher, remembering to keep tops of ports curved for ring-life, and not getting too close to the ring-gaps.

    matching transfers/ports does help but not much, these engines all suffer from asthma... mainly on the intake. notching the piston to clear the port at TDC helps but theres a fine line between enough and too much. once again, the wider you can get it, the better...
  4. crassius

    crassius Well-Known Member

    I like the clone carb with chinese markings (never had to adjust one in the couple years I've been selling them) - first thing you should do is put the new float in that carb and see what happens.
  5. Diamond Back

    Diamond Back Active Member

    i did put new float in and adjusted to 21mm... stop leaking,, but no difference in 4-stroking... i think i need to put a .064 main jet in,,, plus i got play-dough today and am going to case match bottom end ,, you put the play-dough in tightly so no metal shavings get in crank.... some one said to take a small copper wire thru the main jet and ball it on both ends so it dont fall thru and that should lean it out... i know you can solder and re-drill but i dont have solder or drill that small.... also said on the stock NT slide on back side it is slanted,,, the said you can sand down slant a little to lean it out... but that wont help WOT... only main jet does that.... do you have a main jet you can spare...i can mail you a few bucks if you want to help a veteran out
  6. Diamond Back

    Diamond Back Active Member

    Thank you Buddy... i have owned and lived with 2 strokes my whole life,,,CR80, KX125, Fourtrax 250R atv's and trikes and 49cc i am familiar.. do you know the 15mm walboros used on the 49cc chinese scooters,,, they are great with the high and low,,, i would have to make a custom intake and line up and drill hole for pulse hole right thru and under intake on cylinder.... do you know of an intake that may work or get me close... and is 15mm to small
  7. crassius

    crassius Well-Known Member

    before making permanent changes, I'd lower float level some to see if that is the direction to go - at 21mm your main should just be peeking thru float - bend prongs till it is just sunk in float & see what happens
  8. Diamond Back

    Diamond Back Active Member

    how does changing float hight help in running performance... that float just opens and closes the needle under fuel inlet to allow gas in the carb bowl and shuts it off when there is enough so it doesnt overflow out the back,,, what does it do by having low or high... dont get it,,, explain please
  9. HeadSmess

    HeadSmess Well-Known Member

    the fuel sucks up through the jet via venturi action. the higher it has to lift the fuel, the harder it is... lower fuel level= leaner. you get a slight bit of adjustment using that trick. go to far and the jet starts clearing the fuel when you hit bumps... more prone to fuel frothing too...

    the (super fine) wire through the jet works, dont ball the ends up cus they can block the jet, just tack one end to the base of the jet with some solder.

    keep adding a wire at a time til you go to far then remove one.

    sets of microdrills are cheap, but you do need a pin chuck for them... worst case, hold jet in a drill, hold the bit with pliers. you also need a 3mm to remove excess solder from the jet where its bored out. the length of jet orifice also affects the mixture... a short, small hole can flow just as much as a long fat hole, due to friction.

    walbros are great, aim for an 18 though, lets you open the intake port up a tad.

    some have external pulse feeds(usually on chainsaws), or you can simply drill/tap and add a nipple to the cover(or even the manifold), with another nipple on the crankcase somewhere convenient...

    manifold for walbros should include some type of plasticy thing to prevent heat from getting to the carb, which causes vapor lock... why theyre always mounted on those nylon/epoxy manifolds.

    only bad feature with them is getting the cable attached nicely if theyre standard butterfly types that normally run wire linkages.

    also lets you stick the fuel tank anywhere :)
  10. Diamond Back

    Diamond Back Active Member

    Thank you man,,, your a cool person to help me like this,,, you seem very smart so I going to ask you a few more questions,,,sorry, but i trust your input...what do you think of Banana pipe with out silencer?? is it a top end or bottom end pipe,,, would putting a small 2-3 inch header help.. maybe cause my bike dont run right now in high rpms,, but the bike has alot of power,, i never peddle,,, but i dont get that powerband nitrous effect like on my KX125,,, why is that...

    Also should I widen a hair, not overdue it the exhaust port and intake port,,, also some say to raise exhaust port a hair and some say cut piston skirt on intake side just enough to be open at TDC... a guy just told me that he lost power doing this,,, what should i do..??? this is what I mean,,, one person says and does one thing,, and a nother guy says NO,, you will lose power,,, its so hard to get the truth and hard facts
  11. HeadSmess

    HeadSmess Well-Known Member

    a kx 125 has virtually all of the cylinder wall turned into holes for ports...different timings, different intakes, and a lot of pre-production design followed by lots of testing... these things are never going to perform the same way. EVER.

    on that note, a good tuned pipe makes all the difference. i never bought any pipe as i make them myself, but all the talk on here says the banana pipes are pretty lousy. apparently adding a few inches (up to 8 or more?) to the header does wonders, as they seem "designed" for about 12k or more rpm... i say "designed" cus they arent! no, i cant point you to a decent pipe but there have been links to a few that look like the real deal on here... compare the basic outline of whats on the kx to whats available and with some experience, you can see the similarities. you want something that resembles the ktm50 type pipe, but even longer for the lower rpm... pipe design is complicated but they all follow the same pattern, so to speak. angles affect the strength of the powerband, length affects the RPM it comes in at, and length then changes the angles... confusing! the banana pipe is just designed so its easy to make, no more.

    trimming the piston skirt does help, but if you remove too much, you need a new piston. start by just removing 1-2mm at a time, you really still need a few mm left exposed when pistons at TDC. i usually scribe a line while its at TDC and trim to within 2mm of it. remember to chamfer the trimmed edge slightly, a curved notch (use half round file) is better than flat. overdo it and you lose all the low rpm power.

    fiddling with the length of intake manifold also helps, try to do those mods with no exhaust pipe at all... or set it up with a over sized header into a large auto type silencer, so you dont get any effects from the exhaust pipe.

    the parts are cheap, so there is no harm in experimenting, but when doing port work, start with the least work possible until you know what works for you... make one change at a time so you know whats doing what, be prepared to push home occasionally... i gave up on porting too much as my custom built pipes were always based on standard port timings... too much work to redesign a pipe and make it each time!

    dental drills off ebay are way better for porting than a can get right into the transfers etc and remove all the casting flash.they are also cheap :) you do need a source of compressed air, about 30psi, to run them... a full kit on ebay is still cheaper than a dremel.

    ramping the top of the piston helps in making decisions on port changes before hacking into the cylinder itself, but the ramps in the piston affect combustion characteristics as well, so dont use a ramped piston instead of changing the cylinder if happy with the results.

    get a degree wheel if you want to do porting...

    here we go again...

    reading material. read it a few times...
    Diamond Back likes this.
  12. Diamond Back

    Diamond Back Active Member

    I am not going to do anything major,,, just clean up the casting slack and case and port match to get the best flow possible,, i will try to take 2 mm off piston skirt and make both ports a 1 mm wider on each side and just straighten up exhaust port,, its smaller on one side then the other,, not going to touch transfer ports,,, was also thinking about doubling base gasket cause that will raise ports and if no good just remove it,, no harm done,, i tried piston ramps on the transfer port sides on a 49cc scooter and lost a lot of low end.. just going to do a nice polished matched clean up... and am going to put a 3'' header on pipe... need to get carb straight first,, do you have any main jets you can spare,, i can put a few bucks in a envelope and mail it to you if you want,,, if you do can you just put one or two in envelope and mail to me ..please,,, not being rude or looking for free stuff,,, its just that i am a disabled veteran from 18 months in Iraq and have no income right now till VA gets there act together,, also thank you bud,, you are very nice to take your time to help me,,,
  13. HeadSmess

    HeadSmess Well-Known Member

    jets i also cant help on... always just soldered them up and drilled. once again...try wire, which you find in headphone cable etc. super fine stuff. tack solder one end to the base of the jet so it doesnt fall out(gotta scrape the enamel insulation off). plus the postage from AU is a killer...