whats the best plug fot top speed on 49cc 2stroke

Discussion in 'General Questions' started by WINDY CITY BOB, May 27, 2016.

  1. I started changing plugs and went with different recommendations from different people and I'm not getting speed I'm losing speed

  2. Rob H

    Rob H Member

    I'm pretty sure that a spark plug is not going to get u speed but I haven't dealt with a 49 I run a 66 I would look into tuning what type of carb u running
  3. KCvale

    KCvale Motorized Bicycle Vendor

    Do you have a slat head, or does you plug point straight up?
    If you have pulled your head, did you use a torque wrench to torque it back done at ~5# per bolt in an X pattern until they are at 15#?

    I put NGK 5944's in every slant head 2-stroke I build, they are slightly longer so they get bit more compression but they won't fit in a vertical head, and regardless, you can't just guess at head torque, get it right first and you possible causes of poor performance drop to easy to checks and fixes.
  4. I have a slant head, I have it torqued to 12# in a X pattern as instructed by the manual.
    I was told by someone here that NGKB5HS. 6HS,& 7HS Depending on the temperature of my climate. I live in Chicago and its FRIGGIN HOT! HIGH 80's up to 100 plus.
    The only one that rocked was the 7 but I was unaware of how hot the plug is? I'm totally ignorant when it comes to that area I have quite a bit to lrarn about these engines, & engines all together I know enough to get myself in trouble LOL.
  5. butre

    butre Well-Known Member

    try an 8. bigger the number colder the plug. stock is equivalent to a 6.

    make sure the gap is right too
  6. HeadSmess

    HeadSmess Well-Known Member

    if it comes out black, its too cold.

    if its melting electrodes, too hot.

    if its tan, great...
  7. Thanks last night I found a plug that matches up With ThE NGK5,6,7,8 my bike took off like a rocket again! But I blew another muffler off I new it was coming it was leaking at the top weld and the brace holding it was shitty. Lets say it wasn't even there. But that (E3.32) worked great! Now I gotta run it a little while and check it out. Thanks for tjr easy color chart. Very much appreciated. Bob :*)

    P.S. IM GONNA make my own muffler, I already got it designed in my head I'm going with copper and gonna heat wrap(MAYBE)?But that's another thread.
    Last edited: May 28, 2016
  8. KCvale

    KCvale Motorized Bicycle Vendor

    I do mine to 20# in a X, 5# at a time for slant head 66cc's.
    If you have any head compression leak issues, try 20# , works great for me.

    The higher the number, the more heat the plug dissipates out, lower numbers keep the heat in.

    I use the NGK 5944 (BPR7HIX) Iridium plugs in my builds here Phoenix, it should be perfect for those wimpy Chicago temps ;-}

    80's is considered a nice cool Spring morning like right now, all the doors and windows are fully open, nice light breeze, and I'm in my boxers and a T-shift doing my morning surfing ;-}

    100F isn't considered hot here, we will hit that this weekend despite an unusually cool spring and I'll have to close up the house sooner.

    110F is simple hot, that's when the low doesn't get below 80F so the AC is always on.

    115+F is 'break out the oven mitts' real hot and 7's work fine though I have a couple 8's around in case we get to 120F, record is 122F and I was out in it, I might of even wore sun screen that day ;-}
  9. Wolfshoes

    Wolfshoes Member

    My two motorbikes are fitted with forum recommended NGK BPR7HIX Iridium plugs for my northern Illinois farm road riding. This plug works well in 70 or 80 degree F temperatures. The engines seem to run sluggish at 50 or 60 degree temps and strain to attain full rpms, but I am not ready to blame it on the plugs. If I had to buy the plugs over again, I would buy the resistor version, believed to be the BPR8HIX version to reduce electromagnetic interference blamed on problems with electronic rear view mirror/recorders and my Bicycle Computer brand electronic speedometers which had a problem remembering tire diameter. I am successfully using the $15 Bicycle Computer speedometer on one bike by using double magnets on the spoke and mounting the CDI (ignition module) on the seat post instead of the down tube.

    In reading posts on the other motor bike forum site, there was entries (years ago) made by a apparent house mechanic of China Girl motor supplier. The opinion of this mechanic appeared to be that the stock engine worked well without aftermarket parts, but he would like to know what caused the engine at high rpms to skip firing every other stroke and behave like a four stroke engine. Long after the fact this mechanic has probably already determined that the stock spark plug as currently provided cannot keep up with even a stock rpm speed of a stock compression two cycle engine. Iridium plugs are effective at keeping the engine firing even at the high rpms of a stock compression two cycle engine. Probably also high performance engines, but not from my experience. To prevent this four cycling problem in the stock HT engine, I had to make one other change. Any air leak between the throttle cable sheath entering the top of the carburetor needed to be eliminated. A piece of black tape if a rubber boot is not provided is all it takes. By adding the little piece of tape I picked up 2 to 4 mph and had stronger perfomance even near idle.

    It is my opinion that if the China Girl engine was EPA tested with the Iridium plug, intake leak eliminated and using Opti-2 gas 100:1 additive instead of 32:1 two cycle oil, the EPA would have a higher regard for its use and perhaps their BS bike weight limitations would be lifted.

    I have not heard from any motorized bike rider from Cook Co. or in Chicago itself. If you are riding these areas, it would be interesting to hear how it's going.
  10. Steve Best

    Steve Best Active Member

    Wow, guys. When it gets over 86F here, we consider it an exceptionally hot day.
    So we head the 10 miles to the ocean, where it will be in a tolerable 70s again...

    Platinum and iridium plugs have a very hard and fine center conductor that is not prone to wear.
    Usually a bit more money goes into their construction also, so sometimes the insulator is more foul proof.
    Their main advantage is longevity, although some claim fire advantages also.

    Plug heat range (the number on NGK plugs) has to do with the heat transfer path out of the plug and what temperature the tip electrodes are kept. The ideal temp is about 500C. This is on the verge of glowplug like pre-ignition so that the spark has the "tipping point" energy to fire the mixture off. Too low a temp and it might misfire (often called "4-stroking"). Read it all here:

    So how do you read if you have the right heat range?
    Look at the insulator and the ground electrode. Here is an excellent graphic:

    If you have the slant head on these engines, you have the option of facing the plug forward or back. This makes a difference, mostly in the heat range of the plug. If the plug faces forward, you will likely need an NGK 7 or 8 plug. If you face it to the rear, you will likely need a 6 or 7 plug. I found a slight power advantage with the plug to the rear but others have contradicted this, so don't worry if your frame limits you. Just check that the heat range is right.

    The "R" in the NGK plug name is for "resistor".
    I know we think "I don't want to resist any of the spark!" but it doesn't work that way.
    The resistor tend to hold off the spark until it reaches a threshold energy. It sparks hotter, more Volts.
    It also acts to dampen the after EMF oscillation which affect radio frequency (RF). This protects electronics.
    In my crude testing I cannot feel a performance difference between resistor or non-resistor plugs.
    Resistor plugs have a theoretical advantage and don't cost any more so I use them.

    The "P" in an NGK plug name is for PROJECTING electrode.
    It can make a big difference. It typically helps low speed operation by projecting out into the center of the mixture but certain high performance, high turbulence engines, or engine using high oil ratios (16:1, 24:1) can snuff out a "P" plug at high RPM. A standard or even recessed plug may be needed. Only testing will tell and this will affect top speed.

    Regarding outside temperature and its effect on power, it is mainly jetting mixture related.
    Colder temps lead to more dense air and tends to lean the mixture. COLD NEEDS RICH JETS
    This is why engines often work better at cooler temps. It is also why 2 strokes often burn up at cold temps from leaning out.
    Air pressure and humidity are factors as well, as well as cylinder cooling in hot and cold temps.
    Sounds like a nightmare, doesn't it? You cannot figure it out, you have to learn to read the plugs.
    Refer to the chart above (you may have to cut an almost new plug apart to see the smoke ring).
    Just remember that temperature is the biggest variant on your jetting. Jet for the temps you run.

    As for the carb leaking at the top? Don't worry too much about it.
    It should be sealed to keep out dirt but unless the throttle barrel is very worn it does not let air in or affect mixture.
    Same with the float bowl. These 2 areas are designed to be at ambient pressure, not vacuum.

    So what plug is best for high speed? You have to test it yourself.
    Find a consistent location you can test at and try to allow for conditions (temp, humidity, pressure).
    Ideally your location would be a slight hill to test power under load and another steep hill also.
    You need a consistent measuring tool, speedometer, tachometer, or time between points.
    You cannot master what you cannot measure.

  11. KCvale

    KCvale Motorized Bicycle Vendor

    Here is our 10 day forecast for memorial day today.


    This is just the begging of summer so it's still nice in the morning (71f today), but this to0 shall soon pass.
    For me real heat is when I am up before the sun and go outside to get my paper and it's still 90F.

    Small price to pay for year round riding and never having to shovel snow though ;-}

    Yes, the NGK's are a bit longer so you get a bit more compression.
    This is why you can't use them in a straight head.
  12. gus.

    gus. New Member

    One thing I try was the gap size 30 the bike was slow 25 it's much better (ngk b7hs) hear in PR is over 80 all year long..
  13. Greg58

    Greg58 Member

    The best plug depends on several things, as stated above on NGK plugs the lower the number the higher the heat range. I run a straight plug head on my 48cc and have run a bp5hs or bp6hs with no problems, the b6hs is one of the best overall.
  14. HeadSmess

    HeadSmess Well-Known Member

    snow? i have shovelled up piles of chicken feet, but sometimes...only very seldom, do i yearn to shovel snow... closest i ever seen was more like micro hail stones, about 1mm balls :p

    only just got our first frost for the year here... *does maths* ((71-32)/9)x5= erm...

    21.6C ... yerp. winter just begun and cus of the rain, thats about what it was today... lil chillier now its dark though. came rather late this year, hit 34 the other day!

    recalculates... 34/5x9+32= round about 86 for you ;) not bad for late autumn.

    *demands all temps be in K from now on*o_O

    so, back on track... even on days when the mercury (or non-toxic replacement) hits um... 44C(pffft, kelvin...), i never bothered much with different plugs. usually just use whatever is handy, be it the b5-6hs or similar, cj8, rcj7y bmr6hs, etc etc...

    the hotter/lower numbers are preferable on the two smokers due to the oil in the fuel.
  15. Steve Best

    Steve Best Active Member

    Ah, snow...
    I'm gonna dig me a home...


    Trying to test the bike in -10C temps. Hate it.
  16. Dude, you crack me up! Even Grampaw Stanley hates the snow..........
  18. KCvale

    KCvale Motorized Bicycle Vendor

    I might have to try this NGK '8' heat rated plug in this 2-stroke I'm testing...
    Did I say me? I meant my 'Joe'.

    It's a pleasant day here in Phoenix today, only 103F (38.9C}
    We don't even bat an eye at that as it gets down to 75F overnight.

    That time has past until Monsoon.
    This is from this mornings news.


    Fathers day will be ~118F (47.8C), and Monday morning I will be greeted with a 89F (31.7C) morning low and ~119-120F (~48C) high.

    The hottest day ever was 122F(50C), I have no desire to see a higher record set!

    I call this time of year the 'Oven Mitts' time.
    If you need to touch anything outside, best be wearing a pair ;-}
    Last edited: Jun 16, 2016
    WINDY CITY BOB likes this.
  19. Frankfort MB's

    Frankfort MB's Well-Known Member

    Here in KY the temperature never rises above 100F but the humidity hits 60% and it sucks because when you get wet, it never comes off lol
  20. KCvale

    KCvale Motorized Bicycle Vendor

    Hehehe, ya, right now it's a 'Dry Heat', like a blow dryer, maybe 20% humility, but that changes come Monsoon.