Discussion in 'General Questions' started by Diamond Back, Jul 9, 2016.

  1. Diamond Back

    Diamond Back Active Member

    I have a GT5 66cc 38mm stroke engine with 160 miles on it, Run Bel Ray Synthetic oil 32:1 and a little bit of octane boost on Sunoco 93..

    The problem is my motor wont rev past 7,560 rpm at 32.7 mph, have tach and speedo,when I first got the engine I took it apart and cleaned up the cylinder of any casting slag and port matched exhaust to Banana Pipe and intake to intake pipe, I also removed casting lip on transfer ports and widened exhaust and intake ports a hair on both sides,, I did not raise or lower anything,, I believe I got a pretty good motor cause it was pretty clean to begin with not to much to clean up ,, I also case matched the lower end where the cylinder meets lower gasket and case, trimmed gasket then beveled the case edges to gasket and lower part of cylinder jug so its an even flow,,

    using only Banana pipe and stock slant head it 4-stroke like crazy,, under load or hill climb it would run better but still 4-stroking,, running really rich moved needle clip to top and did nothing, so I took a pointed center punch to the left side of main jet hole and tapped it to indent the soft brass fitting and close up hole a little and it worked,, engine ran better... any way to make a long story short I was able to rev out to 35.7 mph and still had more to go but let off due to breaking motor in still....

    I went and bought a RSE compression 6cc head and hour/tachometer,,, I also was given a micro drill bit set sizes #60-#80...which i dont get cause they show 80 as the smallest and 60 as the biggest..I installed head with copper gasket and hooked up hour/tach, and the main jet was a #71 and soldered and drilled using a smaller #67 drill bit...

    Right off the bat I noticed more compression and low end midrange power and it 4-stroked a lot less but still there,, but also felt different in the top end..i had some pinging and detonation at low rpms so i was worried and added a nother head gasket and used a smaller #62 main jet,,, bad Idea ....very hard to start and keep running and noticed big lose in power..

    so I took head gasket out and just use one aluminum one with copper gasket seal liquid,, and drained gas and added new 93 octane gas and lucas octane boost,,, that got me my power back and no more pinging/detonation, but this is where the trouble with throttle and high revs started,, if i gave it gas like 3/4 throttle at 4,500 rpm the bike took of and planted you in seat,,(NICE).. but if I gave it full throttle it bogged like a dead spot, if I backed off 20% off full throttle it ran better and reved up a little more. So I increased the main jet back to 67 and it helped with the full throttle but not 100%.. I can only twist throttle to 90% percent and then hit bogging dead spot from 90% to 100% and the bike 4-strokes again now a little more but not to terrible but did a plug chop and it is running rich,, moved clip to top slot on needle using 5 position needle which is longer,, have to use choke for about 15 seconds then take off and let warm up for 3 minutes then good to go but still 4-strokes with no load half throttle cruising 5,000 rpms but still have that dead spot at 90-100% throttle,,, so what should I do ????????????????????????????????

    I am hooking up a 15mm WALBRO in a few days which should be awesome,, also going to add a 3-4 inch header section on Banana pipe and make a side mount stinger which is longer and seal up end of pipe,, or should i just extend stinger at the end into a dual mini mufflers.. will get a dirt bike pipe soon like a KX 65,,, should i increase main jet some more or what,, and I also have a slotted magnetto key so I can advance or retard timming,,,have not messed with that,, still stock 12:30 position,,, should I retard it,,,

    so the bike has plenty of low mid grunt and torque , still runs rich black plug NGK B6,, cant really feel a kick in the butt on the pipe POWERBAND cause pipe design is to high needs a long header section to lower when powerband kicks in... BUT on a flat wide open road cant get it to rev past 7,560 at 32.7 mph,,,,,just wont budge any more,,,,

    what should i do to stop 4-stroking and rev up to 10,000 rpms,,, but I still want all my power and torque low-midrange so I can put a 36 sprocket on...... thank you for your help,,, will try anything,, CAM00217.jpg

  2. Frankfort MB's

    Frankfort MB's Well-Known Member

    If you have a regular eBay kit then 7500 RPMs isn't bad..... I'm sure it's vibrating pretty good at that speed.
    On the new 2015 grubee skyhawk they balanced the cranks a lot more than the older engines.
    You could possibly try lightning the piston by drilling small holes in the side.
    You also need to lean out the carb more of your still 4 stroking
    Just leave the one head gasket on it, your compression isn't high enough to cause problems (shouldn't be;))
    Plus, are you tires mountain bike tires? They will use a lot of the usable power because of friction. You will be able to get a faster speed with slicks aired all the way to their max pressure (less rolling resistance) I've seen a drop as much as 3-4mph with low tires
  3. Steve Best

    Steve Best Well-Known Member

    Frank is on it with rich mixture leading to 4 stroking, gotta solve that.
    New carb is not the answer, just more problems.
    I run MTB tires and see 38-40mph before maxing out on rpm. If you got the power, you will do it.
    I also tried one of those banana pipes, better torque but less speed.
    Also lost speed but gained torque with the RSE head.

    For rpm, go for wider ports, taller exhaust port or lift cylinder, small squishband on a shallow hemi head and set squish gap to 0.030".

  4. Diamond Back

    Diamond Back Active Member

    Thank you guys for your replys, and good to hear from you again Steve,,,,,, I know i wrote a lot and can be confusing but you guys missed totally what I am saying is wrong,,

    ONE it is not e-bay junk kit,, I said I got it from and the cylinder was in really awesome almost perfect shape,, a little ridge on transfer ports and I DID WIDEN EXHAUST and INTAKE PORTS... MY cylinder also has a thick 4mm base already compared to most and lifts it pretty good compared to others i see..guess i could try a little more..add a base gasket...WHAT SHOULD I SEE FROM THAT ??

    MY PROBLEM IS...... I have this big dead spot at full throttle ... it will rev (TWIST) throttle and if it is warmed up and under a slight load it will pull strong up to where I am at full twist 100% throttle (slide) OPEN,, and it will bog down almost, feels like brakes got put on, and it starts to drop rpms and power,,, but if I twist the throttle back to 85-90% it will start to rev back up again till 7560 and not go any more,, this is with a .67 main jet,,, it was even worse with a leaner .64 main jet, could not twist throttle past 70% or it was like holding kill switch in, so leaning carb out just makes it respond worse...

    This is why i am putting a WALBRO on it, so I can adjust high and low seperate from each other,,, I just need to get longer bolts to go thru composite isolator adapter block and drill and tap fitting for pulse in the crank case that will then connect by small tube to carburetor pulse hole to operate diaphragm fuel pump and I have a T fitting and gas line reducer fitting for the fuel inlet line and fuel return line. got this all done for $30 bucks,, not bad huh ?? and it should run and adjust like my gas 50cc scooter,,,, that thing never 4 strokes and has tan colored spark plug always,,,

    using the NT carb, the engine has the power to pull past 7,560 rpms no doubt,,, but it is acting like a blocked wall at that rpm,,,, want to know what is causing the WALL..!!! I think its lack of gas at WOT,, The NT carb really lacks a midrange tune ability,,

    I have read all of DRAGONFLY stuff on build expansion PIPE and JENNINGS book cover to cover 3 times.. THE Banana pipe is so so WRONG for these engines,, I never ran the stock pipe so I dont know the difference,, But from what I read the Banana wont even come on till 11-13,000 rpms.. For every 1 inch header pipe I add to it will make it come on 500 rpms earlier...So I am going to weld on at least a 10"inch header which will be very hard to clear front tire and not hang to low,, I could get flexible conduit pipe and route pipe a whole different way but not burn my legs and will have to add hangers to it and also the diffuser/stinger is way to short and small diameter, will have to add a max of ten"inch and 3/4 diameter... Also really thinking about sealing end of pipe up and running a side bleed,,, it drastically changes the sound to be way mellow,,, youtube it and you will see what i mean by the one video...

    STEVE you and me have very similar setups or did at one point,,, Can you send me pictures of your bike and as much info on all the stuff you did to it .PLEASE..PLEASE,,, I want to know all the little details and problems and corrections,, and modifications you did,,,PLEASE>>PLEASE
    What is your current setup and future things you will be doing,,, thank you Steve,,, write back asap buddy
  5. Frankfort MB's

    Frankfort MB's Well-Known Member

    Gasbike kits are the same engine as eBay kits lol
    I've used both before, and no difference. Gasbike says they have real skyhawk engines but they don't. They can't buy true Grubee products and haven't been for years (can't remember why not)

    You could be too lean on top end? Or too rich? I'm not the expert tuner by no means.

    Adding a base gasket will adjust the timing as well
  6. mrbg

    mrbg Member

    It sounds like a airleak,take it out for a ride and shut the petcock off while riding if it runs good right before it's out of gas you have a airleak get some carb cleaner and spray the motor @ the base gasket,intake manifold while it's running.if there's change in rpm there's your leak
  7. Frankfort MB's

    Frankfort MB's Well-Known Member

    I doubt it's that because he says it runs good until 7500 RPMs
  8. Diamond Back

    Diamond Back Active Member

    its not an air leak,,, i would have idle problems

    yeah,,, its a fuel issue,, it got better as i increased jet size from 61 to 63 then 65,,, now at 67,,, been raining so havent been able to try a 69,,, that should fix the dead spot,,,, i think my rev rpm issue where i cant go higher then 7560 has to do with the timing,, RSE head, and the shitty Banana pipe,,,, that pipe has no header or proper length diffuser/stinger,,, it technically wont even kick in/ come into the pipe till 11-13,000 rpms.... i bet you if i put a nice KX 65 style pipe on it with the Walbro carb I have and retard timing a little,,,, she will hit 10,000,,, maybe even add a second base gasket and i still havent modified piston skirt on intake side that hangs in port window at TDC...the crappy pipe is killing me...its got torque up the wazzoo,,, but no hard hitting in your pants revving powerband like a KX80...
  9. butre

    butre Well-Known Member

    without raising the exhaust port you're not going to see much past 7500 anyway.
  10. jaguar

    jaguar Well-Known Member

    consider this: if the pipe is tuned to come "on" at 11,000rpm then what you are experiencing is the dead zone right before it comes on the pipe.
    chunk that crap and put the stock pipe back on.
  11. Diamond Back

    Diamond Back Active Member

    Hey JAG,, good to see you buddy,,,I got to talk to you about the status of my WALBRO conversion kit,,, but in private some how,, dont want all my secrets out there,,, but it came out awesome,, start testing tommorow if no rain,,, you have a regular e-mail that i can send you pics and talk about sales???

    ok..on the bike issue,,, dont know why i am stressing it since we are getting a carburetor upgrade tomorrow but i just want to know and learn i guess.... i dont have stock pipe,, i traded it for the RSE HEAD which i thought would be crap,, but i have learned to love it,, my midrange is grunty and got some snot to it,, if i am at 3000 rpms and gas it, it wants to lift front end almost but it pulls you back hard and is at 7500 in seconds,,, just have to run some lucas octane boost with sunoco 93 and its CRISP..

    i have read JENNINGS guide and everything else i could find on expansion pipes,, at first it was confusing but then it was pretty easy and straight forward,, the Banana pipe really just needs a 27 mm diameter header pipe and they say the Banana pipe comes into the pipe around 9,000 rpms. not 11.. POWERBAND really only hits hard for 2000 rpms then drops off so by tuning the header length you can control when you want it to hit,, My RSE head gives me crazy torque and pull from 3,000 to 6,500 so i want my pipe to hit from 6,500 t0 8,500...(i want my Motor to live reliable and healthy))) so i am picking 8,500 as my final let off WOT redline. So i need to drop my pipe 2,500 revs.... for every 1" inch of straight header pipe you drop 400 rpms,,, so i need to splice in 6.25" of 1" inch diameter pipe, that will make the pipe start working at 6,500 rpms and out to 8,500..perfect !!!! also need to due some flange mount opening to 1"inch, and the length measurment starts from piston skirt..

    also by cleaning your piston top and make about (2)) 1 mile WOT speed runs you can see where the burnt section is happening on your piston,, if the burnt marks are in the exhaust port then your pipe flange is too narrow and creating to much suction and needs to be increased till the burnt marks are in middle of piston,, to big and you dont have enough suction to pull up your transfer ports,,, it all has to be just right or suffer lose in power,, I looked at my piston and my burnt marks are in my exhaust port cause my pipe flange and header are 3/4" inch, so i need to increase to 1" inch ..

    NOW THIS PART IS COOL,,,(THE STINGER))... the banana pipe has a 1 inch nub stinger,, sucks does nothing.. The stinger should be half the size diameter of the header,,, so 1/2 inch and by making it longer makes the powerband come on earlier too,,, you dont want to make it to long cause the longer it is the more pressure and HEAT you create in the pipe and can ruin everything you just worked on building so perfect,,, your best bet is to play around with different sizes and lengths,,, but a good stinger is the same length as header and half diameter.. THE PART I AM GOING TO DO IS SEAL IT UP,,, I WANT A SIDE BELLY BLEED OFF ,,, BY DOING THIS YOU CAN SEVERALLY SILENCE YOUR EXHAUST TONE BY 84 %%%%%% AND IT SOUNDS COOL HAVING IT THIS WAY,,, CHECK OUT VIDEO .....
  12. Diamond Back

    Diamond Back Active Member

    SO JAG,,, HIT ME UP SO WE CAN TALK ABOUT SELLING WALBRO KITS AND PUTTING it all together from getting inventory,, making the parts and adjustments needed,,, and advertizing,,, and selling price...... my regular e-mail is please e-mail me there so i can send you pictures and info , prices, and performance issues good and bad,,,, start testing in a few hours..

    i am thinking of calling my company DYMOND BACK THRILL CYCLES. I BUILD THEM TOO STARTING AT $350-400 BASE MODEL ON UP..
  13. jaguar

    jaguar Well-Known Member

    Actually changing where the powerband hits is not as dead set as you make it sound. Best to use my free expansion chamber software to know how long to make the header for it to harmonize with your plans.
    you wrote "if the burnt marks are in the exhaust port then your pipe flange is too narrow and creating to much suction and needs to be increased till the burnt marks are in middle of piston,, to big and you don't have enough suction to pull up your transfer ports"
    One factor that figures in to this is the angle of the transfer ports. Stock isn't angling backward enough which also tends to let intake charge go out the exhaust port. I explain that on my site and tell how to use JBWeld to angle them correctly.
    Last edited: Jul 10, 2016
  14. Steve Best

    Steve Best Well-Known Member

    Diamond, one principle in industrial troubleshooting is to go back to "as designed state" before modifying out of problems. Get it running "as designed" rather than modify because the modifications often add new problems and it becomes impossible to solve what is causing what.
    1) Put it together as simple and as close to stock as possible and get it running well,
    2) Do one modification, get it running well and see what the effect is
    3) try another modification, get it running well and see if it performs as expected.
    This is the way. Just like cooking.
    You cannot pour dry noodles and tomato sauce in a pot and expect spaghetti.
    Gotta follow the recipe and know a little about the process.

    The NT carb is very tunable in the mid-range and top end. Mid is the needle and top is the jet.
    I'm running 10,000 with it, you can too. Keep playing with jets and cutting plugs apart to check WOT result.

    I love the RSE head too, for same reasons as you, but it limits rpm to 9000 or so. Pipe may bring it down too.
    The limiting effect is that of "hitting a wall". No amount of carb tuning or size will overcome resonant tuning.

    Hope there is some help there. Look in my past threads for picts on my bike and things I've tried.
    I'm on the road and cannot repost much now.

  15. Diamond Back

    Diamond Back Active Member

    Your right JAG... I am shure it is not cut and dry as you read it in books,,, plus every book has its own variances to differ with,,, i can settle on learning the ball park for now and tweaking as i go,,, they are all different monsters,,, i will play with the Banana Pipe for now and seal up the end and have a side belly exit bleed off to quiet it down too,, I will try a 6" header and see what the results are....

    JAG how do I get to your software program, ????? eventually I will just get a pipe... i cant make one from scratch,,, sorry JAG but i can buy the right idea and tweak it.........I am upset cuz I missed a 2003 KX 65 stock pipe and silencer for $20 bucks last night,,,, so they are out there and i will find one with in a month
  16. Diamond Back

    Diamond Back Active Member

    OOHHH,,, SHOULD I play with my off set magnetto key,,, and should i retard 5 degrees, or advance 5 degrees ????
  17. jaguar

    jaguar Well-Known Member

    Just go to my page explaining how expansion chambers work and click on the download link from the next page about the calculator.
    Stock ignitions need to be retarded.
  18. gary55

    gary55 Well-Known Member

    I have one of Jag's cdi's with tunable advance curve on one of my bikes, and it works real good. I've had it for three years and no problems. You do all the same mods that I do on my personal bikes. I also deck the jug around .030 or 2.710 base to top and ramp my pistons at the exhaust and transfer ports then trim the skirt for the clear intake shot. On my best running bike I broke down and ordered a real sha 16\16 from delorto. Almost 100.00 with shipping. Night and day diff. from Italian Delorto and chinese knock off. I had to mess with jetting but now I have a chocolate colored plug and no 4 stroking. I would really like to hear how the walbro setup works for you. they look like quality and I'm guessing they are probably easily tunable.. My next build is tha exact same bike in this pic. but with a nuvinci 360 rear hub, and one of those gas bike engines with the 3 port jug and the giant assed reed valve and carby. We'll see.
  19. jaguar

    jaguar Well-Known Member

    Gary tell me if your cylinder has stock exhaust port height and max speed on level ground with your pipe.
  20. Steve Best

    Steve Best Well-Known Member

    I've used ramped pistons to gauge where to cut expensive cylinders (on other engines, sleds and CR250).
    The cut cylinder always worked better than the ramped piston.
    I think the reason is because of the effect on squish turbulence.

    I noted and like the idea of the ramp helping to smooth flow into the cylinder, but my personal results differed.
    I too have been decking the jugs. First about 0.010" to get them flat, but recently did one almost 0.030" to see effect on powerband when lifted with base gaskets and teflon headgasket. Even that little made a difference but I do like the low rpm torque of a low exhaust port. There is a problem with the exhaust port not being completely uncovered. Your ramps may help that, but so will lifting the cylinder.
    If the intake and exhaust ports are wide enough as they open, there is no need to change port timing unduly to get rpm.

    I have a couple of the Delorto "RT" clones. Sad to hear they are poor copies. How do they differ from the real thing?