PLEASE HELP SOMEONE, DEAD AT 7,560 rpms

Diamond Back

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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,,
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Frankfort MB's

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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
 

Steve Best

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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".

Steve
 

Diamond Back

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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".

Steve


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 GASBIKE.net 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
 

Frankfort MB's

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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
 

mrbg

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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
 

Frankfort MB's

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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
I doubt it's that because he says it runs good until 7500 RPMs
 

Diamond Back

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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


its not an air leak,,, i would have idle problems
I doubt it's that because he says it runs good until 7500 RPMs


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 s**tty 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...
 
D

Deleted member 12676

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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.
 
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