I wonder if I filled in those holes if it'd run fine again, being that I've lowered the vacuum difference the transfers dont fire into the cylinder as quickly and significant amounts of exhaust gases make their way back into the cylinder at low RPM's, causing the poof and firing issues. Only two ways to find out: fill in the holes, or get an expansion chamber. apparently 1.5:1 case to cylinder volumes is ideal, "stuffing the crank" causes transfer pressures to be increased and fire fresh mix through the cylinder and straight into the exhaust at low RPMs, wasting fuel and ruining scavenging, causing a narrow power band. Because of my increased compression head and lowered compression crankcase, I've ruined that ratio, likely by alot, probably around 2:1 now, pushing the powerband so narrow and high the motor cant get past the low RPM vibrations of the drilled crank.
Did I screw up my crank?
- Thread starter mew905
- Start date
this^^^
I honestly think these motors are better off with just a mild port here and there, bit of a shave on the piston, deck the head and jug just to make it flat again then add your tuned bolt on's.... all on a well maintained bike.
That is usually the safest combo.
I really don't think the cranks are that bad, tho there is def. your fair share of lemons out there. You'll know when you have a good one might be your 1st might be your 5th luck of the draw with these china's. Usually Id just throw mine over the shoulder when they started eating magneto seals just couldn't be bothered with them.
You can pick up the motors themselves relatively cheap once you get to know the guy that sells them.
In my case my magneto seals were fine. My crankshaft was not out of true (alignment), but I suspect the balance was bad.
But to be honest I have no clue how much vibration for these motors is considered normal, or abnormal, but if I was to rate my motor compared to other motors, it's was like a gravel sifter.
My next motor I may run the crank as is, maybe it will be a fluke and run smooth, but when I rebuild my old one, it's going to be balanced as best as it can be. Then I will disassemble and balance the replacement motor.
It's not just about, "Oh these are disposable motors, if it breaks get a new one" It's about fixing the one we got and being happy fun lucky time with it, hence 'HT"
Or maybe we just want to see how good and smooth we can get a motor to run and have it be more dependable instead of being a bearing crusher and muncher machine, and the feeling that 'I pulled off some mad skilz and my motor sounds like it too'
And besides, we are too wasteful a society, we should try to fix things instead of just tossing crap out, there was once a time where nothing was wasted and everything was repaired.
Bad consumerist mentality.
Oh, and breno, your signature is pointless.
But to be honest I have no clue how much vibration for these motors is considered normal, or abnormal, but if I was to rate my motor compared to other motors, it's was like a gravel sifter.
My next motor I may run the crank as is, maybe it will be a fluke and run smooth, but when I rebuild my old one, it's going to be balanced as best as it can be. Then I will disassemble and balance the replacement motor.
It's not just about, "Oh these are disposable motors, if it breaks get a new one" It's about fixing the one we got and being happy fun lucky time with it, hence 'HT"
Or maybe we just want to see how good and smooth we can get a motor to run and have it be more dependable instead of being a bearing crusher and muncher machine, and the feeling that 'I pulled off some mad skilz and my motor sounds like it too'
And besides, we are too wasteful a society, we should try to fix things instead of just tossing crap out, there was once a time where nothing was wasted and everything was repaired.
Bad consumerist mentality.
Oh, and breno, your signature is pointless.
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I'm reading through Gordon Jenning's 2-stroke tuner handbook. It's quite interesting. It states the bigger the exhaust port is, the more power you will make, no exceptions. The issue is with a bigger port you run greater risk of catching the piston rings and destroying them, you could put a vertical bridge in the middle of the exhaust port but because there's less material it'll expand disproportionately and potentially put pressure on the piston, as well as less contact with the [relatively] 'cool' cylinder wall thanks to a bigger exhaust port will cause the piston to endure more heat. However 62% of cylinder bore is a good safe zone, some high performance motors go up to 70% and some bridged ones even overhang the transfers (maybe 80% bore width to make immense power, but because of heat, sacrificing RPM. To put it simply, our motors have a 47mm bore and run a 24mm exhaust port, or 51%. According to Jenning's studies, we can safely widen the exhaust port to a whopping 29.14mm wide (internally). If you choose to do this, curve AND bevel the top and bottom of the port to push the piston rings back in place as it goes by. Before such an endeavor, port match your intake to 24mm (if you're running a Super rat or GT5-a, then you dont need to, it's already done). As for crank pressures, the optimal ratio between the cylinder and case crank pressures is around 1.5:1 cylinder to case. More or less you're simply losing power, so stuffing will only help if you've raised your cylinder compression (shaved head or aftermarket head) or if the ratio was off to begin with. This came about because of an old moped that had a ratio that was way off (about 1.2:1), and people discovered stuffing the case caused increases in power, this carried on through today as people assumed it worked for every motor. Modern motors are already affixed to this ratio, whether or not the HT's are, is another story. The last thing you can do to these motors without having to buy aftermarket parts is correct the transfer port shapes. A sharp angle transfer port "roof" increases high end power (our HT's appear to use almost an 80 degree roof). Shallowing it out to say 60-75 degrees will lower the power band. Also aiming the transfer ports toward the intake will help the "loop scavenge" design, pushing the exhaust gases out more efficiently. You'll notice only one half of each transfer is actually aiming toward the intake port, the other half is nearly aiming directly at the opposite transfer port (it's actually a few degrees toward the exhaust port). You dont need much, but JB weld in the top of the transfers to aim it toward the intake side will help make scavenging more efficient. That said, I DO plan on creating a steel insert (or perhaps a base gasket with the inserts on it) to correct the transfers for those who are less dextrous or more feeble about modding their motors.
The thing I am wondering about mew, is the chromed lining, how brittle is that stuff?
When I was doing porting work I tried to stay away from that stuff as much as possible.
But if you can grind it safely I would assume one would use a very fine bit, so as not to cause chipping due to much material removed too fast.
When I was doing porting work I tried to stay away from that stuff as much as possible.
But if you can grind it safely I would assume one would use a very fine bit, so as not to cause chipping due to much material removed too fast.