Ported the cylinder today hitting 8500 RPM.

[Backwoods]

At first the engine would not get up over about 28mph. After this fairly easy drill, opening up the exhaust port and tightening squish to 0.030 has the bike doing 36. It's a 70cc. (69.5)

I got the bit from local hardware store. Just simply, well opened it up. I can stick my whole index finger through it, and I have fat fingers.

These engines cant breathe, they need porting. The engine really loves the higher rpm now though.

So I was having alot of issues at first. Like alot. It made me try things I wouldn't normally try, trying to outsmart the engine. The problem was traced back to a Champion spark plug, a B6HS.

This damn plug was like a ghost in the engine, giving just enough spark for you to rule it out, but not running 100%, ever. I chased a bad plug for probably days. Just couldn't get it to run correctly.

I put in a proper NGK BPR6HS. The champion, as noted was a B6HS, no resistor, no protruding tip. Installing the correct plug made all the difference. The champion wouldn't ever even have a proper color on it, making me think its rich, or lean. Some cold days it wouldn't work at all. So theres that.


Moderators Note:

"Champion spark plug, a B6HS"......"The champion, as noted was a B6HS"

It cannot be a Champion B6HS spark plug simply for the reason that Champion does not use the B6HS numbers in its numbering system...That is an NGK plug number, Not a Champion plug number...The OP must be referring to a Champion spark plug, (Number Unknown), he had substituted for the NGK spark plug number he is referring to.

 

[Karl Snarl]

I'm not sure about out smarting the engine, but porting correctly is a bit more than just opening it up.

You got lucky you didn't ruin the port timings. 2mm is enough to turn a good engine into a dead horse.

If you wish to learn to port and map the cylinder correctly, we have some racers and ex racers on here that will get your engine spinning twice as fast ( and last) as I spin mine.

I will be building one next year, using the advice and instructions I received from the people I just mentioned. They helped me get my Nitropropane bike running right, an its a sweet machine.

Jog through the pages of threads, or use the search feature, and look up the correct way to port. It makes a world of difference, especially when you add a good pipe in the mix.

 

[HumanPerson]

and redirecting the transfers angle helps tremendously.

 

[inspectorcritic]

What's that lip behind it?
Seems like that has got to also.

 

[Backwoods]

I left the lip there and thick because taking it off meant the port may be too low, allowing exhaust in the crank at TDC. I left it thick to handle the heat, I basically went almost all the way in with the bit, then opened the top up with a smaller bit. I opened up the intake side to side. Today I was able to hit 42.3mph with a 41 tooth sprocket on the flat. If my math is correct it should be close to 9500rpm. Not quite, but almost. It never would rev that high before I opened up the ports. Jugs are cheap enough to play around a bit with them. I've replaced a few already. I usually set the squish to 0.030 and It keeps the piston really clean. 5.5cc head. It was a 65 jet that did it. I also shave the piston skirt a few mm. It seems to help vibration. Usually 2-3mm on intake side and atleast 1, never more than 2 on the exhaust side. The beefy JRL top end bearing also helped with vibration, alot. They are like $5.

I was actually seriously considering some version of the yd100, but seeing that only one peice designs are all that's available doesn't allow me to adjust the squish, easily. I just don't think that I'm going to be able to get 9500 out of the yd100 in its current form. Theres just not many aftermarket heads available either. I'll wait another year or so.

 

[Karl Snarl]

I left the lip there and thick because taking it off meant the port may be too low, allowing exhaust in the crank at TDC. I left it thick to handle the heat, I basically went almost all the way in with the bit, then opened the top up with a smaller bit. I opened up the intake side to side. Today I was able to hit 42.3mph with a 41 tooth sprocket on the flat. If my math is correct it should be close to 9500rpm. Not quite, but almost. It never would rev that high before I opened up the ports. Jugs are cheap enough to play around a bit with them. I've replaced a few already. I usually set the squish to 0.030 and It keeps the piston really clean. 5.5cc head. It was a 65 jet that did it. I also shave the piston skirt a few mm. It seems to help vibration. Usually 2-3mm on intake side and atleast 1, never more than 2 on the exhaust side. The beefy JRL top end bearing also helped with vibration, alot. They are like $5.

I was actually seriously considering some version of the yd100, but seeing that only one peice designs are all that's available doesn't allow me to adjust the squish, easily. I just don't think that I'm going to be able to get 9500 out of the yd100 in its current form. Theres just not many aftermarket heads available either. I'll wait another year or so.

You want the LD100.

 

[Potato_In_Exhaust]

Yo, I plan to do all dat stuff to my motor! Portin' n' Polishin', curtain window piston, enter the 36th chamber transfer portin', squishy band thingy, trench foot, etc etc! And also add a Honda CR500 FMF Fatty Platinum Series Pipe! Oooh wee, can't wait to impress the ladies once I get the beast kicking harder than a mule on steroids after my modifications!

 

[Sidewinder Jerry]

I left the lip there and thick because taking it off meant the port may be too low, allowing exhaust in the crank at TDC. I left it thick to handle the heat, I basically went almost all the way in with the bit, then opened the top up with a smaller bit. I opened up the intake side to side. Today I was able to hit 42.3mph with a 41 tooth sprocket on the flat. If my math is correct it should be close to 9500rpm. Not quite, but almost. It never would rev that high before I opened up the ports. Jugs are cheap enough to play around a bit with them. I've replaced a few already. I usually set the squish to 0.030 and It keeps the piston really clean. 5.5cc head. It was a 65 jet that did it. I also shave the piston skirt a few mm. It seems to help vibration. Usually 2-3mm on intake side and atleast 1, never more than 2 on the exhaust side. The beefy JRL top end bearing also helped with vibration, alot. They are like $5.

I was actually seriously considering some version of the yd100, but seeing that only one peice designs are all that's available doesn't allow me to adjust the squish, easily. I just don't think that I'm going to be able to get 9500 out of the yd100 in its current form. Theres just not many aftermarket heads available either. I'll wait another year or so.

I'm going to assume you have 26" tires and a 10t sprocket on your engine.

(9,193×26×π)÷(1,056×4.1×(41÷10))=42.3008261269