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Heads and Cylinders All about your porting, compression, rings, cylinder and piston modifications to your bicycles engine

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  #1  
Old 07-18-2016, 09:34 PM
StevenMain StevenMain is offline
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Default My mods, and performance, how am I doing?

Hi, my name is Steven, I'm a highly experience auto mechanic, Have owned several race cars and crewed on the Main and Martinets Top Alcohol cars that my dad has owned or co owned.

NEW to two stroke.
Being as I'm using my GI bill to finish school I don't have money for projects any more so I built my bike since they are good cheap fun.

I have the 66 PK80 ebay motor kit on my Kent Del Rio and here are the mods
NGK #6 range plug
HD CDI kit
Stuffed Crank case
Arrow Reed Block
PZ20 Keihin Carburetor
Expansion chamber pipe (no name ebay but very Large high mount type)
Transfer ports widened, de burred, aimed at intake, roof angle elevated to converge just below deck.
Intake Widened, 3rd Transfer port added, port matched to reed block
piston windowed, added transfer ramp bevels to top
Exhaust port widened, port matched to ported exhaust flange.
Raised Compression (not measured but nearly removed the sealing ring around stock head, its about .008" left)

The mods have REALLY woke this little 66 up, it starts and idles fantastic, has amazing throttle response even with the large 20mm carb.

Running the stock gearing (10/44) I get 41mph @ 9,212 RPM
I was curious if this was respectable "race" performance or not. The best part is I got so lucky on balancing the thing is silk smooth with no vibration whatsoever. Im 185 lbs.

My friend has a stock PK80 with raised compression, a banana chamber with muffler, and delorto 14 Carb, and short billet intake on his bike. I can leave his ass like hes standing still. He tops out at 29 after a LONG time. Mine gets up to 41 in (aproximation) 8 seconds or so?

Wondering how to estimate HP on these things!

Last edited by StevenMain; 07-20-2016 at 01:29 AM.
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  #2  
Old 07-19-2016, 10:40 PM
Sam90lx Sam90lx is offline
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Default Re: My mods, and performance, how am I doing?

If you are hitting 41 mph with a 44 tooth rear, I would say you are running damn good.
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  #3  
Old 07-19-2016, 10:53 PM
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Tyler6357 Tyler6357 is offline
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Default Re: My mods, and performance, how am I doing?

8300 rpms is very fast for a 66cc....definately race performance on these things.
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Old 07-19-2016, 11:37 PM
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sbest sbest is offline
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Default Re: My mods, and performance, how am I doing?

My 66cc got up to 40mph, and that seems to be a barrier with 44 tooth rear sprocket. Further mods would not get it to rev higher or higher top speed, infact 35-41mph depending on what pipe or head or intake. What the further mods did was make it accelerate faster as you mentioned. My math seems to indicate a higher rpm than 8300 at that speed. How did you measure?

Steve
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Old 07-20-2016, 01:28 AM
StevenMain StevenMain is offline
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Smile Re: My mods, and performance, how am I doing?

sbest thank you for pointing that out!
I confirmed speed via 2 separate gps apps on 2 separate phones.
The rpm was done with math, but my math was in error!
I thought the front sprocket was 11 tooth, it's not it's 10 tooth! So 27" tall tire doing 41 mph is 510.68 wheel rpm * 4.1 * 4.4 = 9,212 rpm holy cow that's screaming. I think the only other significant performance gain I could get can only be obtained through some type of aerodynamic fairings. I will edit my original post!
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Old 07-20-2016, 01:36 AM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

One thing I failed to try was I welded the totally stock muffler to the stinger of my chamber because the bike is so loud unmuffled, my 41mph run was done with the muffler in place. It blew an exhaust gasket doing the quick hard sprint maybe I'll remove the baffles and try again unsilenced for a better top speed, I also lowered the seat and welded a bracket to extend it 2 more inches rearward, added rear footpegs, and lowered the front handlebars, this will give me a better tuck. Unfortunately I'm running my speed tests in full motorcycle leathers which are very bulky and draggy.
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  #7  
Old 07-20-2016, 11:16 AM
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sbest sbest is offline
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Default Re: My mods, and performance, how am I doing?

Steven, actually check your wheel diameter, or more accurately, check how far it travels in one rotation with weight on it. You might find your revs are closer to 10,000 rpm.

Steve
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Old 07-20-2016, 01:02 PM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

Good idea, I'll sit on the bike and get an actual back tire rollout measurement and report back.
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Old 07-20-2016, 01:32 PM
StevenMain StevenMain is offline
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Default Re: My mods, and performance, how am I doing?

Right on again with 80.25" rollout measured I must have been doing 9,732 rpm. Not accounting for possible tire growth.

Removing the muffler could help, beyond that is my motor probably maxed out as far as performance potential?
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Old 07-20-2016, 10:42 PM
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sbest sbest is offline
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Default Re: My mods, and performance, how am I doing?

Yeah, I found mine would not rev any more even going down a very steep hill. I do not have an expansion chamber pipe yet but I am running a short pipe that is port matched and was picked out of all the ones Grubee sells for its ability to rev.

I ran the bike for months and hundreds of miles at WOT at speeds of 30-40mph up and down hills. I can feel some play in my lower rod bearing now, so it does come at a cost. Probably better to run it at 8000 rpm, but darn, that 44t sprocket is pretty much perfect for hills, start away, and enough speed. I don't want to go higher or lower without giving up too much on the other end.

I just finished building a jackshaft shifter with a 48cc motor. Not completely pleased with it either. The start away and low cruise rpm are nice, but more parts and complication, and starting the engine is more difficult. I like pedal assisting, but the shifter ratios (engine to pedal) make this difficult.

Oh, removing muffler is generally not much help with a 2 stroke tuned pipe because backpressure on the stinger pipe is useful, that is why the stinger pipe is so small.

I liked the single speed system as a tool for engine development because it focused totally on looking for broad power out of the engine. I'd say you found it Steven.
Maybe time for you to look into a jackshaft shifter?

Steve
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