What can i do to get more power from my china pk80 motor

[Sambo]

I have a pk80 china motor that I ordered on eBay. I installed an expansion pipe exhaust, a 19mm hp carb, and a 32 tooth sprocket but I cannot seem to get over 30mph I have a #75 jet that bogs out at WOT, haven't had a proper look at the ports but will in the near future, any tips on how I could get more power?
Cheers Sambo

 

[mapbike]

So many things play into potential top speed.

engines crankshaft balance
port timing (Which im no expert at)
carb jetting
Exhaust flow
Gearing

First thing that stands out to me is 32T rear sprocket because it takes a very well tuned and set up engine to pull that gearing on a 26" wheel.

I'd suspect that you could move up to a 36T and may gain a couple MPH since your engine hasn't had any internal mods done to increase its power and balance.

Youtube is a great source to find what others have done to get better performance from these little engines.

if you have the tools to do some simple port cleaning and reshaping, that can definitely help, but carb tuning, exhaust flow and proper gearing for your setup are very important.

There are many youtubers who turn out good content, I think a fella in Louisiana with a channel called LA Hover Bikes

here is one of his tuning videos I think he does a fune job for us typical builders wnd riders

There are others but I like tye way this guy explains things.

hope this helps in some way.

best wishes and ride safe.

 

[Sambo]

i don't even know where to start, such a thought full and helpful response, il wack a 36tooth on and see how she does, iv heard of port work and after looking at how blocked the transfer and exhaust ports are i think its a must, based on what iv seen people use Dremel tools is there any specific bit i should use or just go at it

 

[mapbike]

i don't even know where to start, such a thought full and helpful response, il wack a 36tooth on and see how she does, iv heard of port work and after looking at how blocked the transfer and exhaust ports are i think its a must, based on what iv seen people use Dremel tools is there any specific bit i should use or just go at it

I get my dremel type tools from the website, sanding drums and polishing wheels also.

they also have the number micro size drill bit sets that some of us use to drill our main jet once we solder it closed when we're fine tuning our carburetor.

pritty much all things dremel/rotary tool they have and at a good price.

There are several simple things that can be done to these little engines that can add performance and in some cases some extra reliability.

I specifically went to the section on the website where the carbide burr bits are, ese will give you the best result and they last a long long time.

The ball end type, the barrel shaped and also the one that is kinda arrowhead shaped.

Dremel Rotary Tool Carbide Burr

Popular Dremel Tungsten Carbide Cutters like 9901, 9905, 9910, and 9936. Shape, smooth or grind hard materials. Use on hardened or stainless steel, cast iron, nonferrous metals, fired ceramics, plastics and hard woods. Structured tooth cutters have fast-cutting, sharp teeth for greater material...

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Also make sure you have a good supply of base gaskets and one thing that some of us do sometimes is we add an extra base gasket which raises transfer and exhaust port just shy of 1mm, of course the intake port is raised also, so you can mark bottom of intake port and then with the dremel you can either remove the 1mm that way, or the simpler way is to mark the 1mm on bottom of piston skirt on intake side and simply remove that 1mm of material there and get the same result.

this (can) allow the engine to rev higher if all else is capable of the higher revs.

Hope this helps a little and Im looking forward to what your results are Sambo

just be safe, have fun and be patient knowing there is a learning curve we all go through with these engines and if you work at it and do the correct mods, you'll get better than out of the box performance feom you engine.

 

[Hopper108]

what worked well for me was expanding the carbs air inflow, U can go on amazon and buy a racing carb for about 18.00 depending on where you live. When u get the carb, take the cover off and bend metal sheeting on the inside so it's a tube and fits snug. then cut the tube so it will fit where ur putting it. then take a 3/8 drill bit and drill about 32/48 holes depending on the size. like with actual engines the more air u can get in the engine the better. This worked for me but depending on the bike and engine it might not work as well. If not I recommend better fuel, making sure all your chains are oiled well with nonflame. also any weight u can take off will help, but don't affect the structural integrity of the bike

 

[Larry Trotter]

You don’t say what size wheels you have but a 32 tooth sprocket may not be letting your engine get into its power band. My stock China girl on a 700c makes over 30 with a 44 tooth sprocket. In your case a larger rear sprocket may get you more speed especially with an expansion chamber exhaust.

 

[Larry Trotter]

I also have a 20 inch that makes 30 using a 32 tooth rear.

 

[Sambo]

This is the motorised bike i forgot to specify that the pipe is an ebay mz65 clone. i tryed running the 36tooth and got less speed but more tourque. i fixed the terrible 4 stroking because someone decided a #75 Jett would be a safe size for a motorised bike carb by putting a 65jett in, it bogs a little bit at WOT on flat but it runs way better. I got a dremel for birthday and am in the process of fixing the crusty casting in the cylinder on the intake and exhaust ports, i also want to have a go at cleaning up the transfer ports but i dont know if im confident enough in my dremel skills to not scuff the bore of the jug. At TDC there is around 0.8cm of piston showing on the intake side, i have heard talk of skirting the piston so there is full open flow but idk if that would unbalance the piston, keen to see if the porting will make a difference and i have swapped the 32tooth back.
Cheers, Sambo

 

[Sambo]

Here are some of the pics of my light porting. something i also forgot to mention was on the block you can see how part of metal has been broken, i dont know if this will reduce performance but i assume it would. i am trying to port match my intake flange atm but i dont know how i would port match the exhaust when u can see the metal is thin and cannot be opened much

 

[Greg58]

The small amount of piston showing on the intake side should not hurt power, when porting intake and exhaust you can widen them without changing port timing. And for cleaning up the transfers I use a very small rotary tool from harbor freight, it's easier to use and lacks the torque to jump around and damage things. I've got two of those pipes and both were too loud and lacked back pressure for low end, I made silencers for each and that helped bottom end without hurting top end.

 

[Greg58]

When porting you want to go slow and take it easy, if I try to get in a hurry I tend to make mistakes.

 

[Greg58]

Here’s the pipe and silencer on my cranbrook.

 

[Greg58]

This is a stock muffler I cut down to use on my o.p. cruiser.