"Speed limitation 38km/h"

seanbbolson7878

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Aug 21, 2019
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Was reading the manual on my china girl 66cc and it says there is a speed limitation on it. I know it's not an electrical because I have swapped the coil and magneto on it. Could it be because it's not a balanced crank? It will top out at 38km/h and not go any faster even down a hill. Was just wondering what would cause that so I could maybe get a new part to replace it.
 
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Zeldanumber17

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Oct 26, 2019
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There shouldn’t be. I don’t know how they would use a governor. You may be hitting a lean wall. Check for air leaks.

Does it idle high or have a hard time keeping consistent idle?

On the other hand I have had bikes that just won’t wind past 6000rpm because of horrible balancing.
If it’s running good you may have got a dud. Also check for 2 base gaskets. Some engines can come with sometimes 2 or more. that can lower the compression ratio and that causes it to have worse top end.

Sorry for scatter brain. just trying to think of everything
 
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seanbbolson7878

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Aug 21, 2019
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26 inch wheels and 44 tooth but it says in the manual that the "speed limitation" is 38kmh which is 24 and that's when it just starts to 4 stroke and not go any faster. Spark plug has perfect coco color and runs awesome before it hits 24.
 
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indian22

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Dec 31, 2014
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What fuel mix ratio are you running? Have you ridden this bike enough to use two or three tanks of fuel? 4 stroking a that starts at that speed is something I've never encountered, but too much oil in the mix is a typical cause of 4 stroking. 32:1 mix ratio is what I run after engine break-in period. Mix ratios are guidelines and several factors can and do require mix adjustments. Just remember that going much thinner on the mix increases friction and heat, though the engine will produce more power.

This may be more than you want to know but what may be occurring and acting as a speed limiter is that the four stroking is happening at the worst possible time, just when your motor and pipe combination is trying to develop a bit of sonic boost and come up on pipe the 4 stroking prevents it. Even running the kit pipe & muffler there is a weak sonic bounce that aids the motor to get past the rpm bottle neck. With the 44 tooth on 26" and a port timed kit motor 25 mph is about where I've seen them choke and that's on motors that didn't 4 stroke at all at any speed and some of these motors were very pretty highly modified, but what I found was that at around 6,000 rpm, for example, if I held it there for several seconds the motors came to life and blew through this bottleneck, smoothed out and the modified motors would pull hard to over 10,000 rpm. If 4 stroking had been a problem on any of these bikes at 25 mph none could have ever broke past that rpm bottleneck, no mater my modifications.

Running a smaller rear sprocket will get a bit more speed but you end up pedaling a lot and the motor is still stuck with four stroking at the worst time. I actually don't mind some low rpm four stoking, but not during hard pulls through the mid range band. You should be seeing thirty or more likely, thirty five from your setup once it's broken in and off break in mix ratios.

Rick C.
 

seanbbolson7878

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Aug 21, 2019
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I'm on my 3rd tank of gas and I'm running 24:1. With the stock nt carb I had lots of issues with 4 stroking and wouldn't go any faster than 10mph. I just bought a hp carb and it still 4 strokes but finally climbs hills and goes a decent speed. This hp carb has a 78 size jet and the nt had a 65. The bike runs awesome when it's cold and when it warms up it runs a little worse. It still idles and all that stuff just a little less off top speed. I thought that would be the other way around. With the hp carb it starts to 4 stroke where that nt carb did but then it kicks out of it and runs better. Right now my 1/2 throttle and full throttle still only both produce 24 mph. So I dont know if I just need to break it in more? My 4th tank is going to be 32:1. Right now I'm running with the cap off my exhaust and it's still running perfect and the spark plug is the perfect color. When I had the cap on it would 4 stroke worse at less speed. My muffler isnt clogged or anything I can blow through it and everything.