Porting a china girl

Frankfort MB's

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Apr 3, 2016
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yes it is a lean fuel/air ratio that is the main culprit of high head temperatures.
But when you have the right ratio if you increase the compression then that increases temperature
which is why I recommend not raising it too much.
going from 90 to 135psi is enough. most aftermarket heads bring it up to around 160 which is way too much.
Cheap and easy upgrade though:D
 


jaguar

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Nov 25, 2010
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actually it was pretty tough on my arm. I almost stopped to finish grinding it down the next day, it was tiring out my arm so much.
But I'm older than most of the people on this forum.
But why buy a new head when it's not too hard to modify the stock head for the perfect compression.
 

Steve Best

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Sep 22, 2012
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actually it was pretty tough on my arm. I almost stopped to finish grinding it down the next day, it was tiring out my arm so much.
But I'm older than most of the people on this forum.
But why buy a new head when it's not too hard to modify the stock head for the perfect compression.
Agreed!
I use a coarse file to take off metal fast, easy to take 0.020" off in about 10 min.
Then use 150 grit sandpaper taped to flat glass sheet to get perfectly level. 5 min.
I made a cutter up to cut the squish taper out to piston diameter, but did it with sandpaper on a piston dome in the past.

The modified stock head works pretty darned good, but I am having problems with warpage.

Steve
 

jaguar

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probably you have too lean a mixture or a leaking seal which allows excess engine heat.
having the stock timing too advanced in combination with high compression causes excess heat also.
 

45u

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Feb 18, 2016
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actually it was pretty tough on my arm. I almost stopped to finish grinding it down the next day, it was tiring out my arm so much.
But I'm older than most of the people on this forum.
But why buy a new head when it's not too hard to modify the stock head for the perfect compression.
You are 4 years younger than I. By the time I modify a stock head I could have worked on a bike and bought a aftermarket head the cools better!
 

jaguar

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Nov 25, 2010
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Show me an aftermarket head that doesn't raise the cranking pressure to more than 135psi
 

jaguar

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Nov 25, 2010
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I've been on this forum a long time and every time someone gauges the cranking pressure with an aftermarket head it gives really high pressure.
High pressure means high combustion pressure which means high engine temperatures and too much ring pressure on these poorly plated cylinders.
The plating then flakes off. Not good.
135psi is enough for a good boost in power but not enough to cause problems. That's why I recommend not exceeding that pressure.
And since no aftermarket head fits that bill I can only recommend people modify their stock head. I did that and my engine reved to 9000 and the head never got too hot, never cracked, and was just fine. Some glass and sandpaper and some elbow grease and you have the best head there is.
 

Frankfort MB's

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Apr 3, 2016
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Have you done experiments with Puch heads? I've read that they are pretty good for cooling and don't have a very high compression ratio (this is the interweb I'm talking about so it's probably wrong:);))
 

butre

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Dec 2, 2013
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I've not tried the puch standard 70cc head, but the hi hi definitely is not low compression.
 
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