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Old 11-25-2008, 12:26 AM
RLorange RLorange is offline
Motorized Bicycle Elite Member
Join Date: Jun 2008
Location: Australia
Posts: 127
Default Re: Scratches in the bore

I have a similar looking bore. Running too rich after break in can leave carbon on the side of the piston above the first ring. this is what scratched my bore but they are not deep and don't seem to have affected the engine beyond normal wear and tear.

I have found that after break in you need to progressively lean out your fuel both oil and the needle clip setting to prevent build up. I got some tiny washers which I put underneath the pin clip so I could get in-between clip settings. I started off at the 2nd from top notch and have settled with a tiny washer between that and the top notch.

As my rings and bore have worn I have noticed the engine is less responsive to slight fuel/mixture adjustments. With my current level of wear (and scoring) There is a slight drop in power from its peak around a month ago and the engine takes longer to develop full power once the engine is warm.

I have tinkered with this engine a lot and push it hard, 60km per week up big hills.

After break in I removed the head gasket completely by smoothing both the head and top of the cylinder surfaces flat using wet-n-dry on a piece of glass with water. By progressively using finer paper I achieved two perfectly flat and smooth surfaces which seal without a gasket. The 1/2mm removed boosted the compression. The process took a couple of hours and ever since my engine trucks up big hills with a massive gain in top end power.

Be very careful if you attempt this because this mode leaves the piston very close to slapping the head. Your bottom gasket may slowly compress and shrink this clearance to dangerous levels. My local engine dealer has heavy duty gaskets to prevent this. Also every engine has a different clearance. My friends engine actually had the piston bashing into the head gasket! My old one had a good 1mm of clearance and of course ****ty compression.

These engine are low compression around 6.2:1. The gasket mod is about all you can do to boost it.
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