Scratches in the bore

Mr.2Tcycles

New Member
Nov 13, 2008
44
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0
Hawthorne CA
I just took off my exhaust pipe to "gasketize" the gasket with some copper permatex due to a leak. Had the opportunity to look into the port and look at a section of the rings, port and the piston crown. All really clean with some very slight granule like deposits on the piston crown and the black coating of the rings about half worn off showing silver, I am on my second tank and running 20:1 gas/oil. What disturbs me is some major scoring, vertical stripes ( duh!! ) on the bore on the sides of the intake ports. Don't know how deep they are. Engine runs fine, been running it really rich at break in, plugs are more opaque than brown, more like dark gray. I have noticed a lot of the casting impressions on the intake ports. Could it be that they left a lot of machining metal files in the crank to be inducted into the chamber? Or is this just something to be expected at break in. Does anyone know what the bench mark compression ratio of these engines when new? When run in? Either way with the price of these engines and the fun factor, can I really argue? I'm just gonna drive it til it quits/granades/melts, going to do some endurance testing when I go to the Grand Canyon, AZ the later part of week. Gonna really see if they can get 100 to 150 MPG with GPS. Also going to be testing if running 20 minutes at 80% to 100% throttle or more are going to seize or destroy the engine. Should be fun.
 

RLorange

New Member
Jun 21, 2008
127
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Australia
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.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
236
63
up north now
If you load a two stroke engine, or rev it up too much before it warms up enough, the piston skirt can rub in the bore and cause the lines you see. It's called "cold seizure".

Also, if your plug is grey, or closer to white, that's too lean and hurtful too. Too much oil in your mix can make the fuel/air mixture leaner, and that's not healthy either.
 

sprocket

New Member
Nov 7, 2008
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Amherst MA
A picture is worth a thousand words...

Jim
Hi there, nice picture. Since you took the crankcases apart you could probably know what of the distance between the conrod centers. I needed that number to make some calculations. I estimated it to be about 88mm I think. I bought some computer software for tuning 2 stroke RC boats engine and I'll use it to do some mods to my HT engine. First thing I noticed when I took the cylinder apart were those superficial scratches on the bore, on not so superficial, you can feel it with your nail, I guess they leave debris inside the engine when the put it together.
So I noticed 3 areas that can be improved a lot in these engines, one is course compression ratio, I calculated mine to be 5.4:1, pathetic. The other thing is the blowdown time, i.e. the time in crankshaft degrees the exhaust remains open before the transfer ports start to open. A good figure for this is 30 degrees. I measured mine to be about 13 degrees only, that is pretty bad, an this value is as critic as compression ratio on a 2-stroke engine. If the pressure at the cylinder is above the pressure at the crankcase when the transfer ports open, then burned gas flows from the cylinder to the crankcase trough the transfer ports, instead of fresh gas flowing to the cylinder in the oposite direction, not good.
The other thing is the time in degrees the induction port remains open. I measure mine to remainopen for only about 75 degrees, that's also pretty bad, it can easily doubled or more. I've seen people dremel about 2mm of the bottom of the piston skirt on the induction side to increase this opening time. Acording to my calculations we could dremel a 12mm window on the bottom of the piston on the induction side, and still be on the safe side.
So my cylinder has 2-3 mm of piston clearance on the top, so what I plan to do is lower the cylinder 2mm, by removing material from the bottom of the cylinder of the top of the crankcases. That will remove about 3.5 cc of the combustion chamber and boost compression, and then by lifting the exhaust port about 5mm I will get the right blowdown timing, plus the window dremel on the piston skirt, that should improve the engine performance a lot. Also I noticed some metal protruding from the tranafer ports where thet meet the cylinder sleeve, very awful, all that needs to be smoothed.
I;ll let you know how it works ! (if it works ! hehe)
 

Upshifter

New Member
Dec 27, 2008
30
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Oregon
Generally, anything that you can do to smooth the ports, inlet and exhaust, and remove rough castings restricting flow is a good thing, and can increase the power by about 5%.

The fuel mixture is used for more than fuel, it also cools the top of the piston, and lean air/fuel mixtures are harmful.

Usually, grinding material from the ports and increasing compression will add power but it will leave flat spots in the RPM range. Otherwise, it may have good peak power, but have mediocre mid or low end power. To me, it's irritating to have flat spots in the power band.

It seems that to get the greatest satisfaction from engine modifications requires going all the way. That means increasing compression, changing port timing, optimizing timing and ignition, improving and enlarging the carburetor, and fitting an expansion chamber designed for the rpm range that the engine will be operated in. Often, the engine improvements will increase the peak power and the upper RPM range.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
236
63
up north now
Bumping compression will increase power across the range.
Smoothing the casting left overs will help as well.

Neither will leave you with flat spots.