GN125H - Piston, exhaust valve, exhaust port, completely caked in soot?

**h0dgep0dge** · 17th October 2015, 15:03

My partner and I recently picked up a 2012 GN125H on the cheap for her to get her licence on (It's just been a scooter for her until this point). As soon as I rode the thing I knew it needed work, but one oil change, one new spark plug, one carb cleaning, one timing adjustment, and 2 valve clearance adjustments later, the problem was still there. I decided to get serious with it, and I removed the cylinder head to take a look (ended up buying a new socket just for this; as it turns out, 1/4 inch drive shafts don't play nice with head nuts. go figure.). Not fifteen minutes ago, I got around to taking the valves out to inspect them, and as it turns out, the exhaust valve is fouled to crap (I've suspected a compression problem for a while), along with the piston, combustion chamber, and exhaust port. The exhaust port has a layer of carbon 1 or 2 mm thick, crazy.

Cleaning out the carbon is no big deal, but the question remains, what's likely to have caused this? Is there anything I should do to prevent this happening in the future?

**Drew** · 17th October 2015, 17:04

The seating portion of the valve? 1-2mm?

That seems...unbelievable frankly.

Only way to build that much crap, is to be getting a shit load of oil in the combustion chamber. Go another step, and whip the barrel off.

Have a sniff of the oil, you're checking for petrol in it.

**h0dgep0dge** · 17th October 2015, 17:53

Originally Posted by Drew

The seating portion of the valve? 1-2mm?

1-2 mm in the exhaust port, the valve seat is substantially less than that.

Go another step, and whip the barrel off.

I'm reluctant, because I've never dealt with the business with the piston rings and oil rings, and I'm not confident I wouldn't fuck it up. I might get in the gear I need to replace the rings and lap the cylinder, then have a crack at it. I'll take a look at the oil when I get a chance.

**Grumph** · 17th October 2015, 18:10

Was there still a seal on the exhaust valve guide ?
Before you pull the barrel off, tell us if there are any marks on the bore...long deep scratches would indicate a broken ring.

**ducatilover** · 18th October 2015, 00:01

Check the valve stem seals haven't been typical Chinese shit and leaking.
Then pull the barrel, shit on it, put it in the bin, go drink a beer

**Akzle** · 18th October 2015, 06:01

run premum fuel.

Pics. Help. Cos, like. Stuff.

A leak down test woulds done. A'fore the head was off.

**nodrog:Me** · 18th October 2015, 07:11

All that carbon is probably making it run better due to increased compression.

**h0dgep0dge** · 18th October 2015, 11:30

I have pictures!

They include, picture of the top of the piston, pictures of the bore from both sides (i also ran my finger around it; it feelts smooth all the way around), picture of the valves, picture of the valve seats, and picture of the oil seals. No clue if that last one was necessary, but oh well.

**Drew** · 18th October 2015, 11:52

You've got a burnt out valve. Simple to fix, problem solved.

**h0dgep0dge** · 18th October 2015, 11:58

Originally Posted by Drew

You've got a burnt out valve.

Is that to say the valve is damaged? Or just covered in combustion products? I ask because it doesn't look damaged, so does that mean I can get away with just recutting+lapping the valves?

**Akzle** · 18th October 2015, 12:09

Originally Posted by Akzle

Pics.

Originally Posted by h0dgep0dge

I have pictures!

Originally Posted by Drew

You've got a burnt out valve. Simple to fix, problem solved.

yw .

**Drew** · 18th October 2015, 12:10

Originally Posted by h0dgep0dge

Is that to say the valve is damaged? Or just covered in combustion products? I ask because it doesn't look damaged, so does that mean I can get away with just recutting+lapping the valves?

That's the action alright.

**husaberg** · 18th October 2015, 12:44

Originally Posted by h0dgep0dge

My partner and I recently picked up a 2012 GN125H on the cheap for her to get her licence on (It's just been a scooter for her until this point). As soon as I rode the thing I knew it needed work, but one oil change, one new spark plug, one carb cleaning, one timing adjustment, and 2 valve clearance adjustments later, the problem was still there. I decided to get serious with it, and I removed the cylinder head to take a look (ended up buying a new socket just for this; as it turns out, 1/4 inch drive shafts don't play nice with head nuts. go figure.). Not fifteen minutes ago, I got around to taking the valves out to inspect them, and as it turns out, the exhaust valve is fouled to crap (I've suspected a compression problem for a while), along with the piston, combustion chamber, and exhaust port. The exhaust port has a layer of carbon 1 or 2 mm thick, crazy.

Cleaning out the carbon is no big deal, but the question remains, what's likely to have caused this? Is there anything I should do to prevent this happening in the future?

The fact that the valves look like that at bugger all km's suggest it at one time likely had zero valve clearance.
PS buy a new valve they are cheap enough