Running very hot after rebuild SL125

[MrBob]

I'm not sure what you mean when you say the engine begins to "feel tight" but some type of seizing isn't hard to imagine.
I wonder if a piston change would be worth it to bring peace of mind.
I don't know what stock clearances are on your engine but I recall the numbers you're working with being questioned by several posters.
Back in the day I can clearly remember getting stuck on stuff like this and I remember the feelings of relief and confidence when I figured out the fix.
How much are you spending on gaskets?

[Bendernz]

From the factory manual:

The standard clearance between the piston and the cylinder should be 0.01 - 0.05mm (0.0004 - 0.0020in) at the piston skirt.

[Sniper X]

Did you measure and set the proper ring gap?

[concours]

Originally Posted by Bendernz

From the factory manual:

The standard clearance between the piston and the cylinder should be 0.01 - 0.05mm (0.0004 - 0.0020in) at the piston skirt.

I can't imagine it, but factory book trumps. To the OP, is that a Wiseco piston? Or another aftermarket brand? Forged? We have issues with them in 2-strokes, OEM uses cast piston, aftermarket forged pistons have a greater expansion rate, and require ADDITIONAL clearance (read that rattly sloppy when cold) to live.

[Ernest T]

I think you need measurements to figure out what's wrong. From my Clymer manual:

Standard bore with standard piston is 2.2047-2.2051
Piston diameter 2.2035-2.2043
Piston ring gap
top and second 0.006-0.014
oil 0.0118-0.0354

You need to add .5 mm or 0.019685 inches to compensate for your overbore.

[john479]

Originally Posted by MrBob

I'm not sure what you mean when you say the engine begins to "feel tight" but some type of seizing isn't hard to imagine.
I wonder if a piston change would be worth it to bring peace of mind.
I don't know what stock clearances are on your engine but I recall the numbers you're working with being questioned by several posters.
Back in the day I can clearly remember getting stuck on stuff like this and I remember the feelings of relief and confidence when I figured out the fix.
How much are you spending on gaskets?

Originally Posted by Sniper X

Did you measure and set the proper ring gap?

Originally Posted by concours

I can't imagine it, but factory book trumps. To the OP, is that a Wiseco piston? Or another aftermarket brand? Forged? We have issues with them in 2-strokes, OEM uses cast piston, aftermarket forged pistons have a greater expansion rate, and require ADDITIONAL clearance (read that rattly sloppy when cold) to live.

When I say feeling tight, I just mean I lose power and thottle response gets less. From what I can deduce it is the piston to wall clearance getting smaller and smaller until the skirt is rubbing. As for my clearances, I'm just going by what the manual says. I've also checked against new Honda CB manuals and they're clearances are the same.

The ring end gap I measured and they were within spec, although I think it was the second ring was right on the lower limit.

The piston is Mitaka. From what I've heard and read about them I've found them to be a good quality jap piston, I'm pretty sure they are cast unless someone else can correct me? As Mr Bob said about trying another piston, I could get an oversize Honda piston and see what happens with that. I agree that the clearance seems tight and that adding extra clearance probably would help but would that cure the problem? I don't want to take more off if its going to be out of spec. I'm just going by what the manual says.

Thanks for the replies everyone, much appreciated.

[Al Tuna]

Have you checked for restrictions in the exhaust system?

[john479]

Originally Posted by Al Tuna

Have you checked for restrictions in the exhaust system?

Yes, nothing in there at all. Its not a modified exhaust too, just bone stock.

[MrBob]

Originally Posted by john479

Yes, nothing in there at all. Its not a modified exhaust too, just bone stock.

What's your next move?

[john479]

Originally Posted by MrBob

What's your next move?

Well, I think I'm going to look out for a NOS/used genuine Honda piston, the right diameter of course. Don't want to spend a lot if its not this but its all I can think of right now. Don't know what it could be, clearances are right, hone is good, oil to head is good.

One quick question, how much play should there be in the camshaft to bushing? I can move mine up and down a bit and can fit in a 0.05mm (0.001968 inches) feeler guage in and after that there is no play, so I guess I have about that much clearance. Can't see that this small amount of play would affect timing too much, especially with the cam chain keeping it pulled down. Just curious though.

John

[MrBob]

Originally Posted by john479

Well, I think I'm going to look out for a NOS/used genuine Honda piston, the right diameter of course.

One quick question, how much play should there be in the camshaft to bushing?

The NOS Honda piston seems like the least intrusive thing to do. Now that you know the specs of your cylinder you should be able to match them to another piston.
One of the experts around here should be able to address your question about camshaft clearance.

[john479]

Something else I thought of:

I'm running 10W40 semi-synthetic oil in the bike. Used to be 10W30 but went up to 40 after I had a piston blow from detonation on a very hot day and never had a problem since (I checked the timing obviously after). Anyway, could this thicker oil have an effect on it? Its pretty cold at the moment (seems to hang around 40 degrees)

John

[MrBob]

Originally Posted by john479

Something else I thought of:

I'm running 10W40 semi-synthetic oil in the bike. Used to be 10W30 but went up to 40 after I had a piston blow from detonation on a very hot day and never had a problem since (I checked the timing obviously after). Anyway, could this thicker oil have an effect on it? Its pretty cold at the moment (seems to hang around 40 degrees)

John

Typically, you're told not to use synthetic oils in newly rebuilt engines until they've been run for several thousand miles because the superior lubrication of synthetics interferes with proper seating of parts. Not sure how this applies to semi-synthetics. I'm still running dino oil in my rebuilt Volvo engine until I reach 3,000 miles on the rebuild, per instructions of the rebuilder.
I would definitely use the 10/30 for now, again, to help parts seat and with your reduced clearances. I was instructed to do my first oil change at 250 miles, then 500 miles, then 1500 miles, and so on. YMMV.
You understand that oil questions often set off shitstorms of opinion and I like to look here for factual information:
http://www.bobistheoilguy.com/

[concours]

Originally Posted by john479

Something else I thought of:

I'm running 10W40 semi-synthetic oil in the bike. Used to be 10W30 but went up to 40 after I had a piston blow from detonation on a very hot day and never had a problem since (I checked the timing obviously after). Anyway, could this thicker oil have an effect on it? Its pretty cold at the moment (seems to hang around 40 degrees)

John

The "piston blow from detonation" needs more explaination, need to figure out WHY that failure happened.... to a usually bulletproof engine design. There is more going on here....

[john479]

Originally Posted by MrBob

You understand that oil questions often set off shitstorms of opinion and I like to look here for factual information: http://www.bobistheoilguy.com/

Yeah, I've seen a fair few forums getting rather tense when talking about oil. Thanks for the info and link.

Originally Posted by concours

The "piston blow from detonation" needs more explaination, need to figure out WHY that failure happened.... to a usually bulletproof engine design. There is more going on here....

I had another thread on here about the blown piston but something I must say first. I started the thread saying it was my second blown piston. I did have the first one blow due to detonation after I was running at about 50mph at around 7k rpm for about 5 mins. It was a very very hot day. I suddenly lost all power and there was a masses of vapour coming out of the crankcase blower pipe. Got it home and found the piston like it was in the pictures. The second blow out that I refer to wasn't actually a piston problem. The choke flap had broken off inside the carb and had got sucked inside. Luckily I pulled the choke in immediately I head something and only the piston was slightly marked because of it. Nothing else.
I think the reason it blew was because of timing not set properly and as I found out recently, the main jet was very lean (92 instead of 105). I rode for miles after this and never had a problem apart from the one below...

Link: http://www.advrider.com/forums/showthread.php?t=702642

Before the rebore, I had this problem where if I was riding along and I came to a hill, the bike would start to lose power and I would get a lot of blowby through the crankcase breather. I assumed it was because it was working hard, more heat was generated and because it needed a rebore, gases were getting by when the cylinder expanded. This happened also after prolonged riding on any road really where I would have to work the engine quite hard. I remember on a couple of runs I had to stop a couple of times to let it cool down because I lost so much power. However this was different to my new problem as there was no seizing, I was down on compression (I could feel the difference when kicking it over)