Running very hot after rebuild SL125

Hi, please excuse my ignorance, i'm on a learning curve here
After having a my cylinder rebored, a new piston, cam chain, guide and tensioner my 73 Honda SL125 fired up and sounded the best it has ever been.
The piston is now 56.5mm over the original 56mm and is a piston from a CB/CG/XL 125 with the slightly domed top which increases the compression from 8.0:1 to 9.0:1.
The cam chain felt much tighter when I put it on that the one that was one. (The tensioner is on the slackest that is goes) Is this normal?

After doing a few heat cycles (warming it up, just in the garden, and then letting it cool) I took it for a little ride. (Only around 1-2 miles I saw that the plug looked like it was getting very hot. There was also a tapping noise under acceleration, which sounds like the valves. I checked the clearance and adjusted them to 0.05mm when I was assembling.
The stock carb setting pilot screw is 1 1/2 to 1 3/4 turns out from closed and to get a brown plug it has to be 1 turn out. Gives you an idea of how hot it is.

Taking it easy I went out again today and its still the same. 2 times there was a horrible noise and a loss in power that felt like it was seizing up :eek1 but can't be sure. I pulled the clutch in immediately and let is sit for a while to cool down. It started up fine both times and carried on fine.

What could be causing this kind of heat and tapping? Could they be linked?

Thanks very much if anyone can help.

John

EDIT
I have now done around 15 miles in total on it and its still the same

It isn't unusual for freshly rebuilt engines to run hotter, initially. There is greater friction as new parts seat, and with an overbore there is more surface area to cool. You can use a hand-held infrared thermometer to get actual measurements of engine temp.
The tapping noise under acceleration could be valves, but could also be bearing noise - not good. How about pinging? Are you using premium gas with your increased compression?
Are you certain that the timing is right on? Too advanced timing can also lead to heat issues.
A brown plug doesn't necessarily indicate high heat but a photo would be helpful. Better to run a little on the rich side during breakin to help with engine cooling (at least 1 1/2 turns of the adjustment screw).
Frequent oil changes are important right now and use a relatively light weight oil.
I don't know factory specs on this engine but are the tappets normally set that tight?
Hopefully, someone else will help you with your question on the cam chain tensioner.

Don't forget to check for an air leak between carb and cylinder- might be causing lean/hot condition.

If it made a horrible noise AND felt like it was seizing up, it probably was. Is the oiling system functioning? Did you measure the piston ring end gaps to 100% VERIFY it was correct, rather than just relying on the machine shop to do it? When you say the plug looked hot, you mean the coloration was whitish? Can't rely on that, unleaded fuel doesn't really color a plug, certainly not in a short time. Did you shoot the cylinder head with a IR gun? The cylinder?

did you check for oil to the top end after rebuild? Loosen the right rear head nut when engine is running and verify that oil is coming up the stud. The oil slinger (located on the right hand end of the crankshaft) can become clogged with metal particles reducing oil to the top end continued use will ruin the rockers, camshaft and the head. And the oil pump gears may need replaced on that type of engine. The gears get metal run through the pump ruining the gears and the pump housing.

I use regular unleaded with a valve protector additive (as it should run on leaded fuel), which I have never had a problem before with. In the UK, I'm pretty sure its 95 octane.
I will double check the timing, but I am pretty sure it is right. I am running it very rich at the moment. Stock is 1/1/2 - 1 3/4 turns out from fully closed and I am running it at 1 turn out (further in is richer).
The manual says tappet clearance is 0.05mm so I'm assuming that is right.

valve clearance is .002 in intake and .003 inch exhaust.

I shall check that but should be ok

I didn't check the ring end gap :huh, stupid me. The plug did look whitish, I will check the temp with my IR gun and will let you know.

I replaced the kickstart spring whilst I had the top end off which meant I had to split the bottom end. Whilst in there I gave everything a clean, the slinger and oil filter. I will do what you said to see if there is oil getting to the head but I have pretty sure it is.

When I put the new piston in it did feel much tighter than the old one and when I kicked it over I head much more rubbing that the old one but maybe that's just me. I haven't had to run in a bike before.

Thanks for the help.

Thanks sporthog, if my exhaust valve is set too tight can this cause the tapping?

If these measurements are right then I had better change it. In my manual it says both at 0.05mm (0.002 inches)

John

Go wider on the exhaust valve clearance than the intake. .002 and .003 sounds good. And be sure to run some type of lead substitute in the old SL. I used to use CD-2. A little dab'll do you.

I have just thought of something, not sure if its right but hey. Would the rebore (+0.5mm +2cc) mean the main jet needs to be changed? Compression has also increased but I'm not sure by how much.

ZDDP is what I use, and the Diesel Rotella has increased amounts of zinc and phosphorous added - sold at your friendly Walmart.

Sounds to me like the dude who bored it didn't give it enough clearance. My other sneaky suspicion is the piston is too high a dome for the valve train and it is hitting valves after it gets hot and expands.

You could be right about the dome being too high but it is just a standard piston for the larger 124cc bikes so I'm not sure if it would hit it. Also, the tapping noise starts from cold and doesn't change, it doesn't get worse as it warms up.

Just a guess, since you effectively have a larger displacement with a higher compression ratio, you stock jetting should be lean. Therefore increase the fuel supply (larger jets) to get the air/fuel ratio back in balance.
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If the engine feels that it is starting to seize, then I would be very worried. If you can confirm you have good oil pressure, then my next guess would be the rings. If the ring gap was too tight, then as the motor warms up, the gaps will close 100% and expand larger than the bore. Hope that is not the case as it will require tearing the motor down and high chance of scoring the cylinder walls.

I have a CB125 which has been rebored to 149cc with a powroll.

Stock jetting is fine, you don't need to rejet.

It did generate a lot more heat initially but it has setted down now after about 100 miles.

My Honda factory service manual (actually it's an SL engine in the CB frame) say 0.05 on inlet and exhaust valves, although I do have a manual that says -.02-.03. I run 0.05

If it is clattering and overheating it is a more serious problem than jetting and valve clearance. I can't offer an insight, sorry, although it sounds more like an oil feed problem or perhaps the piston enlarging and hitting the valves. Although I know someone who does that conversion using the CG piston and has had no trouble.

It's pretty simple to pull the cam on those - pull it and check the right hand side bearing for wear. If you've had oil starvation it will show there as the cam runs in the head alloy rather than a dedicated bearing.

PM me your email address and I can send you the service manual on PDF - it covers all the OHC 125 series.

I may have found the source of my peril...
I've been researching jetting and carb stuff and I never realised that the top of the main jet comes off and the long part is only the holder for it .

I had a look for a number on it and from what I can tell, it is a 92 main jet. :huh My manual says 105 stock main so if this is actually is a 92 then I am way off.

When I was riding, I was not going to full throttle on it but it was still getting hot. Would the wrong main jet still affect it at a lower rpm? The slow jet is stock at 38.

Here's a pic of it.

I am also going to replace the intake spacer as when I was trying to get it off, I snapped some of it off. It was well and truely stuck. I did notice however that there was a slightly darker area, touching the intake boot, of dirty metal on the cylinder head. Maybe indicating a leak, but not too sure on that one.

Didn't you say it was .5mm larger in diameter? Was it bored or just honed?

You are on the main (partially) as soon as you get off idle. In a sense, a carb is on the main (via the needle) after you touch the throttle and start getting on the gas, not just at WFO which then is all main jet.