Running very hot after rebuild SL125

John,

I'm one of Oregon Coasts OSR Teammates and saw that you were asking if anyone had any ideas regarding your overheating after the rebuild. I did experience similar situation several decades ago, after rebuilding the top end on my 63 BSA Starfire C15, a small 250 cc single four stroke. It was a little low on power when first reassembled and when I took it out for a ride the exhaust pipe got super hot, almost red. I took it back apart and discovered I had the cam off by a couple teeth. I think it was allowing the exhaust valve to open too soon and thus bleeding off the exhaust gas before the combustion cycle was completed. As soon as the cam timing was corrected it was back to normal.

I quickly read through your whole thread, but need to ask, does it have as much power as it did before the rebuild? Is it improved or worse?

Hi, thanks very much for taking a look.

I can safely say that there is a massive difference in power. Much more powerful and while it's not at a stupidly high temp its better than it's ever been. I did notice a big difference when I changed to a larger main jet though, more powerful again.

John

The double image ignition issue is in a lot of the manuals.

Ignition versus carbs? It all depends. On bikes that have points and a running issue? Your nuts IMO not checking the points first. Other than that I have seen a lot more carb issues than ign. issues in my experience.

John, take a look here at my thread where I was fighting some run-abilty issues on my daughters SL100 (bike is very similar to your SL125 and was supposedly "bored out"), take special note of the carb postings, I think the needle orifice may be worth looking at. http://www.advrider.com/forums/showthread.php?t=698196&page=8

I'm also concerned about your timing chain tension, when you describe how tight it is now. May be worth double checking adjustment?

Take care

Bump

OK, first can I apologise for not updating the thread. To be honest, I've been extremely busy and not had much time to work on the SL.

Anyway, I've been constantly thinking about what could be the problem. I've had the engine out again and took the top end off. First thing I notices was more play in my camshaft. It's not stupidly bad atm but it is definitely worse than before. So it made me think that my new cam chain was too tight. I thought it was too tight when I put it on but it appeared to loosen a little. So I've put my old chain back on to see what it would be like.

Over the last 2 months or however long it's been I've been looking out for cheap cylinders to see if it was the rebore. I got 2, one was OS 0.75mm and the second was stock, 56mm still with cross hatchings on! The os one came with a piston too. I thought about putting in the stock size with my old piston to see what would happen but I decided to just try it with the old cam chain.

So, cam chain adjusted and battery charged it started first kick. I ran it around the garden for a bit and it seemed fine, plenty of power and the temp was around 180 fahrenheit. Went round the block and messed around on some soil heaps, opened it up a couple of times and took a temp reading after one fast acceleration. It came to 275 fahrenheit.

So, although I haven't ran it for as long I did before, it seems to be running cooler. I've had no signs of seizing so far but I'm not getting my hopes up just yet. I'm going to try running it for a good couple of hours and see what happens.

John

Basically yes, I've just put the old cam chain back in. I've kept the rebored cylinder and piston the same as before. I'm not completely sure that it is sorted but i have a feeling that the cam chain wasn't helping. The extra wear made be think that because the chain was too tight, there would be more friction in the head, contributing to the heat. If that is the problem, I'm not sure what I will do about a new cam chain as it could do with one really, but I'll see what happens.

I think I will also get a new set of points for peace of mind. The old ones are fairly worn and they're pretty cheap so might as well.

Had a good ride today. Some slow speed trials riding and it ran well. Then pushed it quite hard down a dirt track and seemed to go well. Highest temp recorded was 275-280 fahrenheit. I then did a stretch of straight road, not very long, kept it at around 45-50mph and seemed ok but I could tell it was still getting hot and throttle response got a bit worse. No seizures, recorded temp was 320. Clearly my problem isn't sorted yet but I think I will keep doing some trial riding on it and open it up from time to time and see how things go.

I do have a quesion however I hope someone can answer. Due to the slightly domed piston, my piston to head clearence has been reduced. Not hitting the valves obviously but if the space up there is really tight, what sort of symptoms could I get? I know it will increase compression but can it cause overheating?
How does putting an extra 1 or 2 head gaskets on and seeing how things work sound?

Thanks in advance

John

I would think if the piston & valves were hitting, you would hear it. Too high compression would first show up as pinging, and yes that would lead to overheating. 2 head gaskets is not a good way to go. They are too thick, and would eventually leak. Way better to double-up the base gaskets, or even machine up a sheet metal spacer for the base if required with a gasket on each side of it. Anything you do to raise the head/cylinder could increase tension on the cam chain, if your adjuster cannot handle it.
Are you seeing any signs of wearing on the cam bore in the head? These were notorious for wear.

Easy way to know if the cam is wearing out the machined bearings in the head - the seal behind the points will start leaking as the cam is not staying centred.

Thanks, I think I will try the base gasket idea when I next strip it down. My cam chain will have enough slack to take the thicker gasket so shouldn't be a problem.
I'm not getting even a drop from the cam oil seal so my wear can't be that bad so I think that's good for now.

Will keep you posted on any updates

John

Still trying to pinpoint the problem

My mind is going crazy thinking of all different things of what it could be.

So, I'm hoping someone can make head or tail of this.
After opening her up quite a bit, I found that she did eventually start to lose throttle response as expected so it's still overheating, >300 fahrenheit after almost WOT for a couple of minutes. I pulled the plug and it was sooty black.
This is where I am confused, if I'm getting a sooty plug, then surely the plug isn't getting hot enough to clean itself but how can this be if I am reading around 300 fahrenheit at the plug after a WOT run. I am running a number 9 plug instead of an 8 to try and prevent the overheating but it is clearly getting too hot and yet not hot enough to clean the plug. If the plug is black then surely the overheating isn't caused by ignition timing, is it? Urg BTW, it's not oily black, just sooty.

Also, I'm not hearing any knocking/pinging which I would have thought I would hear at high temps. This then makes me think again if its just the piston to bore clearance, even though I'm within factory spec. If the clearance was greater, then I don't think I would have this 'seizing' issue, but then again, the overheating problem may still not be solved... if there is one. ARGHHHHHHHHHH

Anybody have any opinions on this.

Thanks, John

Have you double checked the oilways?


Your problem could be reduced oil feed. Pump problem or blocked oilway? How is the filter? There are also oilways through the crank that can silt up over time because it acts like a centrifuge.

One thing I have seen is a base gasket that did not have the key-shaped cut out for the oilway that emerges from the crankcases next to the front left cylinder stud. It was a new gasket that had been manufactured incorrectly and it is a very obvious problem - but they are out there.

I'm definitely getting oil to the head. I've cracked off the back right head nut and had oil coming out through there. I also took the timing plate and cam sprocket etc off and checked the oil hole at the end of the cam. Plenty was coming from there.

I've never checked, or even knew that there was an oil feed through the crank, I though it just got thrown up below the piston from the sump.

If some of the hidden oil ways need cleaning what would be the best to do? I've heard that Sea Foam is very good at doing this. Anyone have any experience at cleaning out oil passages on a bike?

Cheers,

John

Major improvements chaps.
I know a lot of people are going to think I told you so but I just didn't expect it to be this and I still don't know why it is the source of the problems.

So I bought a new set of points which didn't fit immediately so I had to file down the cam follower and make one of the screw holes a little bigger. When I ran it I noticed heaps more power and did a fairly gentle ride and the hottest it got, when I did some WOT acceleration runs was 260 fahreneheit. I then went for a long run basically wide open all the time and hottest it got was 340 but was cooling down whilst I was taking the temperature (still running). It never would have done that before. Also, there was never a drop off in performance throughout the entire ride.

So it seems that the problem all this time was the damn points that were on it. Very frustrating but I'm over the moon I've found the source of the problem.

I still have to set the timing on it more accurately but I'm having a couple of problems with this but I'll create another thread on this.

I want to thank everyone who helped me with this. I know the bike much better now, it runs better than ever before, at least since I've had it, and all that I've done to it means that it it pretty much perfect now and I've sorted out potential problems that could have affected it's reliability down the road.

Thanks once again everyone.

How is it running now? I had the exact same problem; trying to put the 124cc top end in the sl 125's 122cc original cylinder. It never worked and I did all of the same things you did with the same results. The compression was too high for the piston head combination. It always sized when it got warmed up. If you used the two piece Honda head, it might have been ok. I went to a stock 122 cc cylinder bored .50 and it runs perfect all day long.