1972 Honda SL125 - won't rev above 2000rpm

[Colin Jay]

A question for the Honda 4 stroke gurus.

I have just about finished the restoration of a 1972 Honda SL125, and I started the engine for the first time yesterday, and it won't run above 2000rpm. It will idle perfectly at 1200rpm and will take a small amount of throttle and runs quite smoothly until it get to about 2000rpm them it just coughs and farts and will die completely if you try to hold the throttle open. If you close the throttle, it will drop back to 1200rpm and sit and idle perfectly for as long as I care to leave it running.

The engine, while not fully rebuilt, had the cylinder honed and new piston and rings fitted. A new exhaust vale was fitted and the valves re seated. The carbie was ultrasonically cleaned and fitted with a carbie kit. The bike is fitted with a new battery that is fully charged (6.2V). As the fuel tank is still at the spray painters, I am running the bike using a small dummy tank (modified lawnmower tank) that I have been using for the last 35 years to bench run and tune my bikes. Fuel is normal unleaded bought on Thursday.

I have double both checked the cam timing and the ignition timing, and both are good (points gap is 0.014"and timed with a timing light).

I have rechecked / reset the valve clearances (0.002"IN and EX)

I have blown through all the air passages in the carbie with compressed air and they all appear to be clear.

I have tried replacing the new jets from the carbie kit with the old ones, with no improvement. (carbie now has the new jets reinstalled)

I have tried the carbie off of my TL125, but got the same result, won't rev above 2000rpm.

I have tried another ignition coil and condenser, even though the ones fitted tested ok, and the engine still won't rev past 2000rpm.

I have tried several new spark plugs (NGK D8EA) with no improvement.


The only problem that I know of is that the rectifier is faulty so the battery is not being charged by the alternator, but I can't see this being the cause of the problem as the ignition runs from the battery, and the battery voltage has been above 6V ever time I have run the engine.



Some, please put me out of my misery and tell me what I have missed.

CJ

[Gears]

Is there a strainer in the fuel tap that might be blocked?

[Colin Jay]

Thanks for the suggestion, but No the bike is being run from a test tank that has no strainers or filters and has a good flow rate.

[smed]

Is the mechanical advance throwing out?

[Tomas]

Well it may just be the maximum rpm your motor will do. It is a 4stroke. They dont rev  hard   

[mick25]

Have you changed the air box or taken the lid off , I took the lid off one of my bikes once and it didn't like it got to much air and wouldn't rev out it needed more fuel bigger jets.
Or have you left a rag in the air box and its sucked into the boot .

[TT5 Matt]

hi Slob
try using the old points as the new points could be suffering point bounce [had that happen on 2 victa mower after replacing the points], also check the flywheel key for being sheared as if your using that for cam timing/ignition timing everything will be out, if not its back to basics and check the exhaust/airbox pipe for blockage

[cz400]

cams not flogged out in the head

[Tim754]

Have had a similar problem with one of my (modified) SL125's years ago. It did rev to about 4000 though. Turned out to be (after much investigation and copious verbal abuse) to be a broken exhaust valve spring. yeah weird . Tim754

Of course Col you could just give up and send the SL to me to love and cherish   

[vmxrider]

Compression test?

[jimson]

Is the pin in the cam shaft where the points are ? Had the same problem with a honda CB350 some years ago. Jimson

[ty4]

flat battery

[OverTheHill]

Is the mechanical advance throwing out?

My first thoughts above so i'll second it. I'll just add though--is the valve timing pretty close to the mark?. If a tooth either way it'd still get past 2 grand though. Mark the alternator rotor with some white twink then strobe it running to see if it's advancing up. Might have to lean it away to stop getting a face full of oil while strobing it. Video with sound of it running & doing what it's doing is always a good idea if you can organize it. Thanks.
ps, gotta tell ya--assembled the first one around here & took it to a farmers field day to demo it & they'd marked a track out through a forest bit. Thrashed the living daylights out of it all day & she survived unscathed. In more recent years [still probably 25-30 years ago] we used those motors in our early bucket racers with megacycle cams & high comp pistons out of something else & apart from locking up bigends when the cage broke from old age mostly, they were bullet proof. Sorry--that's not helping eh!!.

[Slakewell]

With the timing advance there is a fiber washer that sits behind it. If it's not there it can cause it to cut out as revs rise. 

[Colin Jay]

Hi all,

Thanks for the tips.

This afternoon when I got home from work I spend 1 1/2hrs trying some of the suggestions.

Cam and cam bearings in the head are all ok, this was one of the first things that I checked before I even considered restoring the poor little think.

Valve springs were all ok when I refitted the valves, and since it hasn't really run over 2000rpm I don't think a broken spring is the issue. However, I will check this if it get to the stage of having to pull the engine down.

These little Honda engines don't use a key to locate the timing sprocket on the crank, the sprocket is heat shrunk onto the crankshaft.

Fiber washer is present on both the advance units I have (both are stamped 107 which I am sure is the correct advance unit). Both advance units had been cleaned and re-lubed both and they are assembled correctly and move freely (can be put together 180 deg out, but engines generally don't run at all if they are).

The locating pin for the advance unit on the cam is in good order and no signs of excessive movement when the advance unit is slipped over it.

I have tried another set points, fitted to another plate assy, points cleaned, gaped and timed correctly.

I have tried running it both with and without the air cleaner connection boot fitted and there was no difference was between either option.

I have been unable to do a compression check as I don't have a small enough adaptor for my compression tester to fit the 12mm spark plug.

SOME PROGRESS HAS BEEN MADE

I took the muffler off and ran it with just the header pipe, with a bit of coaxing able to get it to rev all the way to 9000rpm momentarily. (Tomas, these little 4 strokes rev higher (9500rpm redline) than a lot of their smoke belching, plug fouling  2 strokes contemporaries from the 1970's).   I reconnected the air cleaner, and was back to not reving over 2000rpm. I then removed the air cleaner connection and it was back square one and refused to reving out again. I have tried another exhaust, with a bit of a muffler on it in the hope that a bit of back pressure might help, but it wouldn't rev with this pipe on. Bike is very hard to start with out the muffler and will not idle very well and generally stalls after a few seconds.

It looks like it is a combination of both carbie and exhaust issues. I will carry on with more investigation tomorrow.

@Tim; No you can't have it! I am a stubborn old pr#%k, and I am not going to let this bike get the better than me. There were days when I would have consigned the bike to the "Spar-lash Locker"(i.e. thrown it into the dam), but those day are now past, and beside there are a great little bike WHEN THEY ARE RUNNING!!

CJ