Cg125 engine rebuild woes.

[Ian616]

Hello everybody. I've not been on a Internet forum before and I'm new to mechanics so please have patience with me!
I replaced the cylinder barrel and piston on my 1990 honda cg 125 br-j following a seizure (a seeping oil leak and too many miles without checking the level).
I got the engine running again, but now the idle speed is ridiculously high and the throttle makes no difference to the revs. It only starts with the choke on, if the choke is opened once it has started the revs somehow manage to get even higher.
I don't know much about engines, but I know all these high revs aren't great for the new cylinder and piston!
I'm as sure as I can be that the carburettor isn't faulty.
Whilst stripping the engine I did make the foolish error of taking off the cam drive gear without first marking its position. I lined up the dot on the gear with the centre of the woodruff key slot but am not certain that this is the correct position.
Could this be causing the high revs and does anyone know what the correct position for the cam drive gear is?
I really don't fancy going down a possibly endless trail and error route!
My many thanks to all that can help!

Ian.

[delsol]

Either buy or download (free if you search enough, sorry cba searching for you) a Haynes manual.

For someone who is new to mechanics, you did not dip your toe in the water by starting with engine internals, good for you, but you need a workshop manual.

[Ian616]

thanks but I have got a PDF version and a hard copy of the Haynes manual, and I'm getting advice as I go from my dad who worked on his own bikes, but I stupidly went a bit off piste when I took the cam drive gear off. It does say in the manual to mark it's position before removal, but doesn't offer any information about what to do if you omitted to do so.

[tomh]

I'm sure there is a dot on each gear that you need to line up with each other.

However I don't think that's the problem, it sounds like you've put the throttle slide in the carb wrong.

It's very easy to get in the wrong position so it's not fully seated and can cause your problems. Remove and rotate until you feel it go further down, at this point there should be little resistance from the spring when screwing on the cap. Hope that makes sense.

[skatefreak]

I hate to mention it but is the engine burnt its oil and you replaced only the barrel and piston have you had a look at the bottom end?
Oil starvation would be very detrimental to the bottom end as well and there is a chance the crank bearings/gearbox bearings could have been damaged
This being said, it would be highly unlikely this would cause the issues you have mentioned and I agree this would be more carburettor linked.

Did you check the bottom end atall?

You need to find a manual and you will find there should be marks on the top end cam gear and usually the flywheel which should get your timing correctly set.

Once set, put a pencil or something that wont damage the piston/valves with the rocker cover off and turn the engine over by hand, you will be able to see when where the piston/valves are through the cycle and make sure they don't interfere.

Good on you for jumping in and getting your hands dirty .

Best regards

-Jvr

[N cee thirty]

cg is overhead valve with no timing gears to set up while doing a topend rebuild. you have either an air leak or the carb idle is too high
____________________
'00 Aprilia RS50 > '92 Honda CG > '99 Yamaha Fazer > '91 Yamaha RXS > '79 Suzuki X5 > 01' Honda Cg > 07' Honda Cg > 82' Kawasaki Z200 > suzuki gsxr 400 gk73a > honda vfr 400 NC30 Mod 2 Passed 09/06/2011

Jewlio Iglesias wrote: I actually did vote BNP once

[Ian616]

Thanks for the help guys.
Yes I checked bottom end for sideways movement and I split the crank case to look for signs of wear. It all looked alright apart from few bits of broken gudgeon circlip and some aluminium sludge from the barrel and piston which I cleaned out.
I've got the dots on the two cogs lined up, I just don't know if the smaller, bottom one is on the drive shaft in the right position as I removed it (without marking it) in the mistaken belief that I had to to split the crankcase.
I've got the engine out and partially dismantled at the minute so I'll have a look at valve/piston cycle and see if I can spot a problem. Then I'll put it all back together and have another fiddle with the throttle slide.
Thanks again for the help, I'll let you know how it goes.

[Ian616]

Air leak? I'll look into it. Cheers.

[Ian616]

...and she's running again!
Yep, throttle slide it was. Bloody thing! No matter, I'm back on the road again and am a happy bunny.
Thank you all again for your advise

Ian.