Honda CG choking at high revs

[8nisd10a]

So it's a bit of a story what has happened I will try to summarise briefly.

Grinding noise in engine, took it to a mechanic he told me the problem is in the bottom end. I stripped the engine down completely and put it back together, put it in the bike but it didn't run. Had a look at the cam gear and realised that somehow I had set the timing wrong (I don't know how I did this after putting the whole engine back together correctly!). The inlet pushrod bent and there was a black mark on the piston from the valve hitting it but the cam gear looks undamaged and crankshaft is still silky smooth. I did a compression test and found with a bit of oil the compression jumped from 110 to 125 so I changed the piston. Fired it up and it sounded good.

Jumped on and rode it it around but as soon as I open it up at all it chokes. It doesn't stall but it won't produce any more power. It feels a lot like a problem I had doing high altitude driving and just like then the spark plug gets black very quickly.

I have done everything I can think of to diagnose and fix it. Ran it without air filter in, cleaned the carb, put a new spark plug in, changed the fuel, adjusted the carb floats, changed the exhaust gasket, nothing makes a blind bit of difference, it just can't handle any kind of load on the engine. I will change the head gasket but I don't see how it could cause this problem.

Any ideas? I'm really stuck as to what to do next but I love the bike!

[jaffa90]

If you are happy with the valve timing what about the ignition timing and the advance and retard mechanism?

[8nisd10a]

Sorry I should have said at the top, the bike is a 2007 model, it doesn't have a contact breaker the ignition timing is set using magnets I believe, but it isn't adjustable and is set by the flywheel position which fits on a keyed shaft, so I don't think this would have changed. Also surely it would not start at all if there was a problem with the ignition timing?

[jaffa90]

[BusterGonads]

It sounds like it might be a carburation issue (poor running, sooty plug) but also, I would be concerned about the valve that hit the piston. Is it bent? Is it seating correctly. If the stem is out of true it may well seat for a low speed compression test, but fai to do so at six thousand revs. These engines are reputed to start misbehaving when the tappets are out even a small amount so a possible bent valve should be investigated maybe.

If you did anything at all to the fueling system (filter, and its box) undo it.

I've got one of these CGs and am interested in the causes of failures. What happened to cause the grinding in the bottom end? Was it a lubrication failure, lack of maintenance, over revving or just very high mileage? These motors can run to very high mileages if given even a little care. There was a commuter guy on Facebook who told me he'd sold one in perfect working order after doing over 80,000 miles on it and that he had another one which was perfect and had done 62,000 miles. Neither of these bikes had had any internal engine work. Only normal maintenance and thousand mile oil changes.

[8nisd10a]

Hi thanks for the reply. The valve really does look fine to me it doesn't, look bent at all and I did regrind it pulling it with a rubber hose and a drill with grinding paste in to the seat, but I suppose it could be a tiny bit bent, perhaps I will try to replace it. I was thinking the only other thing that could affect the timing is if the pinion on the crankshaft had shifted from the impact, I can't remember how it is located on the shaft and I don't want to take the engine out of the frame again if I don't have to.

The bike is great it has always been highly reliable I used to commute to work in London every day on it then I decided to take it for a longer ride and it just seized up when I changed up to top gear. The oil looked a little bit creamy I think the oil cap nay have allowed some water to enter. Very hard to tell but I think in hindsight I should have just flushed the engine through with oil. I didn't find any obviously damaged parts in the engine but there was some damage to the inside of the case right at the crankshaft pinion.

I am wondering about the air breather. I used oil in the cylinder several times to check compression. Is it possible for a blocked breather filter to cause this problem?

[8nisd10a]

By air breather I think What I mean is actually called the air valve. It comes from the rocker cover.

[Robby]

Well it sounds like this was your first rebuild, and you don't know what you're doing, so it could be lots of things.

Its running, but not making power. You didn't mention what the compression was after the piston replacement, or if you honed or replaced the barrel.

So, things I would be suspicious of:
1. Air leaks, inlet side.
2. Head gasket not sealing - gasket surface wasn't clean enough, head not torqued down, re-used gasket, knackered mating surface.
3. Head damage - bent valve, bent/broken valve guide, valve seat problem.
4. Fuelling - carb filthy, full of water, generally blocked.

That's without thinking about the bottom end. You didn't mention what you did to stop the knock, like getting a recon crank.

[8nisd10a]

Compression before changing piston was 110 or with a bit of oil closer to 125. After is 125.

Just changed the head gasket and it didn't make a difference. Then I ran it with the choke half on and it immediately revved higher and I rode it around and it zipped along pretty good. Then I noticed smoke coming from the weld in the exhaust. I guess this is my problem, the exhaust must have got damaged while moving the bike some when. Hopefully a new exhaust is all it needs, but why does half choking it make it run better? Is it because it can't create as much pressure and draw as much air through the engine if there is a hole in the exhaust?

[Nobby the Bastard]

You replaced the piston but not the valve after a timing fuck up?

I'd be looking at doing it the other way round.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[8nisd10a]

Put the valve in a drill gave it a spin its not bent at all, reseated them anyway

[Robby]