Topic: diagnosing oil burning of a torn down engine (cb650)

[davesee]

i rebuilt the top end of my cb650 about two years ago because it was burning oil. i replaced the valve seals and rings, had the cylinders and pistons scoped by a machine shop (cylinders honed as well). when i brought the cylinders and pistons to the shop. the guy said that there was very little evidence of blowby, for whatever that was worth. i was as careful as a first timer could be when i did the rebuild, and i had a some experienced help.

last year about 15 000 km's (9 000 miles) later, the bike began to smoke again, so i've torn into it again. my dilemma is whether or not to replace the rings again, and whether or not to have the cylinders honed again (there is still pretty good evidence of it's last hone).

what does evidence of blowby look like?

are there any other tell tale signs to look for that the rings might need to be replaced?

advice and tips are appreciated...here is a picture of one of the pistons.

[ekpent]

Can you see from the head and valves if there is one cylinder or more that has unsual deposits that looks like it is burning more oil than others. May not be all 4 bad for instance.

[HondaMan]

It's probably NOT from the rings. The rings outlast the valve guides on all of the post-1975 SOHC4 engines. Check the clearance on the valve stems, particularly the exhaust side. Almost 80% of the post-1975 engines I see have worn exhaust valve guides, and about 50% also have worn intake guides.

The 650 guides are still available from Honda (because they also fit some smaller single-cylinder bikes). If yours are worn, you'll need a valve job with new guides. The clearance should be no more than 0.0032" side-to-side valve wiggle on the CB650, when the valve is lifted up 6mm from the seat.

[davesee]

none of the cylinders look particularly clean, but #4 and #2 are especially gunky, especially for a bike that was rebuilt a year and a half ago.

i tested the valve guides for wiggle as per yer suggestion, hondaman. i didn't feel it was necessary to get the micrometer out, because there was a pretty obvious wiggle in some of them, and one of the guides is scored pretty badly. also, as you can tell from the picture, there are huge oily deposits on the backside of the valves. again, i had replaced the valve seals and cleaned the valves thoroughly a year and a half ago. i am assuming that the guides need to be replaced, so i'm on the lookout for a replacement head before i visit any machine shops.

[dave500]

im with honda man and say guides,if after a while of traffic stop start riding or a long down hill if it starts smoking it;ll be guides,if its not really blowing any smoke once your underway the bottom end will be ok.

[davesee]

those are my smoking symptoms exactly. oddly, the smoking stopped for around 8000km's (5 000 miles) after my initial rebuild. i guess fresh valve seals we keeping the smoking at bay.

[dave500]

yep they can only be new for so long used on worn guides,as they get pushed over with the valve stem slop theyll let in oil and then smoke happens,riding country miles it wont smoke so much but will still use more than normal oil,its getting burned as you go along,with slow traffic riding it builds up and shows as smoke.

[Fritz]

i am assuming that the guides need to be replaced, so i'm on the lookout for a replacement head before i visit any machine shops.

I think you should listen to Hondaman. Any used repleacement that you can buy might have the same or even worse problems, so I'd rather have the head serviced and just enjoy the next smoke free 100.000 miles or so

[HondaMan]

Sadly, the situation we face today is due to the introduction of [only] non-leaded fuels AFTER these bikes were out there, beginning in 1984. The guides in particular suffer from it, as they were designed to be used with leaded fuels, which specifically lube the guides and top ring on the piston(s). So, after the engines were broken in with the 'right stuff', the 'stuff' disappeared and the wear cycles increased a lot.

I started using top oil in [almost] every fillup, and after 140k miles my guides are just approaching a worn state, not at max yet. And, it doesn't burn oil yet, either. But, the 750K0-K3/500-4 and 550k1-3 had the Stellite guides, which fare much better than the later cast iron types. Starting in 1976, all of the guides are cast iron, and during the years of MTBE additive in our gas, these guides were eaten by the acidity of that additive. I haven't seen a 650-4 or 550F head since 2000 with good guides in the exhaust side, and most of the 750F bikes are in the same boat. 

Modern bronze guides will help a lot, and I've also worked out a way for a local machine shop to install bronze liners instead on the bigger 750 heads.

This is, if  I can get my turn 'in line' in less than 10 weeks to get the heads actually done...

[davesee]

i think the wait time at my local machinist is also a factor, and so is the 300$ for getting it done.

i found a low mileage engine for sale locally and for pretty cheap, so i'm going to take the head from that, put new valve seals in it and lap the valves, and put it on my engine. at least i'll have a ride for the start of the season.

what's top oil?

[HondaMan]

i think the wait time at my local machinist is also a factor, and so is the 300$ for getting it done.

i found a low mileage engine for sale locally and for pretty cheap, so i'm going to take the head from that, put new valve seals in it and lap the valves, and put it on my engine. at least i'll have a ride for the start of the season.

what's top oil?

A very light oil, like 5w or 10w oil. I use Marvel Mystery Oil, which has some detergent in it to help keep the valves and carbs cleaner, too. You can also use plain old diesel fuel (stinks), so long as it is not bio-diesel, but it takes more. I use about 1 ounce in a fillup.