[Cschapin]

I have an 82 Cb900f that i got running after it sat for 15 years. Got it running and everything works no major leaks but it burns oil on start up and under throttle. I’ve been told that it could be a number of things from timing being off to rebuild the entire engine. One issue on my motor is the front cam timing chain adjustment bolt is stripped and i would need to fix it or replace the head if i rebuild it. I now have the opportunity to buy an 81 Cb900f engine not burning oil and was dyno tested with 20k miles. Would this be an exact swap lining up and are there any differences internally like valve sizes or timing or anything? Thanks for any information.

 

[Tobacco Road]

You've probably bought someone's old bike of which someone lost interest in years ago, and probably at a rock bottom price. Seemed like a deal at first. Now, you're expecting to fire it up, have it run great and ride it for 50K miles. Not going to happen..That oil burning could be caused by many things right now and by other things that would take time to develop. Seals dry out and leak, and everything just gets slowly out of whack the longer it sits.

Personally I wouldn't feel comfortable riding it further than around the block. If you like the bike, tear it down and go over everything. Do it right or get rid of it. Sure you may get it running but ya just can never tell when it will decide to take a crap some day.

 

[Retired Guy]

If it's smoking at start up, after sitting a while, I'd suspect oil is seeping past your valve stem seals if they're like the old Chevrolet V8's. As long as the smoking stopped after a few seconds, I didn't worry about it in my old cars, but I kept an eye on the oil level.
But that's just a guess.

 

[Cschapin]

Thank you both for your replies. I do like the bike and plan on doing it right i just wonder if it would be a quick fix for right now to swap out an 81 engine that only has 20k miles vs my 50k bike that i did get for a rock bottom price of $300. I knew i needed to rebuild it but have never worked on bikes before now. I did rebuild the carbs brakes Petcock and gone through my electrics everything now works. I just haven’t rebuilt an engine ever.

 

[Retired Guy]

The '81 engine might bolt right up, but you might, because of some kind of design change, have to use parts from your current engine to make it work.

 

[Unkle Krusty]

The clue for me is it has been sitting for 15 years. I have a mint low mileage 1980 XS1100. When I finally ride it, there will be some smoke. The reason is, it has been sitting for more than 15 years, and the compression is low. The rings are not doing what they normally do. Check the compression is the first thing to do. I am sure it will be down a bit, the same as my bike. Ride it gently for a few months, and check the compression often.

The manual will probably tell you what it should be. Mine should be about 150 pounds, and I think I am at 125. As a comparison a bike with 10.5 to 1 compression will be around 180 or more. My 41 Ford tractor is 110, and will still run at 90.

Usually when an OHV engine burns oil, it is the valve guide seals, but you do not have them. Without checking you have an OHC or DOHC.

With the older bikes it is often easier to just buy a complete bike as a spare. I have spare engines for my XS1100, not the mint one, and a spare engine for my XS400.

UK

 

[Tobacco Road]

On V8 engines that have sat for a long time I would drop the oil pan, clean out any and all sludge, the oil pickup screen and all related items. Also looking for any bearing or crankshaft damage or rust. Rings can be stuck and not working properly.

If you have not changed oil and filter I would warm it up and at least change oil. etc...Oil passages can get clogged and burn a bearing. All things are possible with any engine that sat for many years. Seals and such are the smallest part of the problem, but could be the only part of the problem. Ya never know.

Before the internet we talked to old guys with experience who knew something about engines, someone like Uncle Krusty. There were books and then there was taking it apart and see what you find. The best way to learn.... And then you may have a bike that sat long term that has no problems, I've see it both ways...

 

[Jmacd62]

Don’t know.
I would.
Look at the specs for 81 and 82 bikes. See if any noticeable differences.
eg. If the sales pitch says increased power, something is different.
Check the serial no on engines match to years. To confirm both years. Honda should be able to tell you.
Honda should be able to tell if there were any significant differences.

Manufacturers often use the same engines on different platforms with slight differences in casing or other components. Also small things change from model year. Or between higher spec models.
Honda should be able to tell you if your engines are the same and it will fit. Or if not what parts you may need to keep or modify.

Buying a used engine may be cheaper. But it’s still a used engine. 20 000 miles for a 40 year old engine is not a lot. You might be better in the long run, getting your engine rebuilt.

I’m not particularly knowledgeable about bikes.
My general experience. An engine which has sat unused. I would suspect a possible issue with the rings.
Doesn’t mean it’s not the valves just. Just I would think rings.

Why the engine hasn’t turned over, it didn’t seize but it did deteriorate. How much? Unknown.
I had an issue with my old Kawasaki, I used to work offshore, for months at a time.
I had to get the rings replaced and the cylinders honed.
Oil was getting past the rings, burning and carbonizing the inside of the cylinder.
The shop referred to it as “pick up” fouling the inside of the cylinders.

Try a slightly heavier grade of oil.
A heavier oil might not get past the rings quite as easily. Or the valve stems.

 

[SemiFast]

it sat for 15 years. Got it running and everything works no major leaks but it burns oil on start up and under throttle.

My first thought would be stuck rings caused by carbon turning to rust, along with rust on the cylinder walls to.
Yamaha used to sell some stuff that was said to free stuck rings, never tried it thinking it was nothing more than a snake oil. The sells guy said it worked. I still don't believe it. Maybe somebody here has tried such stuff and can report.

I would drop the oil pan, clean out any and all sludge, the oil pickup screen and all related items.

^ This ^

If you have not changed oil and filter I would warm it up and at least change oil. etc...

Me? I'd change the oil at least a couple of times.

. I now have the opportunity to buy an 81 Cb900f engine not burning oil and was dyno tested with 20k miles.

Honda is notorious for changing things seemingly for nothing more than the sake of change. If that is true with your '81 and '82 year models, I don't know.
You can find an online parts supplier and compare engine case part numbers and any critical parts where motor mounts and such are located. If the part numbers are the same for both year models you're probably in business with your engine swap, probably.

S F

 

[Unkle Krusty]

Or you could check the compression now, run it for three months and check the compression. Might cost you 2 quarts of oil, and you got some riding in. If you open up the engine, how many people do you know, that can competently work on it. I am guessing none based on your opening statement. I used to know all the guys that worked in bike shops from Vancouver BC to Portland OR. There is only a handful that can put new bearings in the crank of a Honda like yours.
I have had more than my share of engines apart. I have no intention of working on a 4 cylinder engine like your or mine. I paid $100 for a spare engine, frame and rear end, and $200 each for two more intact bikes. One for spares, the other to tidy up and run. I was primarily looking for 4 into 1 exhaust pipes. They are worth $200 for a good set.
If you can find someone to take apart your engine. How much will they charge.
The last time I asked an old mechanic if he wanted to look inside my spare XS400 engine, he said no thanks, I would rather sit on the porch with my coffee. Another capable guy I know, would tell me to pound sand.

UK

 

[SemiFast]

There is only a handful that can put new bearings in the crank of a Honda like yours.

True that.

S F

 

[Critter]

I have heard (never tried it) that if you pull the plugs and put a couple of ounces of Marvel Mistery Oil in each cylinder and let it sit for a couple of days, it will free stuck rings. Just make sure you crank it over and push most of that oil out before you put the plugs back it.

Once again I have never tried this on any engine.

 

[hogcowboy]

put a couple of ounces of Marvel Mistery Oil in each cylinder and let it sit for a couple of days, it will free stuck rings

It works on tractor engines but it might take more than a couple days. It might take several weeks depending on how badly the engine is stuck. And even then some force might be required. But you are going to be rebuilding anyway on tractors so a little force won't hurt too much.

 

[rhorst]

If it's smoking at start up, after sitting a while, I'd suspect oil is seeping past your valve stem seals...

That's my first thought too, if it's not smoking all the time. If it runs well otherwise, it might just need a valve job, or even just the valve seals.