The International CBX Owners Association

If one or both of the special o-rings that are in the oil tensioning mechanism have somehow come apart or decayed, your primary chain would be extremely loose. When cold, the oil is thick enough to provide some tension and normal running. But when hot, the oil thins out and pukes out of the bad o-rings and the chain loses tension. This would cause erratic timing as the primary shaft in relation to the crank would be 'rocking' back and forth. Could also be why the primary shaft seemed to have jumped a tooth. The chain is very slack when untensioned. Time for a look in the bottom end. Are you up for that?

Dave

It wasn't a total loss since I found a worn out brushes and changed them. I have a new valve cover gasket coming so when I get it off what should I do? I may need to remove the head so I can get a good look at the tensioners even though I already looked. I did notice that pulling up on the verticle tensioner was harder than usual it did snap back into place and when I push down on it I can feel it springing back where it belongs. The other tensioner is also working freely.

I checked the timing while it was hot and it was steady on. Is it normal for the intake chain to show slack when turning the crank backwards like mine is doing? I thought the tensioner was supposed to maintain a tight chain either way. The primary chain has been tight every time I checked.

First, you should never turn the crank 'backwards'. It does cause the cam tensioners to go slack and is also not good for the starter drive system. The manual cautions against doing it. I assume you do have a shop manual.

I'm curious as to how you know the primary chain is staying tensioned. There is no way externally to tell that. It's buried deep in the crankcases. You don't happen to be confusing one of the camchain tensioners for the primary chain tensioner?

The cam-to-cam chain's tensioner sits horizontally right below the chain and is easy to tell if its working with the valve cover off. The front adjusting bolt works that one. The cam-to-crank chain's tensioner sits vertically on the inside the chain and it's adjuster is the 10mm acorn nut under the carbs behind the cylinders. It's also easy to check with the cover off. Per the manual you can also check that the cams are timed correctly to each other and to the crank while you have the cover off. I'd also make sure you don't have a couple of tight valves while you're in there.

Again, I'd drop the sump pan and check to see if there is much debris in there also.

Dave

You're right, when you said primary chain I was thinking the exhaust cam chain. I have been wondering about the primary chain in the crankcase myself because of the intermittent scraping sound I am hearing between the alternator and pulse generator. When the parts arrive I will pull the cover and check the relationship between the cams and cams to crankshaft and pull the pan and see what is in there. How do I check for tight valves? Would a compression check while the noise occurs tell that?

I am not hervous about going into the crankcase, just trying to avoid it.

Just follow the rather simple procedure in the manual for checking valve clearance. With this type of head design, the valves wear tighter rather than looser and if you end up with zero clearance, you'll burn a valve.

Once you have correct clearance on the valves, then carb balance and timing can be properly set.

Is your scraping, grinding noise mainly at idle when hot? If so, that's probably just the famous '79 clutch rattle. Sounds like a handful of gravel in the crankcase. It's nothing to worry about. Wish someone that really knew CBXs could take a listen.

Something else to check is that the engine didn't hydrolock a cylinder sometime in the past. Take the plugs out and measure each cylinder pair (1-6, 2-5, 3-4). Measure down to the piston top in each pair and make sure each pair is the same. A slightly bent rod will show up clearly.

Dave

Thanks for your help. Looks like I have my work cut out.

The pistons are the same height measured in pairs so that is eliminated.

twinegar wrote:The pistons are the same height measured in pairs so that is eliminated.

That is good news....

I just changed the oil and filter on my 72 CB750 and used Mobil 1, 15-50. 2 down, 11 to go. Mobil 1 label says 15000 miles. How many miles on a motorcycle?

I've had oil temp guages on 3 different CBX's. All with the sending unit in different places. All got over 300f on a typical Kansas (warm) day with normal use. I change my X's oil at 2000 miles. Other bikes I'd go to 5000 miles for changes. I don't know how far over 300f the CBX's would get because the guages all stopped at 300. That's hot.

Personally, I change every 3000 miles.

With full synth Amsoil I change every 7,500 miles in the Hayabusa that has over 25,000 on it and every 5,000 the 82, that has nearly 90,000 miles on it, mainly because it runs hotter than the Hayabusa. With Valvoline conventional motorcycle specific oil in the 79 with nearly 50,000 miles on it, I change every 2,500 to 3,000 depending on what it looks like by then. The plugs in it always look oil fouled no matter what so when it begins slowing down or smoking, I'll have to redo the head and probably all its components. At that time I plan to do the rings and internal chains and guides, if not a complete rebuild..depending. Seems to me that if the head is repaired I should be able to get to 100,000 miles w/o splitting the cases. The 82 is on its way at 90,000 but its had Amsoil in it since breakin, which may make a difference.

I doubt I would put even 2K on any one bike in a year so I guess I will just change the oil on all of them every spring.
Thanks.

twinegar wrote:I doubt I would put even 2K on any one bike in a year so I guess I will just change the oil on all of them every spring.
Thanks.

If you change every year because of a storage period, you should actually change in the fall BEFORE you store the bike to get all the moisture and agressive fallouts out of the bike.