Cam chain tensioner and the big decisions ...

Hi all,

I've done a search but cannot find a post that fits. I was given my 1983 XJ750R, all original with just 17,000 miles on the clock, but it hasn't run for at least fifteen years, and was just sitting in a shed.

I've already done the carbs and won't ride it without doing the brakes, tyres, etc. so that side of it is all good. Now I'm into the engine, valve clearances etc. and was chatting with a mate who has an XS1100. His cam chain tensioner disintegrated, the chain broke and a couple of valves crashed into the pistons, which also broke. It made a hell of a mess.

He advised changing this before I do anything else. Given the engine is 28 years old and that part is some kind of plastic, what's the chances mine is going to do the same thing? I'm thinking of doing a ground up resto on the whole bike anyway but I'd love to get it going and take it for a cog. That's a bit of a dilemma, where do you stop once you split the cases?

Now seems like the ideal time to bite the bullet, pull and strip the engine, freshen it up with a hone, a set of rings and bearings and a valve lap, repaint the frame and coloured bits, service the forks, brakes, clutch etc. and just generally go the whole hog.

Any first hand experience on broken cam tensioners welcome, cheers, Ian

 

Welcome, Ive never had a problem with mine and Im sure Ive ridden it as hard as anyone could.

mines a 82 and has the auto cam chain tensioner

 

Hi Ian,
I did a complete engine rebuild on my 82 750 Seca when the local water department turned it into a submarine. Once you split the cases, its mostly just a case of clean and inspect and measure - not much to redo or replace unless you see obvious wear in the gear train. Be sure to use anaerobic sealant on the case halves when you reassemble - weird lookin stuff, but easy to use and works very well.
You may want to take the opportunity to clean and paint your cases and heads if you like that cool black and silver look like the 750 on the cover of the Haynes manual.
My bike had 70,000 km on the clock and the main bearings still plastiguaged well below factory spec. A quick deglaze of the cylinders and a new set of rings rounded out the major 'rebuild' work. Inspection of the cam chain tensioner showed extensive wear, and the plastic chain guide showed damage, but neither had failed (although I'm sure both were close). Chain, tensioner, and guide were replaced (possibly the sprockets too - its been a while, I can't remember exactly). Combustion chambers were a little carboned up, but cleaned up nicely, and a check of the valve sealing was performed by pouring oil into the heads while upside down on the workbench overnight - no leaks were noted, so I didn't even bother lapping them.
The carbs were thoroughly cleaned and rebuilt, and I was completely stunned when the bike fired after about 7 seconds on the starter and idled fairly decently - turned out to be a little lean because of the K&N I installed and forgot to oil first!
Follow your manual, perform all the measurements accurately, and give everything the 'ole hairy eyeball when inspecting, and I'm sure you'll be fine - as for riding with the current chain tenioner, only you can make that call - but remember, this part was redesigned, which gives some indication of how the factory felt about the original.
Good Luck, and shoot me a PM if you have any questions on my rebuild.
Dave

 

The Yamaha Auto Cam Change Tensioner is not likely to fail.
It's design is Simple and employs a minimum of Parts.

Nothings Plastic or fragile.
Major complaint: Oil Leak on Bikes with Gaskets not treated with Sealant.

Occasional "Sticking"

 

My mistake, the damaged part was actually the cam chain center guide, which certainly looked and felt like plastic (clearish white in color, snapped easily between my fingers when flexed - nylon?), and certainly wasn't metallic, not any portion of the auto tensioner. The front cam chain guide was made from the same material.
The obviously re-designed part I referred to was actually the rear cam chain tensioner blade, which differed significantly from my (very worn) metal-backed original. Apologies all 'round if I caused any confusion - I plead defective memory, rebuild was done in '99.

 

LoDollarDave,

I think that I may have the same issue. I bought a used engine from a 83 XJ750 Maxim that was supposedly good late last year. Well, it turned out that it was from a Seca because the gearshift has a different design. I did a compression check, first dry and then wet. It was low at first but when I put a couple shots of oil in each cylinder I had one cylinder that had over 200 PSI. About two hours later I checked it again and couldn't get over 45, wet or dry.
I decided to do a top end overhaul before installing the engine. I pulled the oil pan yesterday so it could be cleaned very good because I am painting it as it is put back together. When I pulled the pan I found a small piece of what looked like plastic, about 3/4" X 3/4" and about 3/8" thick that was broken off something. I can't see that anything is wrong with the chain guides.
Looking through the Haynes manual I can't find anything that resembles the piece of plastic, nylon or what-ever it is. I think it is the same thing you found. Do you know exactly what is was?
I hate to not do a bottom end overhaul while it is apart but looking at the prices of the bearings and all, it will be about another $500-$600 to do. That is a lot of money to put into the bike engine that is not available. I am going to take a butt-load of pictures of the complete process of changing out the engine and bike dis-assembly and re-assembly. It is now decided that I will paint the frame along with the complete bike while the engine is out. I am also polishing the clear-coated aluminum.
I need to find out what the piece of plastic is before I pull the bottom end apart. I wish I had the funds to build the bottom end. I have to think about it a bit. It would be nice to know what it is and the condition of it before it is installed in the bike.
Thanks,

 

There's an article about blowing up auto tensioners and flushing bits of plastic out of motor cases in last months BMW Owners News *ducks* Hey, who threw that?
anyway, generally the plastic rail thingers don't ever break unless they get slapped around. That only happens when/if the tensioner fails to take up the slack the way it's supposed to. Since you can SEE the tensioner from a dozen feet away without doing any digging and since you can take off, inspect it, clean it and all with one allen wrench and one crescent wrench... I don't see any harm in doing so. (working too hard is one thing I consider harm btw)
I had heard that early on when yamaha started doing the auto tensioner that they weren't so good but had them plenty sorted by the time our XJs came around.

 

Yamaha sports plazaHurst check out this and look at part #23 that is more than likely where the piece you are describing came from, the pieces can also fall down into the shifting drum and cause the bike to no longer shift.

 

Hi All,

Thanks for your feedback. I checked with my friend who had the problems with his XS1100 and it was not the tensioner at all, but the chain guide. Sorry for the red herring but after looking at my engine, the manual and the posts here I can see that the auto tensioner which I have should be a simple and reliable device but the guides may cause some bother. Does anyone have any ideas about guides, etc. Can they be replaced without splitting the cases? I think perhaps not.

Cheers, Ian

 

Guides:

Cam Chain
> Removable after the Cylinder Head is removed to allow access.
> Once the Cam Chain is Slacked and the Head Removed.
>> The REAR Guide can removed after the Lower-end is freed from the Slotted / Locking-Channel which the Lower-end is carefully positioned to allow the Rear Guide to Pivot, ... locked from other travel.

Hi-Vo Chain Guide
> NOT Removable without Splitting Cases.
>> This Guide has a history of failing and breaking-off ... Possibly due to its forward mounting tab becoming fatigued and breaking-off.
Other issues with this Guide are linked to age.
The Hi-Vo Guide which sits for years dries-out and disintegrates.
Chunks of the disintegrating Guide find their way to blocking transmission moving parts.

 

iandmac, What I was saying is that guides only ever fail because of inadequate tension, causing them to get slapped by a slack chain when going from on to off throttle. The guides last forever (longer than the chain at least) so long as the tensioner works fine.

Carry on.

 

QUESTION: Does the lower guide act as a chain tensioner for the lower chain? I haven't separated the cases yet but notice that the chain has a lot of slack in it. What I asking is do I need to replace the chain when I separate the cases to replace the guide?