cam chain slippage

[Stralya]

Hello again,

Does anyone know if the cam chain on a '78E can jump a tooth on the crank sprocket?

Dan.

[TopCatGr58]

Yes, quite easily as a matter of fact, IF the cam chain tension isn't set right!

You'll want to pull the valve cover off, rotate the engine to the "C" mark and then verify the cam's alignment dots are pointing essentially straight up at 12 O'clock. They may be slightly off of 12, but you should be able to tell if they are a whole tooth off!

Have you done a compression check IF you are not able to get it running? Why are you suspecting this?
T.C.

[79XS11F]

TC ... my manual says to set the timing plate to TDC on #1 for the cam installation and dot alignment? Surely if my manual was wrong I would have a pile of bent valves? This must be a type "O"?
Rob

Originally posted by TopCatGr58
Yes, quite easily as a matter of fact, IF the cam chain tension isn't set right!

You'll want to pull the valve cover off, rotate the engine to the "C" mark and then verify the cam's alignment dots are pointing essentially straight up at 12 O'clock. They may be slightly off of 12, but you should be able to tell if they are a whole tooth off!

Have you done a compression check IF you are not able to get it running? Why are you suspecting this?
T.C.

[prometheus578]

"Back to Sesame Street for T.C."

T.C.,
Yes, I know the letter's "T" and "C" sound similar...
But I believe you meant to write:
"...rotate the engine to the "T" mark..."

[TopCatGr58]

DOH!

Thanks guys, had the Cam Chain Tensioner setting procedure in my mind at the same time That's what "we" are all here for, to help keep us straight!

Hope you got that Stralya, sorry if I caused any headaches!
Perhaps I need a CT?
T.C.

[Stralya]

No drama

No worries TC, I knew you meant 'T'

Petejw and I have been doing major work (top end rebuild) on my '78E

After that work I rode the bike about 10 miles to his place in order to synch the carbs (he has a ripper four dial carb synchroniser)

The bike ran like a champion to his place with a little throttle dwell and lag down low (pilot circuit) which I thought would be put right after synchronisation.

Anyhoo, after synchronisation, the bike would hardly run at all, sometimes firing on 2 and three, other times on all four, sometimes on three and four....just a total mess.

We pulled the carbs, quadruple cleaned them, replaced the mixture screws and even ran them on Pete's '81 Special, they ran perfectly on his bike but mine was still a mess.

We inspected the coils and leads, pick up coil wires, vacuum advance etc. We bypassed the ballast resistor and perfomed the ancient Aboriginal 'Startyabastard' dance all to no avail.

We pulled the cam cover off and checked the dots and arrows (after setting the timing plate to 'T') and found it looked off by a tooth.

We unfastened the cam sprockets and retarded the chain by one hole. We then found that the dots and arrows lined up perfectly. We put it all back together and the bike fired briefly but then nothing.

We were wondering if there might be an issue with the crankshaft sprocket.

It was late in the day by now and we were too frustrated and angry to carry on so we haven't re-checked the dots and arrows but it's our strong sudpicion that the cam chain has slipped again even though the tensioner has plenty of travel left in it.

The cogs on the cams look great but we can't see the cog on the crank. We don't want to split the crankcase if we can help it but we're at the end of our tether. It's been in his garage now for over two weeks and it's all getting a bit stale.

I don't see the point in replacing the cam chain if there's still plenty of adjustment in the tensioner so I'm thinking that the chain is slipping off the crank sprocket.

Are we thinking correctly?

Dan Tyson

[prometheus578]

"Interesting..."

Ok, set it all up perfect again.
Keep the valve cover off.
Pull the cam chain tensioner.
Stick something in the tensioner's hole, like a 6 inch socket extension. Push inwards on this to keep tension on the chain and rotate the engine to the "C" mark... keeping and eye on the chain so that it doesn't jump a tooth.
You are now all ready to adjust the cam chain tension....
But first....
With the tensioner in your hand, loosen the lock nut and then the bolt. Did the plunger move outward under spring tension? I'm thinking that maybe, there's a burr on the tensioner rod, and that when you're adjusting the tension... the tensioner's piston/rod isn't fully moving forward.

It may be binding, or the spring may be weak or broken.
If you think it's functioning ok, retract the tensioner rod, lock it in place and reinstall the tensioner onto the cylinder. Lossen the bolt so the rod moves forward, and then tighten the bolt and the lock nut.
Now... get a wrench and rotate the crank backwards and forwards while watching the chain on the cams, etc.
There really shouldn't be any slack in the chain, though you might feel a little when rotating the engine backwards to it's usual rotation.(When the crank moves, the cams should move...)
I doubt there's any problem with the gear on the crank. I suspect a defective tensioner.

Or, with the tensioner still in, rotate the engine to the "C" spot. Feel the chain... there should be no slack at the rear of the engine, and very little slack on the chain at the front of the engine. If the front is slack, then the tensioner isn't doing it's job.
And just what do you consider "Plenty of adjustment left in the tensioner"?
How much is left? A half inch? A quarter inch? Remember that the further the rod sticks out, the less spring tension it has behind it to push it forward to take up slack. This spring has been under tension for about 30 years, too.
(When I hit 30, I had very little "spring" left in me to push my rod out, too)
What?

[Stralya]

Hey prom.

We adjusted the cam chain tension as described. There's about half inch of adjustment left in the tensioner.

There was enough slack in the chain to install the bridge between the two cam sprockets.

Is this too much slack?

Dan.

[Ken Talbot]

With that much slack, I wonder if the chani has slipped off of the crankshaft sprocket. It can do this when you're messing about with the camchain, and it gets caught next to the sprocket. Can you get a peek down there with a good flashlight to be sure?

[prometheus578]

I don't know what to tell ya...
I have an open engine outside and I just went to look down the cam chain cavity. really doesn't look like there's ll that much room for the chain to get offa and along side the gear, but I may be wrong.
Chain between the cam gears: slack is a subjective term. I would think there shouldn't be enough slack between the gears to slip the bridge in. Did you slip the bridge in before you adjusted the chain?
Really not too many variables here, I think maybe you just did something out of sequence.
Start over.
Turn the engine over by hand and line up the crank at "T". The cam gears follow each other due to the chain. There really shouldn't be too much slack between them.
Turn the engine to "C" and redue the tension.
See if the amount of freeplay/slack feels the same on both runs of the chain(between the intake cam and the crank, and between the exhaust cam and tha crank. Should be about even)
If the exhaust side is looser, then there's a problem with the tensioner.
How many miles on the engine? You may just want to replace the chain anyway.

so we haven't re-checked the dots and arrows but it's our strong sudpicion that the cam chain has slipped again

Maybe, maybe not.
Inspect the dot alignment again. If everything's still as it should be, do a compression check. I don't know if being one tooth off would do it, but you may have bent a valve or two when the chain initially skipped.

[petejw]

thought i should chime in here,
i no ive set the tensioner up properly
as ive done it numerous times before,
in fact i learnt the hard way, when i first got
the bike i stripped the threads on the tensioner
and bent 4 valves. so im very meticulous in setting
the cam timing correctly.
I know theres no binding of the tensioner rod,
there is about quarter/ half inch left on the plunger,
when it has been removed.
and im sure the chain was fitted correctly to the crankshaft cog.
i hand turned the motor over several times to recheck alighnment
and all was good there.
prom made a good point, i also am now thinking
that the tensioner spring has lost its tension, after
setting the chain up, i was checking on the chain slack
between the cams when i was able to lift the center
bridge out.
ill have 2 take the cam cover off again to recheck the alignment. the bike wont start and is backfiring thru both carbs and exhaust whilst cranking.
wat ill do nxt time is remve the tensioner plug and apply pressure
to the back of the plunger and retension.
i dont think and am hoping there are no bent valves, we dont have a compression tester, but with a finger over the plug hole there is good pressure and suction coming from the cylinders when cranking.

[pggg]

You guys didn't happen to replace the advance unit 180 deg out of whack, easy to do

[TomB]

You guys didn't happen to replace the advance unit 180 deg out of whack, easy to do

My thoughts exactly, its easy to rebuild the advance 180 degrees out but not install it wrong, only one peg/hole. Did you have the advance unit apart? The easy check is swap the feed wires to the coils and see if it fires up OK.

It still wouldn't account for the camchain slipping one tooth

If you remove the camchain tensioner end plug you can use a small bar and exert more pressure on the tensioner rod in case you suspect the spring being weak, just to check mind, I wouldn't recommend running for long distances with too much pressure on the camchain, (thats my arse covered )

[petejw]

no didnt replace the vac advance unit,
ill readjust the camchain tensioner with the
cam cover off so as not 2 put xsive pressure
on the chain. itll b a few days/week b4 i can check
it all out again.
the bike has over 90 000 ks on it and the chain
is worn a bit, but it was apparently running ok
b4 we got it apart.
thanx 4 all the responses.

[skids]

I know that if the chain has just a little slack, and the crank is rotated backwards, the cam chain can slip a tooth. Maybe that is indicative of excessive wear on my own bike!

Originally posted by petejw
no didnt replace the vac advance unit,
ill readjust the camchain tensioner with the
cam cover off so as not 2 put xsive pressure
on the chain. itll b a few days/week b4 i can check
it all out again.
the bike has over 90 000 ks on it and the chain
is worn a bit, but it was apparently running ok
b4 we got it apart.
thanx 4 all the responses.