Cam chain woes

[datsun fan]

I'm having a right old headache with my cam chains on this 1979 xl250s,

as in this thread,

http://www.xrv.org.uk/forums/bike-te...tml#post335376

After dropping a valve, I did a topend rebuild, and I suspect this may have been due to a stretched camchain putting the timing out and allowing piston contact with the valves.

I have replaced the cam chain with a Dave Silver pattern, after 200 miles there is knocking from the top, so strip down and check, the camchain is slack, tensioner rubber was replaced, and there was no adjustment left on the chain tensioner, the chain was mega slack, so changed to a genuine Honda, and now all was fine, for again approx 200 miles and it is the SAME!!!

Another strip down, the chain is slack, and if you loosen the tensioner off and push it down to the maximum travel the slot allows - ( experienced tool twiddlers will know what I mean - ) so that there is maximum deflection of the rubber guide, (it was replaced at the last change), the chain is STILL slack!!! there is no way of getting the chain tensioned, you can almost pull it off thecam sprocket, it is that bad!

I can't see that is a problem with the chain, tensioner , or guide rubber,

Tedmagnum mentions timing, but the XL250S - same as RS250 is, as far as I know Fixed timing, no adjustment available.

It runs perfectly when put back together, no misfires at all.

I am pi$$ed off with this and although I can now change a cam chain in about 4 hours, (most of that is gasket and face cleaning!!?) I am looking for ideas what I might be overlooking???!!!

Any suggestions welcome - other than scrapping!!! and buying another machine!!

[AlanH]

Based on experience of high mileage CB250RSA motors (I've had 3 of them and despatched two to death) I reckon there are at least three possible causes of rapid camchain wear:

1) If the cam journals are iffy and partially binding then that puts excessive strain on the camchain. These motors are famous for top end wear and the cam eating its way into the head. Are the clearances spot on for the cam journals? Any signs of scoring/pick-up?

2) Worn crankshaft teeth - will rapidly kill a new chain and only usually a problem at really high mileages (40k plus or abused engines).

3) Overtensioned tensioner - if the tensioner is forced in an attempt to quieten a slight rattle, the additional force can stretch the chain rapidly. However, as you've done the work yourself this is not likely to be the issue

Incorrect cam timing isn't going to cause premature camchain wear - that's more likely to result in bent valves etc.

Incidentally, did you change the head and the cam box after it's valve-dropping mayhem?

[StumpyFingers]

Does this type of engine have a guide on the other side to the tensioner.
I thought some engines used two tensioners one fixed ( just sprung to take up slap) and one which is adjusted.

[AlanH]

Does this type of engine have a guide on the other side to the tensioner.
I thought some engines used two tensioners one fixed ( just sprung to take up slap) and one which is adjusted.

Yes

[StumpyFingers]

Does this type of engine have a guide on the other side to the tensioner.
I thought some engines used two tensioners one fixed ( just sprung to take up slap) and one which is adjusted.

Yes

In that case is it possible that the front fixed one is missing or incorrectly fitted?

[datsun fan]

Thanks for the input -

the front guide is fixed, and I can't get at it because teh head will not separate from the block, no matter how I try, so I leave thwm together.

It does make some things awkward, but I get by with most of it - what can't be done is anything to the front guide, so it remains inplace untouched.

Without wanting to pre- empt anything - I know what it says in my handbook, what method do you prefer to tension the cam chain, - some say with the engine running - others stationary?

Thanks - Pat

[sledgi]

HI I have the same problem with my xl250s and think I have the solution can you tell me if you have resolved your problem.

[jasonbc]

Thanks for the input -

the front guide is fixed, and I can't get at it because teh head will not separate from the block, no matter how I try, so I leave thwm together.

It does make some things awkward, but I get by with most of it - what can't be done is anything to the front guide, so it remains inplace untouched.


Thanks - Pat

If the parts are designed to be assembled them you must be able to dis-assemble them. My first thought reading this is that the previous owner has done something, cocked something up or found something wrong with it and them botched it up and passed it on. My first interest would be to find out why the parts can't be separated. Once you get that done you can have a proper look inside and see if anything more sinister is lurking it's ugly head

[sledgi]

if correct my chain is making a right noise after removing the top end and bottom for the 5th time replacing the cam chain from another bike, changing the front and back rubbing strip the chain is rubbing on the crank case and there is a small mark where the chain has rubbed on the rocker cover.
this is not confirmed yet but I believe that the cylinder head cam bearing diameter is worn and when the engine is running the cam jumps when the valves are trying to open making the chain go tight the slack when the cam is on the firing stroke this will cause the chain to stretch back and forth each time the tension is adjusted it makes it worse and the starin is put on the chain.
as yet I dont have a quick solution other thank trying to install some kind of half bearings or trying the cam with needle bearings but this is very costly.
I will be testing this, this week by removing the rockers and putting the cam in the head and putting my finger in the rev counter hole and checking for play.
hope this helps Stephen

[datsun fan]

Hi Stephen, as yet, still from May this year, I have been disillusioned and the XL250s has been left to sulk in the shed, I have had a new camchain since then but can't be bothered to put it in!

The head just WILL not split - all the bolts are definately out and a chisel just mashes into the alloy, there isn't the slightest beginnings of an opening, so seeing as I can do all that is needed , it is best left together.

I did try to remove the cam rockers from the head, but these wouldn't come out either!, there doesn't APPEAR, to be any loose play on the cam rockers and the alloy cam bearings in the head are mint, clean, bright, smooth, no scoring or discolouration

As you will know it gets very tedious so it's days are numbered

I will probably have another go now the winter is here, (got a bit to do on the AT), but if it is going again will MOT it and sell it for a newer disced, mono 250,

Let me know how you get on with your investigations come to shed any light on the problem, maybe we can get to the bottom of it between us. Pat