Drifting timing belt

[Nuvo]

The patient is a 10v engine which I've recently replaced the cylinder head on account of it blowing a spark plug.

I've aligned the timings marks on the cam and crank pulleys and tensioned the belt as per the factory manual. The belt initially is mounted in the middle of all the sprockets.
When I either hand crank the car or start it up the belt can be seen to drift toward the outer edge of the cam sprocket.

The tensioner and coolant pump are new parts and the belts will be renewed soon.

I fail to see what is causing this. Has anyone seen this happen before? Any ideas what I need to do to fix this? I can't leave it like this for fear of it slipping off and destroying my efforts.

Maybe I just need to increase the belt tension a little more to prevent this. Could this be a possibility?

Here's what happens after running the engine for a few minutes...

[Knight Rider S4]

Getting there Rob..:-)...Check tension but sure you have this right (twist belt so you can only turn 1/2- 3/4)

[Nuvo]

Just a few more steps and I'm done Steve, it's been an education thats for sure.

The tension of the timing belt is set so that I can twist it ~90 degrees between the cam sprocket and water pump as per the somewhat imprecise description quoted in the factory repair manual. I'm thinking it might just need a little extra increase in the tension to stop that worrying drift I'm seeing.

[Error404]

in my experience mate, that'll be too tight. I've found the belt needs to be able to turn approx 120ish degs or it makes the distinctive whirring like is supercharged. Someone had this problem recently with an ABY IIRC which was to do with a washer (or lack of) behind the crank pulley I think.
My belt on my CQ doesn't sit perfectly in the middle of the pulleys, but at the front edge (not overhanging though).

[Knight Rider S4]

Thats right..the belts not catching or rubbing on anything?

[stiglet]

Seems to me to be too tight as Error said.

Also, just to put your mind at rest, i'm pretty sure the KV is a non interference engine, so snapped belt won't harm anything. It'll just stop running!

[Error404]

are you sure the crank sprocket is mounted properly as that has the lipped belt drive on it, the belt will always follow the fore / aft alignment of the crank sprocket AFAIK

[DJones]

The belt position on the cam pulley is normally dictated by the crank pulley. Have you changed anything at the bottom? added a washer between the pulley and crank nose that was not there before? or used a different front pulley etc.

You may recall that the toothed crank sprocket has a lip on the rear stopping the belt from slipping off towards the engine and the front pulley creates a step to stop it coming forward, hence the bottom pulley cant be in the same place as when it came off, if your happy that all is ok at the bottom you can alternatively add a spacer to the cam pulley to bring that forward just check the purchase on the water pump.

Hope it helps..

[Nuvo]

@Alex, I think you could be right. I'll ease of the tension a little bit and see how it behaves then. The tensioner is making and noise at it current setting so it not excessively tensioned if you follow me. I've done a bit of searching and read that it fairly common for the belt to sit on the outer edge of the pulleys. So if I can accheive that, I'll be happy.

The crank sprocket hasn't been touched by me for now so it should be fine, I'll still take a look to see if anything looks out of place all the same.

@Steve, the belts have a clear unobstructed path and I had the belt sitting centered on each sprocket intiall to keep the belt in a straight line plane so as not to bias it.

@Stiglet, Too tight sounds right the more I consider it. Its the NG rather than the KV which amounts to much the same thing although I am reasonable sure that they are interefernce engine although I'm happy to be proven wrong on this.

[stiglet]

I am reasonable sure that they are interefernce engine although I'm happy to be proven wrong on this.

Ok fair enough, I used to have a KV engined series 2 and I always thought they were non-interference, pretty sure i read it somewhere but i guess i could be wrong too! Kinda does make sense as i think they are pretty low compression with a low valve lift? Would like to know if this is the case or not if you do find out either way

I also seem to remember my timing belt did sit pretty close to the edge on the top pulley too.

[stiglet]

Ok just did a check on audifans which is where i think i read it orginally, apparently ALL NON-multivalve engines are non-interference EXCEPT the 2.3e due to raised compression. The KV is non-interference along with the 2.0L 5 cylinder, i think the JS or something? However the 7A, ABY etc are all interference, so apologies you are right as yours is an NG it is an interference engine

[Nuvo]

@DJones. I haven't touched the bottom end at all. Well, the only thing I did was to remove the lower belt guard to clean up the oil pump area that was all.

No, I didn't know that the crank pulley dictates the belt position on the cam pulley until Alex(Error404) and you mentioned it.
It's looking like I've been a little over zealous when tensioning the belt for fear of it slipping off. The irony of this isn't lost on me by the way.

[BRY]

Are all the dowels in the cylinder head / block?
Did you have any issues with them?

[Nuvo]

Ok just did a check on audifans which is where i think i read it orginally, apparently ALL NON-multivalve engines are non-interference EXCEPT the 2.3e due to raised compression. The KV is non-interference along with the 2.0L 5 cylinder, i think the JS or something? However the 7A, ABY etc are all interference, so apologies you are right as yours is an NG it is an interference engine

That sounds about right to me. The 2.0L (on a series 3) has the code NM.
Code JS (I think) is found in series 2 cars.

[Nuvo]

Are all the dowels in the cylinder head / block?
Did you have any issues with them?

The two dowel pins to assist in the alignment of the head gasket and cylinder head?
I don't remember any being fitted to either head or block. The only dowels I recall were between the two halves of the inlet manifolds.

I used a pair of head bolts with there heads removed to assist in the placement of the head on the block. I had considered the possibility of the head being slightly out of alignment but I don't think this is the case.

You've got me a little worried now.