Lotus Elan

More Timing Chain Slip

PostPost by: sk178ta » Fri Jun 03, 2005 2:47 pm

On inspection the timing chain has jumped off the exhaust cam sprocket. The engine was rebuilt only 17oo miles ago with new chain and sprockets. The tension was correct and there was no undue slap or rattle. How could this happen? The suggestion was that I`d over-revved it, but I haven`t, and this seems an unlikely cause anyway.
With another new sprocket all seems fine but I haven`t had chance to give it a good spanking yet. I`m worried that the cause may be more fundamental. Has anyone any likely suggestions?
Thanks,
Jim
sk178ta
Second Gear
Second Gear
 
Posts: 132
Joined: 08 May 2004

PostPost by: theelanman » Fri Jun 03, 2005 3:01 pm

spank your purple +2 instead...!!!!
lol
G
User avatar
theelanman
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1521
Joined: 17 Sep 2003
Location: Peak District

PostPost by: john.p.clegg » Fri Jun 03, 2005 3:19 pm

You should check the sprocket alignment in relation to the crank sprocket
John
User avatar
john.p.clegg
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 5682
Joined: 21 Sep 2003
Location: Manchester

PostPost by: carrierdave » Fri Jun 03, 2005 3:21 pm

Hi Jim,
In considering the problem if the chain was to jump then it would be the exhaust side as this has the longest single section of un supported open chain.
Personally if the chain has jumped then there can be only two reasons, incorrect adjustment causing excessive free chain (What is the chain slap plate like? Much wear?) If the chain was loose then this would have taken the brunt of the wear.

And secondly alignment of the sprockets? Is the crank standard? Does it have the correct sprocket on it with the correct spacing? Was the exhaust sprocket bolted on square? They are a pain in the rear to get back on square when rebuilding the head. Was there something that caused the sprocket not to sit square against the end of the cam?

Lots of what’s and if’s I know, but I think we are all puzzled as to how it could happen.

Good luck

David
carrierdave
Third Gear
Third Gear
 
Posts: 333
Joined: 23 Sep 2004
Location: Rochester, Kent

PostPost by: types26/36 » Fri Jun 03, 2005 3:57 pm

Is the spring inside the plunger in good condition?not broken?
The brass quadrant has a habit of bending and affecting alignment of the chain, is that O.K.?
Is your engine built on a 711 (1600) block? more about that if it is.
Did you personally verify that the chain had jumped? or you accepting this from another source?
Was the sprocket bolt tight?
Was the pivot bolt tight?
Was the locating dowel fitted/broken?
Who built the engine?
Brian
Brian
64 S2 Roadster
72 Sprint FHC
User avatar
types26/36
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 3655
Joined: 11 Sep 2003
Location: U.K.

PostPost by: sk178ta » Thu Jun 09, 2005 7:39 am

Further news: another suggestion is that on starting the engine coughed in the wrong rotational direction throwing off the chain. In fact the dizzy was found to be loose so maybe that loosened on cooling and on start up shook wildly out of timing causing this.
what do you think?
Jim
sk178ta
Second Gear
Second Gear
 
Posts: 132
Joined: 08 May 2004

PostPost by: GrUmPyBoDgEr » Thu Jun 09, 2005 9:15 am

If for any reason the engine rotates in the wrong direction the crank sprocket will load the timing chain on the tensioner side and cause the tensioner to retract. This will result in the chain going slack on the side which would normally be held under load, this being the exhaust side. If there is slack in the chain it would fistly attempt to jump across the exhaust sprocket.
John
Beware of the Illuminati


Editor: On Sunday morning, February 8th 2015, Derek "John" Pelly AKA GrumpyBodger passed away genuinely peacefully at Weston Hospicecare, Weston Super Mare. He will be missed.
User avatar
GrUmPyBoDgEr
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 3151
Joined: 29 Oct 2004
Location: Burnham-on-Mud

PostPost by: rgh0 » Thu Jun 09, 2005 12:24 pm

I would suspect with an engine recently rebuilt that it was like that from the start. Why did you check the timing was there a sudden change in engine behaviour or was it just a routine check?

If chain tension is correct then reverse rotation will not cause any problem. The spring in the adjuster is almost full compressed when properly adjusted and does not allow enough movement for the chain to jump even when the adjuster is on the tension side of the chain.

Rohan
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7487
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: type26owner » Thu Jun 09, 2005 1:14 pm

In fact just the other day I measured the 1/2" of slack in the chain from fully tightened and found it's only 3/4 of a turn of adjustment screw. Now I can do the adjustment without removing the camcover to measure it each time. The sprocket on the crankshaft is half the size as the cams and that's where it will likely jump a tooth first.

I've been having chain noise issues since I installed the harmonic balancer and have been paying close attention to the dynamics of the unit. First off the manual's instructions are written by someone with a glass belly button. The stock setup on the tensioner only exerts 10 pounds of force from the tensioner onto the chain but at a configuration that has very poor mechanical advantage. That stock spring loading is low enough that the inertia of the system is causing the timing to stray too far while in overrun. In order to increase that force one must insert a spacer in between the spring and the plunger. I'm at 25 pounds loading of the plunger now and the chain noise has finally gone away and so has the little bit of popping exhaust. I like my engine to behave like a sewing machine without making any annoying noises like backfiring and so forth.

Like Rohan stated if the slack is adjusted correctly there is no way for the chain to jump a tooth even on the crankshaft.
-Keith
type26owner
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1616
Joined: 18 Sep 2003

Total Online:

Users browsing this forum: No registered users and 15 guests