Back to that irritating tapping. One for the rebuild and overhaul buffs I think

[Moraystag]

Some of you will remember my previous posts regarding tapping noise coming from cylinder 6 or 8. As you know I had all new cam buckets fitted and reshimmed. I had this double checked by local engine rebuild company and all clearances are correct. Still the tapping / rattle persits, so I asked the engine company to double check again and I was there at the time, so I'm not loosing my marbles and the clearances are as they should be. So where is it coming from, I have checked dizzy as I thought it may be coming from that area but nothing untoward there and no excesive play on shaft or gubbins.
Still annoying me today so I removed the plug lead from no 8 chylinder, no drop in revs, although there seemed to be a spark.
Removed plug leads from 6-4-2- cylinders and on each occasion the revis dropped slightly.
I removed and replaced the spark plug on cylinder 8 and repeated the process again, but to be honest the drop in revs was negligable, repeated the same again on 6-4-2, and same again slight drop in revs.

I have no fluctuation in revs while idling and would go as far as say she has never idled so well. There seems to be no smoking from the exhaust.
Any ideas guys as to why the revs do not drop when the plug lead is removed from 8 but no drop in revs, and what would this point to
I'm now wondering if the piston rings are shot on this cylinder.
Any ideas greatfully accepted


Mike

[dasadrew]

Could the noise possibly be a small end?

[GDPR]

busted valve spring, probably exhaust - pull the cam and lift the buckets out from the dead cylinder then look down..

Had a few do that ......

[martin]

I'm with Russ on this, I've had one, and come across a few more, one (at least) valve isn't closing.
(sorry if that's the kiss of death Russ) Martin.

[Kevin Garrod]

As someone who doesn't know any better can I suggest that if there was a broken spring you wouldn't be able to achieve the correct valve clearance which the OP says has been double checked?

[martin]

As someone who doesn't know any better can I suggest that if there was a broken spring you wouldn't be able to achieve the correct valve clearance which the OP says has been double checked?

If it's been broken from the beginning you would ! Martin.

[Kevin Garrod]

I said I didn't know any better, it just seems to me that you would need a pretty thick shim to take up the slack of a broken spring.
I'm sure you are right though.

[GDPR]

A broken spring can still close the valve, just not well enough. The last one I had, the clearance was fine, but the broken loop was catching the inside of the bucket and making an awful row....

[Kevin Garrod]

Thanks Russ, its only by asking silly questions that I know anything at all.

[flying farmer]

I agree with Kryten on this one. If it was a piston or ring problem it would be smoking.
If you can't see anything obviously wrong with the valve springs, it may be one has gone very weak, or you have a slightly bent valve which is sticking in the guide.
You can remove the valve collets with the head still in place. Remove the plug and poke some soft rope down the bore, then turn the crank until the rope keeps the valves shut (on the compression stroke obviously, or you will bend a valve!)
You will need to press the spring cap down to remove the collets. I think I leaned on a large screwdriver to do this, using tweezers to remove the collets, I would recommend eye protection because if you slip the collets will come out like a bullet, I lost one once on a 1300 triumph back in college days doing this (collet not eye that is)!
Once the spring caps are removed you can compare the two springs, make sure the lower spring collars are present, that the valve guides are firmly in the head, and that the valves are not sticking in the guides. Also check that the collets are sitting at even heights in the valve spring cap before you dismantle it. On my 1300 triumph the collets had pulled into the cap somehow which lost tension on the valve spring causing a missfire, the loose valve spring had pounded the lower collar into oblivion, and the bits fell down and scored a cam follower which at least can't happen on the Stag
Finally make sure the cam bucket has been resting on the shim not the ridge around the valve spring cap as this means the cam bucket is pushing on the spring cap rather than the valve. This leaves the collets loose on the valve stem which eventually wears away dropping the valve into the cylinder. This actually occured to someone I know a short while after rebuilding their engine
At least you have narrowed the problem down to one cylinder!
Neil

[cosmicwheels]

You can get a kit that with a valve and a 14mm plug thead holds the valve shut with compressed air, I have used it many times for changing rubber stem seals.hope this helps,John

[GDPR]

only problem with that John, and I have seen this happen, is if the cracking of the taper that holds the collets in done in a cack-handed way.... usually done with a tap of a hammer just to loosen them with the spring compressory(?)bit of the tool and get the cap and spring moving down, and the tap lifts the valve off the seat losing the air pressure holding it up.....and the collets flick out..... and the valve falls in......

Neil's (and my preffered) method of a physical method of holding the valve up using rope and tdc is VERY much the way to go.... doesn't need an air supply, and it's cheap

Worth noting is if the spring is broken, then the valve will not have been opened in a linear manner due to the spring thrust bias and the valve stem and guide will be royally knackered too.... rock the stem before you remove the valve cap and you will see.....

[dasadrew]

Seems like you've earned your keep today then Russ!

[Guest]

Seems like you've earned your keep today then Russ!

Maybe he should write articles for the magazine

[SUTTY1]

I agree the rope method works but like John i've used the compressed air tool in my business for many years and if you've already got a compressor the tool is fairly cheap.

If i remember correctly the instructions say to wind the piston towards TDC and if you do this there is no way the valve could fall in, steve