Exhaust patter

R

restojon

New Member
Hi all,
I'm currently in the process of recommissioning my 2000SC and I've had a recurring "patter" from the exhaust of the car from before I laid it up. I thought it may be carburation related and as the old carb was quite worn I got a good S/H replacement and fitted it today. After a bit of fettling getting the mixture and timing in (dizzy had been disturbed during camchain and main bearing replacement) I started the car up and it still has the patter from the exhaust, it sounds like a bad mixture but when I try to dial the mixture either way the sound doesn't get any better, it just gets more or less even in it's frequency but is always there. I can't help thinking it could be an ignition issue as I've fitted a Pertronix and ever since I've had trouble dialling the timing in. It really isn't happy running at anything less than about 14deg, I've got it at about 4deg @ 950rpm at the moment but the car is not as smooth as it should be at this setting.
I've done a video of the exhaust note and wondered if there's any light anyone could shed on this for me:

http://s783.photobucket.com/albums/yy11 ... CT0089.mp4

All help appreciated,
Jon.
*Edit*
One of my theories on the ignition being at fault is that I wondered if the discharge from the H/T could be affecting the electronic unit in some way, could go some way to explaining the timing problem. Also, when I had the car on the road before lay up when you got the ignition timing happy for idle, when you revved the engine the advance was coming round to about 40deg., when you brought the timing back to knock back the max advance it would pink something rotten under load.
 
EeeeK! Compression tester time I think, I must say in the back of my mind I had a sneaky thought that it could be an exhaust valve gone.
 
hermione149 said:
Sounds to me like an exhaust valve has gone,....Hermy made the same sound when number 4 exhaust valve split,.. :shock:
Click to expand...

Sticking my nose in as this problem has me intrigued! :mrgreen:

Assuming your thinking is correct, the patter is actually a mini-backfire through the exhaust?
 
To my mind not so much a backfire but a combustion leak down the exhaust as the mixture fires....... I think :D
 
I don't think that being intrigued is sticking your nose in Darth :LOL:

If you think about it,on the compression stroke some of the mixture gets forced through the crack in the valve and detonated in the exhaust manifold,this creates an uncontrolled explosion in a confined place,which,in turn,creates a pressure wave that travels through the exhaust system and out the tailpipe,making the rythmic "pap pap" sound that you hear in Johns video.

I,ve owned 11 SC 4 pots in my lifetime and 10 of those had valve trouble and all made the same sound,number 11 snapped the bottom chain on the motorway :LOL:

Since we're on the subject of valves,maybe someone can answer me this,why were some 4 pots fitted with valve rotators and some not :?:
 
Did you measure your valve clearances? When they are too tight, with a hot engine, they also produce this symptom.

What are the valve rotators Kevin?
 
I think I may have got somewhere with this problem, I found I may have a duff "lucas" dizzy cap. Was not firing on 2 properly and I think that the misfiring fouled the plug on 2 and this caused at least part of the problem, it's still there every now and then but whilst the plugs were out I did a compression test (hot) and they all came out 160psi so I'm not sure with the valve crack, however I do have a endoscope which I'm going to have a look into the cylinders with. Anyhow must go or I'll be late for work, will be back with more info later.
Thanks again for your replies, all very helpful.
:D
 
restojon said:
I did a compression test (hot) and they all came out 160psi so I'm not sure with the valve crack, ...
Click to expand...

That actually rules out the cracked valve. But i insist that you should have a look at the clearances.
 
Have brought the feeler gauges home from work today to do exactly that. I shall report back with my findings. Thanks, Jon. :D
 
Don't forget that they need to be checked when the engine is stone cold, preferrably after it's been left standing overnight.
 
Demetris wrote,
What are the valve rotators Kevin?
Click to expand...
As I understand it,they are fitted to the exhaust valves to rotate them in the guides to help prevent valves seats burning,i've come across them in some Mercruiser marine engines, not noticed them in a p6,... yet, :LOL:

Quote from the HBoL,"The exhaust valve on SC engines is fitted with a sleeve and special spring cap to enable the valve to rotate".

I still wouldn't rule out a duff valve just yet,the valve in the pic produced a reading equal to the other cylinders on a hot compression test and only 5 psi difference between the other three on a cold test,
Here's a different pic of the same valve,

Copy of hermione's valve 002 by CharvilChancer, on Flickr

It was the spark plugs and the zero clearance on cylinder 4 that finally gave the game away though :wink:

2006_0430hermione0006 by CharvilChancer, on Flickr

In decsending order,cylinders 12&3 are the same grey colour whereas cylinder 4 is brown,
 
Whoa! The state of that valve has really shocked me! How come it's eroded/burnt away just (more-or-less) on one side?

Just realised something. The head in the picture is a SC head, but your Hermy is now a TC, is she not? Was it because of that valve you changed it over to TC?
 
darth sidious said:
Whoa! The state of that valve has really shocked me! How come it's eroded/burnt away just (more-or-less) on one side?

Just realised something. The head in the picture is a SC head, but your Hermy is now a TC, is she not? Was it because of that valve you changed it over to TC?
Click to expand...

I've no idea as to the cause,faulty materials maybe :?:

Yes,Hermy is a TC now,the decision to convert was made about a year after the valve incident simply because in SC guise she struggled to pull my trailer :D
 
Hi as pm , this is the 2000 sc head i took off a scrap car , miles showing were 86,000, also the 2 fan blades i have yellow off a 2200 and red off a 2000
hope this helps :D
 

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