When they go wrong....

R

restojon

New Member
I've had running problems with my car now for some time, I've been in there with all sorts of gadgetry and have dialled the engine round in circles temporarily getting somewhere only for the car to frustratingly get worse shortly after. Recently the car has been down on power and audibly sounding like it was on three cylinders, I pulled the plugs and number two was much oilier/sootier than the rest and not having a compression tester to hand I poked a bore scope (endoscope)down the cylinder whilst turning the engine over to open the valves and bring the pistons up to TDC, as soon as number two exhaust opened it was very obvious what the problem was, visible large cracks around the outside of the valve head. I stripped it tonight and thought I'd post the pictures up to show what can happen when it goes wrong.
DSC00065.jpg


A while back now somebody posted a similar pic of a valve that had gone worse than this and the symptoms I experienced seem to definitely indicate this type of failure although with a compression test carried out some time ago on this engine there was only a small variance between cylinders, not enough to shout valves or rings at you at any rate.
I have a s/h head on it's way to me and I intend to convert the head in the above picture to unleaded and possibly treat it to some posh stainless valves but I shall have to look into compatibility/suitability first, need to get this one put together first as it is my daily transport.
 
It occours because the valve is not completely closed at times when there is exhaust gas still in the cylinder. The $64k question is why that hppens! There are some obvious candidates; poor materials in the valve seat or the valve; valve sticking in the guide (possible sideways load from the way the valve gear operates? possible poor lubrication of the guide/valve stem? poor selection of guide material?); poor adjustment of valve clearance; cam timing poor as designed; cam timing moving due to timing chain stretch; excessive exhaust back pressure (eg due to internally collapsed silencers); etc etc

Suffice to say that all 4 cylinder engines are succeptible to this.

Some of these possibilities can be controlled by the owner. Keeping on top of valve clearances is obvious. Less obvious is regular attention to the vernier adjustment of cam timing provided by Rover.

I think if I had a head in damaged condition I would want to update the valve seat materaial and valve material to modern standards. This is pretty standard when a head is converted to unleaded - but still worth checking! I think I'd also want to have valve guides made up from sintered iron and then to miss out altogether the valve stem oil seals.

I could also be persuaded to fit an exhaust system with better flow characteristics - I'm sure the standard Rover one is excessively restrictive. This solution doesn't necessarily mean a louder exhaust, just free-er flowing.

Any other views on this?

Chris
 
chrisyork said:
I could also be persuaded to fit an exhaust system with better flow characteristics - I'm sure the standard Rover one is excessively restrictive. This solution doesn't necessarily mean a louder exhaust, just free-er flowing.

Any other views on this?

Chris
Click to expand...

Indeed, when the standard rear silencer failed in my TC, we sectioned it in my friendly exhaust shop, to get an idea on the design.
We couldn't believe how restrive it was! So we decided to replace it with a straight through glasspack, with the largest permissible can diameter.
Guess what, it was more quiet than the stock item, and the free flowing nature of the system went well with the slightly richer needles of the 2200 carbs. On the other hand purists will miss the characteristic rortyness of the standard TC exhaust.
I must point here that the system also incorporates a small front straight through can, similar with the standard factory late 3 can system. Perhaps a rear straight through silencer may be a little noisy in a 2 can system, but i didn't had the chance to find out.
 
Glad that you found the problem and it shouldn't be too hard to fix. The following info on "Why Valves Burn" might help?
http://autos.yahoo.com/maintain/repairqa/engine/ques053_1.html

So, in addition to all the things Chris suggests, I wonder if it could just be normal engine wear, too lean a fuel mixture, carbon build up leading to overheating on the damaged valve, pinking/pre-ignition on that cylinder again causing overheating?
 
Running on unleaded causing valve seat damage ?
Hardly the car's fault considering it's age and that it was built to use 4 star leaded
 
They burnt out valves on 5*, so although unleaded may add to the problem it's nothing new. It's always a combination of factors.
 
One thing I did notice when I used a Lambda tester on the car was with the standard needle the carb is very lean at the top end, dangerously so in fact. I got it into the lowest end of the acceptable range of mixture at the top end causing a very rich mixture in the lower revs. I'm going to experiment with different needles and the Lambda tester when it's back together again. May go some way to possibly explaining this :?:
 
That most certainly won't help!

As all models were susceptible to the problem, I don't think we can be looking at a 2000SC specific design defect here. Perhaps another explanation as to why yours is weak at the top end is required. How about incorrect fuel level in the float chamber or a weak fuel pump? At low revs you'd be able to compensate for these with the mixture screw. At high revs the mixture adjustment has relatively little impact - hence why you'd be way over rich at idle if you did manage to compensate it out.

Chris
 
webmaster said:
How about wear in the carb piston, causing less lift.
Click to expand...

Or incorrect grade of oil in the dashpot.
When I had the HS8's, it didn't matter if I put 3in1 or 20/50 in the dashpots, the fall times were much the same. With the HIF6's, if I put 3in1 in them, the fall time is pretty much 9.81 m/s2 (!!!), but if I put 20/50 in, the resistance is so fierce, it hurts your fingers to lift the piston! The result of this was spark plugs with a clean electrodes, but sooted around the outside, suggesting that the adjustment of the jets was good, but the slow lift/fall times of the pistons caused exessively rich running for short periods and sooted everything up. This was noticable in the driving, but disguised by the fact that after a few minutes at idle the spark electrodes were 'cleaned' sufficiently (by correctly adjusted mixture) to disguise the problem. It might be worth checking this (along with the other areas). As Chris says, the mixture screw only really affects the idle, as it is the lift and fall times/rates/extents of the carb piston (cm movements) that affects richness more than the relative height of the jet (mm movements) when putting the engine under load.

For what it's worth, I'll be running 15/40 in my carbs to test the difference, and going down to 10, or even blending if necessary.

Michael
 
Hi Michael. The wrong oil in the dashpots won't affect the final level of the piston, just make the throttle response odd. So it shouldn't affect the high rpm mixture. I seem to remember someone saying that auto transmission fluid is just right?

It could be worth doing some plug cuts - ie steady speed, say 60 mph, and kill the engine and drop the clutch so as to preserve the plug colour when running. I was thinking about the Lambda sensor result - if it was taken off load simply by revving the engine, it might not be a very good indicator.

Chris
 
Hi Chris. I see what you're saying about final piston height, but my concern is that with it taking at least 8 seconds for the pistons to fall (cleanly & smoothly) from top to bottom, the piston height changes won't necessarily synchronize with throttle position (and vacuum) if these changes occur before the piston catches up. To my mind, accelerating up through the gears caused the pistons to rise and then fall too slowly to weaken the mixture sufficiently before the next up-change, causing very rich running in town conditions. This was further highlighted to me after the sparks were clean and healthy after a steady 60mph run on the bypass. It's basically the same as the pistons being momentarily stuck. I tried it with 3-in-1 and the sooting disappeared, but the throttle response was too erratic for my liking.
Was I onto something there?

Michael
 
You're idea of 15/40 sounds good - but as I said, others seem to rate the auto trans fluid. Sorry, but i don't know what viscosity it is, and you won't have any, will you!

Chris
 
i believe the auto trans fluid was good because its a single viscosity accross the temp range. It was either SAE20 or SAE 50 - cant remember which offhand!
 
Rich is right. ATF is SAE20 and doesn't change viscosity with tempearture, but my argument is that if there is THAT much heat across the carb bridge, you should be addressing something more fundamental than the dashpot oil!

At the risk of getting too far off-topic, I think the issue with my carbs is that the spacers on the end of the damper plungers are slightly too big to allow SAE20 past without excessive restriction. I'm going to experiment with different oils, but if not I'll try and source some smaller billets.

Michael
 
Because one of the arguments for using ATF in place of 20/50 is because it doesn't change viscosity with heat. But if there was sufficient heat in the dashpot to cause the oil to thin, I'd be more worried about fuel vapourisation than damping. it was just a side-point really :) michael
 
As Harvey says, plenty went like that when they were a current car on leaded :)

Another example of why a cylinder leakage test tells you much more than a compression test IMO

It was my hunch though, due to the exhaust note on tickover.
If you'd pulled the head earlier it wouldn't have looked quite so gruesome but still would have needed the valve at least.

For a daily user it lasted well and you'll be very impressed with the correct performance :)
 
You must log in or register to reply here.
Back
Top