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Tips

Cylinder number 3

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boc

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Hi Folks,

The spring rains have cleaned the roads of salt and we now get a glimmer of sunshine from time to time.... time to wake the sleeping beauty.
I took out the 4A for a little shakedown run about 2 weeks ago and felt a noticeable drop in power .. a bit sluggish. Checked the carbs, fuel delivery, ignition components... all fine. As the engine has about 650 break-in miles, i dropped it off at my mechanic for a once over and a compression check and just got a message from him that the #3 cylinder is way down at around 80 versus 150-ish for the others. Now the question is what could be causing this? All internals are new. Maybe a valve is not seated well.. or worse, the head is leaking ie. warped? Any comments are appreciated.

many thanks,
Brett
 
Certainly a stuck valve could do this and (I think) might occur after a long sit.

I've seen a mechanic smack a valve with a hammer (valve cover off, engine running) to get it unstuck. Not recommending anyone try this but it did immediately resolve the problem.
 
Could also be a ring issue. A bit of oil in the cylinder and re-run the test should confirm.

They could also do a leak-down test.
 
:iagree:
Not likely to be a warped head or leaking gasket, IMO; you'd see other problems before seeing that much drop in compression.

My guess is stuck ring, but as noted doing a "wet" compression test will tell you more.

I'd also plan on checking valve lash, as a stuck valve would probably show up as ghastly wide lash on that valve.
 
Not likely a valve or the compression would be near zero with a bad rattle. I vote stuck ring.

Try carb cleaner down the plug hole and crank it with the ignition off. (carefull as it will spray if you don't cover the hole with a rag while cranking). Next put in a good lightweight detergent oil and run it for a while.

John
 
Agreed, likely a stuck compression ring, usualy it's caused by fuel varnish or carbon in the ring grooves, as noted above, carb cleaner down the plug hole, turn it over with HT disconnected to spread out the cleaner, let it sit for couple of hours then get it on the road and get the revs up near red line for a few seconds, that generally does the trick!

Might be a good idea to change the fuel and the engine oil after that.
 
Pessimist that I am I would check the valve train to make sure the valves in # 3 are going up and down. You may be looking a a worn lobe on the camshaft. Bob
 
My head cracked between 2 & 3 causing low compression in cylinder no. 2.
Had it stitched and welded up and all was well again.
 
Andrew Mace said:
what is the currently accepted method to free said stuck ring ... short of disassembly?
Click to expand...

My choice would be to soak them in a suitable solvent, something like Rislone or Seafoam. Remove the plugs and dump a generous portion into each plug hole. Let soak overnight, then spin the engine with the starter before reinstalling the plugs.

Wouldn't hurt to do an oil change afterwards either.
 
Ok, have done the oil change, emptied last years petrol and refilled with 100 octane. Now to remove plugs and inject a bit of oil. I will let that sit a couple of days and then give it another try. If that doesnt work then its time to check the valve lash.

Thanks again for all your tips and suggestions.
 
while its sitting you can pull the valve cover and check and see if you notice anything out of the ordinary, I had an old volvo I was driving and about 1 mile from home it started running poorly, pulled the valve cover off and It had a broken valve spring on one valve, its worth investigating while you wait for the ring to hopefully get unstuck then you can rule out springs

Hondo
 
Sorry Hondo- i took a day off work to do the oil change and empty the petrol from the tank but have not been able to do more yet. Sitting in an airport tonight trying to get home for some leftovers. After digging in to the domestic chores tomorrow i will try to slip away from the wife to do some more work on the 4.
 
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