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MG MGA - Ignition knock?

My car has a high compression (8.8:1) 18GB engine which I run on 91 octane fuel. I use a 25D4 distributor with a Pertronics Ignitor and have the timing set at 14 BTDC (vaccuum disconnected). The car has been running like a dream all summer.

Yesterday I was down to under 1/4 tank and stopped to fill up at a local Shell station, where I often buy fuel. Filled up with what I thought was 91 octane V power. Drove off and all was fine. Perhaps 3-5 miles later I started to notice a pinging noise on acceleration. By the time I got back home (about another 5 miles) it was pinging on even mild accelaration, even in low gear.

My initial thought was that either the pump at the Shell station had pumped out low octane fuel in error, or I had accidentaly selected the wrong fuel grade. Either way, I figured I had a tank full of low octane.

Today I siphoned out all but about a gallon or so and transferred it to my SUV (which runs on 87). Then I went to a near-by PetroCan station and filled up a 5 gallon can with Ultra 94, which I brought home and dumped into my MGA tank. I also retarded the timing by about 3-4 degrees and took the car out for a drive.

I expected that within the first mile or so the pinging would gradually disappear as the higher octane fuel made it way to the carbs. Unfortunately that does seem to have happened. I drove gently for about 3 miles and still the same thing.

Idling is fine. Reving the engine while stopped is fine. Under even mild acceleration in ALL gears I hear a metallic rattling/pinging noise which I assume to be ignition knock. I also noticed that the engine temperature seems to be about 10 degrees above where it normally sits.

I'm at a bit of a loss right now to understand what's up or what to do about it. Any ideas?
Andy Bounsall

Andy, presumably you have enough coolant and the pump is circulating it?
Retarding the ignition may have caused an increase in running temperature, so that could be a red herring. Pinking could be caused by something in the combustion chamber glowing hot and pre-igniting the mixture. Check your plugs for broken insulators or a build-up of carbon.
Lindsay Sampford

A lean mixture will cause higher cylinder temps and may cause pinging. Try gently pulling out the choke to richen the mixture and see it the pinging diminishes.

- Ken
Ken Doris

Check that the mechanical advance is free and lubricated. Very commonly neglected, especially on cars with electronic ignition; it still needs the drop of oil every 3000 or whatever. If it is sticking, you can wind up setting timing with the cam stuck in any possible position in the range, giving very large errors. These then become greater when/if the thing moves whilst running, a random variation. Also possible that an advance spring has broken, but that is fairly rare.

FRM
FR Millmore

Lindsay: Coolant level looked good. Topped it up just a bit while I was there. Checked the plugs, they look fine.

Ken: Tried pulling the choke while driving to richen the mixture. No noticable effect.

Have tried changing the distributor and Pertonix for my spare. Also retarded the timing by 8 degrees. Car idles very rough like this (obviously). Pinging is lessend a bit(didn't hear it in 1st gear), but it's still there.

Could it be due to low octane fuel (which remember I don't know for sure was the case to begin with) even after having mixed 5 gallons of 94 with maybe 2 gallons of at whatever was in the tank?

I'm really kind of stumped at the moment.
Andy Bounsall

Andy, before you invest to much time and effort into the pinking, it would probably be worth draining the tank and refilling with the highest octane unleaded available just to see if this fixes the problem. You can always use the old fuel in a petrol lawn mower or something.

Or you could try putting some octane booster into your tank to see if it helps.

Colyn
Colyn Firth

When troubleshooting, make one change at time and log results before next change.. Recommend that you set the timing back to 14 BTDC (vaccuum disconnected)and go from there.

Russ
Russ Carnes

That's excellent advise about making 1 change at a time and recording the results Russ. I am doing just that.

I believe I've pretty much ruled out the low octane fuel possibility. Adding NOS Racing Formula octane booster had no noticable effect.

Re my previous comment about trying my "spare" distributor. Without going too far into it, that part is a bit suspect. I have been able to borrow a known working distributor and pertronix from a friend. Will try fitting that unit tonight and report back.
Andy Bounsall

No idea if this is right - but just throwing it out there and thinking out loud...so flame away...

Broken timing chain tensioner? Might have the same effect of the timing chain skipping a tooth...or in fact allow it to skip a tooth.

Since the problem had a sudden onset, and can't be remedied easily, I would start looking things that might have let go or broken.

Take your dizzy apart to see if you lost an advance weight spring, or the vac advance spring popped off the post on the plate.

The other thing might be to check the coil. Not sure why a failed coil would cause pinging, but there seem to have been a bunch of them going bad, IIRC...weak spark leading to unburned fuel? OK, this one is probably wrong.

Could you have burned up a valve on one cylinder or had some other mechanical fault in the valve train? Even just one cylinder where the valve lash is way out of spec. Try resetting your valve lash to .015" across the board. In fact, do a general tune up to see what you discover...maybe pull your electronic ignition and go back to points for the time being to rule that out.

If you have been running rich for a long time, you might have carbon build up in your cylinder head which can create hot spots and pinging. You can de-carbon the head by pouring a small bit of water through the carbs (there are other remedies as well...I am sure you can find some threads on this). I have been told that running REALLY rich will cause pinging just because of the amount of unburned fuel in the cylinders. (My car is set up a bit rich right now, but I have had no pinging...)

Can't think of anything else off-hand...proabably a good thing.

JIM in NH
AJ Mail

Swapped distributor and Pertonix for known-to-be-working units last evening. Problem persists, so that rules out misbehaving distributor, mechanical/vacuum advance, and ignition module.

My current plan of attack:
1) replace spark plugs. They look fine, but I have another set so it's an easy way to rule out possibility of a hairline cracked insulator.
2) compression test. Won't fix anything, but might provide a further clue.
3) check/set valve lash.
4) check fuel mixture. I have a gunson ColourTune so I'll see how that looks.
5) decoke/decarbonize the engine. I don't know much about this, but recall reading that it can be done by drizzling a glass of water into the carb with hot engine running at moderate speed.

That'll pretty much cover all of the "easy" to fix possibilities I can think of. I have to admit that my gut feeling is that it's something more sinister, although I've no idea what.

Jim, thanks for the ideas. Pardon my ignorance here, but if the timing chain had somehow skipped a tooth, how does that translate to pinging on mild acceleration?
Andy Bounsall

would be like having your timing wayyy off.

You actually use a squirt bottle (like for windex) to "meter" the water into the carb throats very easily. You should get some results out of the tailpipe to let you know you are on the right track. Don't hydro-lock your engine...warm up the engine first and then let it run for a while after. You can end up using a lot of water (like a couple of cups) before you are done...go slow.

The condition of the spark plugs should give you an indication of the condition of the cylinders...but if it's bad, you could pull the head and have a look see - then clean mechanically if needed.

here's a vid - sure there are many out there.

http://www.youtube.com/watch?v=Ugn8TI0TMio



JIM in NH
AJ Mail

Valve lash checked and it's all good.

Compression test results:
#1 => 165
#2 => 150
#3 => 80
#4 => 80

Uh, oh! I guess this is the something more sinister I was half expecting. Does the low compression on adjacent cylinders mean a blown head gasket or could it suggest something else?
Andy Bounsall

There you go Andy, blown head gasket, not uncommon, been there, done that, got the teeshirt! The copper/ asbestos sanwich gaskets seem to be particularly prone to that. Mine went within a couple of thousand miles, despite the head being skimmed. I reccommend the black composite head gaskets that Barney mentions on his site. There were an upgrade for the MGB engine, and although designed for the 1800 engine, will work fine on the 1500.
Lindsay Sampford

This happened to me as well, head gasket failed on a road trip, leaked anti-freeze into cyl 3, started with what sounded like pinking, ended up seizing piston and breaking the crankshaft ! Apparently burning antifreeze in the cylinder gums up the walls pretty bad.

Car then ran for 15 miles with broken crankshaft and 3 cylinders, actually got me home.

Be sure to check condition of cylinder walls if you replace the head gasket yourself.

-Chuck
Chuck Mosher

Hooray - it's an easy fix anyway (trying to look at the bright side). At leas you got an answer!

JIM in NH
AJ Mail

You might want to consider replacing the cylinder head studs if they have been in the engine for awhile. I had one (replaced 1994 - probably from China) break last year and it was a chore to get it out. I ended up replacing them all with high tensile studs, nuts, and washers from APT http://aptfast.com/ . I ended up talking on the phone with them before oredering the correct set. The set cost me around $200 but the piece of mind is priceless. The set came with lubricant and only required 40 ft-lbs of torque using the lubricant. I have really been satisfied.

Good luck.
Don Carlberg
Don Carlberg

To provide closure, the head gasket had a definite blow-out between 3 & 4 (see photo). No other surprises found lurking when I pulled the head. Gasket replaced and I'm back on the road. I just returned from an 80 mile run and she's purring like a kitten again!



Andy Bounsall

OUCH! Well at least it is not too $$$ a fix.

BTW, I filled up at a Shell station last week in my DD.I don't usually stop there but it was a convenient stop. I caught myself accidentally going to fill it with midgrade instead of the low grade stuff I normally use in that car. It seems at Shell, they put the midgrade to the left, the low grade in the middle and the high grade to the right. Possiby in hopes to get a few extra bucks per fillup by the unsuspecting (like me)? Perhaps you did fill with lowgrade if you instinctively filled from the middle selection.
Chuck Schaefer

Chuck, I'm now convinced that I didn't buy low octane fuel. I always run on 91 octane and normally buy fill up at Shell since their 91 VPower is one of the few fuels in these parts that contains no ethanol.
Andy Bounsall

Andy, I find that pic a little disturbing because I think you've used the best MGA/MGB gasket that Barney and others all recommend as preferably to the copper gaskets. I don't think it should have failed like that all things being equal. Have you done a compression test after new gasket was fitted?
J H Cole

JH, compression test following gasket replacement: #1=>165, #2=>160, #3=>165, #4=>160

Perhaps a little background info is in order...Last fall I bought a Judson supercharger which was rebuilt over the winter and installed on my car late in May. Those who followed the thread at the time might remember that I had trouble trying to get it to run properly and was unable to get the pinging/detonation under control. As I wanted to drive the car for the summer rather than muck about with it, I removed the supercharger and reinstalled the SU's early in June.

Now I didn't put alot of miles on the car with the Judson, but on one outing on a VERY hot day it started pinging like crazy. I returned home and the very next morning, pulled the Judson off. The car had been running great since then until this happened. Of course I have no way of proving it one way or the other, but my suspicion is that the gasket might have been at least partially compromised by that episode.
Andy Bounsall

JH, that's the sort of head gasket I have got as well. I hope Andy's HGF is a one off!
Lindsay Sampford

That's a very good set of results Andy, is your engine fairly new? Do you do the test wet, plugs out, throttle open etc. I'm lucky if I can reach 150 psi on my middle aged engine.
J H Cole

JH, the engine was rebuilt spring 2009 and has done about 13,000 miles since then. The test was done dry, plugs out, throttle open.
Andy Bounsall

The Judson is very good at highlighting existing engine faults. And wrecking your engine if it is running too lean or with the wrong ignition settings

If the timing is too advanced, the mixture weak or the fuel octane is too low, predetonation will occur, with devastating results for the engine (either gaskets or pistons)

From my experience

1. run rich
2. Use the highest Octane fuel that is available - I use 98 octane standard and have no detonation problems, with 95 Octane it didn't run too well.
3. Keep the advance in the conservative range.

I am working with a variety of jets in the Holley to try and figure out the best configuration. I even bought a professional exhaust gas tester on ebay to be able to test the results and tune the engine optimally, but have had problems getting the time to get on with it, and have just had to pass the Judson casing for crack testing and a possible welding job around the engine manifold mounts, so am running a bit behind plan. Maybe you should move to a richer jet (easy to change and cheap to buy). I'd be interested to know which jet you are running (it should be stamped on the side of the jet) and where your timing is set

dominic clancy

That gasket failure worries me too. When they fail they usually go between #2 & #3 at the hot sport between two exhaust valves. Failure between #3 & #4 would make me suspect the hear is not perfectly flt there. I suggest you carry the compression tester in the car and do a check occasionally. If it fails again you probably get the head milled.
Barney Gaylord

Barney, thanks for that. The head and deck were both skimmed when the engine was rebuilt. I'll keep an eye on it but is HG failure something that happens gradually? I'd have thought it was an all-or-nothing kind of thing with the failure occurring suddenly.

Dominic, let's start a new topic re Judson setup.
Andy Bounsall

The Judson is probably a very relevant issue here - the extra compression will cause preignition in an engine setup that was apparently OK before the Judson was fitted
dominic clancy

This thread was discussed between 03/09/2011 and 13/09/2011

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