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MG MGB Technical - 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 pertronix Ignitor and have the timing set at 14 BTDC (vaccuum disconnected). The car has been running like a dream all summer.

On Friday 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.

I siphoned out all but about a gallon or two 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 gas 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 a bit stumped right now to understand what's up or what to do about it. Any ideas?
Andy Bounsall

Andy I would try the most simple things first. If its not loosing water or running uneven I would, because of the 10* increase in temperature, remove the thermostat and give it a try. I would also check the total advance at 3000 revs and set it at 32*. If you still have problems you will have to look deeper into it. For it to ping even under light load usually points to a over heating problem. Denis
Denis4

My guess is that your centrifugal advance has stuck in the full advance mode. Try removing your distributor cap and turning the rotor clockwise. It should turn about 30 degrees with very little resistance. If not, remove the rotor and apply some light oil to the wick beneath the rotor. Let it sit for a while and repeat the process. I think that filling up your fuel tank had nothing to do with your problem. At least your SUV got some good fuel oft of the ordeal. RAY
rjm RAY

Andy-
The aforementioned suggestions are good and worth checking out, and here's another one. E-10 fuel makes a potent stripping agent for the old varnish and gum that has accumulated from years of gasoline sitting in the tank. These organic contaminants, once loosened from the walls of the tank, can plug fuel filters and carburetor fuel jets quickly. If you aren't using a fuel filter, some sediment may have worked its way into one of the carburetors and it partially blocking one or both of the fuel jets, causing a lean running condition. Disconnect the fuel lines from the float bowls and spray some carburetor cleaner down the fuel jet at the jet bridge of both of the carburetors in order to clean them out. If this cures the problem, be sure to clean out the float bowls and the fuel lines all the way back to the tank.
Stephen Strange

Hi Andy
You write '91 Vpower' - in Western Europe Shell's Vpower is octane 99 - if it was 91 octane are you sure it was Vpower - of course could be different regulations for Canada. 91 octane seems very low for petrol...just a thought
/Moss
Moss

Moss
Eu an us/canada have differnt ratings.
There 91 is high simulair to our ron 98

Iirc we use the RON (research octane number) and they use the MON (motoring octane number) figure.
Onno Könemann

Thanks for the ideas. Keep them coming...

I have pretty much ruled out the low octane fuel possibility. Added some NOS Racing Formula octane booster and it had no noticable effect on the problem.

Ray: mechanical advance on the ditributor appears to be operational. Rotor turns counter-clockwise a few degrees with little resistance. Checked with timing light and vacuum disconnected. I see the timing mark advancing as engine rpm is increased. Reconnected vacuum advance and checked again. Timing mark seemed to be much less stable and I thought I saw it jump to a significantly advanced position for a split second a couple of times. Not sure what that indicates, but...

I've borrowed a know working distributor and Pertonix unit from a friend. Will try fitting that tonight to isolate or eliminate the possibility of a misbehaving distributor.
Andy Bounsall

Moss: as Onno said, octane ratings are different in Europe and North America. I believe the ratings used here are (RON + MON)/2. Our 'regular' fuel is 87 octane, mid-grade is 89, and premium is 91. Shell V Power is rated at 91 octane here. 94 octane fuel is available at a few select stations (Sunoco in the US, PetroCanada here in Canada).
Andy Bounsall

As I wrote ''of course could be different regulations for Canada''...in the sense that the content and definition of fuel is regulated on a different basis... :)
/Moss
Moss

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.

Steven: I do have a fuel filter installed back by the pump. I haven't checked it yet, but it should catch any bits of junk coming out of the tank, I think.
Andy Bounsall

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

It could be a blown head gasket which you can check when you remove the head or it could be burnt valves. To check the valves support the head inverted on blocks and pour some kerosene into the combustion chambers and see what comes out of the ports. Failing this it is down to bores and rings. If there is little or no bore wear then you may get off with a re ring which you can do without removing the engine. By the way how many miles has this motot done or how long is it since it has been rebuilt?
Iain MacKintosh

My car blew its head gasket between no 3 and 4 back in the 70s. A payen head gasket, Oselli stage II head and a surface block cured that. It showed up as a general lethargic response though, a tune and resetting the valves seemed to cure it until 3 and 4 stopped firing.
Stan Best

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

This thread was discussed between 04/09/2011 and 10/09/2011

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