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MG Midget and Sprite Technical - 1966 1098 engine 'pinging' problem

Hi all, it been awhile since I've been on this BB. I've enjoyed my Sprie for about 4500 miles since the rebuild. A few weeks ago, the engine would momentarily cut out, then recover immediately. I found the spade terminal connection to be loose on the positive side of the coil. Once crimped more tightly, that problem was resolved, but I develped a persistent "pinging" that is typically associated with advanced timing. I continued to retard the timing until the engine ran poorly, but the detonation sound persisted. I added octane boost to assure I didn't have low-grade gas. I then adjusted the rocker/valve stem gap; then coil and plugs.... still horrible detonation. Other than an internal issue, the only other thing I can think of is lean fuel mixture??? Any thoughts.. not sure the SUs have high-speed adjustment? I engaged full choke while driving and it does seem to eliminate the dteonation sound... BTW I looked for air leaks and haven't identified anything obvious.

Thank you in advance.

TS Smith

"I engaged full choke while driving and it does seem to eliminate the dteonation sound..."

With everything else you've already checked excluded, if you can drive on full choke when the engine's hot, then it must running be way too weak off choke.

I Suggest you level the jets back to the bridge, and unwind them 11 flats, then see how it goes from there.

I'd also check for decent fuel flow. Pull the pipe of the carb and see if you get decent flow into a jar.

If that doesn't eliminate it, again with everthing else you already checked excluded, I'd be looking at what your compression ratio might be.

So TS, are you the driver in the picture?
Lawrence Slater

A quick, rough, check for mixture is to put your hand in the exhaust gas coming out of the tailpipe while the engine is idling.

cold and wet is rich

dry and very hot is lean

damp and warm is about right

Malc Gilliver

Have you checked you have oil in the SU damper pots ?
Malc Gilliver

Thank you all.

Plugs are white, engine is running hot and uneven. Exhaust seems hot and dry.

I added a little oil to the pots, made no difference.

When I raised the pistons (before I knew what I was looking for)the engine trys to stall almost instantly, especially with the front carb.

Given your feedback... I'll assume a lean mixture issue and move the jets out a bit.

BTW Lawrence, thats my daughter (driver) and her friend in the car :). She loves the car.
TS Smith

Well, carbs were definitly too lean... richened until lifting the pistons raised RPM slightly, then leveled out. Drove car, still knocked. Intentionally over-richened, still knocked. Retarded timing, much of the knocking subsided, but engine lost power. Pulled plugs, 1,2 look great... 3 is dark and 4 white. Checked compression, 1 and 2 about 100PSI, 3 and 4 about 60 PSI. Oil is clean.

Looks like I'm pulling the head.. starting with the intake to look for a leak... but low compression with a white plug?? And for 3 and 4 to both be low is odd to me with one rich plug and one very lean plug.

Maybe what I think is pinging or knocking is something worse; but it occurs only under load and can be controlled with throttle as would be expected with detonation.

Please share any thoughts or suggestions.

TS Smith

Did you level the jets to the bridge before lowering either of them?

If you've got significant oil in number 3, that would give you the symptoms. Oil leaking down the valve stem or head gasket gone maybe. Yup, as the used to say in the good ol' day, off with it's head.
Lawrence Slater

First thoughts, 1&2 are 100psi, 3&4 are 60psi - that looks like the head gasket has blown between 3&4.

It might be more theoretical that practical, but the did you check compression with the throttles open or closed ?

I'm wondering if the idle adjustment on the rear carb is sufficiently different from the front carb so as to restrict the airflow into the rear cylinders so generating a lower pressure.

The only way to check is to fully open the throttles when cranking the engine to check the compression

Can't think of a reason why one is rich and the other is lean on the same carb unless you have a non standard intake manifold on or a badly gas flowed intake port in the head.
Malc Gilliver

All the compression readings are low, and the 60s are very low. The generally low figures do suggest that you didn't do it with a wide open throttle. You also do the compression checks with a good battery and all the plugs removed and so that the engine spins faster. I always do mine allowing the engine to spin through 10 rotations for each check, and preferably with the engine previously warmed up.

With a head gasket blown between 3 & 4 you would still get some compression reading, and 60 looks about right for that, although it could also be burnt valves. Both could make a noise that could sound a bit like pinging.
Guy W

I'm with Malc. Head gasket blown between cylinders 3&4 or 1&2 gives a bad pinging sound. It's becuase no 3 on firing stroke leaks through into no 4 which is on compression. That ignites the mixture in no 4 early, hence the noise. When badly blown it sounds diabolical.
Paul Walbran


I had an MGB cylinder head gasket go between 3 and 4 years ago and that made a distinctive 'blown exhaust' noise. I didn't consider the relative firing order would be the cause.
Malc Gilliver

Lawrence, not sure what you mean by leveling the jets to the bridge, I simply adjusted the jet (out). My Compression test was with throttle at idle; I will re-test with throttles open. Intake manifold is stock and unmodified.

The battery is strong (and all plugs removed) engine turns over well, but I only turned over maybe half-dozen revs per cylinder check; I'll shoot for twice that tonight and see what I have.

What would be considered normal compression?

Paul, your theory sounds dead on... the 3 plug is dark (weak fire) the 4 plug is white and the pinging sounds like its at the firewall....

Thanks again to everyone for sharing your experience!
TS Smith

TS. In order to establish the correct mixure and balance the carbs, you need to set the jets to the same level. You do that by raising the jets to the bridge of the carbs. The bridge is the flat section that the pistons rest on when not lifted by air passage going under them.

So remove the air filters. Lift the piston and wind up the mixure nut until you see the jet appear flush with the bridge. Do the same for both carbs. Then wind both down 11 flats of the nuts. This is the base setting you start from to set the mixture.
Lawrence Slater

Lawrence, thank you for the clarification; I'll be sure to do that as a baseline. I noticed that when I was working on the carbs, using a "hose" to listen to the air velocity at the throat of each carb, the front carb seemed noticeably weaker than the rear. By loosening all of the linkage hardware and baselining the idle adjustment I was able to get them closer to "balanced". I am real interested in establishing the baseline for the jets as well.
TS Smith

TS, you may find this link useful.

And there are other pages on the same site that you might find useful too.

But if you've got a bad head gasket, or leaky valve, or low compression in one/two cylinders, balancing the flow through the carbs may prove difficult.
Lawrence Slater

I think I'll pull the head as my next step. Once I know everything is back in order, I'll work toward fine tuning.

I had the block decked... but I purchased the head as built; so I'm not sure if it had an issue. Unfortunatly the guy (professional) I bought it from passed away...

If the gasket is blown, I'll probably take it over to the machine shop and make sure its true.

Interesting that I got about 4200 miles on it prior.

TS Smith

Here's a link ( I hope) to a pdf of the original "SU BUTEC Type HS Carburetters Tuning and Servicing" manual I copied some years ago.

When you click the link it'll take a while, as it's 12 pages and scanned at A4 paper size, which is like US letter, but not quite so you might need to resize.

Malc Gilliver

Thank you Malc, printed perfectly. Good instructions.
TS Smith

I find that this site has a lot of good information including a SU tuning guide in the library tab.
Pete Ottewell

All of which is the same information on the SU site I linked below. But that book is a good one Pete. I've just downloaded it.
Lawrence Slater

Thanks Pete,

Nice site, bookmarked.

Malc Gilliver

Those who have been around on this BBS site will no doubt remember that the owner of the "sprite guru" site pays scant regard for copyright, or more importantly even for the basic politeness of asking for permissions before filching stuff that others have worked hard to put together. Add to that in my opinion, there is lots of confusion and inaccuracy on the website. I wouldn't recommend it.
Guy W

Didn't know that Guy.
Lawrence Slater

Sorry Guy, just trying to help. I have an original printed copy of the SU guide, so my concience is clear on the copyright of that one at least :).
Pete Ottewell

Not sure I am truly following events, but white plugs suggest a very lean mix to me. This in turn leads to a hot engine and possibly overheating and for certain increases the prospect of "pinging" or pre-detonation - often heard under load or at a particular rpm.

If this is indeed the case, you need to ensure the mix is correct and possibly enrichen it. Failing that, check the timing and retard it if necessary. By all means balance the carbs as required, but cannot remember which you do first....balance or mixture
Mark O

Pete, I wasn't having a go at you at all. Far from it!
But I was just warning that there may be good reasons for not using that site. Moral ones at least! And given the attitude of the owner I would worry about what else one might inadvertently pick up from there. Anyway I have said as much as I am going to about it.
Guy W

This thread was discussed between 11/08/2013 and 14/08/2013

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