Pinking/detonation

[ndevans]

How do I eliminate pinking as the cause of a noise in my Minor?

It happens when accelerating, most noticeable in 3rd when accelerating up from about 25 to 50mph. I am using 97 octane premium fuel, I have a 1275 with a big valve head off a Metro, HIF 4 carb, NGK BP 6 plugs, ignition timing set to about 3° btdc, 25D4 distributor has 10° mech adv plus another 7 from the vac unit.

[svenedin]

Hello Neil

Realistically all you can really do is retard a bit and try again. With the Lucas 25D4 you do have the micrometer so you have very fine control. I can never remember whether it is 12 or 13 clicks of the micrometer that changes the timing by 1 degree. You could not achieve that precision by moving the distributor in its clamp.

Are you sure it's actually pinking and not some other rattle? I think mine is pinking going uphill in low gear but I'm not absolutely convinced that it isn't just another rattle.

The book figures for the ignition timing are only a useful starting point really because when engines have been rebuilt and changed they may have different compression to when they came out of the factory. For instance, my engine is a standard 1098cc which as a worn engine had compressions of around 150 psi. Now it is more like 185 psi. So it is more prone to pinking with the raised compression.

For a little used car you could use some Tetraboost. This is actual tetraethyl lead and it raises the octane rating of the fuel. The lead also acts as a valve lubricant. It is expensive and would be impractical for a car in daily use. I do use it when I remember as a means of prolonging the life of the engine. Obviously it's not very eco friendly but the number of cars using lead is vanishingly low. Just be aware that the concentrate is poisonous. Wear gloves when handling and immediately wash off any splashes on skin.

My only other suggestion is to try another strobe. I have two of them and they do not agree with each other! Another possibility, if the engine has a different pulley to standard is that the pulley mark isn't in the right place so you think it's 3 degrees BTDC but it's actually more advanced than that. Just a thought as my engine has a damped pulley that I had to make a new mark on as it was for an engine with the timing marks in a different place on the timing cover.

Stephen

[ndevans]

All good points Stephen, I'm not sure it's pinking, hence the question "how do I eliminate it as a cause!"
I just don't know whether it is pinking or some other rattle caused by something in or on the engine or gearbox. All I can say is it happens when accelerating in all gears, but it's most noticeable in 3rd between 30 & 50. I can't tell precisely where it is coming from either.
Regarding the pulley, when I rebuilt the engine I measured the pulley, drew a cardboard cut out replica, mounted it on the timing cover and used a protractor to measure the angle made by each of the five timing marks on the timing cover. They are at intervals of 4° from 0 to 20°. The timing mark on the pulley aligns with the 0° pointer at tdc (I was most careful to check this when I reassembled it).
I have set the static timing to approx 3° and the timing light seems to indicate it advances to about 20° when I rev the engine.
I will retard the static timing back to 0° and try it again.

[svenedin]

It seems that your advance should be within safe limits at a maximum of 20 degrees advance in fact rather conservative. The chap who rebuilt my engine told me that he wasn't too concerned about what the advance was at slow idle (vacuum disconnected) but that the advance should not exceed 30 degrees maximum when revved and vacuum advance operating. Anyway, I'm not an engine expert by any means and there is a lot of variation between engines so my figures may not apply to your set-up and mine is a 1098 not a 1275. For my own issue I am going to clear some stuff out the car because it might be something as silly as something in the glovebox or a loose speedo securing screw. I did ask a few people but they all seemed to have high frequency hearing loss and couldn't hear it! There are lots of rattles that are actually just resonance at the right frequencies and they go away as the engine speed decreases or increases only being present in a certain rev band. I am even contemplating it might be something to do with my exhaust. I'll get the strobe out again later this week and if I still can't sort it I'll ask ESM what they think when they test drive the car after doing the gearbox rear seal. Dan at ESM said they have trouble with engines rebuilt by a certain company pinking due to raised compression. All they do is back off the timing in increments until it stops.

Stephen

[ndevans]

Timing retarded to 0°. Noise still present, possibly slightly better, difficult to judge though, but still audible especially in 3rd.
No detrimental impact on performance, as far as a 6 mile road test on mixed urban and d/c roads will allow.
Bit stumped
I am still not positive it is pinking/detonation, hence the question "how can I eliminate it as a source of noise?"
It's possible that it's coming from the gearbox/remote control area, but I just can't be sure. Having never knowingly heard pinking before, I have no way of knowing what it sounds like.

[svenedin]

Timing retarded to 0°. Noise still present, possibly slightly better, difficult to judge though, but still audible especially in 3rd.
No detrimental impact on performance, as far as a 6 mile road test on mixed urban and d/c roads will allow.
Bit stumped
I am still not positive it is pinking/detonation, hence the question "how can I eliminate it as a source of noise?"
It's possible that it's coming from the gearbox/remote control area, but I just can't be sure. Having never knowingly heard pinking before, I have no way of knowing what it sounds like.

They say it sounds a bit like the bubbles in an open metal fizzy drink can.

Hard to find videos of it but this one is quite good even though it's actually a moped: https://www.youtube.com/watch?v=lGC51mIQylk

Stephen

[ndevans]

I'd describe what I am hearing as a hissing but could be interpreted as a rattling. I've replaced the K&N filter element with a standard paper one, had no effect. There are no leaks as far as I can tell around the filter housing.

[MikeNash]

Dear N,
I've not heard detonation in a Minor but elsewhere it's been a clear "pinging" noise. One test you can do to check if it is pinking is to enrich the mixture when you hear it, which will suppress it. Older members may remember large piston engine aircraft, both military and civilian, smoking on take-off when max power was required with a full load and where an over-rich mixture gave an increased fuel octane rating (so suppressing detonation).
And you might like to scroll down this link https://www.lycoming.com/tips/detonation which describes the same inflight remedy. It suggests a weak mixture might be the (and your?) problem.
Regards from MikeN.

[ndevans]

Ah, so if I pull the choke out a bit when I hear it, that would enrich the mixture and suppress it if it is pinking?

[MikeNash]

Yes.
An advantage of the SU carburettor is that it doesn't use a "strangler" ahead of the jet to enrich the mixture for starting like many others do and which interferes with the airflow, but drops the jet so maintaining the air mass flow through the carb while you're changing the mixture.
A further thought; pinking can exhibit itself in a variety of noises e.g. tinkle, rumble and knock (which was the title of a learned paper
on detonation decades ago) so I could be wrong to describe it as a clear "pinging". And if things speed up or it runs better when you ease out the choke then that would show the mixture's weak at that point.
Of course, if none of this cures it, your noise might be something else, something loose - and you're about to disintegrate!
I hope you find the trouble soon, MikeN.

[ndevans]

I'll give it a go later.
Wonder whether a change of needle will help, if it's mixture related?

[svenedin]

Neil,

It's your thread obviously but just an update on my situation. As I said, I thought my engine was pinking on hills in high gear. Today I got the strobe out. 2 strobes actually and they both agreed the timing was quite a lot too advanced. As you know, you lent me your strobe in France but I didn't manage to set the timing with any great accuracy because the timing mark had lost most of its white paint and the timing seemed to wander. I have found out that my strobes don't give reliable readings on the copper cored ignition leads I was using (too much electromagnetic interference maybe) but today with standard Lucas suppressed leads (from ESM) I got a definite clear reading of too advanced. I have retarded back to about 4 degrees and will test drive.




Stephen

[svenedin]

I looked up in a Lucas technical manual how many clicks of the micrometer=1 degree of timing adjustment. It is 11 clicks per degree.

55 clicks moves the timing 1 gradation on the scale (arrowed) and is 5 degrees timing change

I happen to have a distributor on the table to play with and just playing with it seems to suggest that 1 gradation on the scale is not 55 clicks but rather less. I will research further.

If anybody is interested this is the original Lucas 25D4 distributor from my 1969 car, rebuilt by Distributor Doctor. It is the earlier "long post" type which uses 2-piece contact sets. Later distributors have a short post that uses 1-piece contact sets. Often the baseplate has been changed. The advantage of the 2-piece contact sets is that you can remove the heel contact for cleaning and access the other contact without affecting the points gap. Handy "in the field".

Regardless of the exact number of clicks, there is a lot of adjustment possible via the micrometer and it is far more precise than moving the distributor in the clamp.


Stephen

IMG_3487.jpeg (1.7 MiB) Viewed 3678 times

[ndevans]

Mike's post above prompted a rare instance of both brain cells working together at the same time.

If it's leaning out on acceleration that could be causing pinking, could the carb dashpot needed topping up? Yep, it was all but empty!

So I topped it up with engine oil and went for a run, but the noise was still there. Possibly slightly less audible, but difficult to judge. I tried pulling the choke out on acceleration, as suggested, made no difference.

So i think I'm going to re-investigate the gearbox remote control. I'm wondering if the plastic bush under the remote control lever has slipped out of place? Holding the gear lever when accelerating modulates the noise to some extent.

One more thing - I filled up on 99 octane Esso premium. I was on premium anyway, but there is no doubting it now.

[MikeNash]

Anyone interested further on aviation gasolines can follow me down this rabbit hole https://ww2aircraft.net/forum/threads/w ... ine.63909/ A couple of interesting points; wartime "Pool Petrol" for the ordinary user was about 68 -74 Octane, a bit variable, and which continued in use till 1950, i.e. within the design time of the early Minor engines. And at the end of the war there was fuel called Triptane reckoned to have a performance number of 200/300 and produced in small batches post war. Wow!
For further reading esp regarding the importance of anti-knock development to the overall superiority of allied airforces in WW2 I recommend the excellent book "The Secret Horsepower Race" by Calum E. Douglas (Tempest Books 2022).
Regards from MikeN.

[ndevans]

How do I eliminate pinking as the cause of a noise in my Minor?

It happens when accelerating, most noticeable in 3rd when accelerating up from about 25 to 50mph. I am using 97 octane premium fuel, I have a 1275 with a big valve head off a Metro, HIF44 carb, NGK BP 6 plugs, ignition timing set to about 3° btdc, 25D4 distributor has 10° mech adv plus another 7 from the vac unit.

Resurrecting this thread, I am starting to wonder if it is going lean when accelerating with wide open throttle (WOT)? That is exactly when I hear it, WOT under acceleration in all gears, most noticeable in 3rd between about 35-45 mph, but definitely present in all gears.

A bit of research has uncovered that the choke on the HIF44 does not drop the jet, it works in a different way and under WOT will not operate because the depression in the carb drops.

Two different AI apps (Copilot and Gemini) suggest that the BFY needle I am currently running is too lean in the mid ranges for my setup, and a BDL maybe more appropriate. I think I have one, once I manage to find it in the chaotic mess that I call a garage, costs nothing to try so that's my next step.

[59Elva]

You mentioned the shift lever might be rattling. My ‘77 MGB had a rattle on acceleration. Traced it to the exhaust manifold touching the steering column due to a soft motor mount. Does your car have the stabilizer link at the rear of the engine? Is it loose or have deteriorated rubber? Bad motor or transmission mounts might allow a shift under load and vibration.

[svenedin]

Are the any signs on the spark plugs of the mixture being weak? Some time ago, I tuned my engine to be spot-on at idle but higher up the rev range the mixture was weak. This was visible as very pale looking spark plug electrodes after a fast run. You have to stop and examine a spark plug immediately after a fast run. Of course they will be hot so be careful. It's no good going for a fast run, then driving through traffic and looking when you get home. You'll be seeing the spark plug evidence of an idle/low speed mixture.

When I took my Minor for a rolling road tune, I observed the whole process. The tuner agreed that the measuring equipment showed the mixture was indeed weak at higher revs/throttle wide open. He tried some needles he had in stock but these were the opposite and it was obvious the mixture was too rich (black smoke from the exhaust). In the end the tuner made a custom profile needle.

Stephen

[MikeNash]

N,
I'd follow 59Elva's suggestion; my exhaust goes close to the edge of the exit hole in the bodywork and for a spell, poor engine mount rubbers and a badly adjusted stabiliser allowed the engine to move sufficiently at high torque so the exhaust pipe rattled against the body. Took ages to sort.
Regarding detonation, I'm sorry I mislead you regarding the non-dropped jet on HiF44 carbs; I'm unfamiliar with them and had assumed they're like the older pattern. But a further thought. When you filled the dash pot I suppose you used SAE 20-50 engine oil. You might try a more viscose oil such as SAE 30 or 40, i.e. "straight" not multigrade. (DON'T THINK GEAR OILS ARE MORE VISCOSE. they aren't; the SAE numbers are misleading.) Plain oils are more difficult to find, but if you've a motor mower you've probably already got SAE30 for it. You only need a spoonful so it's a pricey buy for so little. Do you know anyone into stationary engines? They'll have some plain "straight" oils.
MikeN.

[ndevans]

N,
I'd follow 59Elva's suggestion; my exhaust goes close to the edge of the exit hole in the bodywork and for a spell, poor engine mount rubbers and a badly adjusted stabiliser allowed the engine to move sufficiently at high torque so the exhaust pipe rattled against the body. Took ages to sort.
Regarding detonation, I'm sorry I mislead you regarding the non-dropped jet on HiF44 carbs; I'm unfamiliar with them and had assumed they're like the older pattern. But a further thought. When you filled the dash pot I suppose you used SAE 20-50 engine oil. You might try a more viscose oil such as SAE 30 or 40, i.e. "straight" not multigrade. (DON'T THINK GEAR OILS ARE MORE VISCOSE. they aren't; the SAE numbers are misleading.) Plain oils are more difficult to find, but if you've a motor mower you've probably already got SAE30 for it. You only need a spoonful so it's a pricey buy for so little. Do you know anyone into stationary engines? They'll have some plain "straight" oils.
MikeN.

No apology necessary Mike, I didn't understand the way the HIF44 choke works until a day or two ago, so we've both learnt something here!
Regarding engine (and gearbox while we're at it), I've been down that road, all rubbers present and correct, nuts done up but not to the point of over compressing the rubber and the rubbers themselves are in good condition. There is no metal to metal contact at any of the engine or gearbox mounts or the the steady bar.
The exhaust is a manifold lcb and it's clear of the bodywork at all points, I'm happy that it is not the cause of what I'm hearing.
I've got 20W50 in the dashpot, I may have some SAE30 somewhere, not sure. Would jack or damper oil do instead?