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 Posted: May 11, 2022 04:51AM
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CA
I am no expert on needle selection (my Mini tends to run rich).
Any engine (not just ones with SU carbs), if laboured (e.g. driving up a hill in too tall a gear) will make grumpy noises that tend to sound like engine knock. This is a result of the engine not being able to turn fast enough for the fuel/air charge being supplied. If you take the condition to the extreme, the engine will stall - with a clunk!

It is a kind of knock, but not due to pre-ignition - you can induce the same sound on a cold engine. It is not due to mechanical timing advance, because the engine is turning slowly and it is not due to vacuum advance because the throttle is wide open and vacuum would be at its lowest.

Modern, digitally controlled engines do not readily display this sound because the engine management system is listening for knock and fiddles with timing and air/fuel mixture to try to eliminate it. But if forced. the engine will stall the same way.

.

"Hang on a minute lads....I've got a great idea."

 Posted: May 10, 2022 05:07PM
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Dan , about the pinging test by going up hill in 4th slower then gunning it, won't it always ping in that situation even if the advance is correct and who drives like that anyways, I say that because even if I keep retarding the distributor it still pings when bogging down the engine up hill in 4th , could that mean my carb is too lean? i have a HIf44 carb 1-3/4" , needle# BDL , should I try a AAA or AAM needle?

 Posted: May 10, 2022 11:53AM
 Edited:  May 11, 2022 04:36AM
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CA
Nighthack:

I have to admit I don't know exactly what oil I currently have in my HIF44. (Should have written it down...).
However it would be a motor oil 5W30, 10W30 etc. Since my carb is relatively new it works well on an oil lighter than the 20W50 in the sump.



Cheers, Dan

.

"Hang on a minute lads....I've got a great idea."

 Posted: May 10, 2022 11:37AM
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Dan what type of oil do you in the dash pot? I have a 1275 69 cooper s engine

 Posted: Sep 2, 2014 01:12PM
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Thank you Dan. lots of great info.

regarding thermostat, the previous owner didn't run one. I found out while doing a full cooling upgrade (167°F thermostat, radiator, aux fan).  prior to that the engine would run up to 100~105°C during 2+ hr long trips. Now it's pretty steady around 85°C w/out the aux fan

 I will look into each of your points, especially on the ignition dwell.  Thanks again. much appreciated

 Posted: Sep 2, 2014 05:43AM
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CA

Are you by any chance running without a thermostat? That would tend to make the outer cylinders run at a different temperature than the inner ones, resulting in some difference. But i don't see that much from the photo - 1 and 4 seem to have some black in them - I wouldn't call any of them "extra lean".

A good mixture should produce a dry, light beige or tan colour on the central porcelain, but it does take time to accumlate - I see white with black on the left porcelain, and black scuzz around the tip of the threaded, which should be clean. if you have scuzz there, you probably have the same coating in your combustion chamber. (A very light coating of soot is common - it is hard to read plugs after a run because you let the engine idle wile coming back up your drive and into your work space. Some suggest doing a good run, shut off the engine and check the plugs at the side of the road. I don't do that.)


A non-vacuum dizzy is OK, assuming its advance curve is apporopriate for the engine. Vacuum advance only works at idle or part throttle for economy (it permits a leaner mixture for idle). Once you open the throttle, the vacuum dies away, a spring pulls the advance off and then the centrifugal advance takes over, advancing the timing in relation to engine rpms. (Rally or track cars aren't concerned with economy!) Each type and model of dizzy has its own advance curve, designed to suit the engine it was origially installed on. A MK IV with a 998 and a HS-4 would probably have been built with a vac-advance dizzy.

Start with the basics.:

  • make sure the vac port on the carb is capped it should be a small tube sticking straight up just behind the blue heater hose in your engine pic. put a small piece of tight-fitting hose on it with a small threaded bolt screwed into the end of the hose.
  • clean and re-gap the spark plugs.
  • re-install a clean air cleaner (I don't see one in your pic. - you need it for flame arrest as well as air cleaning, and tuning should be done with it in place.)
  • check you ignition dwell, with a dwell meter if you have one or by very carefully setting the points gap, making sure your point contacts are clean and very smooth (mirror smooth)
  • set idle to spec for checking timing. (Note where it says to disconnect and cap the vac advance for this... if you had vac advance.)
  • set the timing using a timing light. If you have a modern one (I don't!) that can check advance at various rpms, check the total advance at about 3500 to 4000 rpms does not exceed spec.
  • re-set idle to spec.
  • adjust idle mixture for good idle. If your HS-4 has a lift-pin, use it.
  • re-check idle speed as you adjust mixture.
  • test drive.
  • If you have poor accelleration, the next step is to try different weight oils in the dashpot damper. The oil controls the rate of lift of the dashpot, which pulls the mixture needle up at the same time as allowing more air past the bridge. A heavy oil will slow the lift which does limit the amout of fuel but more importantly limits the amount of air. A big vacuum is suddenly applied to the bridge area and the top of the main jet, producing a richness to assist acceleration (the accellerator pump effect). The reverse happens when you suddenly lift off the pedal, closing the throttle. Vacuum drops at the bridge with the dashpot way up in the carb and the main needle pulled out as far as it will go. The air flow is all but stopped but the fuel supply is wide open which produces an inappropriate mix - too rich or too lean (I'm not sure), but not good.  My 1275 high compression engine likes a light oil. the 20W50 as used in the engine is like molasses for it.

Miniman's comment about the exhaust system refers to detonation occuring outside the engine in the exhaust system - it should not impact the internal combustion unless there are burn exhaust valves. He's right that it would occur with a rich mixture and that you wouldn't hear it until decelleration. (It may be happening in your case.)

 

.

"Hang on a minute lads....I've got a great idea."

 Posted: Sep 2, 2014 12:39AM
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Thanks so much for the reply guys. I'm learning a lot.

Had a chance to look into the engine this weekend. The dizzy my mini is running seems to be a "non-vacuum advance". are there any advantages running a non-vacuum advance? should i look into changing it to a new dizzy with vacuum advance?

Minimans, i have noticed a bit of exhaust leak, sometimes i can see a bit of smoke under the engine around cylinder 4. i can always smell traces of exhaust scent whie driving. I'll still have to look into the exact location of the leak.

Another thing, when i pulled out the plugs today, i noticed cylinder 1 and 4 seems extra lean while 2 and 4 looks extra rich.  I'm just running a single HS4, what can cause un-even fuel burn? can the exhaust leak do this? 

Thanks again. 

 Posted: Aug 31, 2014 09:30AM
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Check for an small exhaust leak at the manifold or 1st joint, if there is a small leak you may not hear it under normal driving but when on decel. it will suck air into the exhaust and cause the unburnt fuel in the system to detonate causing a small backfire in the system every time the cylinders push a small amount of fuel in on overun..........................

Mini's are like buses they come along in a bunch

 Posted: Aug 30, 2014 06:19AM
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CA

Here's a pretty good explanation of pinking/pinging/pre-ignition/...

Moss pinging video

The hammer on the piston demo is a really good audio replication of the sound.

He doesn't mention that carbon deposits in the combustion chamber (he talks of unburnt fuel) can get hot enough to act like glow-plus in a diesel engine, which causes run-on - the engine continues to turn and rattle after you shut off the ignition.

 

.

"Hang on a minute lads....I've got a great idea."

 Posted: Aug 29, 2014 07:14PM
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sounds like "normal" backfire to me caused by your car being under heavy vacuum using an SU carb. You can try just disconnecting the vacuum diaphram from the carb and plugging the car end (plugging the distributer end does nothing). i belive the poppet valve throttle disc was iplemented to help prevent the suction of lots of fuel under decel vacuum conditions so you would suck some much excess fuel in those situations, but don't quote me on that.

 

Under heavy vacuum, you will pull full vacuum advance. And, if your rpms are high enough and your mechanical advance is, say, 3500 rpm, you could be at 28 degrees of mechanical advance. Then, when you put the car in a vacuum situation such as decel, you will kick in an additional 10 degrees of vacuum advance so you could be at like 38 degrees of total advance. I'm throwing numbes out there at random, so don't thin the vacuum advance unit will pull 10 degrees of advance-- research to see what yours does. BUT that is a situation that frequently burns up cars and why "racecars" don't use vacuum advance units, which are around-town driveability and part throttle cruise lean-out devices.

 Posted: Aug 29, 2014 06:36PM
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I was able to put together a 1 min video to show 3 quick examples of the continuously loud ticking noise when i just let off the gas or downshift. Normally I just put the gear in neutral and brake to avoid this type of noise. I feel like eventually something's gonna break.

//vimeo.com/104768027

My wife and i will be driving the mini down to LA from SF in a couple of weeks, so i just want to make sure if this is not a big deal or if i should look into how to fix this before we take on the road trip. 

What do you guys think? is this pinking? should i be worried?

Thanks 

 Posted: Aug 26, 2014 05:03PM
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Thanks Ian, I will look into vaccum advance. How do i test the vaccum advance to check if something's wrong?

Maybe my terminology for Pinking is incorrect, from my reading on TMF, it was mentioned that "pinking sounds like crickets in your engine".  I found a sound bite on google on pinking, it sounds closely similar to what i hear during deceleration.  https://www.youtube.com/watch?v=GGJZ6kKzLus . 

I'll try to get my own recording video/sound and update again.  Thanks a bunch

 Posted: Aug 26, 2014 03:04PM
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Its not pinking ... pretty much by definition.  Pinking is uncontrolled detonation caused by excessive compression.  With the throttle closed this is impossible as not enough air can get into the combustion chambers..

Possibly (though doubtful) something is wrong with you vaccum advance?  If you were getting massive over-advance as you decelerate then maybe it might sound like pinking ..but I really can't see how the system could physically advance so far......

Maybe its something else like the exhaust (or some other component touching something as the engine tips forward under deceleration???

I think that's where I'd be looking..

Cheers, Ian

 Posted: Aug 26, 2014 02:02PM
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I would like to humbly ask the gurus about a pinking issue i've been observing with my MK4 998 (single HS4).  Is pinking normal during deceleration? When I decelerate, usually occurs when driving on a freeway off ramp and slowing down towards a stop light, i notice my engine is pinking continuously if the car is in gear. whether i just let off the gas while the car is still in gear, or i downshift to the lesser gears and engine brake.  The pinking will stop if i just leave the gear in neutral and glide with my foot on the brake.

some things i've tried and the deceleration pinking still occurs:

- decarb the engine using the ice water bottle feed to vaccum hose while the engine's hot technique (was used to fix my engine stutter after shut off, and it worked for the stutter, but not the pinking)

- change to new ngk 7 plugs (gap at 0.025)

- retard my dizzy by 1-2 scratch line width (didn't verify my timing properly with timing light)

- richen up the fuel mixture from jet adjustment nut

- fill the tank with 91 octane fuel

What can cause this pinking during deceleration while in gear? if it's not normal, what are other ways i can look into fixing this?  Any info is greatly appreciated. Thank you for reading.