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MG Midget and Sprite Technical - Pinking...again

Chaps

Another blast in the mighty blue midget yesterday still revealed some annoying pinking in the mid range. Low down and top end it disappears by the by. It isn't particularly bad, but enough to cause the engine to run slightly hot - it needs sorting.

Therefore

It is fitted with adjustable 123 ignition, currently on setting 5. Setting 4 - I think, reduces the mid range advance further to 16 degrees - is this worth a try, and is my thinking correct in reducing the mid range degrees advance to try and sort the issue? Engine (1275) is modded to a reasonable state of tune, so I have no real idea of what it should be.

Alternatively I fear I may be masking something else here. Could the pinking be something else. My fuel pump seems tired - could this have an effect? Leaks somewhere I guess - but it seems sound, anything else to check before I start fiddling with timing again...

Thanks as ever

Mark
Mark O

Its going to be hit and miss if your doing this in the driveway

There is alot of variables at play, it might be a weak fuel pump, it could be the carb needle profile is no longer applicable...(only 800 to choose from)
it could be the carb spring is the wrong color ( no really it could be you need a blue over red ) it could be a vacume based air leak

If the engine truly is running lean in the middle of its performance curve then My gut feel is the carbs need to be re-profiled, meaning the needle is to fat in the middle and needs to be thinned down..and trust me, that a science all by its self which is in reality is probably more art then anything else

The best way to sort this is to have everything running to perfection...no vac leaks and perfect opperating fuel pump, good new spark plug, wires ect ect

Then call peter burguss and schedual a day on his rolling road / Dyno...he will be able to dial everything in based on computer tech, years of hands on experiance for that scence of just knowing what works and what dosnt

When you think about what he can achive in a few short hours and what you will have to endure over the next 3 years in your driveway plus a 1/4 mile of cop free street....using peter burguss is going to be very inexpensive

Remember your not only recurving a distubter your also re-profiling the SU carbs

Danial stapleton has a couple of good books that details how to re-profile SU carbs. His most recent goes Into some depth of needle profiling


Prop
Prop and the Blackhole Midget

Please remeber about my above thought....the carb needing reprofiled is just my guess,,, dont get caught up thinking this is the issue and start tearing the carbs apart and sanding the needle down with sand paper

My thought is just a suggestion not a fact, a starting point to simple look into and begin the thinking process that leads to the science and math part

Prop
Prop and the Blackhole Midget

Its going to be hit and miss if your doing this in the driveway

There is alot of variables at play, it minto ght be a week fuel pump, it could be the carb needle profile...(only 800 to choose from)
it could be the carb spring is the wrong color ( no really it could be you need a blue over red ) it could be a vacume based air leak

If the engine truly is running lean in the middle of its performance curve then My gut feel is the carbs need to be re-profiled, meaning the needle is to fat in the middle and needs to be thinned down..and trust me, that a science all by its self which is in reality is probably more art then anything else

The best way to sort this is to have everything running to perfection...no vac leaks and perfect opperating fuel pump, good new spark plug, wires ect ect

Then call peter burguss and schedual a day on his rolling road / Dyno...he will be able to dial everything in based on computer tech, years of hands on experiance for that scence of just knowing what works and what dosnt

When you think about what he can achive in a few short hours and what you will have to endure over the next 3 years in your driveway plus a 1/4 mile of cop free street....using peter burguss is going to be very inexpensive

Remember your not only recurving a distubter your also re-profiling the SU carbs

Danial stapleton has a couple of good books that details how to re-profile SU carbs. His most recent goes Into some depth of needle profiling


Prop
Prop and the Blackhole Midget

Great here we go agian... reviving dead edited postings, straight out of the grave

Prop
Prop and the Blackhole Midget

Pinking is most likely in the mid (2 -3K revs), as this is when the cylinder takes in the max air per stroke, which means more compression, faster flame front, and need for less advance. So you should try a less aggressive setting - but first, as suggested earlier, take a look at the plugs to make sure they aren't running excessively weak.

A
Anthony Cutler

Mark

Do you have the distributor with or without vacuum advance? I had a similar issue with a 25D fitted with the wrong vac advance unit, it was giving far too much advance in this rev range. If yours has got vac advance try disconnecting the pipe and plugging it. I won't harm the power as there's virtually no vacuum at WOT but you will lose some light throttle economy.

Bob
R.A Davis

Bob, that is true but pinking is always bad for engine life. Engines which have been pinking for a long time characteristically show greater bore wear where the rings are when they cop the high pressures caused by pinking - around the top of the stroke.

Mid range pinking is usually too much advance as Anthony notes, though there can be other causes so it pays to check them first. Generally, a rise in CR (unless going to substantially more inlet duration) will require a reduction in advance between 2000 and 3000 RPM.

Too much vacuum advance causes pinking on part throttle which disappears on full throttle. I have yet to find another cause of this symptom.

Wrong heat range for the engine's CR can cause pinking, as CR goes up heat range should go down. Telltale signs are dark brown/deep purple-ish insulator tip, also a tendency to develop a bit of a misfire on very long hills.

In NZ with lower octane fuels than you guys have had, emgines have always been touchy about pinking so we've had to do quite a bit of work on how to avoid it. I have covered causes in more detail on my website at http://www.mgparts.co.nz/advice/technical-notes/combustion-problems-and-detonation/
Paul Walbran

Chaps

Mnay thanks - some useful stuff here. Plugs are standard - they also looked fine last time I loked - but I will check again. I could indeed fiddle with the mid-range advance, but would prefer to eliminate any other possible causes first - interesting about the vac adance. Ignition is 123 so maybe a moot point. Will try it anyways...

Thanks

Mark
Mark O

Do you have big carb(s)? That means to drive a "normal" cruising speed the butterflies will be only slightly opened, thus there is vacuum before them, causing the vac advance to come in. At least, that's what I concluded on my 1360 with big twin SU's. Solved this by fitting a vacuum canister which needs more vacuum.
Alex G Matla

This thread was discussed between 01/07/2013 and 04/07/2013

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