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jetjr

Iridium plugs

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gosh jetboy, my Jabiru is 20 years old. Do you think I may have carbon spark plug leads?

 

Well at least there is redundancy in the two ignition systems.

 

How would you tell what sort of lead you have without cutting into them?

 

 

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Your engines reliability is a much higher priority. A resistor MUST lower the AMPS. and it does affect the spark. It may jump the gap but it's much less intense. The plug points last longer because the spark energy is less. Nev

Straight from the horse's mouth : PLUG STUDIO / NGK

 

The resistor is not there to make the points last longer, it's there to stop RF from traveling back into the ignition wires and causing noise.

 

AMPS are not a factor. The ignition system is a high VOLTAGE system (20kV+) with a limited amount of energy (<50 milliJoules) available. That means very low amps. Lets take 1mA as an example. At that current, the loss across a 5kOhm resistor would be 5V. Now how much impact would losing 5V out of 20.000V have? Even if you'd lose 50V (@10mA), or 500V (@100mA), the effect would be negligible.

 

Here's an interesting article about ignition systems, voltages and energy: ignition comparison - EFII

 

 

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You can hardly see the spark from silicon or carbon leads in daylight. It's nothing like as intense as you get with wire . And don't use any of this on magneto's it will wreck one. Plug points do last a lot longer. They hardly erode at all,. with those leads but they aren't for aeroplanes or older motorcycles. Nev

 

 

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Iridiums were supplied as std on CAE engines.

 

Some low grade coils Jabiru supply have problems with them

 

I have run for several hundred hours and have had good results

 

 

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The resistor is what is being considered WITH carbon or silicon leads it seems. Also the wider gaps come into it.. Of course the iridium plugs last longer in some situations but if the ceramic is contaminated it's no consolation that the points are still fine. On a car ordinary plugs go say 40,000 Kms safely and the iridiums are good for 110,000 The ordinary plugs do the equivalent of 300 hours and the others say 800 realistically no one is going to leave plugs in that long in a light aircraft. I suggest they would foul long before that time. Most do a max of 100 hours. and you just change them as they are relatively cheap.. The silicon leads would be unreliable compared with plain wire ones. You would need to test them with a multimeter frequently and if a small break occurs in a lead in the conductor it arcs and creates a large gap and puts a load on the coil pack or magneto coil and will eventually kill it Even if this does not happen the extra load on the whole system makes it more prone to tracking anywhere in the HT side of things. More so at higher altitudes.. Nev

 

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gosh jetboy, my Jabiru is 20 years old. Do you think I may have carbon spark plug leads?Well at least there is redundancy in the two ignition systems.

 

How would you tell what sort of lead you have without cutting into them?

My engine (2003) and those up to about 2010 came with red HT leads which are plastic core carbon type.

 

To be fair, the old automotive carbon leads were much less reliable than Jabs (leads) as the core seemed to be cotton wrapped carbon which would crumble out.

 

Jabiru switched to the black leads which are believed to be inductive wire type - I havent had the opportunity to test them.

 

I use the Bosch ferrite core helical wire inductive leads from SCA. The wire is SS and has a resistance proprtional to length but generally less than the original carbon plastic core Jab OEM red ones. Cars use these leads together with R spark plugs, but that cant be compared to the Jab setup which uses lawnmower parts mixed with a car distributor (variable spark gap right there) so I tested the voltages and spark with different leads, plugs and gaps first.

 

If you wanted low resistance you can buy Magnecore or similar.

 

care needs to be taken if reterminating these leads, different plug caps use diferent ways to spike the conductor.

 

I dont know what these quotes about silicon leads are all about. Silicone leads refers to the outer insulation covering, nothing to do with the conductor.

 

 

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I'm only using the trade names The core was carbon I think and if you are running gas vehicles you are forever replacing ignition leads. to stop the engines backfiring and doing heaps of damage (Even shortens con rods)..They cannot be altered or repaired and if you tug on the lead even once it's likely to fail. I would avoid using this stuff on an aeroplane or a motorcycle with a magneto. I have rebuilt them( Magnetos) for Planes and work on Motorcycle ones all the time. I have world wide connection with "early" motorcycles tech groups and it's absolutely considered to be a NO NO to run resistor plugs and suppression leads on magneto systems. A plug lead coming off will fail or damage most ignition systems. The coils can short out internally almost instantly with a HT open circuit and the risk is there with high resistance leads. The spark at the plug is feeble compared with continuous copper wired ones. As I said you can hardly see it when out in the sunshine.

 

Why run the risk? It's forced on cars etc because of TV interference. and the ADR's .You don't HAVE to with a plane. RELIABILITY is what an aircraft engine is about.. Backfiring will damage a motor. It only needs to be one POP. It can force a lot of mixture into another cylinder and cause damage ( crankcase studs etc)

 

Not having much choice of heat range of the plugs is a bit worrying also . It's never mentioned. How ridiculous to have not investigated it in the Jabiru fisaco .

 

Anyhow a check on the resistance of YOUR leads will find if they are carbon. IF they are and you haven't been checking them, how silly is that.? You make your own luck with this stuff..Nev

 

 

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I,ve tried but coudn,t buy a "copper wire" HT lead to replace my magnito leads that must be Old now, still original.

 

Any "leads" to a purchase point.

 

spacesailor

 

 

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Well that doesn't appear to be easy. I had a big roll which is now nearly gone. I'm sure it can be done with the resources of the Internet. Don't buy the stuff which is cotton wound to look like it was 80 years ago. It has to handle a bit of heat.. Not sag onto the engine and have excellent insulation properties . Today people only stock stuff that moves off the shelf fast.. If you aren't part of the big common herd , it's tough.. Nev

 

 

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Spacey There's plenty on line. Just get the one you want. I will need to do the same myself. There's a choice of outside diameters if you are fitting it into special plug covers that's important to be right. Stainless wire can't be soldered., remember...Nev

 

 

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..Not having much choice of heat range of the plugs is a bit worrying also . It's never mentioned. How ridiculous to have not investigated it in the Jabiru fisaco

I'm confused now. What fiasco? Are you saying that plug temperature is a factor? Where's the proof? It's not for a lack of available plugs, D7EA and D8EA are just as abundant as D9EA, but I have yet to see any evidence or even hints that this may be a good idea. You are not encouraging people to deviate from the manufacturers choise, are you??

 

 

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I have always used D8EA plugs. The guy in the next hangar was a motorbike racing mechanic and he told me to. Since he had been proven correct about the Jabiru needing an oil cooler I believed him. He said the D8EA plugs were a bit hotter and less likely to foul.

 

 

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With all the varying installations and different user techniques that the engine is exposed to it's completely unlikely that just ONE plug type will be suitable for all occasions. The biggest risk is having the plug too hot where it becomes a glow plug and will cause pre ignition and potentially destroy any petrol engine. Other likely causes are the exhaust valve running too hot and carbon on sharp corners. as well as head and piston crown temps. being above reasonable levels. Fitting a heat sensor under the plug will elevate it's running temp. Not having it torqued enough, likewise. Nev

 

 

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With all the varying installations and different user techniques that the engine is exposed to it's completely unlikely that just ONE plug type will be suitable for all occasions.

Plug temperatures have more to do with the ability to dissipate heat in balance with the amount generated by the engine. Heat Range : NGK Spark Plugs Australia | Iridium Spark Plugs | Glow Plugs | Oxygen Sensors | Ignition Leads | Ignition Coils

 

Considering the strict ranges dictated by Jabiru for CHT and EGT, I would suggest that differences in installations are not much of a contributor. Jabiru users usually run their engines just below recommended maximums, or they struggle to keep the engine run cool enough and plug heat range is least of their worries. For EGT the user has no control over how the carb sets mixture unless you start fiddling with the needles and jets, or run a throttle body.

 

 

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If you do a bit of engine tuning and development you might have a better appreciation of what's going on. You won't get it all from books. As I've said, having it a bit too cool is desired and a safer approach.. Most plugs will tend to foul (Soot) if a lot of taxying is done. This requires a special technique to clear or your mag drops will be out of allowable tolerance. The plugs haven't been given the attention they should have in the engine problems analysis. with Jabiru.. The plug should be chosen to suit the most extreme power situations. RPM s and load (mass airflow) affect the temp most.. As you climb the engine gets unloaded by the drop on Manifold Pressure.. Nev

 

 

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Does the unloading of the engine at full power climb always show up on the EGT Nev?

 

I ask this because my EGT's and also CHT's are a max at full power climb . 160 C is the max I put up with before taking action. On reading about it, I reckoned that my main jet was too small so I drilled it out the next size up on my number drill set.

 

This improved things a bit but the higher temps were still there on full power so I went the next drill size up . The temps are still higher on full power climb so I do step climbing, cutting back on throttle and lowering the nose and speeding up to 90 knots until the temps have dropped back to about 155 C. I am reluctant to drill out the main jet anymore without good advice.

 

My understanding is that at full throttle, there is a rich mixture and this should lead to lower EGT's, but this is not what I'm seeing.

 

 

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The richness is determined by the position of the needle. It's shaped to give more flow at full "UP" on the dashpot. That only happens if the engine is not over propped, Is in good condition, (pulls a good vacuum) and you are at or near sea level. If you left the throttle At full (wide open) and kept on climbing, the needle would gradually lower from full open to something less as you climb as the DA (density altitude) and Pressure is dropping. Your mass airflow reduces and the pressure drop (differential) which holds the dashpot up isn't enough.. The enrichened mixture is what causes the lower EGT's. You might experiment with making more of the tip of the needle a smaller diameter rather than drill the jet out bigger. A fuel flowmeter would show what's happening. I'm NOT a great fan of the Bing on those engines but it's available and cheap. The faulty floats were/ are a bit of a concern as you will chew fuel if that happens and it's a fire risk. Nev

 

 

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A key problem overriding all this is the uneven distribution of fuel or spread of egt and cht

 

At best a user has to limit and fuel for the hottest egt cylinder, often meaning others way below limits and on danger side for rich running

 

Without this sorted or at least monitored well, tuning is simply picking a best compromise

 

 

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The "enriching for full throttle " is a common aim of most aero engines. It gives a bit of extra power, cooling and protection. from detonation.. Below 75% power usually leaning is OK .

 

UNEVEN distribution is a problem unless you use injection. to each cylinder. or very carefully design your manifold with perhaps multiple carburetters. The 912 manifolds are no where near optimum even though two carburetters are used.. That engine is cool running and gets away with it for that reason. Running too cool is not efficient but in this instance is more reliable. Nev.

 

 

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Bruce, If you are looking at mixture issues and the behaviour of EGT at full power, a lot of this has been covered by various Jab servvice letters, Jabbachat and finally the "manuals"

 

My engine was a few hundred prior to the 'economy running kit' which after about 5 reissues became the production carb jetting setup today.

 

Note that it only applies to the correct type of prop loading conditions.

 

I spent lots of time swapping / redrilling jets as starting with OEM kit #1 it needed constant updating by factory SB.

 

In the end I got the correct response which is a higher EGT at cruise power 2900 - 3100 rpm and a reduction of EGT from 3100 - 3300 rpm.

 

This is very similar to the behaviour of the MS carb / O-200 my C150 had.

 

Other than prop loading, there are a couple of things that might affect your response.

 

The way the carb float vent line is vented (must be into the air intake after the air filter)

 

Some jab engines were / are supplied with larger float valve seat diameter.

 

There are different float valve spring tensions available

 

There is different fuel pump spring tensions

 

there is different fuel pump plunger lengths

 

They changed the cam profiles

 

they retarded the timing on 3300's

 

 

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The location of the bowl vent tube is critical.. It's often overlooked. RPMs are only determinant at near sea level. Its the mass airflow that matters. Retarding the ignition will also reduce it's vacuum. as will a sick engine mechanically and a too large prop. A lot of things matter but the principle of the needle taper regulating the full power richer mixture is the major issue we are attempting to address is it not? The needle has to go to the appropriate position to work. That is nearly all the way up. If the dashpot doesn't get there the mixture won't change. under the effect of the smaller dia near the end of it. Nev

 

 

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So how do you fix EGT that are 150 deg in variation?

 

Many have tried and have at best REDUCED to 100 deg differences. Even the present EFi systems cant sort it out without individual injector adjustments.

 

No matter the validity of all the options above if you cant fix that, you cant tune the engine.

 

 

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You tune to a reference that is the best available. Usually the leanest if you are trying to be easy on the motor. The location of each sensor affects its reading.. Flames go every where in ports They all vary.. We have been tuning motors that have poor mixture distribution since a motor had a second cylinder.. Don't say Can't when you simply have to. One carb for each cylinder is easy to tune but not always practical. Nev

 

 

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Has been discussed at length, yes probe location is important but Jab engine do lend themselves to pretty good install.

 

No one I know has achieved much better than 100 deg C spread on EGT, which is way from acceptable

 

Hundreds of hours playing and fitting mods hasnt found a solution, even currently available MPI in both mechanical and electronic hasnt solved it

 

This indicated its both air and fuel problem.

 

Far bigger budgets than Jabiru have tried to solve crook fuel distribution in carb fed engines. One reason why EFI has taken over in auto world

 

 

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