Bad Coils? Another question!

[Gary H NC]

Anyone have a coil arc to the block only when a plug wire is removed?
I'm trying to diagnose a cough at idle. I pulled each wire while the motor was running and did hear a difference in the idle speed. Spark tester checked out ok on each cylinder,compression 100 to 105 on all 6 cylinders.Had the exhaust covers off and the pistons and cylinder walls look really good.
There are no visible cracks in any of the coils but on 2 of the cylinders if i pull the plug wire while its running i can hear it arcing over to the block.I can actually see it on one coil.
Should i bite the bullet and replace all 6?
The carbs were just rebuilt so that rules them out as the cause of the low speed cough...

 

[tashasdaddy]

Re: Bad Coils?

Re: Bad Coils?

gary, is it arcing from the coil or the wire, possibly the boot.?

 

[rodbolt]

Re: Bad Coils?

Re: Bad Coils?

and that is why we NEVER want to remove a plug on a running engine.
open circuited the secondry is capeable of roughly 60Kv thats 60 thousand.
most outboard coils dielectric strenth starts failing about that range.
the coils are going south but the cough has nothing to do with it.
check your idle air jets could also be an idle fuel pick up tube clogged.
playing with the idle air jetting out of the box was common for most OMC stuff.
could also be a failed recirc check valve. easy to test once you get to them.
but ign wont create a cough, thats normally created by the a/f mix in the crancase lit of insted of being rich enough to transfer to the cylinder.the expanding gas from a lean burn pushes up on the bottom of the pison, in severe cases I have seen it shear flywheel keys.
the reeds block it from coming out of the carb throat.
I have seen,especially on the looper V motors, the intake gasket blown out.
if it coughs regularly remove the airbox and use a tooth pick to close off the idle air jets, the ones that make it quit coughing are the carbs that need attention. remember its not uncommon to have to play with the air jetting for idle air.

 

[Gary H NC]

Re: Bad Coils?

Re: Bad Coils?

I can see the spark jump from the base of the coil to the block.
It smooths out just above idle.Also after it warms up it seems to just about go away.I was very thorough on the carb cleaning and rebuilds.All fuel lines replaced too.No adjustments on these carbs.
I will try the blocking the idle jets with a toothpick and see what i find.
The carbs were gone thru just to try and solve this...

Thanks guys!

 

[F_R]

Re: Bad Coils?

Re: Bad Coils?

One of the most basic rules of electricity is that it follows the path of least resistance. Normally the path of least resistance is across the spark plug gap. Pulling the plug wire off eliminates that route, so now the path of least resistance is through you (ouch) or through the insulation of the coil. Once it has arced through the coil, it has burned a pathway through the insulation...including the insulation on the internal coils of wire. It may work awhile, but that pathway is always there, presenting itself as the possible path of least resistance. Pathway or plug gap...which is easier at the moment?

 

[rodbolt]

Re: Bad Coils?

Re: Bad Coils?

I am not faulting your carb cleaning, I am saying for whatever reason the air fuel mix at idle is to lean and its burning in the crankcase.
could be a minor airleak almost anywhere , could be the jetting was never relly correct to start with.
would not be the first jonnyrude that had to be rejetted new out of the box.

 

[Gary H NC]

Re: Bad Coils?

Re: Bad Coils?

Thanks! I will test with the toothpick method and hope i find out what carb is doing it.
As far as the coils i think i'll just buy the CDI coils with new wires.It still has all original wires and coils that are 27 years old so i'm sure they are do anyway.
So if its and idle jet problem will i need to step up a size in the jets?
When i did the carbs i made sure i got good air flow through all the passages with compressed air and the jets were all clear.
It has had this cough since i have owned the motor.
What is strange is after it runs awhile it seems to greatly improve.Kind of gets me thinking intake gasket leak,it warms up and expands and gets better?

 

[rodbolt]

Re: Bad Coils?

Re: Bad Coils?

remember we are dealing with an air jet, make it smaller and the mix gets richer, make it larger and the cough will get worse cause its leaner.
a cold engine requires a richer mix, your already border line lean so cold it will cough more than when warm. its also part of why Looper engines run terrible without the t-stats.
cross flows not quite so bad but they still dont like running cold either.

 

[Gary H NC]

Re: Bad Coils?

Re: Bad Coils?

Thanks Rodbolt! Yea i was thinking in reverse on that one. I'll get to testing it one evening this week.
I was hoping the coils could have something to do with it.Oh well,sooner or later i'll figure it out.
I think i will test the carbs first then replace the coils and put in fresh plugs.
I had the inner and outer exhaust covers off yesterday and was amazed by the amount of carbon build up.It was a 1/2 inch think in places..Got them all cleaned up and replaced the gaskets.I have a complete powerhead gasket set so if i end up pulling the intake off thats ok.I hope its still in the carbs though.
Heck if Kitty Hawk was closer i'd pay ya to look at it..

 

[rodbolt]

Re: Bad Coils?

Re: Bad Coils?

it will build up some carbon.
a good example of what I am talking about is tghis: back in 97 a very good friend decided to repower his 26 s2 pursuit from twin 140 omc to twin 175 omc.
I tried to talk him into twin yamaha 200.
anyway a local dealership in VA Beach installed new engines and control packages.
customer hired me to do the initial seatrial with the dealership personell.
so after nioting about 10 rigging deficiencies we fueled it up and set out.
port engine coughing like mad and actually stalled once.
I told the tech the engine wasnt running properly, he told me it was just breakin oil in the fuel.
I reccomended my customer friend NOT to accept delivery until it was fixed, they gave him the same "brakin oil" BS as they did me so they accepted it.
2 months and 6 trips to the dealer he paid me 250 dollars to fix his brand new engines.
but they never coughed after that

 

[Gary H NC]

Re: Bad Coils?-Another question...

Re: Bad Coils?-Another question...

I hear you RB, It does not run bad at all just above idle but that cough drives me nuts.I need to figure it out soon before fishing season gets here.
What order would you check? Carbs then recirculation check valves?
I'll let you know what i find...

 

[Gary H NC]

Re: Bad Coils?

Re: Bad Coils?

Ok,Did the tests that Rodbolt suggested using a toothpick and blocking off idle air bleed jets.No change in the erratic cough.The motor may run smooth for 5 seconds or so then cough once or several times.Anything just above idle smooths it out.
I know i have at least 2 bad coils.Started it up last night and could see sparks jumping from the side of the coils to the block.
So 6 new coils and plug wires are on the way.Any chance that may fix it?
Also where do i get the tool to remove the pins from the rubber plug for the trigger wires?

Also,i thought this was strange.I had a toothpick in my mouth and decided to stick it between the coil and the block...a bright blue spark jumped out of the side of the coil right through the plastic case...Even a wet toothpick is a better path than the plug wire?

 

[wavrider]

Re: Bad Coils? Another question!

Path of least resistance, wet toothpick less ohms than plug wire (grin)

I used a timing light to trouble shoot my coils, intermittent fire and the timing light showed it up. Replaced coil, still had cough.

My 115rude was coughing at idle, just above idle it was great, I took off all the coils and wire brushed the ground connections good and clean, cleaned the connectors at end of wires and re installed, could not get it to cough again, that is after i went through the carbs,twice!

 

[rodbolt]

Re: Bad Coils? Another question!

I think your confusing a lean sneeze or cough with a simple ign miss.
both will cause the engine to shake but the sound is very different,
but in any case gary has some bad coils that need replacement before continuing.
if the ign on a two stroke misses then the AF mix is pushed into the exhausr where it lights up. can create burn issues with the other cylinders as the ex ports are open quite a bit.
both will cause a shake but only one creates a lean cough.

 

[Gary H NC]

Re: Bad Coils? Another question!

Thanks Rodbolt. I'll have the coils but Friday.I'll let you know how it runs after replacing them and the plug wires.