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 Bad Coil??? 35HP 
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Post Bad Coil??? 35HP
Ok guys, posted a topic about carbs awhile back thinking my carb was dirty. Might have been, but cleaned it up and it starts right up and idles good. Go fishing today and run about 10 mins and lose power. Running on 1 cylinder. Idles on 2 cylinders. Wait a few mins and get back up on plane and runs another 10 mins. Same thing. Finally got to where to it would only run one side. Stopped and pulled valve cover. Looked good. Pulled spark plug. Looked super black and charred. (Spark plug is pretty new-couple trips). Another funny thing is it runs fairly well with no load. Put in the water and instantly struggles. Seems to run fine for awhile and when it gets hot it doesn’t run both cylinders. Noticed it was backfiring slightly. Just little pops. Also noticed a clicking sound on the bad side. Maybe about ever 2 seconds. But not constant.

I’m stumped. Thinking it may be a bad coil? Just not sure if that typical when it can’t handle a load. I’ve had a bad coil before on an old long tail.... planning to pull the cowling off tomorrow to check them out.

Any ideas? Also has good compression on both sides, fresh oil, and good gas. Actually brand new fuel lines and filter.

Note: also tried spaying a little carb cleaner in while it was struggling. Did not rev up or anything.


Fri Jun 08, 2018 10:03 pm
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Post Re: Bad Coil??? 35HP
a coil sounds right in line with what u describe. I've only seen a couple fail but it happens. The valve train is also worth a closer look. Make sure valves aren't set too tight and check for full cam travel. It's even possible the carb is messing with you . I had a cdi coil motor recently that was good till 30 minutes run time then it would wig out. Luckily the tach was on the bad coil side and was a dead giveaway. Attach your tach wire to the suspect side. If the tach quits reading that coil quit firing, good luck


Sat Jun 09, 2018 1:12 am
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Post Re: Bad Coil??? 35HP
I thought about that, however I don’t have a tach. But can’t you wrap a voltmeter wire around the coil wire while running and get a reading somehow? I’ve done that before but it’s been years.

On another note, if I can diagnose it as a coil, anyone know where I can get a non cdi coil for good price?


Sat Jun 09, 2018 5:05 am
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Post Re: Bad Coil??? 35HP
Csafisher wrote:
I thought about that, however I don’t have a tach. But can’t you wrap a voltmeter wire around the coil wire while running and get a reading somehow? I’ve done that before but it’s been years.

On another note, if I can diagnose it as a coil, anyone know where I can get a non cdi coil for good price?


The spark is so fast and such high voltage that most voltmeters don't read it. Just go with the tach idea.


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Sat Jun 09, 2018 5:50 am
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Post Re: Bad Coil??? 35HP
I don’t have a tach.... and I’ve done it before on this motor. Just a long time ago.


Sat Jun 09, 2018 6:11 am
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Post Re: Bad Coil??? 35HP
Csafisher wrote:
I don’t have a tach.... and I’ve done it before on this motor. Just a long time ago.


You must have a special voltmeter because when I was trying to diagnose a bad coil, I couldn't get the right reading. These coils have a diode in them and if it blows, you'll still get voltage output but it's far lower than required to ignite. Your voltmeter may read some voltage but it's likely not going to capture the required thousands of volts needed to ignite during that quick burst, so it probably won't do any good.

A tach is needed regardless on these motors. Just get one.


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Sat Jun 09, 2018 8:04 am
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Post Re: Bad Coil??? 35HP
Thanks for the info. But if the diode was blown, wouldn’t that mean it would never get the required spark to run right? Because I can go run it for a few mins and it’ll run normal, then all of a sudden lose power.


Sat Jun 09, 2018 8:36 am
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Post Re: Bad Coil??? 35HP
Since the coils are in question I would swap them both and a diode wire set. Where are u located? U can order them from several places or I can ship you anything u need


Sat Jun 09, 2018 8:43 am
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Post Re: Bad Coil??? 35HP
Im in Corpus Christi


Sat Jun 09, 2018 9:24 am
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Post Re: Bad Coil??? 35HP
Csafisher wrote:
I don’t have a tach.... and I’ve done it before on this motor. Just a long time ago.


You can get them cheap on Amazon


Sun Jun 10, 2018 8:21 pm
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Post Re: Bad Coil??? 35HP
A tach would be ideal, but you can get one of these on the cheap.

https://www.amazon.com/s/ref=nb_sb_noss ... ark+tester


Mon Jun 11, 2018 6:02 am
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Post Re: Bad Coil??? 35HP
Went took the flywheel off today. I set my multimeter to 20k ohm and got 6.6 across both magnetos. Anyone know if that is good? I noticed a small burn in the wire going to the suspect magneto. This would be the diode wire correct? It also seems that the wire harness the diode wire runs to was kinda of brittle. The plastic housing cracked when I tried to pull it apart. However the contacts seemed fine. Also, my kill switch was bad......

I’m kinda thinking it could be shorting out when it gets hot or something? Or would a magneto do that?


Mon Jun 11, 2018 6:01 pm
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Post Re: Bad Coil??? 35HP
I had an issue like this on an older 16hp GDLT. Replaced both coils and wiring harness. Fixed my issues, I was trying to determine if it was a diode, but after getting the cowling off and noticing how rough the harness looked and how much rust was on the coils I just replaced everything.


Tue Jun 12, 2018 4:36 am
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Post Re: Bad Coil??? 35HP
Swapped coils to opposite sides. Problem still occured on the same side so it’s not the coils. Ordering new wire harness


Tue Jun 12, 2018 4:27 pm
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Post Re: Bad Coil??? 35HP
Well guys put new wire harnesses in. It needed them. Still doing the same thing, running on 1 cylinder when it gets warm after about 30min of run time. I’m stumped. Could be a valve sticking??


Wed Jun 20, 2018 6:52 pm
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Post Re: Bad Coil??? 35HP
What is your valve lash set at?

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Wed Jun 20, 2018 7:03 pm
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Post Re: Bad Coil??? 35HP
I wanna say 6 thousandths? Gonna check it tomorrow


Wed Jun 20, 2018 8:16 pm
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Post Re: Bad Coil??? 35HP
So valve lash was good. Compression was 130/135. Was popping and backfiring while running on the bad side. Pulled the head to take a look. Put some seafoam in the intake and exhaust and let it sit a couple mins. Picked it up and the area around both valves was damp and a few drops had fallen. Maybe the valve is the issue?


Thu Jun 21, 2018 1:10 pm
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Post Re: Bad Coil??? 35HP
Probably leaking valves. I put money it will fail a leakdown test.

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Thu Jun 21, 2018 2:29 pm
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Post Re: Bad Coil??? 35HP
So I lapped the valves. They were definitely leaking. After lapping I put water in the intake and exhaust. There was a very slight and slow leak on the intake. But very very little. I put the head back on and when I put the valve cover on I noticed one of the valve cover bolts was stripped. Or should I say the threading in the head is......... yeppee. Fortunately that’s an easy fix with a longer bolt and nut. Anyway I turned it over and noticed air leaking out of the valve cover. Is this normal? That would indicate leaking valves right?


Sun Jun 24, 2018 6:41 pm
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Post Re: Bad Coil??? 35HP
usually the seats move so much you need to recut the seat angles, lapping along wont cure it. These seats are so off square with the guide it is no even funny

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Mon Jun 25, 2018 7:03 am
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Post Re: Bad Coil??? 35HP
Well in the event I have them recut, as I don’t have the tool for it, does anyone know the valve stem thickness and the angle for the valves?


Mon Jun 25, 2018 9:45 am
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Post Re: Bad Coil??? 35HP
Just send it to a machine shop or a guy who does heads. I charge 100 to do a valve job. 3 angles..

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Mon Jun 25, 2018 9:51 am
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Post Re: Bad Coil??? 35HP
So still stumped here. Did a leak down test and determined the valves are not leaking. The rings leak slightly. But it’s an old engine.... still runs bad on the one side. Doesn’t have to be hot anymore, just kind of misses when you rev it up. Small amount of black smoke comes out of exhaust on bad side.


Mon Jul 30, 2018 11:38 am
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Post Re: Bad Coil??? 35HP
What about the vacuum signal line to fuel pump leaking extra fuel into cylinder 1, starter side. I've seen this a few times


Mon Jul 30, 2018 1:33 pm
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Post Re: Bad Coil??? 35HP
swap coils from side to side to see if it follows the coil. use new spark plugs. ONLY if its a non cdi.. CDI is side specific.

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Mon Jul 30, 2018 1:34 pm
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Post Re: Bad Coil??? 35HP
I have swapped coils and plugs. Same thing happens on the same side. Will C, how would you check the vacuum?

It’s the starboard cylinder. Not sure what number that is


Mon Jul 30, 2018 2:28 pm
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Post Re: Bad Coil??? 35HP
Csafisher wrote:
I have swapped coils and plugs. Same thing happens on the same side. Will C, how would you check the vacuum?

It’s the starboard cylinder. Not sure what number that is

Cylinder 1 is starter side (side the fuel pulse line connects to)
Cylinder 2 is the oil filter side (side you are refereeing too)


If it stays on the same cylinder after swapping plugs/ coils around, Check for an intake leak at the head/ intake union, or adapter/ carb for unmetered air/ fuel

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Mon Jul 30, 2018 2:33 pm
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Post Re: Bad Coil??? 35HP
Did you replace the diodes when you replaced the wire harness?

Not sure if it comes with them or not.

If you did, back to Will and Marcus.

If they have not been replaced, that is the next thing I would do. They are cheap.

Yes, it should be a constant problem, but you state it is "brittle". And as with any electric problem, it can be intermittent, or intermittent until it completely fails.

Another test is to disconnect them and see how it runs then. Just remember, the engine won't shut off and you will have to choke it out to turn it off. If it runs fine, then you know for sure that is what it is.


Mon Jul 30, 2018 2:42 pm
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Post Re: Bad Coil??? 35HP
The diodes are on the wires that connect to the coils right? I replaced all the wiring.


Mon Jul 30, 2018 6:02 pm
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Post Re: Bad Coil??? 35HP
I'll second Caddo. Sure fire way to eliminate/confirm electrical problem. Just pull the diode/ground wires off and run it. If the issue is still there it's not in the harness. New harness may have been pinched when you put the blower housing back on. Don't ask me how I know. Also may want to check the spark plug boots for wear. Might be arcing on heat shield . You could run it in the dark and find it pretty quick. Good luck!


Tue Jul 31, 2018 12:56 am
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Post Re: Bad Coil??? 35HP
If any part of the Kill harness is touching ground or intermittently touching ground this can happen.. Especially around the intake / blower housing area.

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Tue Jul 31, 2018 5:17 am
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Post Re: Bad Coil??? 35HP
I pulled the diode on the bad side and nothing changed. I don’t think the kill switch wire is grounding. Has good compression on both sides. About 130. Leak down test was similar on both sides. Out of ideas


Wed Aug 15, 2018 9:24 am
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Post Re: Bad Coil??? 35HP
What year is it? Is it a CDI?


Thu Aug 16, 2018 6:05 am
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Post Re: Bad Coil??? 35HP
It’s an 06 non Cdi. I’m not sure of the hours but it’s gotta be at least 350. All stock other than exhaust

I’m having the valves lapped and seats recut. But the guy I took it too said it didn’t look bad at first glance....


Thu Aug 16, 2018 10:58 am
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Post Re: Bad Coil??? 35HP
Csafisher wrote:
It’s an 06 non Cdi. I’m not sure of the hours but it’s gotta be at least 350. All stock other than exhaust

I’m having the valves lapped and seats recut. But the guy I took it too said it didn’t look bad at first glance....

Leak down test will tell you all you need to know without taking the heads off

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Thu Aug 16, 2018 5:02 pm
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Post Re: Bad Coil??? 35HP
when u put heads back on don't put shields on ,start up and see if problem still exist but don't run long without cover or shields. I think you missed something simple it gets me every time


Thu Aug 16, 2018 6:05 pm
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Post Re: Bad Coil??? 35HP
Got the head back. Valves are good to go. Same problem occurs. I checked all the grounds. Everything is good on that front. About ready to bolt a new engine on


Sun Sep 02, 2018 8:41 pm
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Post Re: Bad Coil??? 35HP
When you say leakdown was minimal on the rings, what percentage was it at on each side? Have you checked the intake gaskets where it bolts to the head? Have you wrapped the plug boot with electrical tape to eliminate any spark jumping that you wont see well in sunlight? What plugs are you using (I dont recall you saying you put new ones in or not, but with everything else you've done, I think you probably have)? Stock carb? Did you ever get a tach? Does act up when not in gear after its warm enough to start messing up? If your compression is at 130, but you have very little leakdown at the valves, then you've lost a significant amount of compression at the rings and might need to consider checking the internals. Piston could be worn enough that when things get all hot and bothered, its leaning funny. Longshot, but at this point the obvious things havent bought you any apples. Also, which fuel pump do you have? If it's the cam driven, make sure the line isnt against a hot spot on the block, head, or exhaust. I diagnosed a prodrive once that the cam pump line was routed in a bad spot and the fuel was vaporizing when the engine got warmed up. At least that's the hypothesis we decided to pin it on for lack of any better explanation lol. After we moved the line, never happened again, so there's that.

Just some spaghetti against the wall for ya. Maybe something will stick.

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Sun Sep 02, 2018 9:03 pm
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Post Re: Bad Coil??? 35HP
So last week I was explaining valve float to my kid. We were messing with his stock 23. I reved it till we head them float . It idled down and went to one cylinder. Well shit, that's what I get, lol. After spending way too much time checking pushrods, rockers, compression release, valve seal, fire etc. we pulled carb, it looked spotless except for the 1 tiny piece of something in the cyl 1 pilot jet. I assumed i hurt something and overlooked something easy. I hope you find it soon


Mon Sep 03, 2018 11:22 pm
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