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Merc 7.5 weak spark, Capacitor?

M

MikeMan

Contributing Member
Hi, I got given a 7.5hp(to fix). I thought it had no spark so i cleaned all the contacts checked the wires and then noticed that it had spark on both cylinders but they are weak. Ive checked the procedure on CDI ignition troubleshooter but it doesnt say anything about a weak spark. Could a poor capacitor be causing this?
It was made in Australia and is a blueband with 75 decal yet is a 7.5 IMG_1768.jpg.I cant find the year for it but serial number is 8004212. I think its mid 70s
Thanks
 
I took off of the parts off and have seen that the plastic insulator which holds on the points is corroded. Also the wires from both points is corroded and i think has had some contact proximity with the base where they twist out of the housing. I suppose this may be the cause, but it would be good to know before i try and order these parts if the capacitor is worth ordering too?
 
Replace those insulators and wires. I had one (71' 7.5 HP) that drove me nuts.
Insulators are available. I purchased HV high temp wire from McMaster instead of OEM wires which were expensive.
 
Hi
Kim im pretty sure its the one in your diagram that you posted at time 3.35.
I get a slight resistance reading between the left orange wires and the right screw where the stator red wire connects too on this red base thing is that normal? I ran the diode test thing shown in CDI electronics and got a value of 0.001 whereas the cdi thing showed a value of 0.000 was normal, all the other figures a roundabouts the same as the CDI checksheet, im just wondering if this one has to be exactly 0.000.The orange wire goes to engine stop switch and ground.

Also Jim mentions HV High temp wire- would coated household wire do, at least to test that this worn wire was the fault?
The wire in the current leads looks the same thickness quantity as the household wire, it does have a thicker coating though.
My camera isnt working but il try and post an image.
Thanks
 
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I tested the two stators and they were in the range you write of.
The ignition coils which im guessing are those two red things that each sparkplug wire comes from werent in that range. The reading jumped around a bit but both settled at exactly 0.1 when on 200scale (i dont know why my scale is in 200, 2000,20k etc)
I didnt get the part about cleaning the external coils with steel wool, are those the same red ones with the sparkplug wire?- i thought that these were meant to be insulated with the ground wire going back to the points?
 
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Thanks Kim, Il clean those areas with steel wool, also one of those Isolators was badly cracked so im having to wait a few days for that part to fly in from overseas. I got the last oneAVAILABLE in Aus/NZ. The other one has a few minor cracks so im not sure if that will stuff the spark as its still pretty much in a robust form. I can get another one but it would have to come from the US
 
I took the coils off and I couldnt see any ground system and then i took a second look and there was a little thin wire that comes out that lies along the base that looks like a fuse wire, you can see it in the photo lying on the back of the coil, its actually sort of embedded in that rubber. I would of mistaken it for a twig or bit of sealant if i hadnt being informed of this ground system and it definitely needs a clean View attachment 5344.im going to try and fix up the insulators with plastic poxy so i can try spark again while awaiting the new insulator in the mail. The wires were both definitely cracked by the stator plate the last time i tried spark. Ive checked the coil tower part and they are in those specs- both had exactly the same reading. I dont know if thats from the cleaning or because i used the digitmeter incorrectly at first
 
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i drilled the bolt holes in each insulator wider and i then filled the bolt holes and the other littleIMG7101.jpg cracks with epiglass. Il redrill the bolt holes tomorrow and sand down the edges. I guess that epiglass resin would be an insulator as the power people here use a glass resin on their tools for cutting trees around 40k powerlines.
 
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I drilled the holes, wiped them with meths and re-installed. I got good blue spark but only on one cylinder. I found out on inspection that i had left some contamination on a point- an old bit of wiperpaper while cleaning them. After clearing that i now have good blue sparks on both. I also re-oiled with a couple of drips the lubrication wick for the cam-lobe.
A couple of things to add for reference is that the ground system is something quite different to what im use to so when you a checking the wiring etc for shorts, what maybe a short on a typical system is actually normal operation on these.
The wires on mine were cracked and thus were shorting on the bends near the insulators. Also if you replace wires, which i think was the main problem for me, be very careful when you reinstall that you tie them down and out of the way where they exit the flywheel as they can easily get caught up in the magnets on the flywheel when its spinning if not directed out of that area.
Thanks for everyones help.
The capacitor seems fine, l will repost later if its not- they are about $45. It had no cracking underneath.
Now to find a tank and check the rest of it over- a rotten impellor is the first thing on the agenda
 
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