Omc 7.5L king cobra 460ci when warm missfires.

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I picked up a 245 four winns and....
Engine 7.5L 460 King Cobra misfires when engine reaches normal operating temp. I already replaced carburetor. Inspected fuel tank. Replaced filters etc. One item I have found is the coil had oil dripping from its internal casing. I replaced with a new coil and it works fine until about 30 minutes of run time than start to miss fire on all cylinders again. When failure occurs I pulled each wire on distributor cap to isolate and found it worsens so it can?t be on any one cylinder. Must be coil again. The coil is warm almost hot. I have seen on other threads here that the resistor wire from the alternator to the coil must be a certain value ohm. I recall reading on a different engine 2 ohms plus or minus .5. Can anyone assist if this could cause my issue of coil heating up and causing a miss fire on all cylinders? I assume this resistor/wire caused a voltage drop and thus reducing the heat of the coil? My other thought to resolve is the esa. Anyone experience an internal failure when heated on the esa thus causing the same problem? Appreciate any help or schematics to resolve issue.
Thank you
Jim

 

[HT32BSX115]

Re: Omc 7.5L king cobra 460ci when warm missfires.

I picked up a 245 four winns and....
Engine 7.5L 460 King Cobra misfires when engine reaches normal operating temp. I already replaced carburetor. Inspected fuel tank. Replaced filters etc. One item I have found is the coil had oil dripping from its internal casing. I replaced with a new coil and it works fine until about 30 minutes of run time than start to miss fire on all cylinders again. When failure occurs I pulled each wire on distributor cap to isolate and found it worsens so it can’t be on any one cylinder. Must be coil again. The coil is warm almost hot. I have seen on other threads here that the resistor wire from the alternator to the coil must be a certain value ohm. I recall reading on a different engine 2 ohms plus or minus .5. Can anyone assist if this could cause my issue of coil heating up and causing a miss fire on all cylinders? I assume this resistor/wire caused a voltage drop and thus reducing the heat of the coil? My other thought to resolve is the esa. Anyone experience an internal failure when heated on the esa thus causing the same problem? Appreciate any help or schematics to resolve issue.
Thank you
Jim

Hi Jim,

Welcome aboard!


Well a few first things..... Please have a look at the "sticky" messages in the forum list.


In general, post the model number (and year) of your engine Although OMC only offered the 460 from about 1987 to 1990, there's some significant differences (probably NOT with "regular" points type ignition though)

If you're using a coil that requires a series voltage dropping (ballast) resistor and you're NOT using one, you will absolutely cause a coil overheat.

Without knowing the approx year model.........specific model number actually, it's hard to make a judgement about the ESA. I don't think the latest model of the 7.5 King Kobra used an ESA.

Even though you did replace the fuel tank, check (or replace) the anti-siphon check valve, in-tank fuel pick-up screen, and ensure the vent hose is NOT plugged/kinked. check/replace the fuel pump and lines etc. You could STILL have a carb problem.

I do hope you didn't pay a LOT for the boat. You can't buy exhaust manifolds anymore except for possibly MORE than you possibly paid for the boat!

If it's a nice boat, the good thing is, you can pull out the entire mess and replace with either a Mercruiser BB/Bravo(like I did) or a Volvo BB/Duo-prop with little or no actual structural changes to the boat.


Regards,


Rick

 

[devee]

Re: Omc 7.5L king cobra 460ci when warm missfires.

Hi Rick

Sorry on the missing info. I am new at this. I did not pay too much for the boat. It was a gift. lol.
It is a 1988 four winns 245 vista. Engine is a 1988 OMC 7.5L 460 ci King Cobra.
Model number 754BPRGDP. What I can read on the old sticker anyway.
I believe the wire acts as a resistor on this coil. The wire is in rough shape. I do not see any bumps or splices from that I think would not be factory. I would like to know the value of the resistor needed for the coil or the original wire resistance. I think this is one of the problems....

Jim

 

[HT32BSX115]

Re: Omc 7.5L king cobra 460ci when warm missfires.

Hi Rick

Sorry on the missing info. I am new at this. I did not pay too much for the boat. It was a gift. lol.
It is a 1988 four winns 245 vista. Engine is a 1988 OMC 7.5L 460 ci King Cobra.
Model number 754BPRGDP. What I can read on the old sticker anyway.
I believe the wire acts as a resistor on this coil. The wire is in rough shape. I do not see any bumps or splices from that I think would not be factory. I would like to know the value of the resistor needed for the coil or the original wire resistance. I think this is one of the problems....

Jim

Well, FREE is sometimes good and sometimes not so good!! It can make you think spending money on it is "OK"!!

Just understand that once your exhaust manifolds fail, you'll probably be money ahead to scrap the entire engine and drive (scrap=sell)

I sold my entire 460 engine, gimbal mount, drive, and all other associated parts separately over 3 years or so on Craigslist, EBAY, etc for a total of $3500+ All I have left now is a couple of stainless steel props!



That went pretty far to help pay for the replacement engine and drive.....

To view parts, go to BRP -- Parts Catalogs
(you do not have to put an email address in the address field)

You can view parts breakouts and part numbers etc. Then you can easily google the part numbers Some are available some are not.

If you want to be able to work on it, I would suggest obtaining an OEM OMC manual. They're available on EBAY from time to time. They cover specific years and model numbers. You'll want the one specific to yours.

In the meantime, have a look at the following link. Boatinfo - The library, containing manuals, brochures and other technical documents from the last hundred years!

You may be able to buy that resistor wire. I would just remove that piece and replace it with either a coil that operates without a resistor (Pertronix comes to mind) OR use an outboard resistor for the coil you got.

I don't remember mine having a resistor. I sold my entire dist/engine harness to a local guy that had a newer 460 OMC with the infamous (and non-repairable) Spitfire (electronic) Ignition. It was the only way he could get it running...( to sell it!)

 

[devee]

Re: Omc 7.5L king cobra 460ci when warm missfires.

FYI. Replaced coil with the correct coil i.e. external resistor and the boat runs GREAT! 460 engine supplies plenty of power. It seemed the previous owner had the wrong part number coil thus causing issues. The wrong coil would overheat and miss fire. I read the temp on the coil with an infrared digital thermometer and it was over 180 degrees when failing. New one is under 130F. Perfect. Boat easily runs 30 kts and then some. Not bad for an ole girl 1988 vintage.

Thanks
Jim

 

[HT32BSX115]

Re: Omc 7.5L king cobra 460ci when warm missfires.

FYI. Replaced coil with the correct coil i.e. external resistor and the boat runs GREAT! 460 engine supplies plenty of power. It seemed the previous owner had the wrong part number coil thus causing issues. The wrong coil would overheat and miss fire. I read the temp on the coil with an infrared digital thermometer and it was over 180 degrees when failing. New one is under 130F. Perfect. Boat easily runs 30 kts and then some. Not bad for an ole girl 1988 vintage.

Thanks
Jim

Hey, you're back in the power!

Mine used to run 60+ mph with 460/King Kobra. There's no shortage of power!!

Lot's of fuel consumption though!


Good luck!



Rick