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4.3 omc not getting boat on plane

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Oxbow26

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My 1986 penn yan 21’ with 4.3 omc has no problem getting up to 4200 rpm’s but seems to drag the transom and doesn’t want to get up on plane. I verified that it has the appropriate prop so I’m wondering could I be getting prop slip or a coupling issue? There’s no noises etc such as bad spline issues but just not enough speed to put the boat on plane. Also considering the SEI conversion to the Merc alpha series but unsure how difficult the swap is. Thanks for any input
 
Prestolite.jpgchanging to an SEI merc copy won't fix that problem, your issue could be fuel related (fuel filter, fuel vent or anti siphon valve) or igntion advance related (these engines use a points distributor with mechanical advance). If the advance weights are rusted and sticking you will not get full igntion advance at the speficied rpm. There is a wick in the distributor shaft that you will see if you remove the rotor, you need to oil this every season just a few drops of motor oil. On our boat (same engine) we had the same problem and it turned out to be a sticking anti siphon valve on the gas tank. Here's a pic of the OE Prestolite points distributor. The wick is in the center of the distributor shaft and the weights and springs are under the points mounting plate.
 
louc said:
View attachment 21390changing to an SEI merc copy won't fix that problem, your issue could be fuel related (fuel filter, fuel vent or anti siphon valve) or igntion advance related (these engines use a points distributor with mechanical advance). If the advance weights are rusted and sticking you will not get full igntion advance at the speficied rpm. There is a wick in the distributor shaft that you will see if you remove the rotor, you need to oil this every season just a few drops of motor oil. On our boat (same engine) we had the same problem and it turned out to be a sticking anti siphon valve on the gas tank. Here's a pic of the OE Prestolite points distributor. The wick is in the center of the distributor shaft and the weights and springs are under the points mounting plate.
Click to expand...

thank you for the valuable information!!! I didn’t mean to imply that I thought the conversion would help but that it was something else I considered doing on top of isolating the problem of failure to plane. It didn’t really occur to me that fuel or timing might be an issue since the rpm’s seemed to be there, but I’m new to this so I will certainly take a look at all of this and see if it helps. Thanks again for the response. Your information was invaluable...
 
For the 4.3 V-6 here are the tune up specs:
base timing: 6*BTDC, full advance at 3200 rpm is 18* (the centrifugal gives 12* on top of the 6* base timing)
ignition point gap: .019", dwell 37*-->41*
spark plug gap: .035"
idle speed in gear, in the water 500-600 rpm

PS if your Cobra drive is in good shape, and the shift cable has been kept in proper adjustment it is superior to the SEI copy of a Merc Alpha. The only advantage of the SEI conversion is that it is cheap and you can probably get a merc mechanic to work on it then, otherwise the OMC is superior quality wise.
 
Thank you as well for the great info. You wouldn’t by chance know the part numbers for the cap, rotor, wires etc would you? I’ve looked but apparently there’s several different 4.3 generations and not sure which parts this motor requires.
 
www.marineengine.com/parts/omc-parts/omc-boat-parts.php?year=1986&hp=4.3&model=434APWXS&manufacturer=OMC+stern+drive&section=Distributor
right here on this site....
OMC used both year and the model #. Model # if you know it is more accurate, but there is no difference in the ignition parts from one model of 4.3 to the other in the same year, for these older engines that have points distributors. If you know your model # plug it in to the catalog but I think all the ignition parts are the same for '86-88 or so.

I usually just pick up a Sierra Marine tune up kit locally, it comes with the cap, rotor, points and condenser.
 
Well per everyone’s suggestions I’ve ordered the pertinent ignition components as well as fuel filter etc to give the motor a fresh kick. Now another query... the previous owner replaced the oil pan as he said the old one was leaking... but he handed me a brass fitting and said that it’s too small to thread into the pan drain plug hole therefore he couldn’t attach the dipstick. Is the drain hole in fact where the dip stick attaches and if so does anyone know the thread size or part I need to put my dip stick back on?
 
Oxbow26 said:
.................... Also considering the SEI conversion to the Merc alpha series but unsure how difficult the swap is. Thanks for any input
Click to expand...

Doing the conversion from the Cobra to a Mer A drive would be rather counter-intuitive.
You would be exchanging one dog clutch drive for another dog clutch drive, and all of the issues that go along with it!

Consider looking into the conversion that allows you to go from the Cobra to the Volvo Penta cone clutch SX or DP-S drive.
 
I'd be curious as to what the boat weighs at 30+ years old. Maybe water somewhere. Start with a compression test but sounds like engine is making power. It's a smallish motor for a 21'. Will need to be in great shape mechanically and tuned to perform well.
 
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