one cylinder cuts out intermittantly

I have a 35 HP 2cylinder engine on a pontoon boat that I just purchased for fishing. It starts good and runs good until it warms up. Then it looses power as the lower cylinder seems to cut out. Is this a symptom of a faulty coil? Not sure of the year. The model is OA988581

IF that motor has two coils mounted on the top of the block, one of them is bad--seen a lot of that lately.

You can tell which one is bad by making them motor skip for several minutes, then shutting it off instantly and pulling the plugs. One of the plugs will be black and oily. Replace that one's coil.

Jeff

Yes, as Jeff notes I have replaced more coils in the past two years then the last 20 combined. It seems like the lifespan has been reached on coils built in the 1980's. They are developing small hairline cracks that are almost impossible to see (unless you see the spark jump from them with the motor running).

Just as a matter of course, and since this is just a twin, I would consider replacing both the coils "just because".

I wouldn't go with OEM (too expensive), instead I have been opting for Sierra coils (list at about 20 bucks each) and they are superior to OEM coils for Merc portables (2 cylinder models)...

galamb said:

Yes, as Jeff notes I have replaced more coils in the past two years then the last 20 combined. It seems like the lifespan has been reached on coils built in the 1980's. They are developing small hairline cracks that are almost impossible to see (unless you see the spark jump from them with the motor running).

Just as a matter of course, and since this is just a twin, I would consider replacing both the coils "just because".

I wouldn't go with OEM (too expensive), instead I have been opting for Sierra coils (list at about 20 bucks each) and they are superior to OEM coils for Merc portables (2 cylinder models)...

Click to expand...

I'll replacing it next week. Would a bad reed valve do the same thing?

The reeds generally work or don't work. Since it runs ok until it gets up to operating temperature it is most likely an ignition issue.

If it isn't a bad coil then either the trigger or the stator could be the culprit.

When a covering of one of bobbins cracks (plastic/epoxy over a copper coil) it will let out the ignition power. When cold the integrity of the covering may be good enough to prevent a short but as the motor heats up the copper expands, putting pressure on the case, opening the "crack" allowing the power (from whichever particular coil it is) to go to ground.

If it's a trigger or stator bobbin it grounds to their mounting plate (unseen under the flywheel). If it's a sparkplug coil it often arc's to the block, which can often be seen....

Graham:

Good info on the Sierra coils--I'll buy them from now on. Thanks.

On "bad reeds": I've never seen a bad Merc reed in over 50 years of working on them. Oh, I know it happens, but...

Jeff

Agree, I have seen some gummed up reeds from sitting with bad fuel etc, but never had to actually replace one because of a chip or break. I'm sure it happens just not high on (my) suspect list.

And as to the sierra coils. Merc made two basic coils for most of their motors under 100 horses. The coils on the portables are mounted using a bolt right through the center of the coil. These coils produce somewhat less power than the coils on the larger models.

Merc did sell (for a while) a high power coil upgrade for the smaller motors. It was simply the "standard" Merc coil and a new mount to hold them (since they did not mount with a bolt, instead went into the (holders). The standard coil puts out approx 30% more voltage. On a smaller motor this translated into better cold start and considerably smoother idle/low speed operation.

Anyhow, Sierra made the coils to the power specs of the more powerful coil but also built them with the hole through the middle to allow mounting on "most" Mercs. They also include a (cap, cover - whatever you want to call it) to make the coil the same format as the regular coils. And you get the whole thing for about 20 bucks.

The current list price for the (small Merc) coil is 59 bucks. The Sierra coil is less than half that price and it's the more powerful coil - so you upgrade your ignition and get both coils for the cost of a single OEM - life is good

Could you possibly tell me what year this motor might be. I got it used and the owner didn't know. Thanks! Serial number OA988581 Black case with a red stripe.

That's a 1986 model..

Finally got it running today. Changed the coil and still had the same problem. Changed the switch box and it smoothed right out. Thanks for the good information, I appreciate it.

Great! Always better to try the cheaper part(s) first, and a bad coil (seen a lot of them lately) can kill a switch box.

Jeff

Fastjeff Rule # One: Never, ever throw anything away!

Has saved my butt numerous times.


Jeff