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OMC sterndrive stutters at high rpm when hot

dbc327

New member
Hi,


I have a 1988 Bayliner with a 5.0 liter OMC Cobra Sterndrive (GM engine).


A few weeks ago, the engine started stuttering badly at medium-high rpm when hot. When cold, everything runs fine for a few minutes. As soon as it hits operating temp (~170 F), it runs extremely rough when given any gas. If I go back to idle, it runs ok.


At the beginning of the season, I tuned it up, replaced plugs, points, condenser, rotor, distributor cap, and adjusted the dwell and timing to specs. The boat ran great all season until a few weeks ago.


Since this problem started, I replaced the carb with a rebuilt carb and replaced the thermostat (which was badly rusted and wouldn't move). I also re-adjusted the gap and timing to spec. None of these changes helped.


I tested the water pump by temporarily disconnecting the output hose - it pumps fine.


I checked the impeller in the outboard unit - it looks fine, though I'm not sure what constitutes bad.


The engine also runs fine, cold or hot, at all rpm's, when connected to a hose at home. I can rev the engine to 4500 rpm (in neutral) without any misses or stuttering.


Since it runs fine when cold, I doubt it is a fuel pump problem, a fuel intake problem, or a linkage-related problem.


The problem only occurs in open water, when hot, and when in gear above idle rpm.


I don't know what else to check or replace!


I'd greatly appreciate any ideas or advice to solve this problem.


Thanks, dave
 
I suspect the carb. In my experience, rebuilt or reconditioned carbs never run like a new out of the box one. You just can't get all the way into those passageways when cleaning.
This seems to me like the main jets are either clogged or fouled somehow. When the thing is cold, the choke is adding fuel so it "seems" good, but when the choke opens fully, the main jets are just not providing enough fuel. On the hose, the engine isn't under load and not sucking down the same amount of fuel. It runs OK on less.
My advice is a new carb or a known good one.
 
Thanks for your response. The carb was my first thought, but the replacement carb behaves exactly like the original carb. It seemed unlikely to fail in exactly the same way.

I took the boat back out to the lake today to look for clues. Today, the problem occurred at higher rpm's when cold or hot, and whether in gear or in neutral. It ran ok at low rpm's in gear or in neutral.

A different thread with similar symptoms suggested a worn distributor. There's a little bit of play in mine, enough that I can see the gap change when I wiggle it. While at the lake, the stuttering also changed when I advanced or retarded the timing. When advanced, the engine ran better but the stuttering was worse. When retarded, the engine ran rougher but the stuttering was less.

Yet another thread that suggested the coil was going bad... and that these types of issues are often electrical/ignition problems.

When this problem originally started, I thought the timing had slipped.

Do either of these explanations make sense? I'm trying to avoid changing any more parts than necessary.

Thanks, dave
 
Well, unless that engine has 3-4000 hours on it I wouldn't suspect a worn diz. However these things are on ebay so cheap that for $20 you can have as many GM type marine distributors as you could ever want, so it would be something I'd try. Since you did the tune up parts it would seem that it would cure the issue. However, if those points are cheapo chinese stuff, they could be floating at the high RPMs.
 
Thanks again for your help! I rebuilt the original carburetor, soaking and cleaning it extensively. When I installed it, the engine failed the same way. I ran it for awhile, tried tweaking the dwell & timing more. At one point, the engine was idling ok, but then stalled and wouldn't start again.

Per your comment about the points (which were after-market), I again replaced the points and condenser with Evinrude/Johnson branded parts, and the coil. Voila - problem solved.

I never would have expected near points & condenser to fail after a few months of light use!

Thanks again, dave
 
Sounds like a defective coil. I had this problem about drove me nuts. When the coil gets hot it breaks down and you never know when. Engine starts to run rough or dies entirely. RPM doesn't matter. When you run it at home I will bet the engine cover is open so the engine room does not get as hot. If your coil is attached to the back of the port head, move it to the transom and get it away from the heat. I went through 3 coils in 7 years before I figured this out.
 
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