1990 3.0L OMC bogs/stutters/misses at WOT

[drumminmandan]

Hey guys - Been lurking on here for a while now. I have been able to fix most of my problems by reading through old posts. So thanks for the wealth of knowledge you guys have provided over the years, and thanks in advance for future help.

For those that do not want to read the whole post, I have tried to highlight some key points by putting them in bold and underlining them.

I have a 1990 Chaparral with a 3.0L OMC. At the beginning of last season, I did a carb rebuild (fully disassembled, thoroughly cleaned, replaced all gaskets) due to a rough idle problem. At the time, I also replaced the spark plugs, fuel filter, and water fuel separator. Once done, I was able to tune the carb in and it ran extremely well at all throttle positions. A lot of fun days on the water last summer Near the end of the season though, I started to notice a bogging issue. It just seemed to occur one day and I could not trace it to any change points. This bogging issue is what I am looking for some direction on as the problem still exists.

During heavy acceleration from idle, the engine bogs real bad, feels like a cylinder is missing. If I back it off and accelerate at a slower pace, the engine still bogs, but the RPMs do eventually catch up with the throttle position. Once it catches up, the engine runs well. Once the boat is on plane, I can back off or advance the throttle with no ill effect on the engine. The problem only occurs when trying to get on to plane (I have been running the same prop since day 1, and there is no damage to the prop.)

- The problem is more pronounced early in the boating trip. Out of the boat launch, it is real rough. After boating for 45-60 minutes, there is very little hesitation and the problem almost 'goes away'.
- I am unable to re-create the problem with the engine running on muffs. Only in the water / under load.
- On the trip prior to my last trip out (about 2 weeks ago), I was able to run through the old gas (did not run it dry, but down to a little under an eighth of a tank) and put fresh gas in. Problem still existed on my last trip out.
- Problem seems to produce symptoms of a bad accelerator pump. I have disconnected the throttle linkage and manually manipulated the throttle connector while watching down the throat of the carb. The accelerator pump produces two strong jets of gas down the carb throat, so that does not seem to be the problem.
- No problems at idle or startup.

Since I have owned the boat (now in the fourth season), I have not touched anything on the ignition side except for replacement of spark plugs. So regardless of whether or not the problem has to do with the ignition side of things, a tune-up should be done given the unknown history of the boat.

Would it make sense though, that the problem could lie on the electrical side of things given that the problem only exists during heavy acceleration under load, and eventually tends to 'go away' as the boating day wears on?

Hopefully you guys can point me in a direction and give me some things to test. I would love to get the boat up and running soon as it is starting to feel like summer is coming to an end here in Michigan

Thanks for taking the time to read this and any insight you may be able to provide!

 

[Idajack]

I am fairily new to the outdrive systems myself but having disassembled my entire drive to change the gimbal plate I noticed that my flap in the exhaust had pretty much melted into a gob of plastic, I would pull the exhaust riser and look into the top of the exhaust pipe and verify that the flapper is intact. If it isn't evident at the top of your exhaust pipe and intact I would suspect that it has melted and may be blocking your exhaust causing a stumble or bog. I'm a novice to I/o boats but this is where I would start, make sure you have good air in and out of the engine.

 

[vinnie1234]

Both ignition and fuel issues make sense to me. I am with you in thinking that an ignition problem makes the most sense. If the problem is fuel related it is most likely to be lean or water in the gas. If the problem is ignition related the plugs should show a little carbon fouling. I would take the boat out and honk it under load. Make sure that you are replicating your issue. Shut the boat down and pull your plugs. If they look fuzzy and black, I would check the ignition system. If not, I would look for restriction in the carb. Possibly some gunk in there blocking the main jet.

Another (perhaps easier) way to diagnose the fuel issue, is to go out with a buddy. Take the doghouse and flame arrestor off, and when it starts to bog down reach over and slowly close the choke plate. If it clears right up, you have a lean mixture, if it gets better but still sputters you may have water in the float bowl. If it stalls, the mixture is not lean and you should investigate your ignition system.