4.3L RPM Fluctuation under load

[sikemd]

Hey guys, I have a 4.3L OMC Cobra I/O on a 20' ChrisCraft. Yesterday we loaded the boat down pretty good with 4 people, coolers, etc... and when I tried to get it up and running above 3,000 RPM, the RPMs would bounce intermittently ~300RPM or so and it just didn't want to get going, at one point I had gotten it on plane and it seemed fine after that. I tried a few other times to get it up and going throughout the day and each time resulted in RPM fluctuation. It also sounded a bit louder than normal when this was happening I believe, with a thumping type sound. The last time I had it out (last week) with just 2 people it ran absolutely fine. Any ideas what the problem may be? I'm fairly certain the issue is only evident when weighed down and under heavy load. What would cause this to happen due to weight increase?

Thanks,
Don

 

[Levinz11]

Re: 4.3L RPM Fluctuation under load

Done a tune up lately? Checked the fuel system lately? RPM's can't really bounce that much due the prop being in the water, right?

 

[sikemd]

Re: 4.3L RPM Fluctuation under load

Done a tune up lately? Checked the fuel system lately? RPM's can't really bounce that much due the prop being in the water, right?

Replaced the plugs, wires, and distributor last year. Carb was cleaned and rebuilt last year as well. I don't think it is a fuel issue. I'm going to pull the plugs and do compression check tomorrow. I believe it may be a timing issue, but I'm not familiar with non-EFI automotive engines. I think the thumping sound I hear when the RPMs are dropping is a misfire and the thump is the sound of the compressed air / fuel being forced out of the exhaust without igniting, however when I had gotten it on plane it was fine so it is certainly related to heavy load.

 

[astuckey]

Re: 4.3L RPM Fluctuation under load

Did you replace the dist with the same dist or did you do some after-market one such as a mallorie unilite or something similar? I find that if you modify these ignition systems with more advanced ones, they sometimes don't work as well. Is it the stock coil?

 

[sikemd]

Re: 4.3L RPM Fluctuation under load

Did you replace the dist with the same dist or did you do some after-market one such as a mallorie unilite or something similar? I find that if you modify these ignition systems with more advanced ones, they sometimes don't work as well. Is it the stock coil?

Actually, after looking at the motor I'm not so sure if we did or didn't replace the distributor. I still haven't gotten around to pulling the plugs. I took it out again today with light load and just 2 people and when we first got on the water it was still miss firing but not as much and on the way back in I didn't notice any issues. It was a short 5 minute ride both ways. At this point I think it may just be a bad plug (crossing fingers).


I appreciate the info and feedback. Thanks. I will be sure to post when I figure it out.