Large thump after fully engaged, what is it?

[mtb1981]

I have a 1990 Four Winns w/ the 2.3L OMC Cobra engine that makes a large thump after I'm underway. I notice this most when I'm towing someone or navigating the boat which would require some forward-then-reverse gear shifting. It seems like it engages smoothly, and I can advance forward without any issues at low speed, but when I throttle it up either slowly or fast, I get this loud thump that shakes the boat.

Please help, and I apologize for any lack of information. Please ask any questions and I'll get an answer back to you asap.

-Mark

 

[bruceb58]

Re: Large thump after fully engaged, what is it?

One thing it could be is your dog clutch slipping out of gear and the engaging again. If this is what is happeninning, it is critical that you inspect your lower shift cable and adjust if necessary. Don't run your boat anymore if this is the case as you will wipe out the dog clutch.

 

[futz]

Re: Large thump after fully engaged, what is it?

could be the rams letting loose, maybe letting the drive to bang down. hows the fluid in your tilt, maybe air.

 

[180shabah]

Re: Large thump after fully engaged, what is it?

could be the rams letting loose, maybe letting the drive to bang down.

Real easy to check, raise the drive some, note the amount of ram exposed, put it in gear, give it a little throttle, after the "bang", shut it off and check the drive position.

 

[mtb1981]

Re: Large thump after fully engaged, what is it?

Correct me if I'm wrong here, but wouldn't that happen only once assuming I didn't touch the trim after the "thump"? This seems to happen more often, and it seems to be caused by shifting between reverse and forward, although the actual "thud" only happens on throttle-up.

 

[bruceb58]

Re: Large thump after fully engaged, what is it?

If it was the rams, putting it in reverse would raise it up a little.

Tilt your drive up and look at the shift cable housing as it enters the drive. See if it has any imperfections. That is where they fail.

 

[180shabah]

Re: Large thump after fully engaged, what is it?

There really isn't a whole lot going on back there, either your hydraulics are shot, allowing the drive to tilt up in reverse and slam back down in forward.

OR

your about to need a complete new drive because it is not shifting properly. Not trying to be dramatic, but the cobras have a reputation for early failure due to shift cable stretch/misadjustment.

 

[bruceb58]

Re: Large thump after fully engaged, what is it?

When the boat is out of the water, trim up the drive a few inches and jump on it. If it doesn't move down, your trim cylinders are not your problem.

 

[mtb1981]

Re: Large thump after fully engaged, what is it?

Ok, it's time to admit that I really don't know what I'm doing in regards to outdrives. I can handle myself when it comes to engines, but I just dont have any experience with outdrives, so please be easy on me.

I checked the cable, and it looks good to me. I have a picture too, but I can't find a host just yet. (Or at least one where I can upload without creating an account).

Also, I checked the fluid level in the power trim pump, and it looks good as well.

 

[mtb1981]

Re: Large thump after fully engaged, what is it?

I lowered the outdrive, raised it a bit, marked the ram shaft with a dry-erase marker and jumped on it 3 or 4 times. No movement at all. Does this mean it's internal now? If so am I going to be able to fix it?

 

[bruceb58]

Re: Large thump after fully engaged, what is it?

This is pointing to a shift adjust issue. The cable can still be bad but it usually shows with the cable housing. You really need to get a real OMC manual to adjust this not a Clymer or Seloc. It is a bit tricky and unfortunately, adjusting it incorrectly can ruin your drive.

Take a look at this guy's site to learn a little bit about what you have here:
http://www.hastings.org/~stuart/cobra/

 

[a70eliminator]

Re: Large thump after fully engaged, what is it?

Your clutch dogs are jumping out, it's been described as hitting a submerged object while underway, a cable adjustment can take care of it provided it hasn't been let go too long in which case the dog eared cogs wear rounded and can no longer lock in.

 

[mtb1981]

Re: Large thump after fully engaged, what is it?

Thanks guys, I really appreciate the help. I read the above links to other threads and to the sterndrive tech info, and both were really useful.