3.0 Cobra knocking noise

[VMAX]

My 1986 3.0 Cobra has begun to make a loud rapping noise(I was boating with the engine cover off) which sounds like a rod knock (could be) which I only hear when the boat is under way on the water. Running in the driveway it sounds normal, which makes me think that it's either load-related, or it's not coming from the engine, but from the drive.<br />It isn't apparent under acceleration, but at cruise speeds above 2000 RPM, and particularly when backing off the throttle momentarily. Has anybody experienced this type of noise before?<br />Could it be coming from a bad alternator, water pump, loose mountings,etc., or should I plan on pulling out the engine?<br />Any thoughts would be appreciated.<br />John

 

[hondon]

Re: 3.0 Cobra knocking noise

This hardly the quietest powerplant on the market but if you are hearing noises that are new I would start with checking the engine alignment.This is regular maintenance and can cause sounds similar to what your hearing if it is out.Exhaust flappers will rattle like a mother on these 3.0's once the neoprene has worn off.How's the compression and how is she running?

 

[Dually Driver]

Re: 3.0 Cobra knocking noise

Don't ask my why, but I would pull the inspection cover on the upper drive that exposes the drive gears, make sure the lube is where it is supposed to be.<br /><br />Mike

 

[VMAX]

Re: 3.0 Cobra knocking noise

Compression is good-about 120 psi on all cylinders, and the engine seems to run strong-<br />It's a 1986 4 WINNS 170 Horizon and it runs at about 38 mph (speedo indicated)with a 17 pitch prop. I'm not familliar with the exhaust valve. Where is it located, and how is it checked? Is this replaceable, or part of a large assembly?<br />I will try to check it this week. I do have a manual, so I will check there for information also.<br />It is concievable that the problem is spark knock, but it sounds too loud and low frequency for that. Also it is most noticeable at cruise and when backing off the throttle. I would think that spark knock would occur at hard acceleration, but maybe I'm wrong. I'll be sure to post again when I find out what it is.John

 

[coupedeville]

Re: 3.0 Cobra knocking noise

Perhaps you should connect an oil pressure gauge to the engine, and see what you get. If the rod are knocking, it's because there is too much clearance in the bearing, which will effect oil pressure. A knock in the bottom end will be a regular thunking noise. Get a length of hose and while under way, place one end against the lower part of the block and listen.

 

[VMAX]

Re: 3.0 Cobra knocking noise

Well, my noise problem is gone now. Thanks to all who have helped. My timing was set 8 degrees over-advanced. Also, the shift cam which actuates the micro switches was not firmly contacting its stop when in forward gear, which allowed it to rattle under load. Re-routing the cable freed it up and allowed it to shift easier and more positively into gear. We took the boat out with the timing set to 1 degree ATDC per the manual, for 89 octane fuel and it ran well and quiet. With 2 adults and 4 children aboard it maxed out at 35 mph. This was with a 17 pitch prop at 3500 rpm.<br />According to my service manual, the 3.0 liter engine max rpm is 4500 rpm, so I have a couple of questions. Firstly, is 35 mph about right for this combination? The boat is an '86 4 Winns 170 Horizon, 17' bowrider. Secondly, should I consider a 15 or 13 pitch prop? Im new to I/O boats, so these pitch numbers seem awful low. (my 140 Merc inline 6 is turning a 21 on my 16'speed boat and it still accelerates hard). If I did lower the pitch to get the rpms up, would it help the top end, or just the holeshot? I could see a benefit in the future for pulling a tube or a skier. Lastly, if I keep it propped at 17 with the max rpms at 3500, would it give better gas mileage than it would at 4500 rpm? I'm going to buy another prop for a spare, and I want to take the opportunity to improve the boat's performance at the same time if possible. Any help would be appreciated. <br />John

 

[Boatin Bob]

Re: 3.0 Cobra knocking noise

VMAX....I think you would have to go down to a 13" prop to bring your RPM's up into the 4000 range which does seem kind of low. Have you checked the throttle plate in the carb to be sure it's fully opened when you have the throttle at full? I would think your boat should be hitting 40+ mph with that setup.

 

[VMAX]

Re: 3.0 Cobra knocking noise

Hi Boatin Bob,<br />I do know that when I was in the boat alone last week, it was doing 38 mph at 3800 rpm, but I had about 500 lbs. less of people and gear at the time, and my ignition timing was set at 8 degrees BTDC, which I reset to 0 degrees to prevent pinging. Maybe I went too far (manual states 4 degrees BTDC with 89 octane). Maybe my tach or speedo are innacurate. I don't have a GPS (yet) to check my speed.I'm also not sure what the reduction ratio of my outdrive is, so prop calculations are difficult. If anyone has the same boat or something similar, please let me know how your performance compares, Thanks, John

 

[DJ]

Re: 3.0 Cobra knocking noise

VMAX,<br /><br />I think your tach. is suspect. That set up should pull 4000 (at least) with a 17" prop.

 

[VMAX]

Re: 3.0 Cobra knocking noise

Good thought- I'll check it against my diagnostic tach to be sure.