Lower gears replaced in my Cobra...why does it hum?

[Liquid_force]

Have a concern that's been bugging me for a while.<br />Last June I had to have the lower gears replaced in my cobra ('89 4.3L) - Drive, reverse, forward, clutch.<br />It's run great since then, but when I picked it up the mechanic mentioned to me that when the prop is spun manually with the drive in neutral there is a slight "hum". It sounds as if the gears are just GRAZING each other when not IN gear.<br />He assured me he's replaced many cobra lower gear sets and has had this happen on many of them with no future problems. Basically, he said he didn't know why they did it, but that everything was measured and set perfectly (even though it wouldn't engage in reverse on my first test run - he had to meet me at the ramp to make an adjustment at the engine. )<br /><br />I'm just curious if any other techs have had similar experiences with the neutral "hum", and if not, what my mechanic did, or might have done incorrectly.

 

[rodbolt]

Re: Lower gears replaced in my Cobra...why does it hum?

sounds like a case of a parts changer<br /> I to have replaced many gear sets on cobras, alphas, bravos and volvos. never once had any problems with gears not engaging when done. course I also change the shift cables and do the maint on the rest of the system when doing any gearcase repairs. is this a hum or is it a clicking sound?<br /> no adjustment at the engine will compensate,coirrectly, for any misadjustments on the unit or in the drive or wear in the cable/bellhousing.

 

[Liquid_force]

Re: Lower gears replaced in my Cobra...why does it hum?

Parts change? As in a gear set upgraded/updated from stock? That much I know to be true. I bought the parts myself, they came directly from the machine shop that mfg's them for sierra. They were visibly different from the damaged ones that came out of it, but after several phone calls from myself to the shop, myself to the mfg, and the mfg to the shop we did finally conclude that they were indeed the correct, however, updated parts.<br /><br />I guess you could call the noise a click if you move the prop slowly, but it's really faint - basically just a bit of resistance as the gear teeth slide by each other (I think).<br />If you give it a whirl it's more of a hum. It sounds exactly like the drive/driven gears just barely rubbing each other.<br /><br />It bums me out a little b/c we're planning to sell the boat in a couple years and the 'ol prop spin is one of those things some people just do - like kicking the tires of a car. 99 times out of 100 you wouldn't think twice, but when you get a strange HUMMmmmm.... from the lower unit it could be a little concerning to a potential buyer.<br /><br />The on the water adjustment that was made was because when I first tried to back it off the trailer it wouldn't go into reverse. It just made that horrible GRIIIIINND we've all heard 100 times.

 

[rodbolt]

Re: Lower gears replaced in my Cobra...why does it hum?

ok<br /> the driven and drive gears dont move. they are always engaged, just the shift clutch moves. right now the clutch dogs are chattering on the lugs on the gear. bad,very bad. its usually due to the shift shaft or bell crank or linkage not set up correctly. sometimes its due to the cltch being installed the wrong way. its not symetrical. has to go correctly. I did a set of GLM gears on a omc v6 case and the groove that goes to the prop end was cut on the fwd gear end. it looked funny so I broke out my trusty calipers and checked it. sure enough it was not machined like the OEM dog. the groove was on the wrong end. had I installed it with groove aft like the OEM dog it would not have worked correctly. I hate to say start all over but the problem needs to be found and corrected.<br /> the horrible grind on the first attempt was a radical misadjustment of something on reassembly and the clutch dogs were wiping themselves out against the lugs on the gear face, will wipe the lugs as well.

 

[Liquid_force]

Re: Lower gears replaced in my Cobra...why does it hum?

wow - <br />even having run well for 3+ mo (50 hrs?) last season you think I'd be wise to have it torn down again?

 

[lakelivin]

Re: Lower gears replaced in my Cobra...why does it hum?

http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=008398#000000 <br /><br />Lf, take a look at the above post. Not exactly the same issue, but similar in principle. I ran mine for only 17 hrs and a couple of months after the problem (always with this feeling in my gut that something just wasn't right inside the drive), and would have torn up the gearcase if I had gone much longer before the drive was pulled and the damage identified. <br /><br />In my case it was clear to independent, objective individuals that the shop caused the damage by grossly underfilling the drive oil, and got some relief frim small claims court. You might have a harder problem getting relief in court (for me there was a clear connection between what the shop did and the resulting damage; I had documentation from OMC manual that damage will occur from low filled drive, statement from the shop that identified the damage that it was consistent with low oil, etc.).<br /><br />Don't know what additional damage you might incur if something is wrong and you continue to drive the boat without first fixing it (hopefully Rodbolt or one of the other mechanics on here can give you a 'risk assessment'). <br /><br />But if that risk of more costly damage is high enough compared to cost of checking it out now, your best bet might be to show the shop that did the original work Rodbolts post above & see what they might be willing to do to help with the problem. The mechanic admitted that there was a 'hum' at time of repair. If you're lucky enough to have a shop with a high degeree of integrity, perhaps you can work out an agreeable reolution on what to do at this point. <br /><br />My advicce is to make any agreement VERY SPECIFIC, including actions to be taken, who pays costs that are incurred, and exactly how much it will cost you for each possible scenario.<br /><br />Actually, first I'd double check the shops level of expertise with Cobras. If they weren't qualified to do the work in the first place, might be better off going somewhere else instead. <br /><br />Good luck!

 

[rybad]

Re: Lower gears replaced in my Cobra...why does it hum?

I don't know about the "hum", but if you couldnt get into reverse, and he made an adjustment at the engine, it sounds like he simply adjusted the shift cable at the engine. If you can resolve the "hum" it sounds pretty harmless.<br /><br />Those shift cable measurements on the Cobra's are very specific.<br /><br />Good Luck<br /><br />-David