Engine revs but does not accelerate - burning smell?

[jeffconnell12]

Hi guys,

Took out my new to me 1975 Holiday 18 with the merc 3.0L for the first time this weekend - after about ten minutes the RPMs seemed to jump and the boat would slow.. felt like a transmission slipping and the prop sounded like it were cavitating. We experienced it twice then stopped and switched to the kicker.

Here are all of the details - I am really looking for some help on where to start!

- Engine recently rebuilt (with documentation)
- Previous owner put a 4 blade prop on and cut out part of the little metal fin that was clearly getting in the way of the new prop
- There was a very obvious burning smell - similar to a clutch smell
- There was a rattle that sounded like a loose heat shield on a vehicle (very metal sounding) when the problem would occur
- The next day the motor would bog down and die if i attempted to accelerate when in gear - but when in neutral it ran fine
- It runs fine at very low RPMs when just engaged in forward or reverse
- Motor was fully trimmed down

Thanks so much!

Jeff

 

[GA_Boater]

First guess is the coupler is shot. The smell is pretty much the big clue. The motor may have overheated when trying to rev the motor cuz the impeller wouldn't be spinning like a top and not much in the way of water flow.

 

[Bt Doctur]

and the outdrive oil is...........................................

 

[tpenfield]

+1 on the coupler . . .

 

[jeffconnell12]

Thanks for the help - my plan is to pull the engine and leg and replace the coupler.

I'd like to order the parts ahead of time as i have to borrow the space to work so i need to be prepared.. if the coupler potentially blew up what other parts might need replacement? I imagine the oil could be contaminated with metal.. could i potentially get by with just doing the coupler and leg seals?

Should i order a Gimbal bearing in case?

Any advice is appreciated!!

 

[Scott Danforth]

you have to pull the drive every year for inspection of the U-joints, gimbal, alignment and the bellows

if you havent pulled the drive every year, I suggest you pull the drive first prior to ordering parts as you will most likely need something else.

dont forget the alignment bar

 

[tpenfield]

Make sure the couple appears to be bad, before you go to the trouble of pulling the engine. You should be able to get a look at it once you pull the outdrive.

 

[jeffconnell12]

HI guys - I removed the outdrive today.

The coupler looks okay to me.. the gimbal bearing spins nicely with no play.

Looks like some old grease with maybe some water intrusion? The old grease looked very rubber like around the coupler.. kind of black and chunky like wet asphalt.

I noticed the two rear bolts that are maybe engine mounts? were not fully tightened. Is this normal? Are these used to set alignment with the outdrive?

Here are some photos.. if you agree the coupler looks okay then what could it be?

Thanks!

 

[nola mike]

Can't see enough from that pic. Splines look pointy but not totally stripped, but your description makes it sound more like the hub spun loose of the rubber. If the rear mounts were loose that makes the coupler even more likely. .. Are those wood shavings or rust around that Mount bolt?

 

[jeffconnell12]

wood shavings in the photo!

can you elaborate on the "hub spun loose of the rubber" ?

I can grab some more photos in the morning.. hard to capture it but i'd imagine the splines would be completely gone if the shaft were essentially spinning freely?

Thanks!

 

[GA_Boater]

The coupler hub is aluminum and the driveshaft is steel. The hub splines wear to points or wear off completely if not kept greased.

A spun coupler is when the bonding between the aluminum coupler hub and rubber shock absorber breaks down. The red circle (?) shows where the hub and rubber are bonded. Just an example coupler and probably not what you have.

Worn/sheared hub splines or spun coupler have the same result - The drive shaft doesn't have a solid grip to turn the prop consistently. A symptom is the prop losing grip often confused as cavitation. The other sign is burning rubber from the broken bond and the rubber is melting and smelling from the hub spinning in the rubber.

You have both symptoms - Supposed cavitation and stinky, burnt rubber smell.

 

[nola mike]

Why are there wood shavings there? I'd be concerned that your stringers are rotten, which allowed your mounts to loosen, which allowed your engine to move, which ate your coupler. Which is an extensive repair at best.

 

[jeffconnell12]

thanks guys!

I believe the wood is from someone drilling above the motor as there has been some recent work done (mounting hardware installed).

The stringers are aluminum with just a wood deck.

The motor does mount to a piece of wood - it seems wet but very solid. It will be replaced when i pull the motor to do the coupler.

Are those rear bolts supposed to look the way they do? Are they used for alignment or should they be tightened all the way down?

 

[rickasbury]

They should be tight but they may have spacers or a spring washer that makes it look like what you are seeing. No hard in giving them a twist, that's for sure but there is a torque setting for them, I think around 45 ft. pounds? It;s not a lot as I recall...