engine revs but don't increase speed

[boxers21]

I ordered a new 1991 4.3L Mercruiser from a local Chaperal dealer here in town. Put a new SEI 116 on the back. Went to water test Sunday and this is what happened. It got up to 40 mph then I herd a pop and the rpm went up and speed did not increase. I stopped the boat checked the oil and there was no water in it. So I started the motor up again it sounds good and idles good. So I increased the speed and when I reached 1500 rpms it reved up to 2500 rpms and no different speed. Then when it reached 2500 rpms it started to get power again for a minute. Then It started cutting in and out and poping out the carb. I'm at a loss. I prolly have several things going on here. I'm open to suggestions. Thanks Randy

 

[dingdongs]

Re: engine revs but don't increase speed

Ttreat it to new plugs and fuel filters and get it running smooth first.It probably never got on the plane and this is where it needs all the power and torque possible.Fresh fuel is good to start with especially if the boats been standing.

 

[gm280]

Re: engine revs but don't increase speed

Sounds like a prop hub rubber seperation...

 

[boxers21]

Re: engine revs but don't increase speed

Ttreat it to new plugs and fuel filters and get it running smooth first.It probably never got on the plane and this is where it needs all the power and torque possible.Fresh fuel is good to start with especially if the boats been standing.

I put new plugs in it on Fri. and I put new mercarb on it and new fuel filter, and filled it up with fuel.

 

[makonnen]

Re: engine revs but don't increase speed

Sounds like a prop hub rubber seperation...

What he says.

 

[boxers21]

Re: engine revs but don't increase speed

Had the prop tested and prop all fine. Adjusted the lower shift cable. Took It to lake and it ran Good 4800 rpm for about a min. The it reved up and hit the rev limiter. When I backed out of it, it wouldn't get on plane and still revs up to the rev limiter. Any ideas?

 

[500dollar744ti]

Re: engine revs but don't increase speed

Try another prop for testing purposes and report back, it's still probably a spun hub.

 

[ktbarrentine]

Re: engine revs but don't increase speed

Blown coupling?

 

[subysti]

Re: engine revs but don't increase speed

I'd say hub also. been there, done that. thought the prop was good when it was "tested" by someone. tried another prop and all was good again. Although its a new aftermarket outdrive so something could be slipping internally. Prop is an easy check though, start there I"d say.

 

[boxers21]

Re: engine revs but don't increase speed

I don't smell rubber burning or see any pieces of rubber behind the engine near the coupling. I'll try a new prop tomorrow and report back.

 

[subysti]

Re: engine revs but don't increase speed

You wont smell it. The hub everyone is talking about it the center part of the prop that the shaft mounts to. It all looks like a solid piece but its a rubber hub. No bits and pieces either, the prop just slips and spins on the hub under load in the water.

 

[littlebookworm]

Re: engine revs but don't increase speed

If the coupler had torn, you wouldn't get any forward motion. I agree that it's probably a torn/spun hub on the propeller. The inner and outer sections stay 'connected' up to a certain load. Then the prop begins to slip. It's a simple fix. Try a new prop. Hy

 

[boxers21]

Re: engine revs but don't increase speed

cool try new prop get back with you in a few days. thanks

 

[ktbarrentine]

Re: engine revs but don't increase speed

Blown coupling?

Agreed...symptoms not all there for that, but just adding to to the list of possibilities. Will also add that the shaft splines could be on their last "legs".
You can do the prop test without changing props, by marking the prop shaft and prop, going for a "spin", then looking for misalignment of the match marks (was able to check these marks with the boat still in the water). What ever seems easier for you, though.

 

[Rockinsince85]

Re: engine revs but don't increase speed

The new outdrive should have a warranty correct? The torque at higher rpms could make the transmission slip..Do you feel anything in the cable while this is happening. I know its near impossible to hear the drive but just an idea. If it was a coupler or hub seperation i would assume the symptoms would be noticed well before 40mph and 4800rpm. And like someone said you wouldnt get any forward motion. If your drive is binding then that could interrupt the cranks rotaion causing a missfire in timing i.e your backfiring. I would swap drives and try again..