Tired Engine, out of tune, wrong prop, or too high of expectations?

[geneseo1911]

Boat in question is an '82 Cobalt 19' open bow. Cobalt brochure calls it 2900 lbs. It is a very deep vee with a 91" beam.
Engine is the original Mercruiser 260 with approx. 970 hours. 1.5:1 gear ratio.
Current prop is a Quicksilver SS 14x21, looks like new; not sure of the model.
Ran this weekend with about 1150 lbs people/gear/fuel. Got 47 MPH (GPS) @ 4100 RPM.
Ran Last weekend me alone maybe 350 lbs operator/gear/fuel and got the exact same speed/rpm.

My biggest concern is pulling power. It planes nicely with everyone on board, but it STRUGGLED to get on plane pulling me on a tube. As in minutes. Although, it was a small tube and I weigh about 260.

I'm also concerned with the lowish WOT RPMs. It seems to me the boat should be a bit faster at the top end with that much power; granted its a heavy, slow hull.

A 14x21 SS is what it came with new. The one on there now looks to be a direct replacement (could it be original?). So I'm wondering if it was overpropped from the factory (doesn't seem like something Cobalt would do) or the engine is just tired at nearly 1000 hours. The engine idles well, has great throttle response, and doesn't burn any oil. Doesn't even turn it black. The manual says max RPM is 4200-4600, so I'm not too far away.

Am I expecting too much of this engine? Should I drop to a 19P Al or maybe a 20P 4 blade Al? Or should I be looking for a something wrong with the powerplant?

 

[bnicov]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

A 19 Pitch prop will probably do you fine, you'll be right in the middle of your recommended RPM range and you'll get a better holeshot but you will probably lose a couple of mph's on the top end. I wouldn't be running flat out anyways. That is real hard on your engine and on your fuel budget.

 

[craze1cars]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

It's a 29 year old boat, with 1,000 hours. Could something be wrong? Only you and/or your mechanic can tell. Certainly at least once every 4 or 5 years (sometimes more often) I go thru my powerplant with a fine tooth comb. Do you confidently know the currenet status of every item on the below list? If not, do them all...you might find some more power:

Check resistance of every spark plug wire and replace/repair as needed, replace plugs, cap & rotor, set fuel mixture on carb, do a compression check, do a leakdown test, check and set base timing, check for proper ignition advance timing, check throttle cable adjustment, check tightness of carb mounting bolts, check for vaccuum leaks, check & reset rocker arm valve lash, check piston wash to identify state of carb tune...and annually I check engine alignment and gimbal bearing, obviously change all fluids and clean flame arrestor. If my boat had points (yours might....I'm not sure), I'd be replacing those at least annually as well....

Do you run ethanol fuel? Your 1982 boat was built and tuned to run on non-ethanol fuel. There is nothing wrong with 10% ethanol (I run it exclusively), but if you boat wasn't set up for it because it's old, it might simply need different carb jetting...and it may be running a hair lean on today's modern fuels and you don't even realize it....

Some say I'm fanatical. I say yes. And my boat runs perfect.....always....

As for making an assumption that Cobalt wouldn't overprop a boat from the factory? Bad assumption...frankly I find that most boats are overpropped from the factory for my taste....

So yes, you may benefit from a 19 pitch prop. You may also benefit by gaining HP after correcting a possible engine problem(s), allowing you to keep your current prop....

 

[geneseo1911]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

Well, a month has gone by, and I've done a LOT of work to this old boat. The maintenance was not as well kept up as I had thought. I'm glad craze1cars encouraged me to go through the motor. I knew that was the right thing to do, but just needed to hear someone tell me. Here's what I found (in case my experience can help someone):

compression looks good:
1 155 2 155
3 135 4 140
5 130 6 140
7 145 8 145

I suspected the advance wasn't working (only explanation I had for the same speed loaded vs empty), and I was right:
The total advance @ 4200 RPM was only 20*, with the initial timing set at 12*.
I disassembled the distributor, and sure enough, the advance mechanism was dry and rusty. I cleaned & lubed it, and now get 32* total @ 4200 with the initial at the correct 8*, which is just right according the manual.

Points were shot (part missing actually), so a new tuneup kit went in.

Plug wires (which some research revealed were '80's vintage Accel "fat stuff") shot sparks out all over the place when dampened, so they were replaced.

The original carburetor was replaced somewhere along the line with an Edelbrock 1409, but whoever did it just cut the steel lines and hose clamped hose to the ends. This of course was to allow for the in line filter, because the spin on fuel filter wasn't enough :facepalm: Anyway, got that all fixed up.

Further inspection revealed that the carburetor linkage didn't allow the primaries to go completely open, so I had to work that over a bit.

Anyway, I can't think of anything else to test, and the things I fixed should make a BIG difference. I'm excited to take her our tomorrow and see what she'll do.

 

[steelespike]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

I think you will be very pleased with your efforts.And the 21 may prove to work fine.
Let us know your results good luck.

 

[TilliamWe]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

You should notice a HUGE difference with all those items you found!

 

[geneseo1911]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

Sure enough, she's a different boat. Leaps out of the water, even pulling a tube. I got her up to 52, and still had some throttle, but the tach was going haywire so I backed off. I'm not sure what that's about. I verified it with a multimeter, and it was dead on on the hose, but once the engine gets over 4000 RPM on the lake, it jumps all over between 4-6K. I'll have to make some long jumpers so I can use my meter at the helm. Now I'm thinking I should try a 23!

The thing that amazed me the most was the difference in the throttle response. Before it took a long time for the RPMS to build, but now it revs pretty fast.

 

[craze1cars]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

I'm glad to hear you found some HP. Now wasn't that a whole lot more fun and productive than just wasting money on another smaller-prop-band-aid for a handicapped motor that was easily and economically repairable LOL??

Sometimes I probably annoy people when they come here looking for basic prop advice, and they make generic statements like "it runs great and smooth, so it can't be an engine problem." And regardless I frequently send them on a massive mechancal/electrical goose-chase. Yours is a prime example of why I STRONGLY believe that often the answer to a prop question has absolutely nothing to do with the prop...even when someone thinks his/her boat "runs great."

As for your tach, that jumping around is CLASSIC failing-boat-tach symptom. It has corroded internal connections and nothing more. Sometimes you can buy a little time by cranking the cylinder the selector knob on the back of the tach around in circles both directions with a screwdriver a few times. But 98% of the time you just need to buy a new tach. My last two boats were 1992 and 1999 models....and both had bad tachs that did exactly what you describe, and both got replaced to solve the problem. Yours is a 1982, so it's way past time for a tach replacement. They're all over Ebay for reasonable cash.

Enjoy your 'new' boat and thanks for reporting back with what you found! Hopefully it helps many others.

 

[TilliamWe]

Re: Tired Engine, out of tune, wrong prop, or too high of expectations?

In the meantime, tap on the tach to see if it'll stop jumping.