Engine / prop question

[Chaparralboater]

I have a 1991 chaparral 18 foot open bow. I have a 3.0 mercruiser engine and mercruiser out drive. I just bought the boat and it came with a 15x15 prop. The rpm top out would be at around 4500 and reach a speed around 35.. I switched to a stainless steel 14x19 prop and changed the plugs and now I can't get over 3800 rpms but my speed is near 40. The gap in the old plugs is much bigger than the new plugs..should I take the new ones back out and gap them to the old ones gap? It starts and runs great tho..and I have a small water leak from somewhere I can't see back to it looking from inside the boat..

 

[sethjon]

Re: Engine / prop question

Check the manual for the correct gap. Old or new they will be the same. Is the new a 3 or 4. That will make a difference aside from only the pitch.

 

[Chaparralboater]

Re: Engine / prop question

The new prop is a 3 blade, same as the old..so I should go back and regal the plugs to the old ones?

 

[Bondo]

Re: Engine / prop question

so I should go back and regal the plugs to the old ones?

Nope,... Regap 'em to the values stated in the Manual,.... It's Free in the Adults Only section...
The motor needs a Complete Tune Up,+ Service,.... Filters, lube, inspection,...
Complete Service....

 

[Don S]

Re: Engine / prop question

You've got too much prop. Put your old prop back on and try it, bet your rpm's are back up.
Once you are sure you can still get your FULL WOT rpm with your old prop, post a question over in the prop forum to find out what size SS prop you should have gotten.

 

[Chaparralboater]

Re: Engine / prop question

Well if 15 pitch puts me at 4600 and I hit the rev limiter with that and a 19 puts me at 3800 then I should get a 17..I am great with engines in cars but have never messed w a boats ..

 

[Don S]

Re: Engine / prop question

You don't have a rev limiter, except for the prop.

 

[Chaparralboater]

Re: Engine / prop question

I can't find out what my rpm is supposed to be at for wot..it's a 1991 chaparral 1800sl ..3.0 mercruiser with a stock outdrive but I don't know what it's called..maybe gen 2?..but anyway. I would like to find out what it is or how to find it. I searched the manual but couldn't find it,

 

[Oldskool]

Re: Engine / prop question

4200-4600 on a 3.0L, and 4400-4800 on a 3.0LX.

 

[Chaparralboater]

Re: Engine / prop question

How do I know if I have a 3.0lx?

 

[Oldskool]

Re: Engine / prop question

Engine Identification
The MerCruiser Model can be determined by looking
at the LAST TWO LETTERS of the engine code
stamped into the cylinder block. This code number is
stamped on all MerCruiser power packages and replacement
partial engines, but not replacement cylinder
block assemblies.
If the engine serial number and/or model decals are
missing, the engine code letters may help in determining
the engine models.
74520
a
3.0L uses the 2.5L cylinder head
3.0LX uses the 3.0L cylinder head

4 Cylinder-In-Line Engine Code (Next To Distributor)

G.M. Engine Code Location
MCM Stern Drive Engines (LH Rotation)
3.0L - RM, RJ
3.0LX - RN, RK

 

[Chaparralboater]

Re: Engine / prop question

Well I have a 3.0 lx ..I put the 15 pitch prop on a took it out with full gas, and 3 people total..it did run till about 4500 then hold there..I switched to the 19 pitch prop and it didn't even plane out for a long time. I will get a 17 and keep the 15 around as well for a back up and toss the 19 pitch..

 

[Oldskool]

Re: Engine / prop question

IF the 15 pitch is getting you to 4500 WOT, that is where you need to be. A 17 pitch is going to take another 300 - 400 off WOT, leaving you below the recommended range and causing your engine lugging.

 

[steelespike]

Re: Engine / prop question

If you read the stickies at the top of the page and the helps on the prop strore page
you'll understand prop selection better.Highe pitch numbers are kindof like ovredrive on your car.Low pitched numbers are more like 2nd or thrrd gear.The 15 is pretty close to low gear.
I think you may have a motor prpoblem.A 15" pitch should just about scream out of the hole and probably quickly over rev.A healthy motor should be able to spin a 21 -23" pitch have decent hole shot and run 40-45 mph.
Give your motor a good physical.Be sure the throttle plates open all the way.
good compression.Be sure the timing is right and it advances as it should.
Try to find that leak if the bellows are beginning to crack they could let go
she could go down.

 

[Chaparralboater]

Re: Engine / prop question

Well if the rpm range is 4200 to 4800 and I get to 4500 and it holds there isn't that right? Or should the 15 inch prop be pushing me past that. I think I may have an engine issue..I am not sure. I may have to have it looked at..

 

[steelespike]

Re: Engine / prop question

Yes 4500 is within rpm range but a healthy motor will easily over rev on a 15.Typically a 17 is suggested for skiing and such with a caution to watch the rpm with a light load. For more speed you were right to go to more pitch but your motor could barely reach rated rpm with a 15.Rule of thumb 1" pitch change = "ABOUT" 200 rpm you lost 800 so for your application the rule was right on.If your motor was healthy the 15 probably would have produced about 5,500.