Is my WOT RPM resonable given the boat, prop and HP?

[deerhuntre]

I'm trying to determine if a problem exists or I simply have too much pitch.

I'm getting 4100 RPM WOT. I would like to know if this is reasonable given the boat, prop and HP listed below. Or should I be getting higher RPM with this setup?

I would like to know if I should look for ignition problems and work on diagnostics, or if I should simply choose a new prop.

The setup:

19' fish and ski aluminum V-hull. Empty 1125 lbs.
I estimate I'm at 1900 lbs with the motor, gear, fuel, etc.

13 X 19P three blade aluminum prop.

115 Mercury inline-6 outboard.

The $10 question is: Is 4100 RPM meeting expectation?

Thank you for any information and suggestions you provide!!

 

[Windykid]

Re: Is my WOT RPM resonable given the boat, prop and HP?

I run an 18' bass boat similar weight, with a 150 hp with a 22P prop and 5500-5800.

 

[Plainsman]

Re: Is my WOT RPM resonable given the boat, prop and HP?

My 1976 115 merc manual says 5200-5500 RPM.

 

[jimmbo]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Something is really wrong. Is the tach accurate? Is the timing fully advancing, and are the carbs fully opening? If so, running at 4100 WOT will, if it hasn't already melt holes in the pistons. Check Compression first. Readings should no more than 15% difference between highest to lowest. Make sure it is running on all cylinders, check 4 spark, check all spark plugs, wires, carbon arcs in/on distributor cap and rotor, etc. If fine carbs are next suspect.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Thank you for the plan of attack. The compression is good. The 'butterfly' in the carb is nearly perpendicular, but there is a very slight amount of room for it to open more. Very slight though. I tried some adjustments but it's as far as the throttle will push it. I'll be testing for spark after I purchase one of those in-line testers. As far as rotor, etc, this engine uses the alternator and doesn't have a cap and rotor.

Thank you everyone so far!!

 

[jimmbo]

Re: Is my WOT RPM resonable given the boat, prop and HP?

A belt driven distributor was used on the inline sixes thru 1978, in 79 they switched to what is called ADI. If your engine is newer than 78, Add 2 switchboxes, stator, and 6 ignition coils to the list of things that fail

 

[ricksrster]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Deerhunter,
You might want to try the Prop Questions forum.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Do you think its ok to re-post the question there? I didn't want to cross-post.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

One more questions, do I need to use the spark testing device at both idle and RPM over 3000?

 

[redjmp]

Re: Is my WOT RPM resonable given the boat, prop and HP?

My 86 115-6 had the same problem, would only get to around 4500rpm with 13x19 and only go 35 mph but I knew previous owner overcooked it because there was metal on my top 2 plugs! Compression was 105 to 135. now even 140.
Punched it 30 over and now flys at 5700 and 50 mph on 15 ' fg about 2000lbs loaded.
Check your plugs and your compression. If ok move on to spark, timing, and carbs.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

I did install new plugs, but last week I accidentally overheated the motor. I was running for a little while then noticed no water peeing. Then the motor stalled and would not turn when I went to restart. I repaired the water pump and at the next attempt to start I had a very hard time starting, but once it started, it ran fine and restarted fine. I didn't check the plugs since the overheating incident, but will do so. Also, before the overheating I had 125 to 130 psi in each cylinder.

 

[jimmbo]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Numbers from before an overheating are worth nothing. You need to do another compression test. One good thing about your merc... no head gasket to blow or head to warp.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

OK, I'll do a new compression test this week. In the mean time, I checked the butterfly and it in fact didn't open all the way, but it was pretty close. So I did some adjustments and now it opens about as far as it will go, with just the tiniest amount of play. So in a few weeks when the rest of the testing is done I'll try again in the local lake and reply here with the results.

Oh, and also, someone removed the screens that cover each carb. How important are those? Should I order them and install them?

Also, redjmp, I'm not quite sure what you are saying regarding "Punched it 30 over", could you kindly clarify what you are saying?

 

[jimmbo]

Re: Is my WOT RPM resonable given the boat, prop and HP?

reference to 0.030" boring of cylinder. will add 1 or 2 cubic inches to displacement and perhaps 1 or 2 horsepower.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Oh my...I hope I don't have to venture down that path. But if I do, it will certainly be an interesting project.

I will do some more testing this week and post some results. Thank you!!

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

Hey jimmbo, Did you plug with metal look anything like either of these:

http://the_web_guy.home.comcast.net/plugs.jpg

These are my top two plugs. The other four either had slight oil and looked ok, and one had a bit of water and was really clean.

I guess its too late at night for compression testing as its kinda noisy, so that'll have to wait until the weekend.

 

[redjmp]

Re: Is my WOT RPM resonable given the boat, prop and HP?

My plugs looked like your grey one and unforunately you have cooked that piston. The metal is aluminum from the piston melting.
You may be able to save the block if only the piston crown began to melt. It is possible to have that kind of damage and still have good compression. But your motor will self destruct if you keep running it. It might only need the one piston and rings replaced and not rebored.
I still find it hard to beleive that all your cylinders read an even 70 if only the top plug looked like that.

 

[deerhuntre]

Re: Is my WOT RPM resonable given the boat, prop and HP?

What am I looking for inside the cylinder when looking at the piston? And I'm not sure what you are telling me regarding the 70 psi....I didn't get to retest the compression because it was too noisy and its evening.

 

[jimmbo]

Re: Is my WOT RPM resonable given the boat, prop and HP?

"Bit of water"? Hmm.

Plug on the right is definitly showing signs of engine damage. Get a small flex type light and put it in the spark plug hole and take a look inside. You most likely see damage to the top of the piston and when piston is down, a decent view of the cylinder walls to check for obivious scoring/scuffing. As red said you might get away with only one or two pistons bad.

 

[redjmp]

Re: Is my WOT RPM resonable given the boat, prop and HP?

I still find it hard to beleive that all your cylinders read an even 70 if only the top plug looked like that.

Sorry, I am thinking of a different post.