3400 rpms at wot? What do I do!?!?

[glennwingman]

I have a 1976 16' monarch boat with a 1980 merc 90 on it, I-6. I believe the prop to be a 17" diameter 20 pitch. I recently installed this motor and today was my shakedown cruise. Idle in the water is between 1000-1100 rpms in neutral.

In gear idle seems a bit low, at 500rpms or so, she died a few times in gear at idle.

My problem is I'm only at 3400 rpms at wot, which nets me 37-38 mph with 3 people (525 lbs) in the boat, 30 and 24 size batteries, 10 gallons of fuel, and a couple anchors. Today's weight was a typical day, minus a few lbs for 6 poles and 2 or 3 more tackle boxes. Also, it was about 40 degrees, if that matters. I really have no idea what the boat weighs.

My question is, what should I do to reach my 5000 rpm wot target? 12, 13 or 14 pitch prop? Will the extra 1600 rpms make up for the loss of speed that will result from having less pitch, or am I just going to try and see?

Thanks for your help guys, its because of this site that I was able to swap this motor for my 76' 650 merc!

 

[trendsetter240]

Re: 3400 rpms at wot? What do I do!?!?

Did you change the setting on the back of your tach? Your old motor was a 4cyl and this is a 6cyl. If the old one was set to 8p then the new should be 12p I believe.


As far as prop pitch goes, I'd hold off until you verify that the tach is working. If your tach is good then you need to look at a 13 or 14p prop. My advice is to fill out one of hwsiii's forms to get the most accurate prop choice.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

the old motor was a 3 cyl. 1976 merc 650 thunderbolt ignition, the new one is a 1980 merc 90 thunderbolt ignition. there is nothing on the back of my tachometer that would allow me to change my settings, its just got the hookups for the wires. I figured the tachometer was operating properly, as my idle speeds on the muffs, in neutral, and in gear are all within the specs I've seen mentioned here on iboats for my motor. It does seem pretty odd to be that far below my recommended wot speed, but like I said, tach checks out ok at other speeds.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

is it possible for the tach to be correct at lower rpms and off at higher rpms? the needle increase is smooth and steady, and the motor sounds as if its running happily on all 6, I'd think if there was a problem with my wot ac output my coils may not fire properly? but there's still much I dont know about outboards, so anythings possible

 

[trendsetter240]

Re: 3400 rpms at wot? What do I do!?!?

Well, I'm not an expert. My understanding is that the tach reads ignition pulses to determine RPM. A 6cyl would have double the number of pulses per revolution that a 3cyl would. Without adjusting the setting on the back of the tach how would it know the difference?

Anyways..do you know the gear ratio of your new motor? Also do you know the exact size/pitch of the prop you have now? Im positive it is not 17" in diameter. There should be a number either stamped on the outside of the hub of the prop or behind the prop nut on the hub.

 

[hwsiii]

Re: 3400 rpms at wot? What do I do!?!?

Glenn, I don't know at this time what your problems are, but if you will fill out my form I will help you as much as I can.

Trend, I thank you very much for your opinion of me as well.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

If you would prefer to not go to this much trouble there are other people who will come along and gladly help you.

Welcome to the forum, I will try to help you as much as I can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out the best you can and I will help you as much as humanly possible, if you would prefer to not go to this much trouble there are other people who will come along and gladly help you. My expectations are that you are willing to expend as much time and energy as I will to find a better prop for your boat.

Just copy this form to your computer and paste it into a word processing program and fill it out, then paste it into your post or just use the numbers for reference in your reply and type in your information.

Iboats Boat and Motor Info

1. Year, make and model of boat

2. Length, width and base weight of boat, look for boat decal on boat

2a.What is the maximum recommended HP for your boat

3. Number of people and gallons of gas normally on boat

4. What do you use the boat for

5. Is it a Deep Vee and if so how many degrees of Deadrise

6. Year, manufacturer and model of motor

7. HP and gear ratio of outdrive IMPORTANT
If you don?t know the gear ratio, you can pull the plugs out of the motor and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor

8. Manufacturer?s recommended Wide Open Throttle (WOT) range

9. Anti-ventilation Plate height above the bottom of the transom of boat if it is an outboard in inches, use a straight edge or a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

10. Is it a bass boat or does it have a pad bottom and is it on a bracket, if so how far does the bracket extend from the stern.

11. Does it have a hydrafoil, dolefin, whale tail or trim tabs

12. Manufacturer, model, diameter, pitch, number of blades and whether SS or aluminum props.
IMPORTANT
1.
2.

13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for less than $ 50
RPM ___________ Speed (GPS)___________ No. of people ____________ Gal. Gas ________

14. Are you at sea level or a higher elevation, give us the elevation in feet __________

15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by only using the control on the console, checked compression, looked at the plugs and checked spark, is the bottom of the boat clean and barnacle free, and have you checked the Tachometer against a mechanics tach, all of the foregoing could be the reason your prop is not attaining full RPM, or too many RPM.

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop.

17. Does the prop show any damage that you can see or feel.

18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

REMEMBER, The prop I recommend will be NO better than the information you give me.​

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[steelespike]

Re: 3400 rpms at wot? What do I do!?!?

I have a 1976 16' monarch boat with a 1980 merc 90 on it, I-6. I believe the prop to be a 17" diameter 20 pitch. I recently installed this motor and today was my shakedown cruise. Idle in the water is between 1000-1100 rpms in neutral.

In gear idle seems a bit low, at 500rpms or so, she died a few times in gear at idle.

My problem is I'm only at 3400 rpms at wot, which nets me 37-38 mph with 3 people (525 lbs) in the boat, 30 and 24 size batteries, 10 gallons of fuel, and a couple anchors. Today's weight was a typical day, minus a few lbs for 6 poles and 2 or 3 more tackle boxes. Also, it was about 40 degrees, if that matters. I really have no idea what the boat weighs.

My question is, what should I do to reach my 5000 rpm wot target? 12, 13 or 14 pitch prop? Will the extra 1600 rpms make up for the loss of speed that will result from having less pitch, or am I just going to try and see?

Thanks for your help guys, its because of this site that I was able to swap this motor for my 76' 650 merc!

The idle seems a little high in neutral yet low in gear.You may need to adjust the idle mixture.You also need to be sure the carbs are in sync.Your speed seems high for 3,400 rpm.If you add 1,600 rpm I think you would be running 50+.
Fill out the form and H will get you set up.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

I'll work on getting all of that information. If it helps, the numbers in the prop are 2036 17. Its def not 17" as I suggested before more along the lines of 13 or 14. I have another prop with far less pitch that came with the engine as a spare, I believe its a 13 or 14, I'm going to do a test run with that prop as well to see if its simply a matter of being over propped. I'll post my results.

 

[trendsetter240]

Re: 3400 rpms at wot? What do I do!?!?

I'll work on getting all of that information. If it helps, the numbers in the prop are 2036 17. Its def not 17" as I suggested before more along the lines of 13 or 14. I have another prop with far less pitch that came with the engine as a spare, I believe its a 13 or 14, I'm going to do a test run with that prop as well to see if its simply a matter of being over propped. I'll post my results.

glennwingman, I really think you should verify that tachometer before selecting a new prop. The numbers you posted for that prop lead me to believe it is in fact a 17" pitch. If that is the case then there is no way you can each 37/38mph at 3400RPM.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

How would I go about verifying it? Also, would a 76 650 merc 3 cyl with a thunderbolt ignition require the same pole settings as an 80 merc 90 straight 6? There's nothing on the back of the tach to allow me to change poles.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

Update: I switched from my 17 pitch prop to a 15 pitch, should have raised my rpms at wot, no dice, same 3400 rpms, just gets onto plane faster. Top speed was a few mph less than before, 35 mph or so. The motor sounds like its running where it should be at wot? After stopping at 3400 today again, I kept it out of wot, because its obvious I have a problem, possibly a faulty tach, so I dont want to over rev it.
The motor is at 500/600 at idle in gear, i sometimes have to choke it to keep it alive when idling, but It may be more limited to when I have the motor somewhat trimmed out of the water due to it being shallow. Can fuel drain out of these carbs when its running tilted?

 

[JaseBosto]

Re: 3400 rpms at wot? What do I do!?!?

I feel your pain with this type of thing, as I have had unexplainable problem after problem with outboards(which is why I am a stern drive man now).

Lets take into account for a second, that your tach IS working. Run a compression test and no matter what the numbers are, as long as all the cylinders are close to eachother, your ok(around 100+psi though)If it comes out good, I think you may have a vaccuum leak. Most likely this is the line from the crankcase to the fuel pump. I had that issue, which gave me a similar problem. Also your carbs might need cleaning. I am not firmiliar with your hull, so I dont know what it is capable of on the water, for all I know 35mph with your engine is about right. I would recommend a new tach to rule that out completely, and make sure you install it by following the directions.

Summary: RUN A COMPRESSION TEST.Go over every line/tube on your motor. Then run it on muffs and check for unusual sounds. New tach, hope this helps.

 

[JaseBosto]

Re: 3400 rpms at wot? What do I do!?!?

Well, I'm not an expert. My understanding is that the tach reads ignition pulses to determine RPM. A 6cyl would have double the number of pulses per revolution that a 3cyl would. Without adjusting the setting on the back of the tach how would it know the difference?

QUOTE]

Being that it has double the pulses, the tach should read twice as high as it should. For example if the engine were at 2000rpm, it would read 4000rpm.

 

[trendsetter240]

Re: 3400 rpms at wot? What do I do!?!?

Being that it has double the pulses, the tach should read twice as high as it should. For example if the engine were at 2000rpm, it would read 4000rpm.

Actually I found the papers that came with my adjustable tach tonight and it shows that his 65hp merc and 115hp merc both have the same setting of 6p for 12 poles.

But that doesn't mean the tach is reading properly, just that he wouldn't have needed to change any settings between the two motors.


What does NOT add up is 15pitch prop, 3400RPM and 35MPH. A 15pitch prop at 3400RPM with a 2:1 gear ratio should give about 20-21mph.

If the actual speed is 35MPH then the tach is definitely not showing the right numbers.

 

[JaseBosto]

Re: 3400 rpms at wot? What do I do!?!?

As I have mentioned tho, I am not firmiliar with this hull, so while 21mph or so is typical,......hull design makes the worlds difference.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

The speed was also verified by my gps. I suspect my tachometer is at fault, I'll have to replace it with a known good tach to be sure, I'll let you all know what if anything I find out. The most troubling part of a site like this is reading about a problem similar to yours and people not posting the resolution to said problem!

 

[JaseBosto]

Re: 3400 rpms at wot? What do I do!?!?

The speed was also verified by my gps. I suspect my tachometer is at fault, I'll have to replace it with a known good tach to be sure, I'll let you all know what if anything I find out. The most troubling part of a site like this is reading about a problem similar to yours and people not posting the resolution to said problem!

My resolution to the engine not opening up was to fix the vacuum leak.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

Do i have vacuum hoses? I'll have to look into this.

 

[trendsetter240]

Re: 3400 rpms at wot? What do I do!?!?

Do i have vacuum hoses? I'll have to look into this.

Your fuel system is gravity fed and operates in a vacuum so that might be what he's refering to.

First thing to check is the tach. Until you are positive of your RPM then you can't diagnose the issue.

BTW since you used GPS to measure your speed I'm now even more certain the tach is out. It's impossible for a 15" pitch prop on that motor to move a boat at 34mph at 3400RPM. Either the prop is a higher pitch or the tach is out.

 

[glennwingman]

Re: 3400 rpms at wot? What do I do!?!?

it was indeed the tachometer. I replaced it with a known good tachometer, which shows me at roughly 5000 at wot. I'll be ordering a brand new one, however, as the one I tested today has me idling at 1000 rpms in gear, whereas with the old one it was the fsm specified 600 rpms. Regardless, problem has been isolated, and I thank you all for your help!