• Welcome to the Checkmate Community Forums forums.

    You are currently viewing our boards as a guest which gives you limited access to view most discussions, articles and access to our other FREE features.
    By joining our free community you will be able to:

    » Interact with over 10,000 Checkmate Fanatics from around the world!
    » Post topics and messages
    » Post and view photos
    » Communicate privately with other members
    » Access our extensive gallery of old Checkmate brochures located in our Media Gallery
    » Browse the various pictures in our Checkmate photo gallery

    Registration is fast, simple and absolutely free so please, join our community today!

    If you have any problems with the registration process or your account login, please contact support by clicking here or by using the"contact us" link at the bottom of the page.

Strange prop observations - variable RPM at WOT

C

customfab

New member
I recently put on a High Five, 12.75", 24P prop. Before I installed it I was thinking I'd have to watch redline as the diameter wasn't big enough but to my suprise my max RPMs were 5500 at WOT. I lost 5 mph to max GPS of 55mph but the boat's handling & hole shot is 5 times better so I was very satisfied with the compromise. Entirely new boat and I was very happy. The original SS prop was a terrible setup for that boat.
So I took it out for the second time yesterday and during the first of the day I was getting the motor into 6200+ rpm range at WOT. Very, very strange. I thought I was going crazy and had made a mistake on my first outing. However, later in the day the RPMs went back down at WOT. Towards the day's end the boating would be running 5500RPM again at WOT. Can anyone explain this to me? It is as-if the RPMs change at WOT due to lake conditions maybe? Or the motor warms up? I have no clue but I am not crazy!!!:rof:
 
Do you have an outboard? That's about the rpm spread I saw when I forgot to put one spark plug wire on. Firing on 5 instead of 6 can look like that.
 
Yeah, Merc outboard 175. Very interested that you speak of a non-firing plug. I wonder if maybe just maybe I have intermitent firing. I don't notice a lack of power though. Bad wires you think?
In terms of load and conditions. Essentially the same load. Maybe a 100 lb difference at most. Same temp, humidity, etc. Really no changes between runs.
 
JW said:
Do you have an outboard? That's about the rpm spread I saw when I forgot to put one spark plug wire on. Firing on 5 instead of 6 can look like that.
Click to expand...

So I like your comment. Stupid question. Would it be running at higher RPMs with all 6 cylinders firing then? Any suggestions on how to diagnose? Likely defective part?
 
Did you get more speed as well when the RPM's jumped? If you just got more revs and the same speed then your prop is slipping. Could be as simple as too much trim, or a spun hub.
If you are getting more top end as well then it is engine related. Heat and humidity would only change RPM's by 2-300 max. Intermittent firing is hard to track down. First, make sure you are running new plugs every spring. Right after you know it is running slower, rev it to 1500 in neutral. Pull the plug wires one at a time and the RPM should drop slightly. (be sure to wear gloves and use insulated pliers to avoid the shock) If the RPM does not drop on a plug you have found your offending cylinder. Usually it is just a bad plug, but it can be a bad coil. Or even a switch box or trigger although they almost always effect a pair of cylinders not just one.
If it is not ignition related it can also be a fuel restriction. Check filters and fuel lines etc.
Mercurys are great motors, if you love to spend your day on the water trying to fix them.
 
Mark said:
Did you get more speed as well when the RPM's jumped? If you just got more revs and the same speed then your prop is slipping. Could be as simple as too much trim, or a spun hub.
If you are getting more top end as well then it is engine related. Heat and humidity would only change RPM's by 2-300 max. Intermittent firing is hard to track down. First, make sure you are running new plugs every spring. Right after you know it is running slower, rev it to 1500 in neutral. Pull the plug wires one at a time and the RPM should drop slightly. (be sure to wear gloves and use insulated pliers to avoid the shock) If the RPM does not drop on a plug you have found your offending cylinder. Usually it is just a bad plug, but it can be a bad coil. Or even a switch box or trigger although they almost always effect a pair of cylinders not just one.
If it is not ignition related it can also be a fuel restriction. Check filters and fuel lines etc.
Mercurys are great motors, if you love to spend your day on the water trying to fix them.
Click to expand...

Good input. Can't comment on MPH as I dropped it back down immediately once I saw the RPMS. I will follow your procedure and also replace the plugs this weekend.
Get back with you guys on Sunday.
 
Mercurys are great motors, if you love to spend your day on the water trying to fix them.[/quote]
EASY MARK THEMS IS FIGHTEN WORDS :poke: :pissed:
:rof::rof::rof:
 
No need to back down on the gas when it hits 6200. Your engine can handle that rpm range. Next time it happends let it run there for a while and see where it goes from there.

How high are you running the 5-blade?
 
You must log in or register to reply here.
Back
Top