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RPM Not High enough

T

Timarkay

New member
Hey guyes I got out on the boat for the first time this year
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And i was noticing that my Rpm's are running at 4000 WOT
I have a 83 convincor with With a 454 mag from what i read here i should be at about 5000
I have a 23 pitch prop.... What should I do to get my rpm's up to about 5000 is it just a prop change
Any sugestions would be helpful Thanks Tim
 
What was your rpm last year?
What is your top speed compared to last year?
Is your prop damaged?

If you can answere these ? we can help with a recomondation.
 
It sounds to me like you may have other issues there and not a prop. Your motor should be able to spin that prop to 5k . Maybe you are down on power.
 
last years rpm's where the same i am doing about 54-55 spedo
vinny what do you mean down on power and what could i do to bring it back up?
The prop looks fine to me
Thanks Tim
 
I agree with vinny. IT SOUNDS LIKE YOU ARE DOWN ON POWER. I could run a 23 up to the rev limiter on my 253 w/ a 454 mag efi. The 251 is supposed to be a faster hull.

here are Ideas of things to check

1. Is the secondaries opening up all the way
2. Is the flam arester plugged up
3. is the timing correct (are you getting full advance)
4. cam could be worn
5. better give it a compression check
6. try a differnt prop of the same pitch
 
Over board
Thanks For The input I will have to check those things I will get back to you and let you know if that is it, If anyone has anything else let me know please
Tim
 
Mine too......

I just got my convincor at the end of last year and I was able to take it out 2 times, once alone and another time with three of my friends and I was only hitting about 3500rpm's and around 53 - 55 mph. I was thinking that maybe the throttle cable just neededd to be adjusted or something. The guy I got it from said he just had the outdrive rebuilt, but Im not sure. The boat has a 21p prop stainless. I dont remember what kind. The prop also has a little play in it when you grab two of the blades you can move it side to side and when I first got it I had checked it and it didnt move at all, but when I checked it before I winterized it I noticed it had the play in it. Im not sure what that is or the throttle problem is but if ANYBODY has any answers I would like to hear them, Thanks.... John. My boat is an 87 251 convincor as well.
 
One more thing I forgot to mention as well ????

:brickwall: When I put it in gear it cuts out like it almost want to just shut off, thats another reason I thought it might be the throttle cable.
 
I have an 89 Ambassador with 5.7 liter mercruiser and alpha one outdrive and EXACTLY the same problem.

My top RPM is around 3500 in gear and running, not in gear it will go up to 5000rpm WOT. Top speed in gear and at 3500 is around 40. I am running a SS prop, but need to check the pitch.

The engine seems to run fine both in and out of gear but once in gear will only top out at 3500. Mine also cuts out when advancing the throttle some times but only when still warming up.

Any thoughts from the experts?
 
Are all you guys buying gas at the same place. What are the engine hours on all your boats. The engines just might be plain tired. A lot of engines will cut out or hesitate while still warming up.
 
Last edited:
RPM not high enough here too!

Hey folks!!

Bought a used Checkmate ZT 230 with a 320HP 6.2L and an XZ outdrive. Unfortunately, I do not have any data on how the boat ran last year.

My first times out with the boat produced results as follows.

Max RPM 4300 (showing on the gauge) at WOT
Propeller is a Mirage Plus SS 23 Pitch
Speed is 53 MPH (GPS)
Gear ratio is 1.5 (this is what regular altitude XZ's come with).
Based on the Mercruiser web site, my Prop slip is 15%.

A couple of weird things happen though that would indicate I am not on the wrong prop. The Rev Limiter is kicking in when I hit 4,300. Is this normal?
I am also getting 10-15 degrees of roll on takeoff. Checkmate recommends this as the right prop for this motor/boat/outdrive combination.

I took out a similarly configured boat (boat, engine, prop) and was able to get 64 MPH GPS at around 4,800 RPM.

I brought the boat into the Mercruiser shop to have them check things out. Hopefully they find something.

If anyone has any ideas, let me know!

Rob
 
1.) Is the ignition in good shape? Timed right, plugs good, wires good, cap and rotor in good shape, ecm (if it has one) rev limiter set right?
2.) Is the carb getting WOT, are the filters new and clean, good gas, air cleaner in working order, is it getting enough fuel? Are the fule lines big enough, are the fittings flowing properly, is anything kinked, pinched, or crushed? Is the vent on the tank not full of junk and venting properly, is the fuel pickup big enough, is the check valve big enough and working properly?

I recomend you run the thing WFO, then cut the throttle, and turn it off at the same time, pull the plugs and check to see what they look like, it may be running out of gas at the top end, and the plugs will show it. Mine had the same problem, the 350 would only spin to 4k. Swapped in a 383 and that only spun to 4k. Spent about 5k on a new top end, exhaust, carb digital ignition etc etc the following season on the 383, and it turned out to be the tiny ass check vavle not flowing enough fuel when WFO. I'm lucky I didn't melt the thing down...
 
"it turned out to be the tiny ass check vavle not flowing enough fuel when WFO. I'm lucky I didn't melt the thing down..."

That is the first thing to throw in the trash on any performance upgrade project. "Not Coast Guard approved" to remove it. It's usually out of sight anyway, and the youngsters boarding don't even know what they are looking at. My boat had one and the hole was less than 1/4" ???
 
Bear Boat said:
Hey folks!!

Bought a used Checkmate ZT 230 with a 320HP 6.2L and an XZ outdrive. Unfortunately, I do not have any data on how the boat ran last year.

My first times out with the boat produced results as follows.

Max RPM 4300 (showing on the gauge) at WOT
Propeller is a Mirage Plus SS 23 Pitch
Speed is 53 MPH (GPS)
Gear ratio is 1.5 (this is what regular altitude XZ's come with).
Based on the Mercruiser web site, my Prop slip is 15%.

A couple of weird things happen though that would indicate I am not on the wrong prop. The Rev Limiter is kicking in when I hit 4,300. Is this normal?
I am also getting 10-15 degrees of roll on takeoff. Checkmate recommends this as the right prop for this motor/boat/outdrive combination.

I took out a similarly configured boat (boat, engine, prop) and was able to get 64 MPH GPS at around 4,800 RPM.

I brought the boat into the Mercruiser shop to have them check things out. Hopefully they find something.

If anyone has any ideas, let me know!

Rob
Click to expand...

methinks your tach is teasing you. i beleive the rev limiter will kick in around the 5500 range, so either, the tach is lying, or the engine is lugging and you are hearing detonation.

i think the revs on that motor are the 44-4800 rpm range - correct me if i'm wrong.

might want to plug the scan tool into the motor and see if any trouble codes pop up.
 
Given the changes in fuel over the last couple of years, I would drain the fuel system, tanks, etc and replace filters before I did anything. This ethanol crap causes more problems than anyone realizes. It attracts water number one, but it also stirs up contaminants in old tanks (both yours and your gas stations). Nearly everyone around here has gone to 10% ethanol. I don't think you can get anything else any more.

I work for a fuel / oil company and hear the results every day of ethanol causing these problems. We have claims right and left of vehicle damage and our service guys spend all day changing the filters at gas stations (some of them over and over again).
 
Boat still in the performance/repair shop

Folks,

Thanks for the feedback. I will have to check some of these things once I get the boat back. The boat is still in the Mercruiser performance/repair shop.

The Mercruiser dealer is doing a bunch of warrantee work to try to correct some of the problems. Not sure they will pin-point why the RPM is so low, but they are putting their "computers" on the engine and also doing water tests.

I will keep everyone posted once I get the boat back and their report.

Rob
 
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