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Old 09-03-2018, 11:56 PM   #1
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Default WOT rpm

We just bought a 1999 Maxum 1800sr. It has the 4.3 Merc and a 1918p21 prop. Just curious as to what I should be seeing for rpm at WOT and what recomended cruising rpm would be? We went out today and it will hit 5700-5800 rpm fairly easy. Is this too high.
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Old 09-04-2018, 01:52 AM   #2
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Yes way to high, should be 4400-4800. The boat is under proped. WOT should not be ran for more than a few minutes at a time.
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Old 09-04-2018, 02:04 AM   #3
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Am I going to to do any damage by using it this way? Sorry for the ignorance this is my first boat.
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Old 09-04-2018, 02:12 AM   #4
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Not if you dont run WOT all the time... Maybe a 24 prop?
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Old 09-04-2018, 02:57 AM   #5
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Just donít operate for extend periods above 4000 rpm. But different props will achieve proper WOT and all around performance.
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Old 09-04-2018, 12:14 PM   #6
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Something is unusual here. You would be going 60+mph at that rpm with 1.81 outdrive. Did you check your speed with GPS during this run? If it was 60 mph then yeee-haa. Many new boaters would not be comfortable going 60 in an 18 foot boat on their first time out.
Check your outdrive and see if you can find the gear ratio stamped on it somewhere. It may be on an aluminum plate or the drive itself.
Did your tach respond normally to throttle inputs? It couldn't hurt to clean the terminals in the back of the tach. I can always tell when my tach terminals need cleaning because it indicates exceptionally high.
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Old 09-04-2018, 12:21 PM   #7
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Tach seems to work fine and respond correct. I did not check with gps but I highly doubt there were any 60mph runs.
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Old 09-04-2018, 01:06 PM   #8
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I'd verify the dash tach is reading properly using a shop tach directly connected to engine. My timing light has an RPM feature and the Faria dash gauges have been off since the boat was new.

WRT the prop, to get to the recommended 4400-4800 WOT rpm, that would need roughly 5in of additional pitch! Something doesn't sound right. Doublecheck the tach and post speeds (from a gps) and the RPM.
Could be a spun hub if the rom's are high and not having the corresponding speed.

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Old 09-04-2018, 04:57 PM   #9
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Agree with the others that something doesn't sound right (5700 rpm?, ck. that lower unit ratio). I have an '06 1800 SR with the 4.3 Merc. and when I bought the boat it had a 21" 3 blade aluminum prop and it was incredibly sluggish.
I changed to a 4 blade 19" aluminum, and have since added a Stingray hydrofoil. Now, I run up to about 4600 rpm, putting it right in between the 4400 to 4800 range.
Net, net, with only myself aboard, i'm on plane and cruising at 2500 rpm. She pops up out of the water very easily, very happy with how she runs now.
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Old 09-05-2018, 05:51 PM   #10
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After talking to a few people, I believe the tach has to be reading wrong. Someone mentioned to me the rev limiter on these engines is 5100 and I have not hit any rev limiter. Also from some reading I am not sure there is a prop out there that would add enough pitch to bring me down 1000rpm to the recommended 4800 which also makes me feel the tach isn't correct.
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Old 09-05-2018, 06:03 PM   #11
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Have you checked the "arrow indicator" on the back of the gauge. You have to set the arrow to the corresponding # of cylinders. Otherwise, you get an incorrect reading.
You have to push in the arrow to turn it. Make sure its on the right setting for the 6 cyl. motor.
Otherwise, I have used Faria Beede repair service before and it works well. The needle on my tach would not go lower than about 1500 rpm. It would go up from there, but never back to idle speed. I think it was $55.
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Old 09-05-2018, 06:11 PM   #12
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does the dash just pop out on these boats or are there screws that hold it in?
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Old 09-05-2018, 06:24 PM   #13
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not sure i follow. the dash is molded in. the gauge sits in a recess in the dash and is held on with a plastic piece that surrounds gauge from behind. nuts, washers secure to the studs running out from gauge.
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Old 09-05-2018, 06:30 PM   #14
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I guess I am asking how do I get to the back of that gauge to check if it is on the right setting?
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Old 09-05-2018, 06:34 PM   #15
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it's all accesessable from underneath the dash.
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Old 09-05-2018, 10:07 PM   #16
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Smitty, look behind the tach and you will see the two mounting studs with brass nuts. You will also see the three-pole harness. The harness is a push-on so you just pull it off. Remove the two brass nuts and the tach comes out from the front.
YOu will need to loosen it from the rear, but there should be plenty of room. Also, you may be able to see the cylinder setting for the tach without removing it.
It's possible that someone installed a tach gauge as a last maintenance item before selling it to you, and it came out of a 4 cylinder engine. I doubt it, but definitely possible.
While you have the tach out, clean the three poles with a wire brush. Put on a little dielectric grease and push the harness back on firmly when you re-install the gauge.
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Old 09-06-2018, 12:10 AM   #17
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So I’m 99% sure I found the problem. Somebody had replaced the tach I guess and set the arrow indicator to 1 which according to the label is a 4cylinder. I changed it to 2 which looks like the 6 cylinder setting and plan to retest this weekend but I don’t see an reason why this wouldn’t fix the problem.
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Old 09-06-2018, 01:59 AM   #18
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Quote:
Originally Posted by Tsmitty4494 View Post
After talking to a few people, I believe the tach has to be reading wrong. Someone mentioned to me the rev limiter on these engines is 5100 and I have not hit any rev limiter. Also from some reading I am not sure there is a prop out there that would add enough pitch to bring me down 1000rpm to the recommended 4800 which also makes me feel the tach isn't correct.
I tend to agree. Get a phototach from an auto store to verify your readings.
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Old 09-06-2018, 03:38 AM   #19
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Ck and make sure that the tach is set on 6 cylinder not 4 or 8.
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Old 09-06-2018, 02:09 PM   #20
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Quote:
Originally Posted by donald View Post
Ck and make sure that the tach is set on 6 cylinder not 4 or 8.
Don
I'm guessing you maybe didn't read the entire thread?
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