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Engine won't go over 3,000 RPM's

Old 06-10-2019, 06:45 AM
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Default Engine won't go over 3,000 RPM's

Hello, I have a 1991 Welcraft 2600 Coastal with a MerCuiser 7.4/V8 Bravo engine. Boat is new to me. the Engine is very clean and appears very well maintained. She starts up with ease and really purrs nice, no issues at all there. but when I took her our and opened her up the most I would get was exactly 3,000 RPMs and max speed of about 17 to 19 MPH. I was out there for a good hour and no change. I came back to the dock checked out a few things. inline Fuel filter was a bit clogged up with crap so cleaned all of that out. Water separator had about 2 inches of water emptied that, throttle linkage appeared to be fine. So took it out again and same thing. IT runs very smooth never bogs down or stalls. just stays at 3k and 18 MPH. People have mentioned the Prop but I think it is the original and trying to contact the original owner for this thoughts as well.
Looking for some more suggestions from the smart folks out here. any ideas?

Thanks as always
Jeff
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Old 06-10-2019, 07:06 AM
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Do the usual diagnostics. Carb or fuel injection?
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Old 06-10-2019, 08:05 AM
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Is is Carb Billinstuart
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Old 06-10-2019, 10:11 AM
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For it to hold exactly 3000 rpm and stop every time, that sounds like a safety on the motor is preventing is hitting the rev limit. check engine manual for a list of safety controls. at least one of them should say that it limits motor to 3000 rpm.
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Old 06-10-2019, 12:22 PM
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Originally Posted by LIBoater View Post
For it to hold exactly 3000 rpm and stop every time, that sounds like a safety on the motor is preventing is hitting the rev limit. check engine manual for a list of safety controls. at least one of them should say that it limits motor to 3000 rpm.
That sure sounds very likely.
Harpoon - you didn't fill in the boating area in your signature which makes it hard to guess if you have mandated E10 gas in your boat tank or other conditions there which might help with diagnostic guesses you request.
. If there is ethanol in that gas and you had inches of water in the separator the gas would have likely have separated into 2 potentially damaging layers.
If there isn't alcohol in there , any water passing past or blocking the separators would lean out the fuel getting to the cylinders.
Some carbed Mercs have a knock sensor that would cause the ignition to limit to 3000 rpm, or less, and I think there are other conditions ( temp , pressure fluid levels?) that may also cause the limit .

When your engine 1st reaches that 3000 limit is there still plenty of throttle lever throw left?
Is the throttle lever on the engine moving all the the way to WOT when the dash control lever is WOT?
Have you checked timing to see if it is in spec & advancing correctly?
Are all cylinders firing?
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Old 06-10-2019, 12:36 PM
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I've never had much luck cleaning a fuel filter. If that's the issue the fines won't clean out but it will stall on you not hold at 3k.

Throttle cable? I don't know of any safety limit that would be at 3k.
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Old 06-10-2019, 04:33 PM
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Restricted intake or exhaust, timing off or not advancing tach inaccurate, wrong prop,...did it ever run good?
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Old 06-10-2019, 05:14 PM
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Originally Posted by harpoon70 View Post
Hello, I have a 1991 Welcraft 2600 Coastal with a MerCuiser 7.4/V8 Bravo engine. Boat is new to me. the Engine is very clean and appears very well maintained. She starts up with ease and really purrs nice, no issues at all there. but when I took her our and opened her up the most I would get was exactly 3,000 RPMs and max speed of about 17 to 19 MPH. I was out there for a good hour and no change. I came back to the dock checked out a few things. inline Fuel filter was a bit clogged up with crap so cleaned all of that out. Water separator had about 2 inches of water emptied that, throttle linkage appeared to be fine. So took it out again and same thing. IT runs very smooth never bogs down or stalls. just stays at 3k and 18 MPH. People have mentioned the Prop but I think it is the original and trying to contact the original owner for this thoughts as well.
Looking for some more suggestions from the smart folks out here. any ideas?

Thanks as always
Jeff
Thats a lot of water.
Is the bottom of the hull clean? Or does it have 2 inches of growth on it
Definitely check you have the correct prop
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Old 06-10-2019, 05:24 PM
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Prop could be overpitched
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Old 06-10-2019, 06:24 PM
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Hey TTaxi, Sorry about that I will work on my profile. thanks for chiming in the try to help. For gas I just put regular 89 Octane fuel. Interesting comments regarding the Water and Knock sensor. The Temp is a very constant 150 to 160Deg, never rising above that. Oil pressure is at a constant 40 and voltage a solid 12 to 14 V.
When it reaches 3,000 rpm my throttle is at it max movement. not more play in it. I will have to check on the timing and cylinder firing so thank you for that idea.
Jeff
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Old 06-10-2019, 06:33 PM
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Jumpsummo, thanks for the reply. I thought of the throttle cable too but it looks to be fine. I will have a mechanic take a better look to be sure.. When I was full throttle in the ocean this past Sunday I had my fishing buddy reach under the cover and push the throttle and it didn't have much more room to go..I may have gained 1k more of rpm and another couple MPH..

Jeff
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Old 06-10-2019, 06:34 PM
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Hi Likestidham, thanks for the reply, I don't know what that means. I am really not much of an engine guy but I know many other questioned the prop size and or pitch. I need to reach out to the original owner and see if they ever swapped it out..

Jeff
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Old 06-10-2019, 06:39 PM
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Hi GetReel, the bottom of the Hull seems pretty good. maybe an inch or so of consistent water but not a big mess.
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Old 06-10-2019, 06:42 PM
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What is what is your current prop? Size and pitch
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Old 06-10-2019, 06:43 PM
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Hey Billinstuart, Restricted intake? like a governer? People have mentioned that but I don't know what that would look like. I don't really know if it ever ran good. the previous owner had it for 3 years but never put it in the water. He is a Marine repair shop and was fixing this up for his wife to use with her friends but they were always too busy so she never had to time to use it so they decided to sell. So I have to think he put some good work into it if it was to be for his wife.














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Old 06-10-2019, 06:44 PM
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Originally Posted by harpoon70 View Post
Jumpsummo, thanks for the reply. I thought of the throttle cable too but it looks to be fine. I will have a mechanic take a better look to be sure.. When I was full throttle in the ocean this past Sunday I had my fishing buddy reach under the cover and push the throttle and it didn't have much more room to go..I may have gained 1k more of rpm and another couple MPH..

Jeff
Another 1k of rpm is a lot and may suggest that the throttle cable needs attention/adjustment. I second on checking the ignition timing. If the timing is not reaching full advance (around 32 degrees total) the the engine will not reach full rpm under any circumstances.
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Old 06-10-2019, 07:10 PM
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take off the air cleaner and make sure your secondaries are opening with the throttle all the way forward. (engine off)

I think you will find the issue is bad gas. was there any gas in the tank when you bought it?
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Old 06-10-2019, 08:28 PM
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Originally Posted by hottoddie View Post
Another 1k of rpm is a lot and may suggest that the throttle cable needs attention/adjustment. I second on checking the ignition timing. If the timing is not reaching full advance (around 32 degrees total) the the engine will not reach full rpm under any circumstances.
Under hard acceleration the vacuum goes to zero. This is also full advance. So when at idle is the highest vacuum. Connect a timing light and observe the timing changing when you connect and disconnect the tube to distributor.

I don't know the history of this boat. Yours a long time. Used to run better? 99% it's caused by someone touched it.

That throttle being pushed by hand shouldn't get you any more anything. Cables stretch. Dont make too tight but just barely at full throttle.

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Old 06-11-2019, 05:11 AM
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Originally Posted by harpoon70 View Post
Jumpsummo, thanks for the reply. I thought of the throttle cable too but it looks to be fine. I will have a mechanic take a better look to be sure.. When I was full throttle in the ocean this past Sunday I had my fishing buddy reach under the cover and push the throttle and it didn't have much more room to go..I may have gained 1k more of rpm and another couple MPH..

Jeff
Went 3000 to 4000? Only goes to maybe 4500...
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Old 06-11-2019, 05:16 AM
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Originally Posted by harpoon70 View Post
Hey Billinstuart, Restricted intake? like a governer? People have mentioned that but I don't know what that would look like. I don't really know if it ever ran good. the previous owner had it for 3 years but never put it in the water. He is a Marine repair shop and was fixing this up for his wife to use with her friends but they were always too busy so she never had to time to use it so they decided to sell. So I have to think he put some good work into it if it was to be for his wife.














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Dirty flame arrestor, sealed engine compartment, high intake air temperature. Start with timing, look at plugs, check for lots of blow-by indicating low compression. You don't get low compression pressures without blow-by or leaking valves. Do a dynamic compression test to see if all the cylinders are pulling their own weight.

There is no governor.
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