missing 1000 rpm?

[jenny1671]

1986 doral classic with merc cruiser 140 4 cyl. boat runs fine ,problem is wot is 3200 rpm and should be at least 4200 or 4600 rpm. Speed with 2 people aboard is 29.9
mph.
with a gps reading . Specs from manafucture say it should run top speed at 38 mph. Boat has been tuned up with all new iginition parts checked with dwell meter for timing ,new wires,,plugs , cap&rotor, new deep cyle battery,its starts good,idles ok ,still has the little bog when you take off but just for a second then your good.
to go.Question is ,what would stop it from reving another
1000 rpm on the water,we do all our own maintance every year ,we run syntec oil in engine and change the gear oil also.It also has new and correct prop 14x 19 .
this boat is mint for the year and always trailer to lake.I've checked linkages ,ran boat with cover off and the carb flap is open all the way,we have run out of idea's ..Has anyone else had this problem with a 3 litre merc 140.............dave&jenn

 

[ross_larter]

Re: missing 1000 rpm?

Try pulling the plug leads one at a time, no change in running means that plug's not firing, you may be running on 3 cylinders

 

[Reel Poor]

Re: missing 1000 rpm?

Is this an ongoing problem or is it a sudden problem ?
Did the boat ever perform correctly with the new prop ?
Did the boat perform correctly with the old prop ?

 

[jenny1671]

Re: missing 1000 rpm?

Try pulling the plug leads one at a time, no change in running means that plug's not firing, you may be running on 3 cylinders

 

[jenny1671]

Re: missing 1000 rpm?

thanks 4 the reply,plugs check out ok ,all 4 have brown to tan color it seems to run ok ,we just don't have to speed due to the rpm .i think there both linked...

 

[jenny1671]

Re: missing 1000 rpm?

Is this an ongoing problem or is it a sudden problem ?
Did the boat ever perform correctly with the new prop ?
Did the boat perform correctly with the old prop ?

hello there ,its an ongoing problem, this is our third summer using it,it had a 17 pitch prop before so we bought a 19 which it calls for ,but no change ,1 or 2 mph tops, adjusting the trim on lake to max speed at wot is still 30 mph at 3200 or 3300 rpm..its a 20 ft doral bowrider,big boat for a 4cyl maybe thats all were going to get out of her,, any help would be great.

 

[searay3]

Re: missing 1000 rpm?

Check the compression. If within specs, I would swap to a new coil and a carb kit w/float (20 yrs old). I would also give the outdrive a good look. Oil condition, ujoints, etc. You mention the 'proper prop'. How is the hull/transom. Could it have developed a leak somewhere and soaked up water making the boat weigh more than it should. My point being, if the engine checks out, there may be other forces at work here holding it back. 100+ horses is not much in a heavy boat.

 

[Reel Poor]

Re: missing 1000 rpm?

its an ongoing problem, this is our third summer using it, it had a 17 pitch prop before so we bought a 19 which it calls for ,but no change ,1 or 2 mph tops, adjusting the trim on lake to max speed at wot is still 30 mph at 3200 or 3300 rpm.

What was WOT rpm with the 17 pitch prop ?

Going from a 17 pitch prop to a 19 pitch should have droped WOT rpm about 400 rpm.

Take a look at this chart.

 

[NW Redneck]

Re: missing 1000 rpm?

Reel Poor is right. Going UP in pitch will bring RPM's down. Approx. 200 rpm difference per inch of pitch up or down. Going to a 15P prop should bring you up about 800 RPM. This of course is after everything engine-wise checks out. You said ignition side is OK, but have you checked the carb? Getting enough air through spark arrestor? Dirty fuel filter or restricted fuel line? Fuel pump pressure OK? Throttle settings OK? Correct jets in carb? Just a few things to check.

 

[searay3]

Re: missing 1000 rpm?

while the prop may well be the issue, with a small motor and large boat, 20 yrs old, I have to think that the boat was underpowered to begin with. The condition of the motor would be my first concern. A lot of hours working very hard would take a toll. I would check compression and see if there is anything to work with before I'd start buying props to bring up rpm. If the hp isn't there, you may get the rpm's up, but it won't push the boat very well. It would be a dissapointment to spend 500 on props to find out you have a tired power plant.....

 

[tommays]

Re: missing 1000 rpm?

I think your tack is off a bit as 30 MPH at 3000 to 3200 RPM is very fast for that boat motor combo

I Own a modern verison of the boat with the same power and prop size and 3000 to 3200 RPM should have you at 22 to 25 MPH MAX

Tommays