Is the mercruiser 140 really that gutless?

Mo' Money

Recruit
Joined
Jul 19, 2016
Messages
2
Good evening all,
After reading this forum for many months, and coming back to this forum everytime I have a question about something I've finally need to ask something specific. Is my 140 really that gutless? I have a 18' 1979 Chaparral with 140 mercruiser. My latest prop is a 14.5"x18 4 blade(at the recommendation of the local prop shop). Since this is my first 4 cylinder boat I have been around/on, I'm not sure whether this boat should be this gutless. The hole shot seems slow, about 30+ seconds to get up on plane, and about another minute to reach my top speed of 28 when I trim right (with only 2 adults in the boat). At that speed I'm not even achieving WOT, I'm only hitting about 3500. I had a 14.25x21 3 blade prop on and hit 32mph and 3800 rpm, but the hole shot was slower. Last year when I got the boat it had a 15 x 17 prop. Top speed was 23, but WOT rpm was about 4300, and hole shot was much better, fairly quick to plane. I just rebuilt the carb this spring and everything seems to be operating normally. So are the numbers I'm seeing with the different props normal, or is there something else I'm missing? I've read forums that say their 3.0's are getting upwards of 40 mph and can pull tubes and stuff no problem. Any advice is greatly appreciated, thank you.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,309
Start with a compression test. Something is amiss if you can't hit 4600 rpm with the 17 pitch prop.

Have you verified dwell and timing? Is the throttle fully opening?

An 18 foot boat is as large a tub I would want to try to push with a 3.0
 

Mo' Money

Recruit
Joined
Jul 19, 2016
Messages
2
I did check compression last August;
Compression #'s
#1 135
#2 135
#3 134
#4 125
I changed from points to Pertronix electronic ignition. Timing is set at about 6* btdc. Also put in new plugs, wires, cap, and rotor. Yes the throttle is opening all the way. I checked just the other night. Starts almost effortlessly, idles fairly smooth. I'm not sure what else to check...
 

thumpar

Admiral
Joined
Jun 21, 2007
Messages
6,138
With my 120 on the 16.5' I was able to make 4500 RPM with a light load and 19 pitch prop. Is there a chance it is water logged. At 4300 RPM and 12% slip (normal) you should be at 31 MPH. Did you GPS the speed?
 

Alumarine

Captain
Joined
Feb 22, 2005
Messages
3,684
Weigh the boat. It could be waterlogged.
Are you trimming in all the way when starting out?
If so, are you sure the leg is going in as much as it can?

It should plane quicker with 2 people.
 
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