Ready to give up on this 650

[Cariboo Milkman]

I have had this 69 650 4 cylinder on a 14 foot glass runabout for about two years now, and have yet to get it to run well enough to plane in less than 30 seconds, if at all. I use it at 3700 feet elevation, it has a 15 inch pitch prop, and it starts and idles well. Boat and motor combination weighs approximately 1000 pounds after adjusting for the weight of the trailer and gear.
Here is what I have done/learned so far:
Carbs have been rebuilt by a qualified mechanic. They seem to be synced as they appear to open and close together.
Plugs have been replaced, and they all look the same after one hour running.
Compression with engine cold is 107 on the lowest cylinder and the other three are 114 to 117.
Spark tester indicates good spark on all four cylinders.
The only thing I can think of to do yet, is Clam's link and sync procedure. If that doesn't work it's time to get rid of it.
Any other suggestions?

 

[bilge rat jim]

Re: Ready to give up on this 650

When the boat does plane, what is your WOT rpm?

 

[Cariboo Milkman]

Re: Ready to give up on this 650

I have no tach, but my i-phone app, which may or may not be accurate, was reading about 4400 if I remember right.

 

[bilge rat jim]

Re: Ready to give up on this 650

First, I would get an accurate tach and confirm the WOT rpms. If 4400 is correct, you are about 800 rpm lower than the recommended WOT rpm. This could be caused by a variety of things- timing, fuel supply, an ignition problem at high speed, wrong prop, and a few others. Start by confirming that the throttle plates are fully open at WOT, and that adequate fuel is getting to all cylinders (spark plug check). Then check for ignition consistency at high speed, using an inductive timing light. Fuel and spark OK? move on to timing, here is a link to timing procedures for your outboard: http://www.maxrules.com/oldmercs/timing/45.pdf
Once all the above is confirmed OK, we are left with the prop. If you have a 15P 3 blade, the diameter is 14. The next most common prop for that outboard is a 13Dx17P, see if you can find one to try- the reduced diameter and increased pitch may help your boat onto a plane a bit quicker.

 

[Chris1956]

Re: Ready to give up on this 650

Jim, is correct on all but his last point. The 15" pitch prop may have too much pitch for the weight of the combo.

How does an iphone app read RPM? What is top speed? Now an iphone could read that....

 

[bilge rat jim]

Re: Ready to give up on this 650

Chris, I think you'll find my prop theory makes sense. What we want to do here is increase thrust, not torque. Reducing the diameter does just that, and if Merc made a 13Dx15P for this outboard, I would suggest starting there, but there are limited props for this model, and my records show that 17P is the lightest pitch available at 13"D. If the 13Dx17P reduces the time to get on plane, we know we are the right track. We can then tweek the prop (probably by slightly reducing the diameter, and adding some cup) to achieve a good balance between hole-shot speed and WOT rpms. This is all dependant, of course, on how accurate Cariboos' phone is as a tach.

 

[wired247]

Re: Ready to give up on this 650

65 HP on a 1000 lb boat? I'd be ditching the 15 pitch prop and moving to a 13 pitch or at least adding a couple of vent holes to the 15 pitch. Aluminum props are super cheap. Open up the idle screws a bit too and increase idle timing to get it to idle smooth IN THE WATER IN GEAR. Maybe even run a little better grade gas an jack up the WOT timing a couple of degrees.

 

[Cariboo Milkman]

Re: Ready to give up on this 650

I'll try to reply in order.
Timing-is 69 not electronic? I was planning to use Clam's Link and Sync that I found elsewhere in this forum.
Boat came with a beat up 17" P prop. Changing to 15" P made little improvement.
iphone app reads pulses from firing and can be adjusted for cyl #s and stroke. Not sure how reliable it is.
Weight of boat/motor combo is 1000 lbs which should mean hull only is around 750 lbs. Correct?
I've tried adjusting carb mixtures with little effect.
I'm running premium gas.

 

[wired247]

Re: Ready to give up on this 650

You dont need to run premium gas with that. Doent help anything unless you jack the timing to match the fuel.

1000 lbs is 1000 lbs.

 

[Chris1956]

Re: Ready to give up on this 650

So you have a 14 footer that weighs 750-1000 pounds? That is a problem. My 16 foot speedboat is build real strong and heavy for it's length, and it weighs in a less than 700#. I expect a 14' runabout to be no more than 350#.

 

[bgc]

Re: Ready to give up on this 650

Cariboo,
The Merc's are quite sensitive to their timing. Have the Linc-N-Sync checked and make sure the Max Advance is set properly. When you advance the throttle slowly through the mid range, does she cough and run rough? Hit a spot where the is little power being produced?