Boat won't plane... I think its a RPM issue

[Will Liston]

So I've been restoring a 1969 17.5ft Glastron Tri-Hull. It has a 1980 80 HP Mercury 4cyl engine. The motor at first sight was it decent shape but after I dug deeper it had all sorts of issues. I've changed all sorts of stuff and it's almost 100%.

Here's what I've changed:
Stator
Trigger
New Switchbox
Rectifier
4x Coils
Harnesses
Spark Plugs
Spark Plug Wires
Impeller
Cleaned/Rebuilt the Carbs and adjust float levels
Fuel Diaphragm

After all that the motor starts on first turn of the key. It Idles like a champ however when under load it doesn't appear as if gas is coming out of the high speed jets in the carbs at WOT. The carbs are cleaned and spotless. If I pull the carb off and blow through the fuel line with my mouth gas blows out of both jets. If I'm going with the current I can kinda get on a plane but the boat only goes 12-14MPH. The boat is not waterlogged or anything.

I'm stumped. The fuel lines aren't obstructed. I don't have a tach but it definitely doesn't sound like its hitting the right RPM's. Could this be a timing issue of some sort? An over propped situation? All ideas are welcome...


Thanks,
Will

 

[Will Liston]

Re: Boat won't plane... I think its a RPM issue

Oh yea and the motor seems to have decent compression... All 4 cylinders are between 133-136 PSI.

 

[Will Liston]

Re: Boat won't plane... I think its a RPM issue

It's acting as if the pulse isn't strong enough from the fuel pump diaphragm which is causing the carbs not to engage all the way. This makes me think its an RPM Problem... Which could be over propping or a timing issue... Anyone got some ideas?

 

[wired247]

Re: Boat won't plane... I think its a RPM issue

You have 2 circuit carbs. Idle and full speed. If the transition from idle to full speed is poor, you will stay plodding along at just below plane. The way those work is you have richen the idle circuit beyond what you would think reasonable and they act like a crutch to get it through the transition to the main circuit. It will affect your idle quality. Then you bump up your idle timing to get it to idle smooth at 800 ish in gear. So...assuming your float levels are where they should be ( critical ) and your carbs are linked sync'd, you should try fattening up your idle mixture . 2 turns out is a good starting point. You may need to go beyond that. Maybe less.

 

[Will Liston]

Re: Boat won't plane... I think its a RPM issue

Yea 2 is the minimum anything below that and it bogs down after 1/2 throttle... I've had them almost loose to the point in which they are falling out. I think my prop is way over pitched. I got the boat as a frankenstein of parts so the engine isn't original to the boat and the prop isn't matched up right... There's no markings on it but from a visual inspection it looks to be between a 18-20" I'm guessin 19" pitch. So I just order a Solas Rubex 13" which will be here Friday. I'll use that as a starting point and work from their. My boat is rated for a up to 140HP and I have an 80HP so we'll see if the 6" pitch change will make any difference in RPM.

PS: I put fresh floats and gaskets in and measured them according to the Seloc Manual for my style of carb 11/16".

Thanks

 

[Will Liston]

Re: Boat won't plane... I think its a RPM issue

Oh and I just got a tiny tach from amazon to have a rough idea on exactly what RPM's I'm pushin.

 

[wired247]

Re: Boat won't plane... I think its a RPM issue

Yeah, You need to wind that motor up fast. You could try venting the prop. 19 pitch is a bit much for a 80 and a trihull although I ran one on a 90HP ski boat that pulled like a freight train.