Looking for engine advice - boat is jerking?

[neddie]

I have a speedboat (96 Bayliner) with a 4.3 Mer Cruiser Engine. Only been used in fresh water. Recently I took it out and at half to full throttle the engine has been losing power and jerking. It will lag and then build the speed back up. It is worse towing people or turning tight. I tried a few things. Tried putting more fuel in and running it on the advice of a mechanic. Don't know if it is the engine timing or carbarators. Any ideas?

[H20]

Hi neddie,

Welcome to the forum.

Is your engine fully trimmed down? Having had recent experience myself it could be the prop ventilating (not gripping in the water). Do the revs increase as you lose power - this would certainly suggest ventilation with the other symptons you mention.

Without having more info I would recommend you try with the engine fully trimmed down and see if it still happens.

Ultimately |I think more information will be needed - What has the mechanic said and suggested? How many people have you got on board? Does the engine bog down or as mentioned before do the revs increase dramatically? Are you getting the boat on the plane? Is the prop in good condition and do you know what type of prop you have.Am sure others will come along with more questions too.

Let us know as much information as possible....

[baasboat1]

Hi and firstly welcome to the forum

Let's start wotha few more bits of info please, does it sound like its misfiring on one or more cylinders? When was it last given a good service and had the fuel filter changed?

It could be many things, but let's use the basics you need fuel air and a spark to make it work, so thinking of fuel - presume you have some and air vents are clear and the fuel filter is newish, is the fuel good or old and may have gone off or gummed up the fuel lines/ filters. Air - is the flame arestor clean and the carbs seem to be working ok? What about spark, are all the leads in good condition and attached properly, is the rotor arm clean?
It sounds like its suffering under load, so is it not getting enough of the three things to meet the demand, try testing each in turn. Also you say that its been in freshwater, have you just started using it in the sea? If so the wave bouncing could have dislodged some gunk in your fuel tank that could have wrked its way througha fuel line to either a fiter or block a line partially.
The above is just my basic starter for 10, I'm sure others will post more later!
Have a look at the op manual and the trooubleshooting section, it will give you a basic checklist like the above to heck out.

Hope this helps, where are from and where do you go boating?

Regards

Baasboat1

[dpb101]

I agree with Bassboat.
Establish fuel supply OK, also checking that the choke is working OK.
Then, before spending any money, and if you have the correct gauge, do a compression check on the cylinders if only to rule out more serious issues.
If all that OK then look at changing the ignition leads and plugs, I have seen the performance of some older boats transformed by that step alone, you can check first by starting the engine in the dark and looking for earthing sparks along the leads......it can be a remarkable sight, but it needs to be proper dark!

[Capn Jack]

Hi Neddie

Welcome to our friendly forum, hope you have a great time here. Where do you hail from?

I agree with bassboat and dpb.

There is a common denominator though for surging and low top speed and that's a leaky exhuast/inlet/carburettor gasket???

What do you guys think?

PS when this has run it's course I'll pop it into the Technical forums - we'll leave it here for now so that neddie can say hello!!