115 merc issue

[Esox_Magnum]

My 1991 115 has a problem mostly at WOT. Will run great most times on flat water but will bog if hitting a wake or in windy conditions. Does not hold prime if not run for a couple days. For the life of me can find the problem. Has been a problem since being rebuilt a couple years ago. Tan great before that. Any ideas?

[Surface Tension]

I'd check the compression and be sure its banging on all cylinders.

How is it coming out of the hole? If there is bog on waves and wakes I'd have to think the hole shot isn't there either. Could be the carbs were cleaned and rebuilt and now the engine isn't running as lean. Your going to notice a drop in performance if your lean condition is gone.

If you have a lower pitched prop put it on. Which brings up the question, what are your RPMS at WOT now?

[delcecchi]

Out of curiosity, is this the 2+2, or the six?

[Esox_Magnum]

2+2. Hole shots fine. Has done it since the rebuild. I have ran both 17 and 18 pitch 3 blade stainless. The 17 before rebuild ran great at about 43 mph tach wasn't working then. Now no matter what prop I run barely hits 40mph and around 5200 with the 18p. Hole shot with either prop is good....

[delcecchi]

This isn't relevant to your problem, but I was told to be careful about WOT RPM on my 115. Something about the crankshaft not liking high RPM. I was running a 21p on mine on a 1775 lund.

[Esox_Magnum]

I used to run wide open from Spring Bay to the dam before rebuild. Now lucky to get to Stailies or however it's spelled.

[delcecchi]

OK, back to immediate problem. Bogs when hitting waves or if it is rough. Started after rebuild.

Not being a mechanic or anything I can think of a couple of possibilities. This is not a fancy motor with computers and all that stuff so that limits somewhat what could be going on. Did it start doing it right away after the rebuild or did a significant time pass?

First idea is that there is a bad or loose electrical connection in the ignition system someplace or a worn spot in a wire that could intermittently short out/open up due to the impact of hitting a wave. Possibly even in the switch or control box?

Second idea is fuel, like float level in carbs, fuel pump, bad hose or check valve. You mention it loses prime so either the fuel is escaping through the carbs or back through the check valve. Any sign of leakage in the area of the carbs? You could try an new bulb with a new check valve. That is cheap and easy for sure. I can imagine an impact causing the check valve to malfunction and momentarily starve the engine of gas.

That is best I can do. Maybe a real mechanic will chime in.

Louie at Timbutu Marine is pretty good if you need help up there.

[Esox_Magnum]

Just got back from there. Have checked wires tried a different bulb and connector. Thinking it could me the male end on the motor or possibly I missed a bad spot in the fuel lines when I redid the floor.

[delcecchi]

Looks like a job for someone who knows what they are doing, not me. Good luck.

[Esox_Magnum]

Ok while waiting for my partner to get the trailer this morning I popped the hood. Found gas leaking from accelerator pump and a fitting loose for something. Tightened loose fitting now to find a pump. Maybe finally found the problem. Not sure why the mechanic that did the rebuild never figured it out he had it back 3 times to fix it before I gave up on him. Maybe why he don't do my work anymore....

[hamms-scooter]

Hope that was it. It is more enjoyable fishing then working on a motor! Moving some fish down there?

[Esox_Magnum]

New pump and fixed loose fitting still not the problem... guess it goes to the shop this winter

[Surface Tension]

So the hole shot is good then starts to bog. Could be a lot of things.

I'd first look toward the fuel level in the bowls and if their keeping up.

Did you replace fuel lines and are they the correct dia?

An undersized line will cause restriction and not fill the bowls when under higher RPMs.

Were the carbs cleaned and rebuilt at the time of the engine overhaul?

If they weren't they should have. Reason being, dirty carbs or lean conditions are responsible for a high percentage of engine failure. Improper float drop and low fuel level in the bowl and the opposite of too high a fuel level. Both would create a bog but since you have a good hole shot and rebuild I'm leaning toward the lean end. Pull the spark plugs, they'll give you a good idea of the whats going on.

Oil injection. Is this intact or are you premixing after the rebuild. If you mix your going to see a loss in performance.

Losing prime after sitting a few days is OK. If its losing prime while running thats another matter. Primer bulb, same as the fuel line it needs to be the correct size.

These things you can do and eliminate as the problem yourself.

If all that checks out I'd look at timing and tuning. Since this has been rebuilt, its possible the timing is off. You could still have the hole shot but when RPMs increase, timing is an issue.

[Esox_Magnum]

Never thought about timing. Carbs were not cleaned at rebuild but will be after seasons over. Gas lines are all good and same size as before rebuild. Sometimes may got 3 or 4 miles without bogging then roll over a few wakes and it bogs. Has me baffled

[Surface Tension]

Before you hit the waves are the RPMs good and running like it should?

A few things change when the attitude of the boat changes.

The fuel level bowls, fuel in the tank, and more load put on the engine.

Your fuel outlet on the tank. Its on the top or bottom of the tank.

In both cases a barbed fitting is threaded into the tank.

On the end inside the tank is a tube normally with a brass check valve and screen.

That tube can rot out and crack. If the fitting is on top the tank you can suck air.

If the check valve is plugged or the screen it will restrict the fuel flow.

Fuel level in bowls.

Usually the fuel level in the bowls is optimal when level, that would be trimmed down.

If your running trimmed up the level in the bowls changes. Hit a wave trimmed up and the bow rises. Between the two plus not running at recommended RPMs, you bog.

Getting on plane with no bog. What is different. The load is greater getting on plane.

Bow rises and levels off.

Your start out trimmed in and then trim out when on plane. The fuel level in the bowls changes.

You'll have to do some elimination to find the answer.

I'd shut down and pull the plugs when under bog.

Are they wet?

At home put the muffs on.

Start the engine trimmed in and let idle.

Any miss or sneeze?

Check the plugs.

Trim up under idle, what happens?

Check the plugs.

This would be a good time to check compression too.