Mercury 70 hp bogs down at full throttle.

[willyyes]

Anyone have a problem wiht a merc that when you open to full throttle it runs fine for a few and then bogs down? I do. It will run fine at half throttle after that. It is snot over heating and it has a new water pump and new fuel filters. The tank is venter and all lines going to the motor are fine. We just bough the boat, used, but the previous owner really kept it as his baby. By looking at it, you really couldn't tell its a 91 key west. Prestine condition. But the motor? He has a mech that works on it and it is in very good condition. I know his mechanic, I use him also. But dollars are few and fare between. Any suggestions?

[Frosty]

Lets just get a few things clear, you say it runs fine at 1/2 throttle continuously? but then bogs "after a few seconds" after opening to full.

This is critical for correct diagnosis.

[pistnbroke]

fuel starvation....carb bowls empty ...check fuel pump.....

Hi Frosty how you going ???

[Frosty]

Hold your horses there Piston we havnt heard yet.

Those 3 pot mercs were bad for this.

[Wynand N]

Originally Posted by willyyes

that when you open to full throttle it runs fine for a few and then bogs down? I do. It will run fine at half throttle after that.

Usually that is the symptom of clogged emulsion tubes (aka main nozzle) - the small holes on the side of tube get clogged by tacky dried two stroke when the carbs stood empty for awhile.
When servicing engines, we found this is the most common cause for a motor to bog down when hitting the loud lever.

The problem also sound like the float levels may be a bit low, but I doubt that it is the cause in this case.
Spark plugs can also cause this type of problem and had such a case just last week with a Suzuki 30hp.

[willyyes]

I took the boat out yesterday so I could fix the trailer. This is when the event happened. I motored just above idle speed to the docking. When I finished the trailer, I took the boat back out and watched the motor as I went to full throttle again to see if it was a one time event. It happened again. I went to full throttle and trimmed the motor to plane the boat. As the engine started to bog down, I noticed the motor went down by itself and I could not tilt the motor up. I pulled back on the throttle and the engine started to run smoothly no bogging down. I tried to trim the motor and it worked, I throttle again and the motor went down and so on. I think maybe I have a short in the engine controls or the cables are slipping. Any ideas? It has new plugs in and the motor was just serviced.

[Frosty]

See Piston? thats what you get for premature diagnosis.

It would appear that it is not bogging down but being loaded by the engine lowering itself.

First check the oil in the trim pump. It should not go down at full throttle.

[willyyes]

Well, I think I solved my own mystery. Believe it or not, it was the fuel line. When I trimed the motor and opened the throttle, the fuel primer bulb would point downward, starving the engine. When the trim went down and I felt the bulb it was firm, so I didn't think it was any problem there. I discovered it in my driveway when I flushed the motor out. I opened the throttle and the same thing happened, but this time, I was watching the action, didn't have to worry about any other boats in my pathway. Sure enough, the fuel hose shifted down and the bulb pointed down. I switched the lines and put the bulb closer to the engine and tried again. This time no problem. Stayed at full throttle and the engine stayed on trim. Simple. But thank all of you who responded to my concerns. Your suggestions were well received and taken to mind. Happy sailing all, and stay safe out there, don't take chances.

[Frosty]

Running the engine in your drive is not full throttle just full RPM.

Don't go out without a pair of oars just yet.

[pistnbroke]

I dont quite follow the trim issue ...agree with Frosty check the oil.....and the moving bulb ....should work anyware in the line and at any positon .....still say its fuel starvation.....guess he has two faults but you can of course do some work and put the fault right without knowing .....

[Frosty]

Originally Posted by pistnbroke

I dont quite follow the trim issue .....

He said when he gives it full throttle the engine goes down, ( trim collapsing)

This will lower the bow and consequently the RPM. This may appear to be the engine bogging down.

[pistnbroke]

So if next time he takes a lump of wood to put between the leg and the transom so it carnt move down ( tuck under) he should be able to prove it ...personally I always put the leg fully under when hitting the gas and then lift it when on the plane ....

[Wynand N]

Originally Posted by pistnbroke

personally I always put the leg fully under when hitting the gas and then lift it when on the plane ....

exactly how it should be done. Only difference my way piston, is when hitting the gas, my finger is already on the trim up button bringing the leg up as it accelerates - found it planes faster that way

[kayant1]

i have a Chris craft amerosport 230 it had a 305 in it i installed a new 350 mercruiser with an alpha 1 drive .everything is new fuel pump to i had the carb rebuilt rodchester it was running great but now it bogs out at 3000 rpm and its getting worse it started doing it at 4500 rpm, but when i'm in neutral it runs great i rev it up sounds good then i put it in gear i get it to 4500 rpm doing great for 20 sec then it bogs down, i have to bring it down to 3000 rpm anyone know what is the cause of this why when its on a load it has a problem

[kayant1]

it also surges a little when it bogs out around 2 to 300 rpm i need Prozac lol