TOPIC: 1968 merc 125 bogging at full throtle

Ok here is the issue. I bought a 125 merc from tim calmes that was rebuilt. This thing starts like a dream but with the last inch or so of throttle this motor starts to crap the bed like its starving for food. I have 2 new cans with brand new 5/16 lines ect.I also am having problems getting to wide open throttle rpm range. it is only showing about 4200 rpm. The boat is about 900 lbs and its a new 3 blade aluminum prop 19inch. any thoughts on where to start looking?

back the idle screw out 1/4 turn and try it.

Three things come to mind:
1) Is the female bayonet a plastic one with a flat disk inside that seals off the fuel when not connected - or is it a Quicksilver or Tempo cast aluminum with a stainless steel ball to seal off the fuel when disconnected?
The flat disk style will not feed an inline-6 adequate fuel for higher RPMs. They won't feed a larger 4 cylinder like the '70's-'80's 800 & 850 4 cylinders either.
2) Check the size of the fuel pick-up inside the tank, and see whether the filter screen in the tank is too small. I would want a minimum of a 5/16" ID in the tank & preferably a 3/8" ID. The in tank screen filter needs to be large enough to allow a healthy amount of fuel to pass through it.
3) Perhaps the prop has too much pitch? How well does the boat go from zero to plane? The Merc should be able to spool up from 1500+/- to 3000+ quite quickly while bringing the hull onto plane. A 13.25 x 17 pitch should determine if this is the issue.
Doc Frankenmerc

Both tanks are new west marine 6 Gallon plastic tanks with new 5/16 or 3/8 lines. I need to go look and see but I am at work and boat is at marina. I will know better tonight. I don't know what the pick up tube is either but will try to take one apart tonight.Are the pickups changeable or can I find a tank with bigger fittings somewhere. and I think it is fuel related because the boat gets up and goes pretty quick it does not feel like it is lugging at all.i run the tank a direct line from the barbed fitting at the tank to the cast piece that connects at the motor.

I found back when they were new that the center pin in the motor end fuel connecter was to short but I would assume they have been all changed. would run fine to almost full throttle then starve and die down pull back on the throttle for awhile and would run full throttle for a short time and die again. just a thought. look at the pin should be about1/4 in or more sticking out.

One other thing to maybe check, make sure one of the floats in the carburetor is not stuck. I had that problem this weekend with my 140 tower of power, it would start fine and ran pretty good, but at wide open throttle it would bog down and then pick back up. You can check this by pumping the primer bulb and looking at the carbs, if one of them is pouring gas out of it, then one of the floats is stuck.

The primary pickup is a tube that goes to the bottom of the fuel tank that in layman's terms sucks up the fuel from inside the gas tank. P.S. if I was a betting man I would say the new plastic gas tank is probable the problem I have heard people have problems with those. That is why I use the metal ones. Hope that helps

I have also noticed that the vent in the tank's fill cap is pretty tiny in some of the new tanks.
I haven't experimented with whether or not the tiny vent hole would restrict flow to an inline-6. I just drill the vent hole or holes out as big as I can without eliminating the ability for the vent to seal when closed, and let the lake pipes ROAR!!!!!

Doc

Well my tank is crappy. I am going to west marine tomarro to get some parts or a new one.there is no screen and some very suspect parts. I hope to get a new pickup and such. I will get back with results soon.

Well I give up. I am taking themotor to tim calmes on sunday. I have a new tank and a rebuilt one and I can not seem to tune this motor. I am sure tim will have it done in no time. I will let you know what we find on sunday.

Well the boat went to see tim calmes sunday and as it turns out I am not the best at tuning carbs but that was not my total issue. Upon inspection tim found that the carbs were not sinked properly. We did not have the right wrench with us to fix it but we went back to his shop and he made the adjustment. because of time constraints we did not retest the motor. On the way home my plate disappeared from my trailor. so you would think that you could just go to dmv and get a new one. NOPE you need to file a police report in order to get a new one. WHAT A WASTE OF MONEY. Update on motor soon.