1978 Mercury 50hp flooding out

[HuntnFish75]

I've got a 1978 50hp thunderbolt that has ran fine for the last 2 years, just this year it started flooding out at WOT, or if you go from WOT to neutral very quick. New fuel tank, new fuel lines

 

[Bondo]

Ayuh,.... Welcome Aboard,.... Sounds like a Carb problem,....

Pull 'em, 'n rebuild 'em,....

 

[HuntnFish75]

Got them rebuilt and they had obvious issues. Got the boat out on the water and at WOT it turns 6000+ RPMs and only got up to 28mph. Not sure what the problem is now.

 

[Texasmark]

Got them rebuilt and they had obvious issues. Got the boat out on the water and at WOT it turns 6000+ RPMs and only got up to 28mph. Not sure what the problem is now.

How about a slipping prop. Pull the prop and scribe a line from the hole in the center to the outer housing material. Run it. Take the prop off and check your line. If not as you marked it, get a new hub or prop.

Mark

 

[quicktach]

Trimmed OK at WOT ?

 

[HuntnFish75]

Texasmark- that's what I thought, but wasn't sure
quicktach- the motor doesn't have tilt and trim

the top of the powerhead, under the flywheel is wet, you can also hear what sounds like a rattling noise coming from the top end. not sure if the top seal is bad or what but I know that last year, the boat ran 30mph and only turned 4000-4300rpms at WOT

 

[Texasmark]

the boat ran 30mph and only turned 4000-4300rpms at WOT

................verses what rpm/speed when everything was running right in the past? Would assume as you mentioned upper seal is bad. If you have rattling that is not good. Could be a bad bearing at the top of the crank or a broken ring on a piston. For an upper seal to be leaking I would assume the engine has a lot of hours on it which would lead one to suspect a major overhaul is in the near future with the engine noise.

Still would certainly check for a slipping prop. If that IS the problem and fixing it returns you to your previous condition, I'd just maybe up the oil a little, like use 3/4 quart of oil to 6 gallons of gas rather than the more common 1/2 quart (pint) to 6 gallons and use a Syn. Blend TC-W3 oil. Pennzoil Marine is just that and WM sells it in the sporting goods section. The syn smokes less (BTDT) and would help in reducing potential plug fouling with the extra oil.

So, you may be able to use the engine as is for a number of years and forgo the expense and trouble of a major overhaul. However, once you determine that you are back to previous satisfactory operating conditions, with proper rpms at WOT, you might decide to cut your throttle back and keep the max rpms at a lower level, like maybe 4500 which might prolong your engine life.

Mark

 

[racerone]

Report the values of a compression test.---Then check if you have good spark on all 4 leads.------Running on a hose in nuetral at 6000 RPM is a mistake , do not do that.

 

[HuntnFish75]

boat has been taken to a mechanic, he did a compression check and all 4 cylinders came back in the 120-125 range. I replaced all 4 powerpacks and plugs back in December. I Never run it at WOT for very long, just enough to get the boat on plane fast and back it off to about 4000-4300rpm. I upped the oil last year and aside from the smoke, it never really changed much. the plugs were changed when I got the boat in 2013 and again at the beginning of december. It had a major overhaul 5 years ago, only parts they didn't replace were the top seal and top crankshaft bearing.

 

[racerone]

Motor should run like a top.----How did you determine that it is " flooding out " at WOT , or is that a guess ?

 

[HuntnFish75]

I could smell gas, and after the motor would die, it took 4-5 minutes of sitting to start. I also had fuel running down the side of the outer case of the motor

 

[Texasmark]

Interesting on what you didn't have changed in the overhaul and current problem.

On that much fuel leaking, the smoking gun should be obvious. Sounds like a flooding problem to me and I'm going to suspect sticking linkage in one of your carbs. Running WOT the needle valve is open all the time and when you shut down, if it doesn't reseat immediately, the fuel pump will continue pumping fuel into the engine until it quits rotating. It takes time to work the excess fuel through the engine and then it will start.

Tell you what. Take the cowl off and front of the plastic air box covering the carb intakes (venturis) so that you can see them. Make a run that duplicates a run that precedes your problem. Shut your engine off and immediately lift it up as far as you can and block it...put a wood 2x4 between the engine and clamp bracket to hold it up. Look for fuel leaking out of the carb(s) venturi. If you see any, you have your smoking gun....carb float not seating properly. Could be a bad needle seat, the float height is not adjusted properly, or float linkage binding.

Mark

 

[HuntnFish75]

Interesting on what you didn't have changed in the overhaul and current problem.

On that much fuel leaking, the smoking gun should be obvious. Sounds like a flooding problem to me and I'm going to suspect sticking linkage in one of your carbs. Running WOT the needle valve is open all the time and when you shut down, if it doesn't reseat immediately, the fuel pump will continue pumping fuel into the engine until it quits rotating. It takes time to work the excess fuel through the engine and then it will start.

Tell you what. Take the cowl off and front of the plastic air box covering the carb intakes (venturis) so that you can see them. Make a run that duplicates a run that precedes your problem. Shut your engine off and immediately lift it up as far as you can and block it...put a wood 2x4 between the engine and clamp bracket to hold it up. Look for fuel leaking out of the carb(s) venturi. If you see any, you have your smoking gun....carb float not seating properly. Could be a bad needle seat, the float height is not adjusted properly, or float linkage binding.

Mark

Mark, I had the carbs rebuilt about a week ago. It runs better now aside from the rattling in the top end and high RPM reading on the tach. I took it out for a little while yesterday and about an hour into the trip, the Tach stopped working.

 

[Texasmark]

Mark, I had the carbs rebuilt about a week ago. It runs better now aside from the rattling in the top end and high RPM reading on the tach. I took it out for a little while yesterday and about an hour into the trip, the Tach stopped working.

Was that after the leaking problem was noticed, or before as I assumed?

 

[HuntnFish75]

Was that after the leaking problem was noticed, or before as I assumed?

That was after the leaking problem was noticed. It doesn't leak fuel anymore, it rattles in the top end and turns 6000+ RPMs at WOT. I ordered a new bearing and seal (hard to find since my serial number doesn't register with ANYBODY) and they'll be put on as soon as they arrive.

 

[Texasmark]

Sounds like she's doing what she likes.....run. However, do you run 6k+ for long periods or short bursts? I used to run my tower at 6k for something like 5 minutes at a time, kind of a sprint thing and never hurt it. Had it about 7 yrs and sold it to the BIL and he's had it for about 15.

 

[HuntnFish75]

Sounds like she's doing what she likes.....run. However, do you run 6k+ for long periods or short bursts? I used to run my tower at 6k for something like 5 minutes at a time, kind of a sprint thing and never hurt it. Had it about 7 yrs and sold it to the BIL and he's had it for about 15.

I haven't ran it at 6k since I saw it do it the first time. I've got a new bearing and seal on its way here. Once it's here, it'll be put on the motor whether it needs it or not. My tach actually stopped working Saturday while idling around on the lake. Took it to the river sunday and it worked right again. I thought maybe the carbs weren't set right. Tried to adjust it and couldn't turn the idle screw. Is there a trick to getting it to turn or is it just tight?

 

[Texasmark]

The engine idle screw nor the carb low speed adjustments are a trick to turn. Spring loaded friction. Turn with 2 fingers sort of thing. Is it the tach or the tach drive signal, corroded connections? How's your battery voltage when the engine is running over 1k rpm.....14 to 14.5 or nor on a fully charged battery?

 

[HuntnFish75]

I guess I'm over my head when it comes to carburetors, I cant find any sort of screw or adjustment that can be turned by hand. not sure on the tach, it has always worked good up until I took it to the mechanic to have carbs rebuilt. battery voltage is another thing that has never been "right" since I got the rig. it might lose 0.2 volts in a day of trolling, but it has never shown higher than 13v even with the motor running at WOT.

 

[Texasmark]

Since you have low charging voltage and tach problems going to bet your rectifier/regulator module is amiss. The grey wire from the regulator powers the tach and it counts the rectified AC pulses from the stator.

Get a picture of both sides of one of your carbs. Want the left (port) side looking forward. There IS a brass low speed adjustment needle with a stainless steel spring around the shank on your carbs.