76 Merc 850 Thunderboldt on Fiberform

[douglasjett]

A few of you guys helped me get my boat trim mech working the other day and it is now functional, Thanks again. I put the boat in the water, it starts, purrs like a kitten... I can even throttle down to a decent trolling speed. The problem now is that I don't seem to have full throttle. I would think this 850 would push the ol Fiberform to about 40MPH, I'm getting about 25.... I opend the hood and throttle cable moves freely all the way in and out, just doesn't seem to accelerate past about 2" of forward throttle, total travel of cable is around 4". Is there an adjustment to shorten cable? Could I have a fuel block of some type? Any ideas would be great!

 

[chum1]

Re: 76 Merc 850 Thunderboldt on Fiberform

Hello Douglas, You may start with the basics, compression, spark, make sure all cyl's are firing at full throttle in the water, research previous post, your problem could be as simple as fuel or bad wiring.

 

[Moody Blue]

Re: 76 Merc 850 Thunderboldt on Fiberform

Have you checked/inspected that the carbs are opening all the way when the throttle is advanced fully? How are you measuring speed, GPS or onboard speedometer?

What is a Fiberform? How big is it and what is the construction, fiberglass or aluminum?

Do you have a tach on the boat? What prop pitch do you have?

There are MANY possibilities.

 

[cannonford57]

Re: 76 Merc 850 Thunderboldt on Fiberform

ok dougster,first and foremost as chum and moody said lets determine if theres not a linkage prob. and everything is doing what it should do.push the throttle all the way down,make sure its all the way because if the cables are old sometimesthey can be stiff the last few inches.with the cover off look down the carb throats and make sure thr butterflys(plates) arefully open.if so you need to get a shop tach or let a shop run it to find out the rpm's..5500 seems right i believe at wot.as moody said it may be the prop not giving ya the gratification your looking for....good luck

 

[douglasjett]

Re: 76 Merc 850 Thunderboldt on Fiberform

No tack on the boat speed by fish finder. If I had to guess I'm thinking carb not opening all the way. Do I need to have motor running to check that throats are fully open? If not does the carb need rebuilding? Fiberform is 16' deep v glass hull.

 

[douglasjett]

Re: 76 Merc 850 Thunderboldt on Fiberform

Update:Started motor at home with water muffs. I opened carb cover, the throats are fully open (within 1/8 of an inch if that matters). I have no way to measure tack but motor screams without load on it. I have trim set in 2nd from transom posistion so I'd say tilted out about 5-10 degrees, could this be problem, should I be running in bottom possition parallel to transom??

 

[Moody Blue]

Re: 76 Merc 850 Thunderboldt on Fiberform

Whoa, you should never run your motor at more than a fast idle (1500 rpm) without load. Lucky your motor didn't self-destruct. With the throttle linkage to WOT, the butterflies in the carb throats should be completely horizontal, or fully open. Try (gently) opening the throttle linkage at the carbs by hand to see if they will open any further. Motor does not have to be running to check this.

The trim setting is trial and error, based on the boat, how high/low the motor is mounted, the load distribution in the boat, the hull type etc etc etc. Without power trim, you have to compromise for an overall best setting. It should be set so that at planing speed the bow is out of the water but not pointed skyward. Ideally, the motor is trimmed fully down for hole-shot, then trimmed up for optimum ride. Without that adjustability, you are compromising both operating conditions.

Your prop plays a big role in the performance of the boat/motor combination. Somewhere on the prop should be a series of numbers stamped followed by a dash number. Something like 48-xxxxx -17P which would indicate a 17" pitch.

 

[Chris1956]

Re: 76 Merc 850 Thunderboldt on Fiberform

if motor is making fullpower, and boat is not waterlogged, and prop pitch is correct, boat should hit 32-36 MPH, IMHO.