850 bogs at full throttle

[massingill]

got a 76 merc. 850, compression is 150 on all 4, carbs have been rebuilt 2 times (same mechanic), runs great at home, runs great in neutral. bogs down and dies when hit full throttle. mechanic supposedly has checked all electronics, timing, and says there is nothing else he can do. it wouldnt even pull the bow out of the water, only moves barely above a crawl. last time i took it back to him, he changed the prop, from 17 degrees to 13, made a huge difference, but still wont plane. any ideas, this guy has had my boat since november, and he is a reputable mechanic, but i cant take this anymore.

 

[Willyclay]

Re: 850 bogs at full throttle

Welcome to the Iboats Forum. I'm not an expert but can assure you the people who are experts will be able to help you. If you have a tachometer, what RPM's are indicated at full throttle? Hang in there, help is on the way!

 

[emckelvy]

Re: 850 bogs at full throttle

If this motor has (4) coils it's a distributorless model. If so, check the stator. This is a very common problem and I did an 85HP just like it last year. Before that had the same issue with a later-model 90HP Inline Six.

If the stator is out-of-spec, it'll start and run fine with no load but will not rev up.

Check the Hi and Lo Speed windings with a digital multimeter. You can find one good enough for this task at Sears for under $20. They have sales on these all the time, too.

This is a real easy job to to, just disconnect the (4) stator wires from the ignition module and take resistance readings with the meter. Note where the wires connect, however each wire color is marked on the box.

Specs are as follows:

Low Speed (blue & blue-white): 5700-8000 ohms.

Hi Speed (red & red-white): 56-76 ohms.

Any readings outside these specs indicate a bad stator. I expect you'd find the Hi-Speed windings have an extremely high resistance.

Recommend CDI brand for a replacement, I've never had a bit of trouble with any CDI stators.

Now, if your motor has (1) coil and a distributor (the 4 spark plug wires leading to a dist. cap should be obvious), I'd say that maybe you have some kind of restriction/obstruction in the exhaust.

Pull the lower unit and check for problems where the exhaust dumps into the bottom. Also shine a strong light up the exhaust tower to look for anything obvious.

Beyond that, unfortunately you'd need to pull the powerhead for inspection of the exhaust area including exhaust ports for excessive carbon buildup.

Perhaps it's time to break in some wrenches and get rid of that mechanic who's been Easter-Egging your problem for months and months!!!

HTH & G'luck........ed

 

[phil46227]

Re: 850 bogs at full throttle

Interesting,
I have a 73 Merc. 850 that kinda does same thing. Although it will just die sometimes at top speed. Can anyone tell me the ohms. for my stator, I have the 2 wire stator.

Thanks

 

[phil46227]

Re: 850 bogs at full throttle

^^^

 

[jamaica06]

Re: 850 bogs at full throttle

I also have a 2.5 200hp that does a similar thing it will take off sometimes then bog down or just wont take off. same thing fuel system gone through completly. anyone have resistance specs for this one?

 

[emckelvy]

Re: 850 bogs at full throttle

This is for Phil, you don't have the same ign issues as the other poster, your '73 850 would have a distributor. So the stator shouldn't have much to do with your problem, unless your charging system is bad and battery voltage is way low. Your ign system won't like that! Usually the problem with that is the rectifier. Your stator should have a very low resistance between the 2 wires, with no continuity (infinite resistance) from either wire to ground.

Pull your distributor cap and check for dirt/debris/arc marcs. If the cap is full of 'trash' or is bad, there's a potential for crossfire and that'll make the motor run poorly.

Your Merc may have fuel issues; it's probably time to renew carb floats, float needle/seat in both carbs, and maybe install a new diaphragm kit in the fuel pump. These parts are not too $$pendy and may fix your problem. One way to diagnose fuel delivery problems is to pump the fuel primer bulb when the motor starts to die out. Also you can hit the choke button for an instant and if the motor picks back up, you're starved for fuel. If it dies, it's probably getting enough gas.

Check your fuel lines and tank for issues too.

If you come to the conclusion that it's not a fuel related problem, you can put someone in the back of the boat with an inductive timing lite and ear plugs (it's loud with the covers off!). Clamp the pickup onto #1 plug wire and make a top speed run with the lite turned on. If the motor has a miss, you'll see it in the lite. Repeat on each plug wire and perhaps you'll spot a cyl with a problem.

Don't forget that all the diagnosing in the world won't help on a motor with poor compression, so rule that out before spending a lot of time or $$ chasing problems.

HTH............ed