'76 850 Intermittent bog out of the hole

[Smig]

['76 850 (85 hp) on a 16.5' Tri-hull. SN in signature]

I have been messing around with this for a while and suspect that I am missing something, so I am trying to put together a good trouble shooting list ...here are the symptoms:

1) Sometimes the engine will bog & die out of the hole, but it doesn't always happen. When it does, it only occurs when I try to accelerate quickly - I can put the motor in gear and accelerate slowly without a problem but if I try to accelerate quickly, it will bog & die.

2) The bog & die happen together - if it I try to back off when the bog happens, the engine will still die.

3) Neutral idle is around 800 rpm, in gear it is about 650. I can criuse forever at 650, but the bog happens immediately on acceleration (ie: not at 2k or 4k rpm). I tried raising the idle (1100 neutral/950 in gear) but this does not change the problem.

4) When the bog & stall does happen, it is with a large load (people on board or a tube/wakeboard in the water.) I have never had it happen with a light load.

5) After a bog & stall, I have to raise the warm-up lever all the way up, otherwise it won't restart.

6) When it's happening and I do manage to get the boat up to speed, the engine might stall when I come back down to an idle.

7) The plugs have always been inconsistent (both cyl to cyl and day to day): sometimes wet, sometimes dry, but usually wet.

8) ...and the most bizarre (perhaps unrelated??) symptom. Last fall while fogging the engine I noticed that when spraying into the bottom carb, the engine did NOT change speeds like it usually does. When I sprayed the top carb, it bogged down, but when I sprayed the bottom carb, it did not.

So this is what I have done:

A) I decarbed, rebuilt the fuel pump & carbs and replaced the bulb (it had failed.)

B) I have linked & sync'd several times (using Clams' method.)

C) Boat is propped for 5500 rpm max.

C) Blue spark on all 4 cylinders can jump 1/2" (on the muffs.)

D) Compression is good/even on all 4 cyls.

So I'm not sure where to focus - fuel or ignition, so my thought is to put together a good troubleshooting sequence (for both driveway and water tests) and start at the beginning, one thing at a time to see if I can isolate the problem.

Thanks
Shaun

 

[Chris1956]

Re: '76 850 Intermittent bog out of the hole

The problem is very likely idle mixture. I would start and warm the motor in the water. Now in gear, try to adjust the idle needles for max RPM and smoothness.

After that adjustment, the motor will probably bog and die as your try to accelerate, however, the carbs are now in sync. Now open the top carb mixture screw 1/8 turn and try to accelerate. If it bogs, repeat with bottom carb. Continue with this sequence, testing acceleration after every adjustment. You will eventually find a setting that gives good acceleration and OK idle.

 

[Smig]

Re: '76 850 Intermittent bog out of the hole

Thanks Chris,
I did do this as part of the link & sync, but since this problem is intermittent, it was while the motor was running fine. One time when it was acting up, I did try opening the mixture screws in 1/8 turn increments but it didn't seem make a difference. I do tend to run the motor slightly on the rich side because I am paranoid about burning it up - could that somehow be contributing to the problem?

Thanks
Shaun

 

[shoestring]

Re: '76 850 Intermittent bog out of the hole

your knowlege seems substantial, you will get it figured out. maybe recheck your float levels, this changes your main circuit timing

 

[Smig]

Re: '76 850 Intermittent bog out of the hole

your knowlege seems substantial, you will get it figured out. maybe recheck your float levels, this changes your main circuit timing

The floats are one thing I have not rechecked - I will have to try that. You mention 'main circuit timing' - what affect do the they have?

Thanks
Shaun

 

[shoestring]

Re: '76 850 Intermittent bog out of the hole

The floats are one thing I have not rechecked - I will have to try that. You mention 'main circuit timing' - what affect do the they have?

Thanks
Shaun

sorry it took me a while to get back on this. picture a bottle of pepsi with a straw in it, if you blow across the top of the straw it will draw the pepsi up the straw and spray it out of the top. the lower the level of pepsi in the bottle, the harder you have to blow to get the pepsi to the top of the straw. same thing in the carburetor. outboards don't have accelerator pumps in them and your float level is critical to time when the fuel starts coming thru your main jet if your float level is a little low then it will take a little longer for the fuel to get to the "top of the straw". and the opposite if the level is too high. the same therory holds true with your transition ports. the little ports in the side or bottom of your venturi. your throttle plate should cover these at idle and will uncover them as you open the throttle. these ports as well as the main jet are exposed to engine vacuum as you apply throttle. the vacuum pulls the fuel into the venturi. if the float level is low, it will take more vacuum (throttle) and time to pull the fuel in. if it takes too long then you will get a bog. it will always take a little time for the fuel to get there and you can make up for this by running your idle circuit a little rich, this will give you a little reserve of fuel in the crankcase for the engine to live on until more starts coming in from the carb. i'm thinking if this was the problem, it should happen the same all the time. but then again air temp and humidity can change how a carb functions. a carb bordering on being out of tune on a cool overcast day of fishing might run ok and a hot clear day of fun might suck.

 

[BoobieBouncer]

Re: '76 850 Intermittent bog out of the hole

I have a 1977 850...

I had to spend a few hours adjusting the idle speed screw and the idle mixture screw to find a happy medium.

My motor was dying when I put it in gear and bogging down in the hole shot.

I recommend turning the idle mixture screws out more to richen the mix for the hole shot and also turning out the idle speed screw keep your idle smooth.

The higher your idle the harder the workout your lower unit gets when put into gear though, so keep that in mind.

Your 800/650rmp's are perfect (i'm jealous!) so start with your idle mix screws...

 

[Smig]

Re: '76 850 Intermittent bog out of the hole

... i'm thinking if this was the problem, it should happen the same all the time. but then again air temp and humidity can change how a carb functions. a carb bordering on being out of tune on a cool overcast day of fishing might run ok and a hot clear day of fun might suck.

Thank you for the explanation - it makes sense and I will see the carb in a completely different light now!

Like you mention, it seems that if this where the problem, then it would be more consistent, but I will give it a try anyway. Besides, after your explanation, it will be interesting to look that the guts of the carb to see how it works.

The floats are old plastic ones, but they seem to be in decent shape. Is there anything that I should be looking for?

Thanks
Shaun

 

[Smig]

Re: '76 850 Intermittent bog out of the hole

I have a 1977 850...

I had to spend a few hours adjusting the idle speed screw and the idle mixture screw to find a happy medium.

My motor was dying when I put it in gear and bogging down in the hole shot.

I recommend turning the idle mixture screws out more to richen the mix for the hole shot and also turning out the idle speed screw keep your idle smooth.

The higher your idle the harder the workout your lower unit gets when put into gear though, so keep that in mind.

Your 800/650rmp's are perfect (i'm jealous!) so start with your idle mix screws...

I think that I will open up the carbs again (to check the floats) and then redo the link & sync. Usually I do this by myself, but this time I will bring along a crowd to put a load on the engine.

I've been running the idle higher (1200/1000) but that hasn't made a difference, so I put it back down to protect the lower unit - is there any relationship between the mixture and idles?

Thanks
Shaun