Mercury 850 Problems

[boatmandave]

Hi,

I have recently (Over the past 12 months) resurrected a 1979 mercury 850 4 cyl 85HP non distributor type outboard.
I have replaced trigger, alternator, switch box, regulator, plugs, one carby, (Rebuilt the other) replaced all fuel lines, tank, checked fuel pump, motor was pulled down by merc dealer and was all ok inside (Only parts needed were replaced) reeds turned, pretty much everything checks out ok. All timing setup ok, carbs adjusted as per manual. The engine and boat sat in a shed for around 15 years.

Now the problem. It starts and runs fine (Will start cold on only a few turns of the key) warms up ok and revs fine. When in the water it has a flat spot at about 1500 to 2000 revs, only way I can get it out of the water is to pump the accelerator a few times, then all of a sudden it decides to go, pulls the boat out no problem up to about 4000 RPM. I know the max RPM for this engine is around 5200 to 5500 but it stops at 4000 RPM? (Stops I mean goes fine at 4000 RPM but no more)

So 2 problems, 1 it has this annoying flat spot and 2 at WOT it will only get to 4000 RPM.
This is a 85 HP motor on a 14 ft Valero Dart (Gross 640kg) so it should fly.
Top speed at 4000 RPM is 30 MPH I would have expected it to go to 5200 + RPM much faster than 30MPH, but it does not.

Compression is 135, 137, 138, 134. Fires on all cylinders.
The prop and all gear is standard out of the factory, in fact the engine has never been touched other than from me replacing all the electrical system and our Merc mechanic pulling the motor down and replacing parts needed. (The mechanic said it was so good inside it did not need rebuild, so we opted to replace only what was required, a few gaskets etc etc.

The reason we replaced the electrical system was due to wires all being brittle and shorting out stator and switch box, the rest was replaced because we felt it best to do so while we had it all apart.

From my working out (I am a car mechanic which means nothing compared to this 2 stroke stuff) it seems to be a fuel related issue but I just cant find where or what?

BTW If I squeeze the primer bulb while taking off it makes no difference, so I do not think it is the fuel pump. The pump also pumps plenty of fuel if I pull the pipe off the carby and turn over the engine.

Naturally I have changed fuel, and oil mix and all the obvious things.
Any ideas on what might be the issue? I am running out of ideas?

Regards
Dave

 

[Laddies]

Re: Mercury 850 Problems

The flat spot could be the fuel bowl vent jets, remove the and test run on light hulls the will cause problems. As far as RPM at WOT was this the motor that camer on the boat or did you install it, if thats the case it might be the prop has to much pitch.

 

[boatmandave]

Re: Mercury 850 Problems

Thankyou but I don't quite understand, you mean remove the bowl vent jets? then test the boat without them?

as for the motor it was on the boat when I bought it, however I cant see any models the same with this engine on it, normally they came out with 40 - 60HP johnsons on it. so possibly someone changed this motor.

Too much pitch? Wouldn't the engine labor if it had too much pitch? It goes straight to 4000 RPM without any laboring or it certainly does not appear to be working hard?

Regards
Dave

 

[boatmandave]

Re: Mercury 850 Problems

Took the boat out for a run again today, removed the Bowl vents but made no difference. I was able to improve it by undoing the mixture screws out more. In fact if I wind them out until the engine runs rough at ide it now pulls right through the the rev range up to about 4000 RPM.

It was a little to rough out today to get WOT for long enough to see if it will rev to 5200 RPM? So I will try again tomorrow if its calm.

Think I might be on the right track, but, is it supposed to run rough at idle but ok at WOT? I have been adjusting the mixture to run smooth at idle, maybe its better to be a little rich at idle?

Regards
Dave

 

[Chris1956]

Re: Mercury 850 Problems

Dave, You are on the correct track. Those carbs do not have accelerator pumps, so the idle mixture needs to be a bit rich in order for it to rev.

Try and set the mixture 1/2 way between perfect idle and really bad idle, and try to accelerate. You will eventually find the correct settings

 

[Laddies]

Re: Mercury 850 Problems

Dave when a engine can't get to it's rated RPM (5500 in this case) it don't develope the rated HP at mid range causing a lag or a unit that has problem getting out of the hole

 

[Motor Boater Bill]

Re: Mercury 850 Problems

You need to adjust the idle with the motor in water and the lower unit in gear. If you adjust the idle in neutral it will be too lean in gear. If your boat is fiberglass, it could be waterlogged--you might weigh it at a truck scale.

 

[boatmandave]

Re: Mercury 850 Problems

Hi,

Thanks for all your help, been great.

I took the boat out again today and adjusted the carbs while we ran it under power in the water. This fixed the flat spot problem, it now pulls out of the water no problem and goes much much better, however it still only goes up to 4000RPM?? I even adjusted the carbs at 4000RPM (Which was fun) and didnt get any more RPM? Strange...

With 4 poeple in the boat it goes 4000RPM, with just me it goes 4000RPM and the speed is 30 -35MPH in both cases.

As for waterlogged? I just had the boat weighed for rego, it was 660KG including the trailor, so I guess at least 150kg for the trailor which leaves around 400kg for the boat and engine, this seems about right. Plus I see no eveidence of waterlogged?

I am wondering if the tacho is not accurate? Maybe 30 - 35MPH is flat out for this boat/motor? (Maybe speedo is out as well?)

Running out of ideas, may try to get tacho calibrated as it is an after market tacho? It may well be doing 5000 + RPM? (Certainly sounds more like 5500RPM)

Regards
Dave

 

[Chris1956]

Re: Mercury 850 Problems

Dave, Adjusting the carbs are other than idle is not worth doing, since the idle jets have no effect above idle.

As long as it pulls smooth from idle to high speed, the idle mixture is set OK.

Your tach may be wrong, or the pitch of the prop too great, which limits the RPM. In addition, you might like to check the max Timing advance and make sure it is 21 degrees BTDC. Also check the throttle stop to make sure the carbs open all the way at WOT.

OK, Is the motor (likely has a 20 inch shaft) mounted on a boat with a 20 inch transom? If ithe motor is too long for the boat, it will cause drag.

 

[boatmandave]

Re: Mercury 850 Problems

Thanks,

Ok yes that make sense, idle jets, effect idle.....
Yes it is pulling smooth from idle to high speed now.

I will take the boat out again with timming light and check that. I checked it but not under load.
I have also got my friend organised to follow me and compare speeds.
Carbs are set to WOT, and butterflies are just off the WOT stop, so I know this is correct.

Good point, 20 inch transom, where is this measured?
The cav plate is in line with the lowest part of the boat.
Here is some pics of the boat.

http://hutchison.id.au/ngallery/albums/Dave_little_boat_project/9.aspx
Shows transom with motor on it.
http://hutchison.id.au/ngallery/albums/Dave_little_boat_project/16.aspx
Full shot of the boat

Regards
Dave

 

[boatmandave]

Re: Mercury 850 Problems

Ok,

Today we took the boat out again.

Got the carbs sorted out now.

Also calibrated the boat speed with a Nokia phone that has gps with speed setting, its very accurate (Tested with our cars first) found out that we are actually going 45 - 50 MPH not 30 - 35 MPH, played with the calibration of the speedo and got it pretty close. So we were going a lot faster than we thought.

Also the tacho was set on the wrong setting, now we have about 700RPM Idle and at WOT its 5400RPM. So the tacho gave us a wild goose chase too.

Put the timming light on it and got it all timmed right now as well.

So thanks to all you guys with all the pointers, you are all great and helped me get my problems sorted out.

Regards
Dave