1976 Merc 850 85HP Maxes Out at 3000RPM

[crowbuttedweller]

First, I have read the other posts and checked the basics.

Compression 1-155 2-160 3-160 4-155
Good Spark
Carbs Just Cleaned, Soaked, Recleaned, Resoaked, Compressed Air, Etc.
New Fuel lines throughout
Fuel tank removed and extensively cleaned.

My problem: The motor starts first turn every time, and feels like it is running well. When I put in gear, it will do okay and I can get up to about 2800-3000 rpm depending on how I adjust the carbs. It maxes out at 3000 rpm at full throttle. The carbs are wide open, fuel is flowing freely. All looks great.

The only thing I have noticed out of the ordinary is that the plug in #1 is black and the others are not.

Any suggestions on the next step?

Thanks in advance.

 

[hkeiner]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Good Spark

How did you test for good spark? Was it done only at idle or are you checking it when the motor is maxing out. Spark can be good at lower RPMs, but fail at higher RPMs. Also, what are you using to test the spark, a weak spark may be insufficient to get higher RPMs but enough to cause an inductive timing light to show a spark. Unless you have positively confirmed that you have a good spark, I would still suspect the ignition. Just my thoughts...

 

[Chris1956]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

What is the pitch of the prop? what size and weight of boat. Did the boat used to get more RPM with same boat and prop?

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Thank you both for your replies. To answer both questions:

1. I used an inline spark tester from autozone.

2. The prop is 21P. The boat is a 15' Monark McFast 5 bass boat. I bought it as seen for a steal knowing I would have to go through the motor. Fairly light tri-hull. Is it too much pitch?

 

[Chris1956]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

I would think the pitch is pretty close. Try a 19" pitch prop and see if the RPM goes up about 400 or so. I would think more toward your high speed stator winding.

 

[CharlieB]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

If that spark tester is a neon 'flash' tube, it is NOT a good enough indicator of your ignition quality.

Joe Reeves has posted a neat do it yourself How-to build your own using a piece of 2 X 4 and a few nails. Drive a couple of nails par way into the wood, bend them over so that you can turn each one to get the ends about 7/16 inch apart. rig one to a ground, the other to the spark plug wire. Watch the spark to see the color. Test each cyl/coil.

Repost with your findings.

 

[ddrieck]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

I went exactly the same problems your having last year with my 78 Merc 500 50.
I purchsed a new trigger, stator, plugs, switchbox, and coils. Did the link and sync in tank and on the trailer backed in at the ramp. And still would not go over about 3000 RPM's.
Gave up an got a rebuilt 1976 85hp Johnson and tore down the Mercury to sell it off by the peice.

Anyway, While taking the Mercury down I discovered that the exhaust tube was almost conpletlty choked off with carbon. There was only an area abouth the size of a nickle for the exhaust to escape. Chisled that junk out, put her back together and wammy full RPM's and jumps on plane like new.

oh BTW, I did no a decard-----> three times

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

First, for CharlieB:

Rigged up the spark tester and checked everything. Getting a strong blue spark on all 4 across 7/16.

Second, for ddrieck:

What is the easiest way to check the exhaust tube? My manual doesn't show anything helpful.

 

[Chris1956]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

You will need to drop the gearcase to inspect the exhaust tube. I would think that this is unlikely. You might ohm out your high speed stator windings first.

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Chris,

Here are results from ohm test:

STATOR

Blue - Blue/White - 6630
Red - Red/White - 67
Blue to Ground - 0
Red to Ground - 0

Also did trigger while I was in there

TRIGGER

Brown - White/Black = 765
White - Violet = 760

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

I checked other postings and found the ohm test results in my previous post to be in the acceptable range.

I have compression, fuel, spark, and a headache.

Anything else that I can check before tearing it down?

 

[hkeiner]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Your symptoms certainly seem to indicate an ignition related problem, but if you are satisfied that you have tested the ignition properly I would then test the fuel delivery system before tearing down the motor, as you plan. You need a vacuum gauge to test the upstream side of the fuel pump and a pressure gauge to test the downsteam side of the fuel pump. The below guides may help. Your shop manual should contain the specs for your particular motor. I provided the one for my 150hp V6 as an example of what to look for in your manual.

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Big discovery. Not sure if it is good or bad, because I don't understand why it is happening.

When I did my inital spark test, it was in the water with the engine running. Has excellent spark on all four. When I pull the wires, there is good blue spark jumping across.

When I initially ran the test with the boards and nails above, it showed spark as well.

Tonight I was going over everything again.

Checked for spark while running - all 4 had good spark.

Checked using other method again. Nothing on 1 or 2, but 3 and 4 were great. Also there was no change in the running of the motor when the wire was pulled. It ran the same either way. This happened on both 1 and 2.

When I pulled #3 the motor stalled until the spark was able to jump.

Checked while running again (to make sure I was not crazy) and spark on 1 and 2 was good.

Checked compression again. Good all the way - 155 -160- 160- 155 just as it was the first time I checked.

I checked the plugs from 1 and 2 in the 3 and 4 holes and got spark when cranking and running.

What causes spark when running and not when cranking over?

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Went down to the dock again tonight.

Pulled plugs and had a friend hold plugs to block while cranking.

Again, no spark on 1 and 2.

Hooked up all the wires and started motor.

Pulled #1 plug cord and had a spark jumping. Same with #2. But, 1 and 2 sparks are not behaving like 3 and 4. 3&4 have the quick timed spark. 1&2 have a more 'solid' spark. They are not sparking as often as 3&4.

Is this simply a timing issue? Ohm'd all components as manual shows and everything is ok.

Help!!!!! Thanks!

 

[CharlieB]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Post the serial # of your motor so that we can look up your specific ignition system.

Something is NOT right, spark should be the same on all cyl, and consistant at any speed, cranking and running.

A weak or intermittent spark on even one cyl will limit power, two, well, you're only running on half of your engine.

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Ther serial number is 4370065.

But, I think it is fixed!

I was checking wiring coming from ignition harness, stator, trigger, etc. and found three terribly corroded and deteriorated. I spliced in good wires as needed. Also replaced a ground wire from control box to top of block.

Cranked & spark on #1! Checked #2 and same thing!

Started it and ran at low RPM on the muffs. It sounds like a completely different motor. I am taking it down to the dock in the morning, so wish me luck.

The worst wire was a green/red. Is that coming from the ignition harness?

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

New Problem. Same, but new.

Everything was fine running great. Put cover and cowl back on and whammy. 1&2 die.

While checking spark I found spark jumping where at #1 wire connection boot to ignition coil (this is no distributor model).

Do these wire unscrew? That is what I assumed but cannot get #1 out. It just turns free. Is it supposed to just pull out? Last time I did that I guessed wrong and it was the screwin type.

 

[CharlieB]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Whenever you need to look up parts, start here

http://www.mercruiserparts.com/SelectProductLine.asp

Follow the links until you get to your serial #

Your plug wires should screw into the coil, either the wire gave up, stripped, or the threaded pin in the coil has come loose, if the threaded pin turnes inside the coil then you need to replace the wire AND the coil.

Glad you finally found the cause of your problems.

 

[crowbuttedweller]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

Am now back to the same problem... No spark on 1&2, but now it is always that way, cranking or running.

 

[86RajunCajun]

Re: 1976 Merc 850 85HP Maxes Out at 3000RPM

switchbox maybe? can you DVA test them?