1985 Mercury OB 70HP WOT Problem

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smplesoft

Cadet
Joined
Jun 27, 2009
Messages
20
I have a 1985 Mercury O/B 70HP and I can't seem to achive 5500rpm @ WOT and I need some help.

This is what I have done so far.
Last season I bent my prop shaft so I replaced it and the prop (10p) in november but didnt test it until this spring.

Started motor w/muffs in my driveway and let it warm up. I noticed the water stoped spitting out of the engine so I shut it off. Replaced the impeller and now it pumps water fine.

I noticed the Tach would drop to zero while engine was running. I checked the rectifier and found it to be bad so I replaced it. Tach is good now. Also noticed broken wire on trgger and replaced that as well.

Took the boat to the lake and noticed I can only achive 4200rpm WOT. I suspected maybe some bad fuel so I replaced with new fuel and rebuilt cabs. Still no change at WOT.

Below are some results from some of the tests I performed on the engine.

Compression:
Cyl #1 121psi
Cyl #2 125psi
Cyl #3 120psi

Spark on all 3 cylinders (OK)

Blue to Gnd 6600 Ohms Range (5800-7000) OK
Red to Gnd 159 Ohms Range (135-165) OK

Current symptoms:
Starts and idles ok. When throttling up the engine breifly hesitates at around 3000 to 3200rpms before reaching 4200rms at WOT. Any sugestion? Please help.
 
Joe_the_boatman

Joe_the_boatman

Chief Petty Officer
Joined
Apr 14, 2005
Messages
482
Re: 1985 Mercury OB 70HP WOT Problem

Did you change pitch when you replaced the prop? When's the last time the carbs have been cleaned?

You may try spraying premix directly into the carb throats at WOT out on the lake. If it speeds up, you know you're not getting enough fuel.

Off the top of my head:
Make sure primer bulb's hard (and stays hard) when running. Check fuel pressure from pump. Check carburetors for clogged passages/jets. Check float bowl height.
 
S

smplesoft

Cadet
Joined
Jun 27, 2009
Messages
20
Re: 1985 Mercury OB 70HP WOT Problem

It seems the prop pitch may be the issue. I need to do some checking on total weight of 1700lbs
 
S

smplesoft

Cadet
Joined
Jun 27, 2009
Messages
20
Re: 1985 Mercury OB 70HP WOT Problem

This is the same prop. I didnt change the pitch. But it might have been wrong from the get go.

Your link doesnt list a Mercury 70HP motor. and mecury website starts at 2000 model year. any ideas? 1700lbs total weight. Need to achieve around 5500 rpm
 
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