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WOT problem

S

smplsoft

Contributing Member
I have a 1985 Mercury O/B 70HP

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.
 
"Timing light on each plug wir

"Timing light on each plug wire while at full power, check for consistent spark. Look down each carb throat at full power, check for flow out of main jet.
Now lets talk prop, what kind of boat has a 10p prop and a 70hp. Maybe the barge is holding water.
happy.gif
"
 
"I have this engine on a Class

"I have this engine on a Classic 1968 MFG Edinboro. The original manufacturer specification weight is 945lbs. I dont know if this includes the original motor or not. When I bought the boat it had a 10p prop on it. Am I using an incorrect prop pitch? If so, what would be the correct prop pitch for the best performance?"
 
"That will be the "dry wei

"That will be the "dry weight" of the boat alone. So add 260 pounds for the engine (280 if it has power tilt), battery, gas, junk/stuff plus the passengers.

So say with two passengers and stuff for the day say maybe 1700 pounds (as a rough estimate) of total weight.

That's a reasonable weight for that motor (which has 2.3:1 gears - optimal for running a little heavy) to push.

The 70's WOT range is 4750-5250 so optimum would be about 5000, and you should be able to achieve that with a little MORE pitch than the 10p that you are currently turning.

Just off the top of my head I would expect a 13 pitch at minimum and probably a 15 pitch should work if loaded a little light.

So as JB suggests, unless your hull is saturated, the motor is definately under-performing.

Your compression sounds excellent, so next steps would be confirm strong spark and then troubleshoot the fuel system.

It could be a problem with the motor not "breathing properly" (reeds, excess carbon, partially blocked exhaust), but best to start with the most likely issues first...."
 
"There you are, I new you woul

"There you are, I new you would help. That 10p doesn't sound right at all, unless its 2 foot in diameter."
 
"In terms, of the carbs at ful

"In terms, of the carbs at full power, they seem to have a nice even spray with no spitting. How do I check for consistant spark with a timing light at WOT? What should I be looking for? Changes in RPM or an inconsistant strobe?"
 
Sound like a 14p prop is neede

Sound like a 14p prop is needed here. Would this be the optimal pitch? What about diameter?
 
"If you can't achive max r

"If you can't achive max rpm with what we think is a 10p its going to get worse with a 14p. We need to verify what the problem is, engine, tach or prop.

Checking the spark at rpm, look for a consistant strobe light effect out of the timing light. Rotate the timing light to the next cyl and check again.

With the cowl off, advance the throttle and make sure the timing base is rotating and not stuck in place."
 
"Re: Checking the spark at rpm

"Re: Checking the spark at rpm, look for a consistant strobe light effect out of the timing light. Rotate the timing light to the next cyl and check again.

At idle or Max RPM?}"
 
"At idle the strobe light will

"At idle the strobe light will be slow, at max rpm it will be very fast, do both. I think you would know if you are loosing a cyl do to spark problems but since the timing light doen't cost any money to check lets give it a try. Can we verify the prop is a 10p and not an 18p or something else. A 10p prop has a flat pitch (pontoon prop), a high pitch prop will hold the engine back from making rpms.

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When you say timing base are you refering to the trigger assembly? Yes, it rotates around the hub changing the timing."
 
"A far better and safer way to

"A far better and safer way to see what's happening is to check the plug colors. To do this, however, you need to use plugs that you can read (ie; not surface gaps). An equivalent plug like an Autolite or NGK that has a conventional plug gap will allow you to see what's going on (and they don't cause any trouble--I run them in mine).

With different plugs, run the heck out of it at WOT, then shut it down fast as you can and pull the plugs. If they are running light tan or gray then all is well. If stark white or dirty black, however, then there's your problem.

Jeff"
 
As Jeff said.. I forgot to tel

As Jeff said.. I forgot to tell you to squeeze the bulb and see if the engine picks up at rpm.
 
Yes. The trigger rotates freel

Yes. The trigger rotates freely around the hub. Going to check the tach against a timing light w/tach as well as test spark sometime later today.

I will get back to you guys as soon as I run these tests.

Thanks for all your help so far.
 
Sorry opps I listed this engin

Sorry opps I listed this engine as a 1985 and its acually a 1982 Mercury O/B 70HP 3Cyl sn# 6164457
 
"The prop is a through hub exh

"The prop is a through hub exhaust prop correct? I'm just wondering if the exhaust is plugged, how did it sound in the drive way"
 
"It is a through hub exhaust p

"It is a through hub exhaust prop, however I didnt notice any unusual sounds. Could a weak spark cause the symptoms described above? I have not performed a DVA test on the ignition yet. I have access to DVA meter if needed."
 
"If you can beg/borrow a DVA t

"If you can beg/borrow a DVA then may as well have a look - at the very least it would rule the ignition out.

If all is in spec I really would have a look at the exhaust tube - I have seen them so carboned up that you can barely get a pencil through what is left of the hole.

I have tried to draw an arrow to where I'm talking about on the pic - not sure how it will post.

287319.jpg
"
 
I havent had a chance to check

I havent had a chance to check anything today but hopefully on thursday afternoon. Can this be checked by removing the lower unit and looking up the exhaust tube or does this require a complete tear down of the mid section? How do you clean it out?

Thanks
 
I would try a different prop b

I would try a different prop before doing any tear down work. This 10p prop just doesn't sound right for your engine. Maybe it was a factory stapped goof up.
 
"Can you tell me if this state

"Can you tell me if this statement would be true? A 11.25x10p prop should not restrict WOT, as it would not be giving the engine enough load? Or am I thinking about this all wrong? If my calculations are right I think I should be using a 12x14p prop.

17.9 foot lapstake hull runabout (945 lbs)
1982 Mercury 70hp ELPT O/B sn# 6164457 (250 lbs)
13g onboard fuel tank
2:83:1 gear ratio
Max RPM Range 5200-5800 RPM
1700lbs total weight nominal(boat, engine, gear, people, ect...)

Would this be the correct prop?
My brain hurts....

Thanks"
 
"Mistype on the gear ratio - 1

"Mistype on the gear ratio - 1.83:1 - but otherwise yes, a 12x14 sounds like it should be fine. Believe an 11 1/4 x 15 would be a typical OEM prop, so with the 10 pitch you should be over revving anytime you run light."
 
"Well, found the problem. Cyli

"Well, found the problem. Cylinder #2 has an intermitent spark. Discovered it doing a DVA test
on ignition system. Missed it durring the initial spark test. Found to have a faulty switch box. Ordered new one and I will retest it when I recieve it. But I wanted to say thanks to Graham Lamb, JB and Fastjeff for your help. I have learned soo much about this engine, probably more than I wanted to but am happy that I was able get the right direction from everyone.

Thanks
Kirk"
 
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