Mercury MK 55 Top Speed

[Retroman]

Im running a 12 pitch, 2 blade prop on this mark 55. Its on a 16ft blue fin aluminum hull. Has to be 400lbs between me and the wife. 2 full 6 gal gas tanks, battery, case of beer, ice, food, bar b que, chairs etc. I had a top speed of 22mph. Does that sound right to you guys. Thanks for all your help.
Jim
http://www.youtube.com/watch?v=nEPmmvnafic

[gstanfield]

Sounds a bit slow to me, but I'm no expert on these old outboards with a two blade prop. What RPM's are you turning at WOT?

[Retroman]

no tach as of yet. that is a big peice of the puzzle.

[Mike Sea]

That is a big boat and a heavy load for the Mark55 with a 12 pitch prop. You may have to go to a 10 or 9 pitch prop to get more speed.

[upspirate]

Didn't sound like it was winding out to top RPM.

One thing I noticed years ago on a friends Feathercraft aluminum boat with a Mark 55,it would get up to a certain speed, and just build RPM but not go faster....we noticed the hull seemed to nose down more at speed.

What was happening was at speed, the aft bottom area was flexing into a hook ,forcing the bow down.

He reinforced that area with several aluminum angles and got more speed out of it.

[Roberta]

I agree with Warren. Sounds like the motor was slowly gaining RPMs and not peaking out. Maybe needs a little less pitch.

Roberta

[TUGMYWAY]

The Mark 55 had a wot rpm of 5400. It looks like the lower unit was a 2 to 1. So 12" p prop would be slipping at 30 persent +-? That would be poor.Would be somthing like 27 MPH at 10 persent.

I would set the boat up light and retest. One person,low fuel, don't take anything other safety gear. If it gets better,then get a smaller prop.

See if you could get a timing light with a tach.

Good point on the bottom flexing.

Cool video,I mean very cool like cold! LOL
Motor sounds like it is lugging.

[Retroman]

ahh, very interesting perspective. You guys are great. im gonna try a 3 blade 10 pitch prop. gonna play with the timing also.

[TUGMYWAY]

One change at a time!

Some times playing with the timing does not work out! I have pistons to prove that.

#1 Try the boat light.
#2 Try the other prop.
#3 If and only if you can find a "OLD MARINE/Merc guy ) reset the timing.
(on the Mark 55 the mag is the big deal) bi#!& to set up with the plate and pointer.

[TUGMYWAY]

RetroMan
Do the light test first!

Then the prop!

Then the setup I sent you. ONE THING AT A TIME!
I will post the high valve setting later. Need to get to the shop and my old book.
Randy

[TUGMYWAY]

Went to the wayback book and their is no listing for a start point on the high speed inlet. It goes through using a test wheel in a tank and adjusting as needed for a rpm of 5200 to 5600 rpm.

After your testing of the light load and other prop ,I would do some wot runs and go rich then lean. Then back to a rich setting.

After getting the high set,then go back to the low and reset for best idle in gear.

[Retroman]

thank you, i ll keep you posted.

[Caber-Feidh]

Listening to your "holeshot" video, I would really bet you are under propped. That's over 5400 for sure. Remember that vintage ignition doesn't have a rev limiter, the engine just hits a point it can't scavenge properly, and goes flat. Typically there will be a slight climbing in RPM even from that flat point.

Just a guess, but from the sound, when it's wound out I am thinking you are running 6.2K+ (provided the "holeshot" video is not speed altered)