260 Merc I/O power issue

CITM

Cadet
Joined
Jun 28, 2010
Messages
8
I originally posted under the prop forum thinking I had a prop issue but now I don't think that's it. I hope this isn?t too confusing but I cut and pasted my original post and also added an update below after checking some items recommended by ?steelspike?, who indicated that the prop I was running should work fine. Here's that thread: http://forums.iboats.com/showthread.php?p=2822301#post2822301


I have an issue that I?ve been struggling with for some time and hope you guys can help. I trashed a prop that worked pretty well and have struggled to find the right one since. The one that worked was a 15 1/4x15 Black Max. I tried a Quicksilver of the same dia and pitch and it didn?t work, RPMs were way too low. Another I tried was a 16x14 aluminum (don?t recall manufacturer but RMPs were too high and speed too low).

My goal is to increase RMPs without losing (or possibly gaining) speed. My current setup is barely OK with a light load but with a heavy load it really struggles to get on plane (any conditions), and stay on plane when it?s rough. Heavy load for me is 5-6 people plus coolers, etc = 1,000-1,200# +/-. I installed trim tabs and they help a little but not enough. It seems to ride high in the bow with the current setup.

My current prop is a stainless 14?x15 Turbo, I?m not sure of the model.

WOT with an average load is 4000 RPM @ 27 MPH. Heavy load is 3600 RPM @ 24 MPH. I believe my max RPM should be 4600-4800 RPM.

Engine is a 260 HP Merc I/O with an Alpha drive. Rebuilt prior to ?09 season.

Boat is a 1984, 26? Stamas Sport Cruiser. Dry weight is 7000# according to Stamas.



Update:

I took my boat back to the shop that did the engine rebuild and compression, timing, and advance checked out OK. Visual inspection of lifters and cam was performed and they ?looked fine?. I haven't verified this with another shop but believe I can trust them.

I talked to yet another mechanic who suggested I check the coil by looking at the spark. Nice blue spark that will arc 1/2" so that should be good.

I weighed the boat on the trailer. It weighed 10,000 pretty much empty when I got it, however I don't know how much gas was in it at the time, this was 5 years ago and I don't recall. It weighed 10,800 full of gear and with approximately 30 gallons of gas in it last week. I weighed items that I could take off/out easily and could account for approximately 600-650 lbs of gear. The trailer weighs 3,000 lbs according to EZ Loader (called them this morning). This seems to correspond with what I understand the boat should weigh (7,000# dry). I wouldn't go so far as to say that there definitely isn't ANY absorbed water but I don't think there is much.

I tried a 15 1/4x15 aluminum Black Max, which is as close as I could get to the prop that was on it when I got it, (they don't make the exact model number Black Max any more). Before I dropped a cylinder (and trashed the prop at about the same time), it seemed to get up on plane and perform just fine (sorry, I don't know what RPM it was running at). I do know it'd do 27 MPH WOT easily (30 MPH in ideal conditions) with a medium load (4 guys) and cruise at 24 MPH.

Last trip out with the 15 1/4x15, I had 5 guys (approx 1,100 lbs) plus 100 lbs of fish and I couldn't even get it up on plane - 17 MPH @ 3500 RPM regardless what I did with the trim tabs. Going out without the 100 lbs fish I hit 20 MPH at about 3600 RPM but it'd fall off plane easily. I tested it the day before with 3 guys but it was pretty rough and hard to tell, still didn't "feel right" though. 2 guys (under 400#) and 2' chop I got 24 MPH at 3700 RPM. This issue seems to have gotten progressively worse.

Other things I have done:
Cleaned the spark arrestor - no change.
New distributor cap (prior to timing check)
Checked throttle, it does open all the way and 4 barrel does open
Changed fuel filters
New (remanufactured) carb last spring
I?ve ran 400 gals of gas through it this year, it shouldn?t have bad gas in it (although that?s kind of what it feels like)

Things I haven't done:
Replace plugs (did look at them though and they looked OK)
Replace plug wires (did this last year, used Merc wires)
Clean fuel check valve - I have heard of similar symptoms with a partially clogged check valve. I?m not sure if mine even has one?

Any help would be greatly appreciated! Aside from the short list of things I haven?t done I have no idea where to go from here other than have it dyno tested or take it to a mechanic and have them try to find the problem.

Thank you,
Gordy
 
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