540Fever

Long story short, I just had the motor rebuilt. Had surface rust and low compression in one cylinder, no tune, wrong plugs that ripped off one of the plug straps, and it had some hours on it. The motor was originally built in 2006 and who knows where all it's been inbetween. I was running 73 before the rebuild with 26p prop would hit 62-6300 if I would let it. The shop has built several of these 540's (Dart 310 heads, hydraulic roller fairly mild in my opinion, 10-10.5 cr, with a 1050) and were running 80-85 in similar boats I was told. The dyno numbers look great, solid torque 600ft. lb from 3k up to 4700 peaking at 640. I had another BB dyno'd here about 10 years ago and the numbers backed what it did, so I don't anticipate a happy dyno but I won't rule it out.

I assumed with the work that was done I'd need a bigger prop. I picked up a labbed 29 for my first outing and couldn't even get the boat to plane 3000 rpm and it sounded like it was going to explode. I ruled out some odd fuel pressure numbers I was seeing and now getting a steady 7lbs. Put the 26p back on the boat and could get the boat to plane, but I top out at 71-72 at 5800rpm, 500rpm less than the old motor would turn it. I checked plugs and everything looks good besides 2 and 5 which are a bit rich, nothing too crazy though. Also checked timing, I have this DUI 7000 distributor and I'm not real enthused with it. Timing is showing 30 at 2200, 40 at 3000. I finally called the shop after all this and they told me these disti's wouldn't be their recommendation due to this. They said at 5000 the timing should be at 34, which is more in line with what I'd expect for a pump gas with moderate compression. I looked around at a few different MSD distis today and will probably pick one up. I don't expect much of a performance improvement, but I don't feel comfortable at 40* timing on 91 which is usually all I can find on the water.

So that was me trying to keep it short I guess my main question, am I chasing my tail trying to find a non-issue and this is just what the boat is? I had a personal goal of touching 80 with this new setup, but maybe that wasn't realistic. Had this motor spun the prop just as hard before the rebuild, I'd probably just write it off.

SB

iTrader: (1)

Yeh, 34-35 deg total more common with modern heads. You don’t need to change dist’s to do this.

Few members here ran relay with larger wire gauge (12 gauge i believe) to feed battery power with to their dui’s an picked up 300-400 rpm.

Keep plug gap at .035”. Don’t go by their “up to .050” gap” nonsense.

We need a ton more info on old vs new engine to help compare old vs now performance.

540Fever

FYI internals on the motor are the same. Fresh machine work, new rings, bearings, valve job, springs, valves, cam, lifters etc. Nothing major other than the low cr cyliner that was fixed,correct plugs, tune, and cam. Those few things are pretty significant however.

phragle

iTrader: (3)

Your spinning a 26 at 6000 rpm and only seeing 73??

540Fever

With the old setup, yes. You could feel the prop slip, it would really shoot up the RPMs. This was the main reason I assumed I’d need a bigger prop with the new setup.

Now it’ll hit 71-72 at 5800 with the same 26.

Helmwurst

That is like 24+ slip!

540Fever

Interesting. I just ran it through a calculator and with a 1.5 ratio it's right there 24-25%. Since it isn't spinning it as high as it was before, I didn't even think to check. Appreciate the input.

What kind of mph am I giving up with that amount of slip? I'll go read through some old threads and educate myself. New to boats as of last year have always been a car guy so still learning all the ins and outs of boats!

Thanks

F14A water jet

I came out of cars also and am still learning. What I did is (physically, with a straight edge) measured the drive(s) when parallel to the running bottom of the boat and marked that as "0" on my trim gauges. Then I run the rpms up to about 3/4 throttle with the drives "in" few degrees. When the boat speed maxes out and the boat settles down, I trim out in small increments until I see rpm rise with no mph (gps) rise...or some porpoising (at which time I bring the drives in a degree or two). Then I max the throttles and do the same thing (until I get slip/cavitation at max throttle) and then tuck it in a degree.

SB

iTrader: (1)

Pretty sure many Fountains (like formula’s) had 1.65 gear Bravo’s vs typical 1.5. Gearcratio should be on upper side if drive.

op - do google seach fountain 27 props top soeed and you’ll see lots of older threads on here snd the old Soeedwake of owners of these bosts with various merc engines discussing propping. Lots of good info

540Fever

I'm right about 3.5 on my trim which is parallel to the angle of the keel/hull. 4.5 seems to be the sweet spot, anymore I start to lose MPH and you can feel the boat getting loose.