[jy118lfd]

I have a 19' aquasport and a 150 crossflow that will not get more than 5100 rpm at wot moderate load. Compression is 80 to 85 on entire motor lync and sync is correct, timing set at 28, plugs are tan when checked right after a wot run, carbs are 1" bore 83 150 carbs just soaked and rebuilt. The motor runs and idles great. AV plate is about 2 inches above the bottem of hull. 15x17 Alumnium prop 42 mph Prop ventilates slightly during holeshot then settles in nicely. Not to much trim needed for top speed. 30psi water pressure at wot.My question is what next? I cannot understand where to go from here. A 15 pitch prop seems way too small for a small boat.

[Dhadley]

Your numbers are pretty close but the speed or rpms may be off very slightly. Anyway, we've seen this with certain early model V6 crossflows. Especially with the tiny carbs. Lets try something. Youre very close on propping compared to what we've seen over the years. Typically we use a 19 on that combo. See if you can find a 19 and try it. Theres something specific we're looking for here.

[jy118lfd]

I have a set of 235 carbs from a later crossflow should I try those i never got to try them last year. The rpms are right both my tach and a tiny tach read the same. What are we looking for?

[Dhadley]

You can try those carbs but it'd be easier to put a bigger prop on. Bigger carbs may distort the test and the results we're looking for.

[jy118lfd]

I had ran it with a 21 and a 19 but it would only get about 4400 rpm with both Alumnium 14 1/4. The one on it now is a 15x 17. This was last year with the same load and carbs

[Dhadley]

Both were the same rpm?

[jy118lfd]

Yep thats what led me to try the 15x 17 The 19 and 21 were a hustler alumnium and the 15x17 is an omc

[jy118lfd]

The speed was slightly different It was 45 with the 21 and 40 with the 19 but 41 at 5100 with the 17

[Dhadley]

OK, so the 19 and the 21 were both at 4400.And the 17 is at 5100.Right?

[jy118lfd]

Yup

[jy118lfd]

I must add that the 19 and 21 were tried with the motor in the third hole from the top. I then tried the 17 at the same height. I then raised the motor and ran the 17 again went from 5000 to about 5100 +-50 rpm. I also ran a 14 3/4 x 17 viper amd it ran at 5100 and 42 mph but must need to be balanced cause it had a vibration so I went back to the 15x 17 and here I am. I could go up another hole but the prop ventilates a little during the holeshot and does not need much trim with the 15x 17. It did not ventilate with the viper so if that works out I could go up another hole

[Dhadley]

So no matter what prop or what height you ran the most you ever saw was 5100. Now we're cooking. OK, now its time for the carbs. the 235's are not the best choice to leave on there but a good set to test. With either 17 -- the aluminum or Viper, preferably the Viper -- run the big carbs at the same engine height. IE -- the only change will be the carbs. Pay no attention to the holeshot or how much fuel it burns. We're only interested in top rpm with all things the same except for the carbs.

[jy118lfd]

You got it will run with the big carbs I have the 15 x17 on it and will test with that. The viper vibrates at aroud 3000 to 4000 rpm do you think this would bad to run to test. I have to have it checked. It looks new no dings or marks of any kind

[Dhadley]

Cant hurt to have it checked. They're usually a decent prop. Just remember -- one change at a time.

[jy118lfd]

Yup will post with the carb test only, tomorrow

[jy118lfd]

Ran with 235 carbs 44 mph 5300 fully trimmed Great hole shot idles at 650 in gear in water. Better midrange power than 1" 150 carbs Idles a little rougher.

[Dhadley]

With which prop?

[jy118lfd]

omc 15x17 alumnium

[jy118lfd]

Bump

[Dhadley]

OK, thats what were looking for. Stick with me here -- youve got the boat on plane and youre cruising in the mid range. Nice easy ride, all trimmed correctly etc. Then you advance the throttle to wot -- does the boat "nose over" (bow come dow) and give you the feeling you just trimmed the motor in (down)?

[jy118lfd]

No, it seems to go faster but not at the pace you would expect for the throttle input. At about 4000 it cruises around 32 to 34 sounds great but then you open it up and it only goes to 42 44

[Dhadley]

In a way thats good. It would seem the hull is in good shape then. Have you ever had a different motor on this boat or this motor on a different boat?

[jy118lfd]

Yea a 235 when I first got it. It ran 62 mph at 5100 with a 22 pitch stainless prop before the motor coked up and tripped the number 6 ring. I was trying to set it up when that happened butit was better that it did cause the motor had corrosion issues and the crank and 4 rods were bad. Only the top main looked good so I changed the midsection and put this used 150 on it

[jy118lfd]

Never ran this motor on a different boat

[Dhadley]

Did the 235 ever get more than 5100 rpm?