Okay, Tried again today. Lovely day on the water mind you!

I put the Mirage Plus 25P on and re-built the carb with new #65 PVs and left the #86 Jets in to see what it would do.

With the base vacuum tuned to around 10 - 11 inches, the screws are about 1 1/2 turns out. Motor ran a lot better with the new PVs and an oil change!

Best I could muster was 68.7mph (GPS) at around 4850rpm. Engine felt like it wanted to go better but seemed to lack fuel?

The plugs were a bit blackened and there was some black residue on the transom so that leads me to think it's too rich?

Base timing is 14 Deg BTDC with 20 Deg adv curve on a HI-6M.

Next test I'll leave the Mirage on and lean it out a touch. Anything else anyone can think of or any other suggestions?

Cheers,
John

ok i think your getting there . my late motor was close to your this is wt i would do . 1 run your motor at 3000rpm set the timing at 34degrees then lock it in ! then run it on the water . you got 68.7 last speed at 4850you need to go down on the pitch more ! it is 200rpm for i pitch size . i would try a 23mirage plus that will give you 5250rpm was your motor dyno????.i last year i had same thing go wrong on my boat i put smaller prop on my boat ! 4900rpm my boat ran 75mph ,but it was a dog ,i went down pitch now 5500rpm now going over 80mph .if you have a dyno sheet look to see where your highes tq number is . lets say your is 4400rpm then you add 1000rpm to that ,that will give you max rpm to prop your boat. good luck

I have a Bravo 1 24P I'll be testing next but for now I just want to get the carb / fueling issues sorted out and make sure I've got the right jets etc in before I put too much emphasis on anything else.

Any thoughts on the Jet sizes?

that 24 prop will be almost the same as the 25 pitch . 3 blade props are faster ! . i ran 84 jets in pri. 88 jets in sec. with a850 holley on my motor .you say your runnig 86 all 4 . do the timming then put the prop that give you the rigth rpm . then run the boat at 3000 rpm for a few miles trun key of and pull 2 spark plugs see wt they look like hoping tan color if they are the pri. jets are rigth! then you got to go full throttle and do the same thing trun key off and pull some plugs .your timming has to be rigth and the motor need to be pro at the rigth rpm . the motor has to be in the power band. that y you are runnig rich. once you get your boat runnig 5200 to 5500 range then you can see wt jets you need . the only other way is put it on a dyno !

Measure your vacuum while running....you will need an assistant. My guess is your PV may be opening too soon. Try a PV that is at the vacuum level at 80% of WOT.

what holley carb are you using anyway? I have an opinion however would like to know what model/size holley you have or did I miss that in another post?
John

What everybody said! just throwing out my o2 here,with a Holley you need to get the carb to operating temps first to make then idle correctly..Also your primary throttle blades shouldn't be touched from the factory spec's to turn the idle up because it throws off the idle circuit and the fuel going into the motor puddles instead of spraying..if you need to raise the idle adjust the secondary stop screw to achieve the correct idle.if its loading up at idle you need to turn in the idle enrichment screws,like stated no more then one turn out,most of my boat engines end up at a half of a turn..you want the front throttle blade as closed as possible..if its open to far then you will get a stumble off idle..somtimes it can be corrected with a bigger squirter in the throat of the carb...

Back to running no thermostats! run a restrict or plate,i make my own out of 316 ss with hole punches in various sizes to get the proper engine temps,you will never have to worry about a thermostat sticking this way from some of the crappy water we run in..

Sounds good to me!
I've checked and re-checked the timing. I'm now set at 14 BTDC (start) running a 20 deg Adv Curve for total timing of 34 BTDC. My other option is to run an initial advance of 24 deg BTDC and run 10 deg adv curve, which might help me at idle / docking speeds? Running the HI-6M makes adjustment easy!

My last test, when coming up onto a plane (or trying to), the engine seemed to stumble around 3600-4000 rpm when on full throttle. If I tried to get it up using less throttle, there was no stumble but it takes longer (obviously).

I'll check the primary idle and try adjusting using the secondary. I didn't know about that one, thanks!

John: My Carb is a Holley 850 D/P HP (4 corner idle, PV protection circuit, no choke, less vacuum ports on the base). I'd be keen to hear your thoughts too!

Thanks all.
John (the other one)

ok john it sound like you on the rigth track . think about when you drive a car you put it in the wrong gear . you have it in 3 gear . you take off the motor slowly pick up power til it hits the higher rpm . you give it gas and it bogs and feel like it has no power. how about you try 2 gear to start out in ? it will bog , but it will pick up better. . well that wt your prop is doing! i beleive if you put a smaller pitch prop so the rpm is higher up in 5200 to 5500 you will not get that stummble at3600 to 4000rpm !4850 is to low for your motor.my boat did the same thing !i put a smaller prop on . i run 5500and the boat is very fast no stummble .once you get the rigth prop on then see wt it runs like

Hey guys,
Sometimes you just have to get to back to basics.
First by my caclulations the motor / carb combo is good to 6276 RPM.
Now with that in mind you are either running to much carb or to much prop.
Now concerning the vacuum issue. A camshaft of this type has a certain amount of overlap / valve timing.
This always creates a less than ideal amount (10-12 inches) of vacuum.
First you must make sure you are measuring manifold vacuum. Most ports on the carb are NOT manifold vacuum.
Once you have determined optimum vacuum at idle, with a Holley carb you take that number and divide it by 2 and that will give you the size power valve you need.
The real trick is to establish baseline numbers. Then make 1 maybe 2 changes then go make the same test run. Record the gains or losses and adjust accordingly.