Full Force

iTrader: (5)

I put 547CI 600hp engines in, I have 800cfm HP500 blue cars, what jetting would I look for to start out?

James

I have a 540's, alum heads, 8.5:1, 35 degrees total timing, Holley 850 with 88"s front, 96 rear and 6.5 PV front and back. A/F is steady 13.5 down to 13.2 WFO, EGT run about 1400

With 90 jets in the front the A/F went up to 14.0 and at WFO 13.8

Twin engine heavy 12,500 lbs boat

blue thunder

Interesting, why would more jet in the primary cause a leaner mix?

To the OP, I always presume if I cannot get to a good a/f number within 10 jet sizes of box stock, I have the wrong carb. So if memory serves me, 72p, 88s is box stock 800cfm marine calibration. I'd start with plus 5 front and back from box stock. I'd start with the 6.5pv and take vacuum readings to validate its correct. In the end though, I think you need a bigger carb.

FIXX

iTrader: (2)

you need more carberator..if your running a pv in the secondary and try to go past 4 jet sizes you going to have to open up the fuel passage behind the power valve...do your self a big favor and call c&s speialty carberator and ask about their aerosol carberator..those carbs are so responsive and idle real good around the docks..

toyo321

Yeah you could get the 850 to work but it require a lot of work $, and you should be running a 950cfm carb with your horsepower.

Call the guys at AED they know they're *hit. And yes they do marine application work



http://www.aedperformance.com/?gclid...FcRlOgodFigAfQ


They flow benched my AED 950 carb on my 515 BBC at 650HP and it runs spot on. It starts easy with one pump on the throttle and it is very well mannered even just at idle while docking.

Full Force

iTrader: (5)

I know I will need bigger carbs, the engines when built (by someone else) were set up very conservative, small cams, and then had 780CFM carbs, I opted to use my HP500 carbs that are set up real nice for now, I plan to change things later for sure, but need to finish the season on what I have now...

Budman II

iTrader: (1)

Force, I'm in a similar situation with my build. I have an HP500 800 carb like yours to run on my 489 with AFR heads, and I realize that I am leaving some potential on the table with that carb, but it's not in the budget to spring for a 950 right now. I'll be watching your thread with some interest.

MILD THUNDER

iTrader: (3)

I'm a big fan of the use of a wideband on the boat. Jets are just a small part of what you can modify for a good, efficient, fuel curve. Things like air bleeds, booster style, power valve restriction size, and many other things factor in.

Ive been tuning on a set of carbs on some blown 800's for a buddy. These carbs have been modified with custom air bleeds, power valve restrictions opened up big time to .0935. Keep in mind a standard holley powervalve will not move enough fuel to support that size pcvr, and a high flow valve is needed. Going larger than .120 PCVR is pointless, as it will exceed the high flow PV capability. His boat cruises at a AFR of 12.4 from 3000-3500RPM. Then, at 3600-3700RPM, the AFR drops to mid 10's, and stays there to WOT. There is no need for a AFR of mid 10's at 3800rpm. My next move, would be to reduce the PCVR's, because when the PV opens at 3600RPM, its dumping ALOT of fuel in. The reason I would rather pull fuel from the primary side(powervalve) , rather than go small on the sec jets, is for fuel distribution. I don't want 75% of the engines fuel demand to come from the primary sides alone, and 25% coming from the secondaries.

The idea of running a 6, 8, 10 jet size difference when not running a rear PV is to keep fuel even distribution wise at WOT. On his setup, in order to get a good cruise AFR on the primaries alone, the jet size is what it is, and cant be changed. However, everything after that should be tailored accordingly. With 88's in the primaries, and 94's in the secondary on his current setup, to get the AFR above 3700RPM to be acceptable, we'd have to step down big time on secondary jets. Then the fuel distribution becomes a issue.

Ultimately, without a actual wideband on the boat, nobody can dial in a carb that well on the bench, on the net, or reading plugs in my opinion. Every boat is different, loads, props, air temp, altitudes, etc. Sure you can make it work, but with fuel prices where they are today, I feel a wideband is a great investment. It will pay for itself.

Everyone associates carbs with $hitty fuel mileage. with todays technology, you can get good economy out of them, but you aint gonna do it the old fashioned way of throwing big jets at it and looking for mohagony colored spark plugs. Many car guys are out there running big block vintage musclecars, getting 20mpg after tuning with a wideband. The old days of slapping a carb on and running it as is should be a thing of the past.

Full Force

iTrader: (5)

I plan to put widebands in this winter... I run that on my blown cobra and its the lifeline to the engine....!!!

It's in the plans for later can't do all at once! Lol

MILD THUNDER

iTrader: (3)

Just a side note, if I were to purchase a set of new carbs today, I would go with a carb that has all the tuneability features, like adjustable air bleeds, screw in idle feed and power valve channel restriction and emulision jets. Tune it with a wideband, and you'll have a carb that will outperform anything anyone can "bench" setup for you.

Tim, sorry for getting away from your original question. To get back to that, I'd start with 86 in primary, with a 6.5PV, and 94 in secondary, no pv. Then start pulling plugs. They used that carb on the HP425 454 with 79/89 jets. Merc set them up safe.