new 800 cfm from holly jetting
 

im replacing my old holly 750 with a 800 cfm holly.

Im wondering how much different jetting should be?

I know my old one has a pv in front and plugged off in rear so ill do that with new set up.

ill post my old 750 jet sizes once i remove...

my setup is a 454 prob 420 ish hp but builder like last builder claimed 500 haha

it pushed a pantera 24 70 at 5000 rpms with tools and props in cabin

Merc used a holley 800 on the old 454/420HP engines. I want to say they ran 79p and 89s or close to that. Obviously rear PV plugged. I'd probably start there and then plug read.

Thanks blue thunder and more ideas appreciated thanks
boat located in s florida so can run on the leaner side

Usually 5 jet sizes up from how they come from holley (72p/87s) on the prim and secondary gets you close. I believe my jetting ended up at 77/90 when my setup was similar to yours. I used an a/f meter to get it dialed in.

I put my 77/87 from the 750 in the 800, but ran into a problem im not getting any fuel out of the port side round part... so the boat wont idle, the pumps squirt fine. Im thinking of taking the front metering block off the 750 and putting on new 800 see if it fixes problem... WtF any ideas

If by round things you mean the boosters, you should not see any fuel coming out of them at idle. If you do, your floats are set too high. If that turns out to be the case smell your oil to make sure you havent got gas in it. That will wreck your bearings quick. I built an engine for a guy a few months back that put on new carbs and didnt get the floats right. Spun most all of the bearings, fuel in oil is nfg.

I spent all day swapping parts I can't get new carb to idle below 1500 rpm, I meant one of front boosters wasn't doing fuel at 2000 rpm when the other one was. Idle screw are out 1.5 turns, went and got more gaskets bc thought there was a vacuum leak, but didn't solve it. I'm going to put old carb on if it works then sending 800 back

What's the fuel psi at?

6-10 the last carb ran perfect at idle and ran great at wot last time out..

I remember talking with you about it last time, and you mentioned the fuel psi was like 10psi? And you were running twin holley red electric pumps on each engine?

Please don't take this the wrong way. I only want to see you succeed in getting this engine running. BUT, it seems for quite some time, you have been having nothing but engine issues in this boat, going on a long time now?. From engines breaking, to incorrect harmonic balancers, to odd fuel pump setups, and all kinds of other things you have posted. This shouldn't be that hard, its a simple, mild, 454 single engine boat.

For starters, 10psi of fuel pressure is too much. And it should not be varying between 6-10. Holley needle and seats are good for 7-8psi. Sure theres a few that can handle a little more, but your typical needle and seat, 7lbs is about what your looking for.

Running 2 electric pumps inline, in my opinion, is just asking for problems. Those holley red pumps are just not a very good pump in general. If this is a Mark IV block, which I thought it was, I would run a basic 110GPH marine mechanical pump, with preset fuel pressure. Or, run a Mallory Marine 140GPH pump with regulator that it comes with. That's a good pump.

On the 800 carb..

Take it off, and make sure the idle transfer slots are pretty much squared off. If the speed screw is turned to far, it just messes the whole circuit up, actually bypassing the idle circuit, and runs like crap.

Bring the idle mixture screws out 1 1/2 turns.

Make sure you blow the idle bleeds out, high speed bleeds, and passages in the metering blocks. Clogged bleeds will cause issues.



install carb and start engine. The engine should start and idle with the above settings I mentioned. Verify fuel psi is not more than 8psi. Be leary of gauges, because I have seen more fuel psi gauges incorrect than correct. Even if its a liquid filled mechanical, get another one and test it against the one you have. Now, set float levels. Next, turn each mixture screw in 1/2 turn at a time, watching the tach. Repeat until the engine rpm starts to drop. When it does, go 1/4 to 1/2 turn back out on the mixture screws. Then, use the speed screw to slightly adjust your desired idle rpm.

Once all that is set, go for a run. Now watch your fuel psi gauge. You do not want it dropping below 5psi at wot, or any throttle position for that matter. If it does drop, you're gonna need to look into Why. That could be a fuel line size issue, anti siphon valve, clogged tank vent, so on and so on.