I am in the final stages of getting my 489 ready to fire up, and I thought I would poll some of the others who have a similar build who are running Holley's to see what their combinations are. I purchased a rebuilt 800 DP from a member here that he said came off of an HP 500 and still has the stock HP 500 jets and power valves. I have not yet opened it up to verify, but that is the assumption I am going with. My combination will have about 8.7:1 CR, iron 088 square port heads with 2.25/1.88 valves, Performer RPM Air Gap (rect port) manifold, Gil exhaust, and a hyd roller cam with .575/.578 lift, 221*/230* @ .050, 114* LSA. The cam is fairly similar to an HP 500 cam, except for the LSA, so I am hoping I will be close right out of the box. The only thing that concerns me is the staggered jetting that they used for the Dart single plane manifolds. Since I am running a dual plane, will that throw things way off?

Is anyone else out there running a 489/496 with the Air Gap and a Holley 800? If so, what power valves and jets are you running? Also interested in hearing from the 454/502 guys, as it shouldn't be that far off from that.

I realize that I will have to get some plug readings to finalize my setup. I'm just trying to get close for the startup and shakedown, and I want to err on the side of caution to avoid leaning out. I would rather deal with some black transom disease rather than burn a hole in my brand new pistons.

Thanks!

I have a Merc manual #9 which covers the 460hp cyclone engine merc used to make back in the '80's. It was a 1/4" stroker with forged bottom end making it 482 cu.in.. It had an aluminum high rise dual plane and an 850 dp holley. They show it used 84's all the way around. I believe most use the 6.5 power valve.

The HP500 carb does not have a secondary PV.

I would start with 78 primary and 88 secondary jets.

Thats sounds lean !

And keep the stock PV in the primary side? I guess the only true way to select the PV would be to measure manifold vacuum.

A 9022 list 800 (which is what a HP500 carb is) comes with a 72 in the primary, an 87 in the secondary, and a 6.5 PV in the primary side only. Keep in mind that the primary venturi on an 800 is slightly smaller than the secondary venturi... thus the greater than normal spread in jet from front to back. If it were me, I'd step it up 4 sizes from stock front and rear as a starting point. With that cam, you're probably ok with the 6.5 PV... but you may need to check your vacuum with the engine in gear... If the vacuum drops sharply when you dump it in gear, and the PV opens, she'll quit every time. Then it may be necessary to tune the PV accordingly.

468" Airgaps with BG 850's 78F 88R squared with 6.5 high flow PV. I have more cam 610" @ 114 but pretty close. Thats at 800 above sea level too. Oh also stepped the squirters up a number too.

Well, I opened up the carb a few minutes ago, and it has the staggered jetting - primary: port - 81, stbd - 75, secondary: port - 89, stbd - 93; 6.5 PV. I think I recall reading that Merc did this to try to even out fuel distribution with the Dart intakes.

So I guess I will have to buy some new jets to get it squared up, since I doubt if the dual plane intake will take very well to this setup.

That's correct, Merc jetted them that way. My reference was to how a 9022 comes out of the box from Holley, which will be closer for your set up. Jetting comparisons really can't be made from carburetor to carburetor, without all other things in the carb being identical... especially low and high speed air bleed size. That's why it's best to use what the original jetting was as a baseline.

Well, I had the chance to buy an 800 that probably had the factory Holley jetting, but it was not a marine carb. According to Holley's documentation, in addition to j-tubes and sealed throttle bushings their marine carbs have different metering blocks for marine use.