Another Jetting post
 

I'm finally in the final stages of my engine build. I'm running HP 500 carbs and I'm wondering where I should start for jetting. Here are the basic specs of the engine-

Merc 420 bottom end
Stock 088 Heads
RPM Intake
Cam 232/237 dur .612/.578 lift

I'm guessing the power should be around 460-475 with this combo and I plan on proping it around 5500 rpms. From what I can see merc jetted the 420 and 440 engines at 84 on all 4 corners. Do you guys think this is a safe baseline or should I jet up a bit from here?

you didn,t say what intake manifold you are useing,if it is a single plane like the hp500,i would start as if it was a hp500.if you do a googel search it will tell you how merc jetted them,they were staggered.

Its listed Mike, I'm using the Edelbrock RPM intake. The hp 500's are different in each corner and I'm afriad if I run it as is with the dual plane I'll run lean.

yep,i missed it,with the dual plane manifold,id forget the stagger,i think a safe place to start,78 in primary 84 secondary with a 6.5 power valve,what are you running for exhaust?

I'm running Gils with the newer HP500 tails.

Last summer I built a similar mill - same heads, intake and carb, but I am swinging a 4.25 inch stroke for a 489, and my cam is a bit milder - 221 / 232 and .575 / .578 and 114* LSA. I am also running Lightnings instead of Gil manifolds. I started with 80's on the primary and 88's on the secondaries, and a 6.5 PV. It seems to be idling on the rich side. I didn't really get past 10 hours on it last year, so have not had a chance to dial it in yet. Just set the base timing and let it rip, without really running it hard up to this point. I had another thread out here about this time last year. I guess I have a summer of plug readings and prop testing ahead of me.

The Hp 500 carbs do not have powervalves in the secondaries. You will have to jet the secondaries about 8-10 sizes higher than the primaries to compensate for no secondary PV. I would start with 80 fronts and 90 rears.

I found crazy horses old post on his 330 on roids engine. That engine is almost a the same as mine except his has a touch bigger cam (234/244) and it's flat tappet and mine is roller. He was running a holley 800 and dyno tuned it to 75 pri., 89 sec. jets. Would there be a big difference in getting from a 800 to the 830 used in the HP 500's?

The HP500's used Holley 800 cfm carbs. I believe the 830 is actually the one that uses annular boosters, and is (I think) an 800 center section with a 750 base plate, or vice versa. I'm sure someone will correct me if I'm wrong, but that's what I recall reading.

I'm not sure either- but I know I've read over and over on here that he hp 500 carb is actually 830 and not 800. Does anyone know for sure?

endeavor, here is my thread from last year, where it is stated that the 800 cfm (list 9022) is what is used for the HP500. I haven't contacted Merc myself to confirm, but I have a couple of friends running the HP500's, and they both said they are 800's. I'm pretty sure that is accurate. Hope it helps.

"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 Merc ran the staggered jetting to try to even out fuel distribution with the single plane Dart intakes that the HP500 employs.

http://www.offshoreonly.com/forums/g...ley-800-a.html

HP500 Carbs are 800cfm...for sure... and are the #9022's as posted.

OK- Good to put that to rest! Thanks guys!

Hey Endeavor, what did you end up settling on with your jetting? I am going to drop down a few sizes on mine, at least on the primary side. Also need to check the float level. With the 100* air temps two weeks ago, the exhaust fumes would make your eyes water. Way rich - pulled the plugs last week and they are black and soot-covered. Definitely too rich.

Let me know what you came up with.

I'm trying 75 Primary and 89 Secondary. I'm not in the water yet, I've been fighting water in the oil and finally found out it was coming from a leaking valve guide. I'm in the act of dealing with the machine shop as I type. I'll post on the jetting as soon as the boat his the water!

That's a bummer. Hope they get it resolved quickly so you can get back in the water. I'm fighting starter issues on mine today.

Just an update for anyone jetting a new build. The jetting that I posted earlier is what I'm still running. This is 75 primary and 89 Secondary. From what I can tell the jetting seems spot on. So for the record the build was stock Merc 454 Mag/420 bottom end, stock 088 heads; edelbrock RPM intake; Roller cam: 232/237, .579/.579 lift @ 110; Exhaust is Gil through transom dry to the tip. I'm quite happy with the results which is right around 440 hp. I have a bit more that I can squeeze out of it as I'm not yet at the proper RPM's but I'm waiting for winter to have the props worked on.

What brand of cam is it that you are running, and what kind of roller lifters? I'm shopping around for a different cam while mine is out.

It's a Reeds cam, I bought them from mild thunder. The specs are very close to that of a 500 EFI. Reed Cams is no longer in business but from what I've seen out of my engine they made a good cam. The lifters are Morel and the springs are Isky SP-8005.

I'm running the same springs in my 489. Where did you buy your Morel's? I would like to upgrade to a set, but it's tough to swallow the $650 price tag for a set. :eek:

I came across a member selling one of Bob M's rollers for a decent price, but it was set up for a 502 running AFR 305 cc heads. I'm not sure if that would be very much optimized for my setup, considering that I am running stock 088 heads with some minor bowl blending and 2.25 intakes. Also considering a ZZ502 cam and just keeping my stock GM lifters, but the lift is kind of wimpy. Hmmm, I feel another cam selection thread coming on... ;)

I bought mine from a member on here. They had like 5 hours of dyno time on them. Check out these, the body looks identical to morel lifters. From what I've heard there are only a few companies making lifters for all of the various brands. I used to work in the ski industry and I can tell you that there are only 3 main factories making skis for every one else. Those are Atomic, Elan and K2- Everyone else's stuff comes out of those factories. I doubt the automotive industry is much different. Either way its something to look into.
http://www.jegs.com/i/JEGS+Performan...20810/10002/-1