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?