Was wanting some help with jetting . I have a 502 ,chiller and a 420 mega . Aluminum heads and 525efi cam. I have 2 750 holleys . I run at almost 4000 ft any suggestions on power valves and jets . Last year I had a 250 mega no chiller and stock heads and carbs were set at 69 primary and 81 secondary.

What model 750's are they? We we built our 454 420 mega blower set up with very similar cam specs with dual 700's and started with 4 sizes larger front and rear and ultimately ended up going down from there. That was my first dyno session where I learned high EGT's didn't necessarily mean it needed more fuel. First pull should have melted the Pistons according to EGT's but plugs showed it was running rich. Had to lean down to get the fire back in the hole so to speak which brought the exhaust temps down a significant amount. Ended up 4 jet sizes sizes smaller than originals. So started off plus four and ended up minus 4. With your 502/750 set up I'd still start with 4 sizes larger front and rear and adjust from there. Is it going on dyno?

No dyno. I put my carbs from last year on 69 primary and 81 it seems very responsive 5.5 power valves wouldnt this motor need more fuel? New heads,cam,chiller which means more hp . idle was nice left a little residue on transom. Should i go up in secondaries considering i will have more hp????

pissing in the wind without tuning with a wideband, especially on a boosted engine

Start out 4 sizes larger than what you had in before as a baseline. Although responsive prior to new builds its still difficult to say. (Where you where then - (rich or lean certain rpm's etc) You've got a lot going on there including expense. Do yourself a big favor and spend another 200.00 on a wideband and weld a bung in the lean side and go from there. Depending on what 750 series carb you have its unknown what size jets are stock however it appears your close to stock jetting of its the carb I think it is. 4 jet sizes up or down seems to be the magic number if you have the correct carbs. Again even with the wideband I'd start out 4 sizes larger than the particular 750's come with.

Without Egt's or wideband I'd be slightly nervous dropping the hammers. Espesily being SC. Chit happens fast as you know. I'm always a stickler for a dyno session on new builds even though I have to drive a couple hours and spend 350-450 it just puts me at ease. If it was just changing from the 250'sc to the 420mb it wouldn't be as much of a concern however I'd still put in a phone call to b&m and see what they recommended. But now you've got more flow etc as well with your other upgrades. Hopefully someone will post who has done similar changes as you have.

I totally agree , I run N/A and spent 200.00 on a Wideband to get my jetting right . Its a no brainer , Why gamble thousands for a couple hundred ?

Thanks thanks thsnks for input

had a issue a while back, boat ran fine..or seemed to when we got the 02 on it, found went dead lean only at WOT and could not fix with the blow thru demon, had to change to a quick fuel, but had we not ran the 02 ?????