Any suggestions for "beefing up" a q jet 4bbl carb on a stepped up 350 with performer manifold, stepped up cam and ported heads, alum exhaust??? I do not really want to replace this as it was recently rebuilt and works fine.

Pick up a copy of HP Books "Rochester Carburetors". It has all the info you'll ever need on selecting jets and metering rods.

Yeah, there are several things you can do/should check. Roe's book doesn't really go into much performance detail for a marine engine, but it has all the basics. The best way to power tune a Q-jet is to take it back to the stock configuration and then do step-wise mods. What was the original application?

1) What's the casting #
2) What is the duration of your cam? That will dictate how big the idle screw ports need to be. If its a marine 350 Q-jet there are probably around .080, which should be fine for moderate cams.
3) What's your current jetting? .069/.042 & CH?

I'll post my step wise tune-up for you shortly. I don't do this for a living, so you'll have to use your own judgment.

220BR,
I'm eager to see what info you got. I'm trying to dial in my Q-Jet, and it's a pain in the butt!! With the stock 350 it seemed to run fine. The jetting was 73 main with 45 rods.
After putting in the 383 stroker, the boat pulls like crazy up to 4500 rpm, then falls on it's face! I thought not enough fuel, so I went to 75 main/ 48 rod. Too much gas! The boat slowed down to 4300 rpm, and I lost the crisp throttle response. Now I'm going the other way. It already has the high flow needle and seat, and the idle passages look to be the larger size. Any suggestions?

Make sure you have between 5&7 psi at throughout the rpm range. The Q-jet is sensitive to fuel pressure. Also, I don't run a filter in the carb body. I use a large flow filiter in the hard line from the pump to the carb.

I can't figure out how to post a link to a word document and the forum gods said its too long to paste here. Send me an email and I'll send it to you. Good luck

I've got plenty of fuel pressure, and I'm using the external filter. Right now, I'm too rich. When I was at 73 main jets it was hard to tell which way to go. I went 75 and it got worse, so now I'm going 71 (with a corresponding rod change), and I hope to try it this weekend. Email the document directly to me: [email protected]

Paul

I'm pretty sure they used the same Qjets with the BBC and SBC. Different jets I believe. I just put one for a BBC on a warmed over 350. I haven't broken it in yet, and only had it out once, but it seemed to run great.

My CaM is a 270/276 advanced duration with 214/220 Duration.
CRANE PN 104124

The casting number a I am not sure of nor do I know what jets other than it was recently rebuilt by a local marine shop who likely jetted it for altitude. It just seems like I could do better at WOT. I had a edelbrock on a V6 before and it seemd to flow better with more fuel. Please let me know if power tuning this qjet is worth it and how to do it????

It's a lot of fun to play with jets and metering rods on a Rochester carb as I had one on a 1980 Vette. I retuned with new heads on up. But it is a lot easier I think to change jets and cams on a Holley and when I finally had enough fun and "caved in" the Holley was a better performer. Rochesters and even the Holley/Weber's on my 350 magnums are not as good as that Holley was. As I recall the Rochester has really small primaries and big vaccuum actuated secondaries and the Holley was better balanced with more juice from takeoff to top end. The Holley may have had mechanical secondaries I can't remember.

Fulloblarney, if you can send me an email address, I'll send you my write up on how to tune a marine Q-jet.