Liberator21

Does any one out there have any calibration info on the Rochester Quadrajet for a small block? I built a 383 stroker for my 21' and put a rebuilt Rochester on it. The engine pulls like crazy up to 4600 rpm then just hits a wall. It doesn't matter which prop I use. I found some old posts with people mentioning the same problem, but no one said what they did to remedy it. I've already ruled out ignition problems. The engine is a 383, 9.5:1 cr
stock heads with port work, and a Comp Cams Extreme Marine cam. I saved my old carb, and I'm in the process of rebuilding it. My old carb is a true marine carb, compared to the rebuilt one which seems to be a converted automotive carb (according to the numbers). I also noticed my old carb has larger orifices in the main body than the rebuilt one. Some jetting and rod selection info to get me started would also be greatly appreciated.

Thanks

Ted G

Q-jets are sometimes limited by the small inlet at the needle and seat and will run out of fuel causing a lean out at higher RPMS it may be what you're up against. Sorry, can't help you with jetting, most engines like that I've played with had Holleys.

Liberator21

Yeah, I know most people are using Holleys. The reasons I'm not, are because I'd have to fabricate a new bracket to reposition my shift assist module, make a different throttle bracket, and bend a new fuel line. Right now, I'm a bit lazy, and I want to enjoy my boat this coming summer, because I worked on it all last summer. I have one of those Holley 650 spread bore quad replacements up on my shelf. Maybe I'll try it if I can't get the Rochester dialed in. I already ordered the big needle and seat assy. and slightly larger jets, and different metering rods from Carbs Unlimited. The old carb (the one I'm rebuilding), ran great on the old 350 (300HP) before it died (the carb not the engine). I ran 5200 -5500 rpm no problem. I hope freshening it up does the trick. Does anyone out there have an old cast iron mercruiser or Aquapower riser they want to get rid of? Doesn't have to be in good shape, just not leaking.
I want to modify it for sensors so I can do some testing.
I hate to cut up a new one.

Paul

John B

Make sure you have enough fuel pressure along with the larger needle and seat. I found my 400 hp small blocks ran better with the secondary air valve set not to go full open. We found this an the dyno. I used a paper clip held down by one of the air horn screws. This restriction of the air valve (at WOT) increased the signal at the suction tube (venturi), and richened it up and made more HP. We tried different air valve wind-up but the cam deal on the AV still caused it to lean out too much. This is a little trial and error, but the paper clip makes adjustments easy.

Liberator21

That sounds like a good idea. I've heard (and read) many different schools of thought on the secondary air valve. I could probably take that one step further, and drill and tap a small hole in the casting where the air valve stop bumps against. Then put a small screw backed by a small spring (Like the idle stop set screw), so I can have an adjustable air valve stop. As far as fuel pressure goes, I have the large Carter mechanical pump on the engine, and I'm planning on putting a fuel pressure guage in this spring. I wanted to run this engine on the dyno before I put it in the boat, but time and cash constraints kept that from happening. I have a good feeling about this other carb though, I just got back in from the garage. I was doing some final clean up
of the body, and taking some measurements. I was going to powder coat the body, but the coating on it now is in good shape, and that would have doubled my work. Thanks for the input.

Paul

220BR

There are a number of tricks to beefing up the Q-jet. Fuel pressure (5-7psi) and the larger inlet valve is one. I'd also get rid of the filter in the body and go with a larger in-line filter.

Depending on the duration of your cam, you may need to enlarge the idle mixture ports. You really need to be able to make the engine run leand AND rich at idle. More duration usually makes it hard to go rich. Rule of thumb is no larger than the largest screw diameter minus 0.030". Usually, its 0.125 needle diameter and 0.095 port diameter max. I've some info somewhere about the recommended port sizes for given cam duration. Let me know if you want it.

Once that's done, you need to make sure the primary side is set up right. Disconnect the accelerator pump linkage (and shift linkage) and bump the throttle. You should get a slight hesitation. If no hesitation, primaries too rich. If big stumble, too lean. I'm running .069/.042 on my 383 which is a little rich for me.

Before keeping the AV from fully opening, I'd open up the secondary pull over tubes like Rochester did for the 427 Shelby Cobra. You drill 4 small holes perpendicularly through the tubes near the end. Doug Roe tells you how in his book.

If you set up the idle circuit, the primary side and the pull overs like above, you shouldn't have an issue. The Q-jet is a fun carb, but it was designed for a different purpose than a Holley, so if you're pushing 450HP, I'd think about switching.

Liberator21

Thanks for that advice. I got rid of the internal filter a long time ago. The idle discharge ports in the carb I'm rebuilding right now are .086" This carb came on my boat when I bought it with the original 270HP 350. It had 73 main jets, and 45 rods, and even that seemed a hair on the lean side. The replacement carb had 70 mains, and 42 rods. On the 383 forget about it! It was way too lean. I put the 73 mains, and 45 rods in that carb, and after 10 hrs running, I still have no color on the plugs. I figure with the carb I'm rebuilding, I'll start with 75 mains, and 48 rods. I'd like that port diameter info if you've got it. That pull over tube mod sounds interesting. I never saw that one.

Paul