While swapping from an old 305 to a 350, I'm keeping the carb and rebuilding it. Just to get me in the ballpark, what are the stock Qjet jet sizes in the higher HP 350s?
Edit: Or should I consider something different/better for a 325 hp vortec-head 350 at 9.5:1, 5200 rpm, and a comp xe264 roller?
If my memory serves me correctly, (maybe), Q jets were either 625s or the monster caddies had a 800. The quick way to tell the difference is in the od size of the venturii. It has been way to long to be sure......I do however have a Q jet book, I think, and will see if your carb is listed. Just post the numbers someone will have the info you need........MP
I have a book, post some numbers maybe I can come up with something???!..
If my memory serves me correctly, (maybe), Q jets were either 625s or the monster caddies had a 800. The quick way to tell the difference is in the od size of the venturii. It has been way to long to be sure......I do however have a Q jet book, I think, and will see if your carb is listed. Just post the numbers someone will have the info you need........MP
They're actually either 750 or 800. 750 was on most stuff, 800 on later (post 75) trucks, bigger engines like 454/455/500, and on some of the higher HP boats. I'll have to get back to you with numbers since the boat is 2500 miles away right now. I just thought there might be an easy answer.
Thanks for the replies
Wait a minute... I don't need the carb numbers for what I want... If I give you the numbers off the side of a 305 carb, you'll give me jet sizes for a 305 application. I need jet sizes for a 350 so I can put the right ones in when I rebuild, or at least get close.You crazy guys, you almost had me there
Being as jet flow is based on airflow thru the venturi the 350 might only need about 1 jet size and 2 metering rod sizes bigger than the 305 for the primary side to compensate for the higher flow volume thru the carb on the 350. A good starting place for a mild 350 is a 72-74 jet and 47-49 primary rods. Not sure what the 305 set-up will have in it. I've dialed in some as low as a 68 jet and 42 rods, but that was a 355, mild rv cam (4500 rpm max), headers (which tend to lean out air charge slightly) and a very well designed exhaust with X over. In a 1/2 ton 71 chevy truck with T-350 and 3.42 gears, he got 19-20 mpg hiway, and a very smooth transition from idle to cruise. It was on the ragged edge of lean, but it was just a highway runner, not a puller and he knew that.
Secondary side is going to depend on breathing capability of the 350 (cam, headers, higher RPM?) and will probably need to go 2-3 steps richer, with a more gradual taper than the 305 secondary rods have. Remember that a lot can be balanced by adjusting the secondary air valve tension, which richens the secondaries when tightened.
A quick call to one of the carb shops that specialize in Q-jets can get you real close if your honest with them about the build and intentions of the 350.
Ok, thanks for the starting point. That's what I needed. I unfortunately know of two shops close to me. One is Jet Performance and they won't tell you the time of day without $300, and the other one has no one who speaks english
The engine is a mild 355, 9.5:1, vortec (haven't decided if I'm using stock castings or if I'll use some that have been VERY mildly cleaned up in the bowl) with a Comp 212/218 roller, aftermarket manifolds 4" risers, through hub exhaust, maybe a switchable thru-transom later. 325 hp, 5200 rpm is my guess.
If your going to spring for Vortec heads get teh ones with the bowl work, cut down valve guide, and upgraded springs. There are also some that have both intake bolt patterns on them now... WORTH THE MONEY!!1
I actually have two sets right now. One is a set of stock rebuilds that came with the marine longblock I bought, so I assume they have inconel valves and all the goodies. The other set I have is off a street engine good for .550 lift and is very mildly pocket ported, but I would need to upgrade to inconel valves and I'll probably do LS1 130# beehive springs.
Then I start asking myself, if I'm going to have to do a valve job anyway, should I just go with 2.02/1.60, and if I'm cutting the seats, should I just go ahead and smooth out the bowl some more... its one of those, "if yer gonna do it, do it right" but in the end it will all come down to 3 mph and how much its worth to me to get that.
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