glassdave

iTrader: (1)

a friend of mine just gave me a pair of carbs off his sonic and i was just trying to get some info on them. they are Q jets off a early 90's 42 Sonic and i believe they were 502's. carbs have #7W8506 on the choke plate and #9776SB3424 ob the side. anybody know what CFM they are? was hoping to run them on a pair of small blocks i am building up. thanks all.

220BR

That's the number on the outside of bowl assembly? Never seen a Q-jet with a number like that. The only Q-jets I know of start with either 70xxxxx or 170xxxxx.

That 9776 num ber looks like part of a Merc number, as in 1347-9776. I don't have my books with me so I don't know. Good luck

Madcow

Those are from 502 mags (390 HP)

glassdave

iTrader: (1)

yea . .thats the #. its stamped on the mounting flange and on a sticker on the side of flange that the choke plate mounts through. it is the stock merc carb that came on that boat. like i said 502's so i imagine whatever came on that motor. thanks

glassdave

iTrader: (1)

thanks jim . . i was gonna call and ask you what HP those were. (they are from his boat)

kinda curious what they flow at, you have any idea?

Big Block Billy

Should be 750 CFM. That's what Quadrajets on big block Merc's were, Until Rochester went out of business due to fuel injection.

Big Block Billy

Oh, I noticed you want to run them on small blocks. 750 is alot of carb for SB. Unless you are reving high. Your motor can only handle so much air. Try This formula.
C.I.D. x Max Rpm
----------------------
3456
This should give you an idea of what carb you need.
You might get by with 650.
A stock 350 Q-jet is 584 cfm. Glad to help BBB

220BR

A 750 Q-jet is different froma 750 cfm Holley or demon, etc... Because of the air valve that controls the secondary meters, the Q-jet will flow what the engine needs up to a max of 750 cfm. Its fine for a 305 up to a big block around 450hp.

Liberator21

220BR is absolutely right. The Q-Jet is an air valve carb. The air valve (if setup correctly), will only give the engine as much air as it can use. They basically came in two sizes: 750 and 800 cfm. The 750 is more common, and pretty much used on everything. The 800 is a little harder to find. It was used on big caddys, and stuff like that. Measuring the venturi size will tell you what size carb you have. I have the sizes at home, so I'll post them later. Look for a number on the side of the main body. It should start with 70, or 170. If the number is on a metal tag, it could be a Weber manufactured Q-Jet.
In the later years Weber made these for Rochester.

Big Block Billy

Very interesting design the Q- jet is, allowing it to be used on many different motors and still run good. I thought it was jetted differently on a Big Block. I always figured that if you could run a 650Cfm and don't need more CFM's why go bigger. If it was me I'd save them for a motor that needs them and put lower CFM rated carbs on the motors. Holley 650's or Edelbrock AFB style with electric chokes would be my choice. I preferable like the squarebore design. I run a Holley Hi-rise squarebore manifold from a 66 Vette along with an 850 Squarebore carb.. I had to open up the bores in the manifold though. With a 454 slightly worked mag motor I turn 5800 Rpm's. No choke either. The best running setup I ever had. Quadrajets don't hold as much fuel in the bowls and tend to boil off the fuel after a good run. I have not had this problem since going to the Holley. It starts up and runs like it has fuel injection, I'm not kidding. If you go with the Q-jets, maybe you could block off the heat riser passages if your heads have them. Also use a phenolic insulator block under the carb. That's what I'm going to do to a boat that I'm selling with a stock 330. It has a Q-jet, with the usual hard starting ,etc due to lack of fueletc... nothing wrong with carb. Hope this helps Best Regards, BBB