Q jet question, 454 750cfm on 350
 

Am wondering if it would really be a problem to run a 454 carb on a 350. Im running out of 350 Q jets, Im workin' on a heavy 25' cabin cruiser that needs a carb, it has a 350.
I've been through all the aggravation, of completely rebuilding carbs, and still having problems.
I feel the bigger carb may help, it has a fuel starving problem.
I started at the tank, no more anti syphon valve,
Put the correct prop on the boat, Gas is good , motor is rebuilt, fuel pump is good, no restrictions in fuel system, even ran on seperate tank.
I've got it isolated to the carb.
I've had three carbs on it, and none were professionally rebuilt, and not known to be perfect.
Tha problem is the boat is in a mooring and far away and is really kicking my a...
Has anyone had any luck with a 750 Q-jet on a 350.
I have several new. Thanks, BBB

Re: Q jet question, 454 750cfm on 350
 

From my very limited experience with qjets, i think one setup for a 454 would actually be too lean for a 350. It can be made to work, of course by rejetting it, new metering rods, etc.

Re: Q jet question, 454 750cfm on 350
 

I would think that that would be the other way around.
BBB

Re: Q jet question, 454 750cfm on 350
 

Thats what I thought too, when I looked into recalibrating a 350 qjet for a 406 I remember :confused: that the parts that were recommended had less openings? From looking at the link below, I just get more confused.

It got way over my head so I said screw it and bought a 750 double-pumper. I have a qjet for a 350 mag collecting dust if you want to try it. I could send it to you as long as you send it back. PM me if you are interested. Not looking for any money here, or could sell it to you. I'd rather have it back though. It ran fine before the motor shattered a valve seat. :eek:

This link may help, or just give you a headache like it did to me, hence the holley;

http://www.boated.com/topic.asp?TOPIC_ID=63861

towards the bottom I copied this;

Here are the OEM data points I used, followed by the way I set up the theoretical Q-jet:

Merc 305
Primay jet= .068, Primary rod=.042, delta=.027
Secondary rod=.0667 (CL-L tip)
Total flow area @ WOT=.027988 sq. in.
%Primary flow area=22.9
%Secondary flow area=77.1

Merc 5.7L
Primay jet= .069, Primary rod=.041, delta=.027
Secondary rod=.0667 (CL-L tip)
Total flow area @ WOT=.027986sq. in.
%Primary=22.9
%Secondary=77.1

Merc 350 Mag
Primay jet= .068, Primary rod=.036, delta=.032
Secondary rod=.0567 (CH-S tip)
Total flow area @ WOT=.029936 sq. in.
%Primary=21.4
%Secondary=78.6

'73-'74 Pontiac 455 (Firebird SD-Manual)
800 CFM Q-jet
Primay jet= .074, Primary rod=.044 (44B), delta=.030
Secondary rod=.0393 (DM–S tip)
Total flow area @ WOT=.033776 sq. in.
%Primary=22.32
%Secondary=77.68

Charlie’s Theoretical Q-jet:
I estimate that you need a total WOT flow area of 0.032 sq. in. This was estimated by interpolating from the Merc 305 (predicted .03229), the Merc 350 Mag (predicted .03225) and the Pontiac 455 (predicted .03190).

I next set the secondary flow at WOT to be 78.5% of total WOT flow, which is the way the 350Mag was set up. Thus, secondary flow area =.02512 sq. in. This means the CV rods ( .02422 sq. in.) or the DA rods ( .0256 sq. in.)

This means the WOT primary area is 21.5% or .00688 sq in. To get this area at WOT you need the .071 primary jets. I would use a mid range delta of 32 mils or the .039 Primary Rods.

Have fun.

:eek: :eek: :eek:

Re: Q jet question, 454 750cfm on 350
 

I put a 454 quadrajet(marine) on a 350mag with vortec heads,comp xm270 cam,performer intake(standard) when the 4 barrels open it runs like a raped ape,plugs look good,idles without loading up, big improvement in my case not sure about a heavy stock boat

Re: Q jet question, 454 750cfm on 350
 

I think a 750 on a bone stock 350 will be a pig

Re: Q jet question, 454 750cfm on 350
 

Qjets are different beasts than other carbs. the primaries are tiny, which allow for good starting, idle, throttle response and economy. The secondaries are HUGE, and thats what gives them the high rating. The same carb (tuned different) was used on pretty much all of GM's engines, from 305's to 454's. There were some 800CFM versions, but I think they only ended up on the hipo stuff like SS's and 442's, etc.

They are great carb if tuned right, but are way more complicated than holleys and other squarebores.

Re: Q jet question, 454 750cfm on 350
 

I currently have a Q-Jet from my 350 MAG disassembled. I could get the jet and metering rod numbers for you. Let me know.

Re: Q jet question, 454 750cfm on 350
 

That might help me. I've seen from 69 jets to 74 in stock SB applications. Thanks, Bill

Re: Q jet question, 454 750cfm on 350
 

Sorry I didn't get back sooner. Was sick all weekend.

Primary jets are 69
Secondary jets have no markings, non-removeable.

Primary metering rods are 33/55
Secondary rods are CH

Hope this helps