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02-28-2018, 12:33 AM | #1 |
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Q-Jet from a 350 on a 454
I have a leaky, slobbering Q-jet on my 74-454 TH400 and found a used QJ in better condition that came off a 74-350 PU. Can I run it on my 454 without any major issues? I won't know if it's as good as it looks until I get it bolted on, but aside from jetting would there be any reason not to use it?
It's from the 4MV family with the divorced choke and looks about the same from what I can tell. Mine is # 17057229 The replacement is # 7045213 Thanks! |
02-28-2018, 12:43 AM | #2 |
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Re: Q-Jet from a 350 on a 454
Should work.
To check if the rods and jets are the same, look in here for your carb numbers to compare them. http://www.digitalcorvettes.com/foru...ad.php?t=88376 |
02-28-2018, 12:52 AM | #3 |
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Re: Q-Jet from a 350 on a 454
Typically a big block Q-jet has the fuel fitting on the front whereas a small block Q-jet has the fuel fitting on the side. If you are not using the original steel fuel line, that won't matter.
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02-28-2018, 01:03 AM | #4 |
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Re: Q-Jet from a 350 on a 454
Great! Thanks for the info.
The inlet is up front pointed to the right side just like the one I have now. I'm not concerned about the fuel line. Someone cut it to add an inline filter so it's pretty much all hose from the thermostat housing on up. |
02-28-2018, 11:24 PM | #5 |
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Re: Q-Jet from a 350 on a 454
My 454 has been breathing through the Q-jet off my dad's '86 350 for at least 5 years now, which is probably about 25,000 miles - no problems yet. If it runs right and your plugs look okay, I wouldn't doubt it.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
03-01-2018, 12:47 AM | #6 | |
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Re: Q-Jet from a 350 on a 454
Quote:
How many miles do you have on the motor? I got mine last year with 103K miles and the PO said he just bought the QJet from a carb guy who built it for that motor. Maybe so but it looks like it's been opened more times than a bathroom door and I have no idea how it's jetted. All I know is that it won't hold steady idle, floods with the slightest tap of the peddle and wets the plugs instantly. I've tried adjusting what I can from the outside and it either runs terribly rich or not at all. I thought about ordering a kit, float and some 64 primary jets but the QJ from the 350 looks much better which is why I thought I'd ask. If I have to rebuild one I at least have the option of going with the better carb. Many thanks! |
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03-02-2018, 09:37 PM | #7 |
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Re: Q-Jet from a 350 on a 454
The carb is on and it runs great - half of it anyway. All butterflies open fully at WOT but the secondaries aren't coming in. I'm hoping a tweak of the spring is all that will be needed for the back air doors to open correctly, but at least I can walk around it now without my eyes watering.
I don't have any history on the carb so I don't know what might have been done or why but it's already better than that leaky bucket I had on it. Idle is smoother than it's been since I've owned the truck and acceleration is smooth with no off-idle flat spots, but it does pick up an intermittent stutter at around 1800 that sounds spark related. The plugs have been swimming in fuel for a very long time so that will be next on the list. I really appreciate the info and help from you and everyone else on this board. It's been years since I did any work on these old motors and some of what I remember is still a little fuzzy. Thanks again. Last edited by BigRandy; 03-02-2018 at 10:02 PM. Reason: Spelling SNAFU |
03-03-2018, 02:09 AM | #8 | |
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Re: Q-Jet from a 350 on a 454
Quote:
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
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03-03-2018, 02:58 AM | #9 | |
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Re: Q-Jet from a 350 on a 454
Quote:
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03-03-2018, 08:49 AM | #10 | |
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Re: Q-Jet from a 350 on a 454
Quote:
Not too long after I messed with the secondaries last year I towed a Jeep CJ7 to St. Louis and my first tank (doing 65 with the cruise set part of the way) was 7.3, and that's the worst I've ever seen the truck get by a decent margin which told me that I'd got the secondaries a little too loose. But anyway, all that is to say, a big block can drink a lot of gas and be perfectly happy, but it's possible to get 10 or a bit better in the right conditions.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
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03-03-2018, 10:40 AM | #11 |
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Re: Q-Jet from a 350 on a 454
Have you tweaked your apt (adjustable part throttle) yet Alex?
Could be up to 3 mpg extra there. You should come live up here where gas costs double what you pay! |
03-03-2018, 12:46 PM | #12 |
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Re: Q-Jet from a 350 on a 454
Uhhh - considering I don't know what it is, probably not! Do tell?
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
03-03-2018, 04:26 PM | #13 |
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Re: Q-Jet from a 350 on a 454
There’s some info in here.
I usually start at 1 1/2 turns from bottom and adjust up from there. The one in the baseplate (older carbs) is a bit more fun to access. http://www.crankshaftcoalition.com/w...4MV_carburetor |
03-03-2018, 06:46 PM | #14 |
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Re: Q-Jet from a 350 on a 454
Is that the doohickey that controls when the secondaries start to open? Mine does roughly 3000 RPM at 60 so would it be correct to assume the back barrels might be coming in? Can't say that I have ever touched one before.
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03-03-2018, 07:51 PM | #15 |
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Re: Q-Jet from a 350 on a 454
The only doohickey that controls them is your right foot.
The apt and secondary air door are separate parts of your carb. At cruise the secondaries should be totally shut. The bottom throttle plate and top air door. Check your spare carb to see how it works. Stroke the throttle and watch how far open the primary throttle opens before the secondary throttle plate starts to open. The secondary air door (top) is held shut by spring pressure. It only opens when there’s enough demand. Under high load conditions like full throttle or almost full throttle pulling a heavy load up a hill. |
03-03-2018, 08:23 PM | #16 |
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Re: Q-Jet from a 350 on a 454
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03-03-2018, 10:10 PM | #17 | |
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Re: Q-Jet from a 350 on a 454
Quote:
The secondary throttle plates on a Qjet open mechanically at a certain point relative to the primaries, but the air valves (top flaps) are connected to the secondary metering rods so if those flaps don't get pulled open by engine vacuum, very little air and fuel goes in and it's still primarily running on just the 2 smaller barrels. They can be adjusted to be so stiff they almost never open, or that they flop open at the slightest touch and pour all the fuel to 'er.
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Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue. |
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03-03-2018, 10:32 PM | #18 |
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Re: Q-Jet from a 350 on a 454
Everything you ever wanted to know about a Quadrajet, by the guy who probably knows more than anyone:
https://www.amazon.com/Rebuild-Modif.../dp/1932494189
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03-05-2018, 11:06 PM | #19 | |
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Re: Q-Jet from a 350 on a 454
Quote:
agree, he literally wrote the book on Quadrajets. I called hs office number at Cliffs Performance two years ago to get a quote for setting up a 4MV for a 408 BBC, 268H Comp Cam, Weiand high rise spreadbore, 350THM...He answers the phone and after finding out this motor was in a 72 C/10 he talked about how much he missed his old Chevy truck for almost an hour! He indicated that he has a Camaro with the same punched out 69 L34 396 motor he'd love to tune my carb on! He said throw it in a box with the motor specs and mail it, one month and $345 dollars later it's back and is still running like a top, never had to touch it other than idle speed. He indicated that whoever installed the primary shaft bushings got them in crooked which explained why the new rebuilt Summit stage two Quadrajet never ran right on my 350 truck. There is a wealth of information on a Corvette site that Geezer shares from time to time, it is as good as it gets for setting up and understanding why and what to do and more importantly, what not to do. I'll see if I can dig it up...
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03-05-2018, 11:12 PM | #20 |
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Re: Q-Jet from a 350 on a 454
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