Q-Jet from a 350 on a 454

[BigRandy]

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!

[geezer#99]

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

[Captainfab]

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.

[BigRandy]

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.

[Alex V.]

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.

[BigRandy]

Originally Posted by Alex V.

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.

That's good to hear.

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!

[BigRandy]

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.

[Alex V.]

Originally Posted by BigRandy

That's good to hear.

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!

129K on the engine. This carb. or the truck's original one have been, at times, capable of a bit over 12 MPG and would regularly get 9.5-10 towing up to 9,000 lbs. until a few months ago when I tweaked the secondaries and took them a little too far. Need to get back under there and tighten them up a little.

[BigRandy]

Originally Posted by Alex V.

129K on the engine. This carb. or the truck's original one have been, at times, capable of a bit over 12 MPG and would regularly get 9.5-10 towing up to 9,000 lbs. until a few months ago when I tweaked the secondaries and took them a little too far. Need to get back under there and tighten them up a little.

Sounds encouraging. I'd be delighted to see 10. It's been getting 5 in the city and the best ever was 7.5 on the highway. It's got 373's but nobody I know of gets that kind of mileage

[Alex V.]

Originally Posted by BigRandy

Sounds encouraging. I'd be delighted to see 10. It's been getting 5 in the city and the best ever was 7.5 on the highway. It's got 373's but nobody I know of gets that kind of mileage

Most don't, so don't get your hopes up too high. But I've spent 55,000 mi. in this truck and have done the math too many times for it to be wrong. Yesterday I'd let it get so low the needle had stopped moving - I pumped $30 into it (didn't pay specific attention but @ no less than $2.55 a gallon) and drove 92 miles, half that in a heck of a hurry doing 70 MPH (3,200 RPM) and passing quite a bit, the other half at a steady 60-65 like I usually do, and when I pulled back in the needle was back below 1/4 but not closer to the 1/8 mark yet. So if I burned 9 of those 11-something gallons I pumped, I still got exactly 10 MPG. That's with syn blend 10w30 in the engine and Amsoil 75w110 in the rear end, and stock everything with the TH400, 4.10's, and 32" high 235/85r16 tires.

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.

[geezer#99]

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!

[Alex V.]

Uhhh - considering I don't know what it is, probably not! Do tell?

[geezer#99]

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

[BigRandy]

Originally Posted by geezer#99

Have you tweaked your apt (adjustable part throttle) yet Alex?
Could be up to 3 mpg extra there.

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.

[geezer#99]

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.

[BigRandy]

Originally Posted by geezer#99

The only doohickey that controls them is your right foot.

That's a size 13EEE doohickey that I'm quite familiar with and probably use more than I should. I'll go have a look at the other carb.

[Alex V.]

Originally Posted by BigRandy

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.

Very interesting about APT - I need to rebuild my truck's original carb. at some point and will go through all that before it goes back on. I've not yet examined jetting differences between the K20's carb. and my original but would like to when I have them both off.

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.

[truckster]

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

[Ironangel]

Originally Posted by truckster

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

Have to
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...

[geezer#99]

http://www.digitalcorvettes.com/foru...ad.php?t=88376

I’ll post it again.