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#1434905 Tue Dec 28 2021 05:13 AM
Joined: Nov 2008
Posts: 48
K
'Bolter
I’m thinking I should have gone with a 650CFM instead of 750 and possibly it’s simply too much carb for a stock rebuild with headers, sensible mufflers and a 90’s era automatic overdrive.

I’ve tried a dozen variations of jets and metering needles included in the 1407 tuning kit, first going lean and then going towards rich and it is still a dog off the line and gets the same 9mpg since we got it in 1981.



[img]https://www.instagram.com/p/CNk8dqjArvB/?utm_medium=copy_link[/img]



I’m not expecting anyone tune my carburetor by description over the internet but maybe someone has experience with a similar combination and might have some ideas.

I’m ready to order a 650 edelbrock or even rebuild the old Quadrajet but unfortunately dad modified it to idle on the cam he was running before I rebuilt the engine and went back to the stock lift.

Thanks in advance for any helpful comments!


Sometimes you got to leave 'em like you found 'em.
Joined: Feb 2004
Posts: 24,502
H
Kettle Custodian (pot stirrer)
One well-built Qjet is worth more than a truckload of Carter AFB "clones". Edelbrocks in general are sort of a "one size fits none" copy of a Carter, with a bunch of manufacturing shortcuts taken that make them virtually impossible to tune without a flow bench, a pile of replacement parts in various sizes, and a few decades of carburetor building experience. I've only been building carbs and teaching the subject since the late 1960's- - - -and I don't feel qualified to do it.

What's your rear axle ratio? It's possible you're working on the wrong end of the vehicle. A little lower gear ratio to let the engine wind up and get into a better torque range might solve the low end bog, and improve the gas mileage by letting the manifold vacuum stay in the "cruise" area more frequently.
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Joined: Nov 2008
Posts: 48
K
'Bolter
I believe it has the standard 3.73 differential.
But you make a good point. I put 32” BFG all terrains on it so that didn’t help either…


Sometimes you got to leave 'em like you found 'em.
Joined: Feb 2004
Posts: 24,502
H
Kettle Custodian (pot stirrer)
Install a temporary vacuum gauge and tuck it under a windshield wiper so you can watch it while you drive. I think you'll be surprised how often it stays down in the sub-10" of mercury "power" range with the metering rods in the full rich position. Lower gears allow the manifold vacuum to stay high enough to keep the metering rods pulled down in the leaner ranges.

With an OD trans, performance and fuel economy would probably be better with a final drive ratio somewhere in the low 4's.
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Joined: Nov 2008
Posts: 48
K
'Bolter
Windshield wipers? This beast has one vacuum powered wiper motor and the arm stays in the glove box so it won’t get lost!
Heheh!
I’ll just watch it from the access door in the firewall!


Low gear on the TH 400 was 2.48
Low gear on the TH 700r4 is 3.06 so I “think” that probably helps to correct for the oversized tires. These gear ratios make my head spin so my thinking may be 180 degrees off!

Your initial advice about resurrecting the old Quadrajet is probably the first thing I’ll try. I really miss the distinctive sound of those secondaries kicking in!
The mileage is really not an issue for me If the performance level goes back up.
I’ll revisit the differential ratio situation after that issue is straightened out. I’m basically using it as a farm truck since I rarely take it camping any more. I usually pull a ‘47 Tour-ette teardrop or a 1965 Mobile Scout 13’ canned ham, both of which are easy to pull although the canned ham has more wind resistance.

I really do appreciate you sending me in the right direction Jerry, thanks much!


Sometimes you got to leave 'em like you found 'em.
Joined: Feb 2004
Posts: 24,502
H
Kettle Custodian (pot stirrer)
Those huge secondary barrels do have a distinctive sound, but if the secondary air valve torsion spring is adjusted correctly you shouldn't be able to feel a difference in acceleration as the transfer from primary to secondary air flow happens. There should just be a smooth change in RPM, but with that big block it should plant your spine firmly into the seat back!

That 454 should be able to pass anything but a gas station! Yes, the 1st. gear ratio on the 700R4 is lower, but the tall tires probably still need a gear somewhere in the low 4's. The rock crawler guys love those 4.56's and 5.13's to go with their big wide, tall tires.
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Joined: Feb 2016
Posts: 679
7
'Bolter
My 1984 C20 crew cab had the factory 454 Th400 3.73 rear end. The Q-jet was rated at 800 CFM for that application. I got a consistent 10 MPG no matter what. I would put a good Q-Jet together, get rid of the headers and tune it as close as you can to stock.


1957 Chevrolet 5700 LCF 283 SM420 single speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto
Joined: Nov 2008
Posts: 48
K
'Bolter
It’s always been scary fast and a lead foot runs in both sides of my family, I always did my best to stay out of the secondaries but dad had it set up his own way, where you pretty much had to stomp on it to get them to kick in.

The only thing I hate to lose with that Edelbrock is the manual choke. I guess I’m a control freak and use the choke sparingly.
I was thinking eventually going with 4.11 gears since “the brakes are good and the tires are fair”… but how much would 4.56 gearing take off the top end?


Sometimes you got to leave 'em like you found 'em.
Joined: Nov 2008
Posts: 48
K
'Bolter
Originally Posted by 78buckshot
My 1984 C20 crew cab had the factory 454 Th400 3.73 rear end. The Q-jet was rated at 800 CFM for that application. I got a consistent 10 MPG no matter what. I would put a good Q-Jet together, get rid of the headers and tune it as close as you can to stock.


I’m big on factory original too with the exception of this Frankenstein dad built. We used to have many heated debates about stock vs original. My argument being that you can wring more HP out of it but at the cost of longevity, expense and simplicity.

That said, I’m curious as to what the disadvantages of headers are? I always believed the common assumption that improvements to exhaust flow was a good thing for both economy and performance. I hope I don’t sound argumentative or unappreciative, I’m just curious about the advantage of going back to cast iron?


Sometimes you got to leave 'em like you found 'em.
Joined: Feb 2016
Posts: 679
7
'Bolter
You can run the Q-jet with a manual choke, they used to make a kit to do just that. Mr. Gasket or some other aftermarket venders had them. Hotrod Lincoln will gently give you his thoughts on headers. Depending on how the internals of your engine have been massaged you may be killing horsepower and economy with the headers.


1957 Chevrolet 5700 LCF 283 SM420 single speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto
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