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post #1 of (permalink) Old 04-25-2000, 11:54 PM
BigMan
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TR Carb tuning?

Hey team rush. Any tips on tunning a quadrajet carb on a cevy small block. I'v always delt with hollies and have never touched one of these but I need to tomarrow morning.

Al
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post #2 of (permalink) Old 04-26-2000, 12:18 AM
 
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Re: TR Carb tuning?

The best way to tune a Q-jet is to take it off, and screw on a Holley...
(OK, so it wasn't much of a joke...)

There aren't any really easy ways to tune a Q-jet I know of...
It's all modify, test, modify again, test again, ECT. ECT. ECT....

Use a vacuum gauge to set the idle mixture just like every other carb, but after that, you are pretty much at the mercy of new needles, 'jets'
and a very precision drill set.

I'm not much of a Q-jet fan. I don't like anything about them.
I'm told they do off camber better than a Holley, but a Holley has it all over them in every other respect...

Sorry I couldn't help.
Aaron.


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post #3 of (permalink) Old 04-26-2000, 12:24 AM
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Re: TR Carb tuning?

I'm not appreciating them very much at this point either. I'm a damm Diesel mechanic, I hate carbs! I have always had good luck with the hollies but it is not possible for me to replace it right now. Tooo many other mods going on at one time! Thanks anyways.
DO you at least know the basic starting positions for the mixture screws and stuff. This thing is so far out that it barly runs.
Al
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post #4 of (permalink) Old 04-26-2000, 10:56 AM
 
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Re: TR Carb tuning?

Same as any other carb...
VERY LIGHTLY bottom out the idle mixture screws, then back them off 1-1/2 turns.
Hook a vacuum gauge to manifold vacuum (not the PCV line).
Adjust the screws until you get the highest vacuum reading.

The screws MUST turn together. Both in or both out together.
I normally do 1/4 turn at a time, and when you get close, blip the throttle in between adjustments.

Be very careful to take it easy on the adjustment needles. They are fragile.

The screws must be turned out the same amount unless you have an exhaust gas analyser and completely separated dual exhaust.
Those little rich/lean indicators are nice for this if you have headers with O2 sensor nuts already welded in.
---------------------------------

CLOSELY check the accelerator pump on any Q-jet.
They used the dumbest possible method to incorporate an accelerator pump, so of course it fails on a regular basis.
Usually leaking fuel out when compressed, and air back into the chamber when you let up on the pump...

This leakage of fuel out is a fire hazard.
The air leakage into the chamber means air is taking up space fuel should.
Air compresses.
So when you hit the throttle, the air compresses, and makes the pump shot late.
With the air taking up room that fuel should, the pump shot is short, weak, and full of bubbles.
More of your shot is leaking out of the chamber past the seal, furthering starving the pump shot...

Stupid and unsafe design.

Causes huge off the line stumble.
That's the 'Q-Jet Cutout'... Stomp on it, and it seems to fall on it's face before it takes off...
--------------------------------------

Everything else is metering rods, bending linkage, lightning weights and drilling holes, all too complicated to describe without pictures.
It's more of an art than a science when you are working with carbs anyway...
Carter AFB's are the worst, then comes Q-Jets, after than comes the knock-offs, like Edelbrock and Webber....
Holley is the most tuneable, but with all the adjustments, it's easy for beginners to go the wrong way...
Inglese (what Webber copied from) is probably the second most tuneable, but it has the same problem with beginners that Holley does.
Predator is probably the most end user friendly carb on the market that actually works as advertised. It only has three adjustments...

Holley just kicked Barry Grant's ass again with the release of the new super tuneable line...
They really are something, but I'm sure with 12 to 15 new adjustments on them, even more people will be playing with things they don't understand....
I just wish Holley would do the Grant thing and make interchangeable venturis for the main body, and I wish they had Annular discharge nozzles on all performance carbs...

I'm rattling on again....
----------------------------------

Good luck, and happy hunting!
Aaron.

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post #5 of (permalink) Old 04-26-2000, 02:15 PM
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Re: TR Carb tuning?

Obviously, TR is not 'with-it' in the off-road 'world'. As carburetors go, clearly, the Q-jet is the best for off-camber situations. FI is the only better option. I would agree with the TR carb assessment for on-road and racing, but clearly not for off-road.

bobH
post #6 of (permalink) Old 04-27-2000, 11:21 AM
 
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Re: TR Carb tuning?

My assessment was that I had been told that Q-Jet did well on off camber situations.
Did you miss that?

Holley will kick any Q-Jet in the butt on everything else. I can get a Holley to do just fine up to about 35 degrees, but it sure won't do it out of the box like a Q-Jet will.

Everything else about a Q-Jet, from the lousy integral filter to the ill designed linkage, is crap.
What were they thinking when they stole bad designs from stromburg and carter to build that piece of crap...?...
(That makes carter an even bigger piece of crap...)

Vic Edelbrock fixed a lot of problems on the AFB, and called the redesign a "Performer"...
Good joke there!!
Just because it doesn't leak all the time, and the secondary throttle bores are actually of some use doesn't mean it works...

Holley is still the hottest ticket out there, and the most tuneable.
They just don't take a twist very well with out doing some tune up work first...

I've been playing with a new Holley with all the tuning extras they just built in, and one of the new Barry Grant Demon carbs...
Holley kicks Grant a$$ again!!
I just wish Holley would do the interchangeable venturi bores like Grant does...
That is the ONLY thing the new Holley is missing to make it perfectly tuneable...

Re-read the first post, I didn't undercut Q-jet's ability to do off camber...

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post #7 of (permalink) Old 04-27-2000, 01:07 PM
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Re: TR Carb tuning?

Yeah, I 'got' it. I still say, it's just different opinions. Your background, your requirements, (I assume), are completely different than my off-road reguirements. I have no problem seeing how 'racer-types', or 'fiddlers', would love a Holley. It doesn't take a genius to observe the abundance of Holley support available at places like speed shops, local parts stores, carburetor shops, and through Summit/Jegs, etc. But, as many of us know, and apparently including you, the Holley bowl and float design is a piece of crap for off-road. I admit, there are things/tricks that can be done to make it more-or-less 'livable', but a person still ends up stuck with the horrible (for off-road) float and bowl design. So, to a person from a racing background, I might emphasize that MY area of interest, engine-wise, is how well it runs from about 500-600 rpm, up to about 1200 rpm. I really don't giva hoot how it runs at 1500 rpm, or 2000 rpm, or 2500 rpm, etc. The vehicle MUST have impeccable manners at idle, and near-idle. There must be no stalling (usually from fuel-slosh, or floats that don't work well with severe vibration, or off-camber). And, obviously, with the vehicle standing nearly on the rear crossmember, the engine must still do 500-1200 rpm, with some throttle applied, REAL well, and WITH NO STALLING. This is obviously where the Q-jet realy shines.
Now, I'm no genius... I never discovered this by sitting down and thinking this through. I discovered this the same as a lot of other Jeep folks have. It was totally by accident. It was one of those late nights, before leaving for a 'rock-trip', and the engine was still apart. I got the manifold on, but I couldn't find enough parts in the garage to make the old Holley work. So, I eyed an old Q-jet, cleaned it up, found an adapter to mate it with the manifold, and did all of the other 'get-ready, make-it-run' things, and took off (towed, not driving). The result was amazing..... The Jeep had never run so well, at off-road conditions.
This was all over 15 years ago. Since that time, I have discovered that the majority of my Jeep acquaintences have likewise discovered the off-road manners of the Q-jet.
'Course, I'm sure you could point out the downside of the Q-jet. But, for those of us that use the Jeep as a 'toy', and almost exclusively off-road, none of the downside problems mean very much. It's kind of like the problems you have observed with the HEI.... an ignition that 'falls-off' at some higher rpm, means nothing to us. All it has to do is work ok at low rpm.
Please don't consider this the start of a debate. It's just a matter of different opinions, and different points-of-view.

bobH
post #8 of (permalink) Old 04-27-2000, 08:53 PM
 
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Re: TR Carb tuning?

To make a Holley go off road at the RPM's you described, it takes a change from those huge floats in the dual line, center hung float bowls that everyone is so fond of....
They just can't give up that chrome fuel line!!

When you go to the smaller side pivot float bowls, with the more or less round floats, and the off road needle and seats, lower the fuel level about 1/16", and lower the fuel pressure to about 4 or 4.5 Lbs, they will do pretty good...
I also use short jet extensions on both ends, and vent tube extensions out the top...

Most guys that go off road listen to the so called 'Speed Shop' jokers, and wind up with a power valve that opens just as soon as they lug it down low... Bad situation...
Jeep already in a bind, and suddenly 30 to 40% of the total fuel that carb can flow is dumped into the intake....

Off roaders need at least two In.Hg difference, and a lot of them need up to five In.Hg difference.
The speed shop will put them as close as one half inch away, and then it's flood time...

I always could make a Q-Jet run OK, but nothing like a Holley...
Q-Jets take constant maintenance also, if you are going to keep them working correctly, and not having them leak...
I don't think I've ever seen a Q-Jet that was six months old that wasn't fuel stained from leaks, no matter who built it.
---------------------------------

I'm going to say it again, just for the record...
I am told Q-Jet's do really well for off camber situations.
I have seen several of them in off road situations that would scare me.

Most people in the know agree, if you can't afford fuel injection, use a Q-Jet.
I don't turn my jeep upside down (unless drunk), so I don't have much call for something that will run on it's side...

I have been intrigued with the pictures of the rock crawling guys though.... [img]/wwwthreads_images/icons/wink.gif[/img]
(maybe next year...)

See you all, Aaron.


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post #9 of (permalink) Old 04-28-2000, 01:33 AM
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Re: TR Carb tuning?

Hey BIGMAN, where did your web page go?! I have it linked on my site, but when someone clicks on it, they just get "Not Found". Send me an e-mail with an updated http. BTW Holley's rule! But I'm an ex drag racer, so I'm a little biased

Mike H.
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post #10 of (permalink) Old 04-28-2000, 10:54 AM
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Re: TR Carb tuning?

Your comment about the power circuit reminds me of a story. Seems like there must be a buncha 'jeep-type' folks that overlook a couple of characteristics of carbs, and either through ignorance, or from 'speed-shop' influences, they run with just about the opposite of what is needed for some off-road conditions. Obviously, you are correct about what happens to some folks, with the power-valve. I agree, if you are going to have a power circuit, it ought to be kept away (hg-wise) from where you are running. To compound the problem, the 'rock'-trip that I like to do, is at 6000-to-6500-ft elevation. This further complicates the power-circuit problem, because I really don't want the power-circuit dumping fuel right at the manifold pressure that the engine is trying to 'cruise' at.
The 'story'.... (This will likely convince some of you that I am 'nuts', as if you didn't already know.) Jeeps of the V6-era came with a Roch 2-bbl. I think it is called a '2-GC' (not sure). Stock, these vehicles have a reputation for running pretty poor in high-elevation, off-road, rocky situations. With stock gearing, it is common to spend an entire day slipping the clutch, loading up, stalling the engine, cussing, restarting, watch the exhaust belch black smoke, try to get going again, and repeat the process over/over. I've lived with a few of these vehicles, and learned to really not like the situation. So, on the last of these jeeps that I had, I started experimenting. I completely blocked off the power circuit. I found that the thread for the valve was standard, and I machined a fastener to fit. (BTW, the power valve lays in the bottom of the bowl, flooded in fuel, and engine vacuum is needed to close it off. So, every time the engine is started (no vacuum), there is a little belch of fuel, until the engine starts and creates vacuum.) Then, since these vehicles require so much bouncing in the rocks (wrong gearing), and the throttle-foot is mostly out-of-control, I reasoned that I didn't want the accelerator pump to work very good, if-at-all. So, I severly hampered the action of the pump, by finding an old leather pump in my junk pile, that was shrunk, and had a section that was turned-over. I nipped a little of the leather out, for good measure. It made a good 'bog', just like I hoped for. And, finally, without the benefit of any real 'science' or logic, I calculated how much smaller the main jets would have to be for running at 6000-ft. I think I remember it was something like .002 or .003-inch smaller. No testing involved, I just put in the smaller mains, and ran it. The results were as you might expect. It ran pretty awful at low elevation, and on the street. But, on the 6000-ft jeep trip, it actually worked pretty well.
So, what's the point? Aaron is correct. One needs less fuel, less power circuit, for some of the low-speed, off-road situations. (sigh... the benefits of FI sometimes look pretty good.)

bobH
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