[David Brown]

You may have a leaking carb, but it also may be too much cam, or combination of both. CompCams lists it as "rough idle" and needing a 2880 stall convertor, with an RPM range of 2300-6500. Might be leaning more to strip than street.

[72LuxuryLeMansLa.]

Originally Posted by scott-462

Most likely, and i've been on cliffs site before. What about modifications to the carb itself, other than just changing gaskets/jets. Modifying the idle circuit? Are there detailed directions on this or possibly youtube videos? Right now i have 8" of vacuum and the power brakes arnt working very well.

His book shows all of this. http://www.amazon.com/Rebuild-Modify...et+Carburetors

[Blued and Painted]

ALL the guys i know running that cam have had to add an electric vacuum pump (not a huge deal) or switch to a hydro-boost set up. 11" of vacuum is border line for running a standard brake booster IMO.

Cliff's book is good. It makes no sense until you have the carb on the bench and identify all the circuits. Air and fuel. The idle tubes are not easy to remove without damaging them. Plan on a few bucks for new ones. Plan on buying some quality U.S. made wire guage drill bits. Instaling the throtle shaft bushings is a pain but it realy improves the operation of the carb. On mine it reduced the opening effort by 50%.

I think Mr. cliff holdes back some trade secrets but its still a good book and will get you in the ball park. It would be impossable to list all the mods for all the carbs and engine combo's.

[Cliff R]

Bushing install is easy once you get the factory staked screws out of the shaft(s). We sell a bushing install kit that makes this process simple, no drill press or holding fixture required, it will bore a perfect hole on center every time so all you have to do is tap in new bronze bushings.

http://www.cliffshighperformance.com/bushings.html

We also sell custom carburetor kits with tuning components based on the carburetor number provided. This takes most of the guesswork out of the equation, and lots of folks have used our book and parts to do their own work with excellent results.

We also provide technical assistance via this Forum and our website. I'm hesitant to help out with calls directly to our shop, especially when you have bought all your parts someplace else, then want me to crawl thru the phone to fix your carb.

I've science out what is needed to build just about any carburetor part number exactly for your application, just takes a phone call to the shop to set one up with what is needed for a good end result.

As far as "trade secrets", there is a lot more that I will put into the first revision of the book. When I first wrote the book, the High Performance Chapter ended up about 80 pages, with the desired book size around 128 pages (nearly as I can remember), but the publishers want 6 or 7 chapters pretty much equal size, so we left out some of the more complicated stuff that would have only been beneficial to racers and folks that really want to get deep into this stuff. What was left is beneficial to 98 percent of the folks wanting to use a Q-jet for high performance use. I'll help fill in the gaps here and there when needed.

The bottom line when it comes to this topic, is that you are NOT going to go faster with anything else at these power levels, and if you want to enjoy excellent efficiency for "normal" driving, the Q-jet can do that as good as of most often better than anything you'll find aftermarket.

Still nothing wrong with going aftermarket, it's a hobby, and to date I've never seen a "rule book" on what you must use to get a good end result. Even with that said, in recent years I've noticed a HUGE decline in the quality of the "shelf" stuff, specifically carburetors and distributors. Plan on spending some pretty decent coin for the better units if you really want a good end result.....IMHO.....Cliff

[scott-462]

Cliff, what about my carb being a remanufactured unit, should i build it or look for another original Qjet? Maybe run the numbers to make sure all parts match??

[PONTIAC DUDE]

I wish someone or some company would reintroduce the Thermoquad carb for use in GM applications.
What ever happened to Cater and this unit?

[Skip Fix]

Ken Demon has reintroduced a Thermoquad but it is only 625 cfm and 750.

http://www.demoncarbs.com/1904.asp

[Cliff R]

"I wish someone or some company would reintroduce the Thermoquad carb for use in GM applications.
What ever happened to Cater and this unit?"

The original Thermoquads were nice units, but they phenolic float bowl was problematic and leaked easily at the bottom seals. There is a nice aftermarket float bowl available, but it is NOT a direct fit for all models, and often requires modifications.

The linkage (choke, secondary, lockout, etc) for the TQ's is also a NIGHTMARE for the builder, it's a bit "hoaky" and complicated, they could have done much better there, IMHO.

The TQ's also came in 1000cfm models, which are highly coveted by many "circles" in this hobby.

The smaller units are basically a Quadrajet with two float bowls, two floats and needle/seat assemblies. Surrounded by a phenolic bowl, the fuel temperatures are cooler, and there is more fuel capacity and fuel inlet volume for high HP applications.

Having one fuel bowl/one needle-seat assembly and one float has always been the downside of the q-jet. The fuel delivery system must be up to par for the power level, or they will give troubles earlier on hard runs than other designs. Once one gets around that deal the q-jet will run right with anything else and we've proved that again and again over the years by direct example.

I've seen the Demon TQ style units, but never to date had one in the shop to do any testing with it. I certainly hope they are better than Holley clones they make, which have been plagued with problems over the years of production......Cliff

[PONTIAC DUDE]

X 2 on the Demon - Holley clones.
I have chased more problems with fewer rebuilds with those carbs then any.

The GM Thermoquad 9800 series carbs had the GM type linkage. Basic swap for the wife's car.

Also read that the TQ primaries are about 50cc bigger and make for a slightly better 4bbl response.

[paul s.]

Originally Posted by scott-462

How well does an out of the box carb work on a mild built 455, lets say a holley for example? It has an XE284 cam. From research it sounds like a Qjet needs lots of work to run well.

Kinda late to the party but I'll throw in my 2 cents...

Generally, the more cam duration and overlap you have for a given displacement and compression ratio, the more you will need a custom built carb. HOW the compression was attained, the TYPE of camshaft, single or dual plane intake will all play into the equation.

As far as quality, any mass produced component will have some issues from time to time due to poor manufacturing processes, but it is troubling to hear some have significantly more issues than others as Dude states. As far as compatibility, a less efficient engine, a high hp engine running low compression, or an engine with a long cam backed by a tight stall or manual trans where the engine is lugged down will like a custom carb.... from my experience.

Also, using a carburetor for an application for which it was not intended will dictate custom calibration.

Cam selection can be huge in those cases.

I've used Jet, Edelbrock, and the occasional Holley with very few issues and no reported issues.

Specifically, for your combination, I don't see a problem as the Holleys are set up to for "performance" applications where low vacuum is often present and a richer idle is necessary.

[CROCIE JR]

Man, I'm just trying to figure out how I got buy all these years without a custom carb. I dont call a Q-jet with a rod and spring change "custom" maybe "calibrated" Basically if you want it balls on, you need a AFR meter. If you want it pretty close, choose 1 of the carbs I posted earlier. 8" of vac. seems short to me with your combo. Try running the vac. adv. to the manifold directly. You probably need 28-36 deg. at idle to make you engine happy. However you want to get there is up to you. Or you could just twist the dist. around till the engine tells you what it likes for base timing. Then make it so. You may have just fixed your carburetor. QF Slayers are at about $300 these days, a good choice on the cheap. I havnt test one yet with AFR.

[scott-462]

I cant recall what my base timing is currently at, but it has an HEI and it idles smoothest/runs best with the distributor turned completely advanced until its hitting the RPM intake.... Havnt tinkered with it in a while but was considering moving all the plug wires one terminal over to allow more adjustment. And yes, i've checked that its correctly wired many times and it is. I put a light on it when i first fired the engine and noted that my initial timing was a bit higher than most peoples.

[Cliff R]

If you find yourself having to run over 30 degrees initial timing to make the engine "happy", the carb is too lean for idle fuel delivery, and you are just "crutching" the problem or don't know how to correctly fix it by running the timing clear off the scale......IMHO

The XE284 camshaft in a 455 with 9.5 to 1 compression will idle just fine with "normal" timing settings and a carburetor that's correctly calibrated for it. Expect right at 11-12" vacuum at idle speed (about 900rpms), with a noticeable "lope". I've tuned plenty of those engines here, and shoot for 10-12 degrees initial timing with no additional timing from the vacuum advance, lots of them show up with MSD distributors that only have a mechanical advance anyhow.

Here's a clip of my own engine where you can here the idle quality using a HR camshaft with over .630" lift and 289/308 seat timing, and high ratio 1.65 rockers on it. 13.5" vacuum at 750rpms with no additional timing from the vacuum advance at idle speed:

http://www.youtube.com/watch?v=6zVdoLR-VzM

......Cliff

[paul s.]

Originally Posted by scott-462

I cant recall what my base timing is currently at, but it has an HEI and it idles smoothest/runs best with the distributor turned completely advanced until its hitting the RPM intake.... Havnt tinkered with it in a while but was considering moving all the plug wires one terminal over to allow more adjustment. And yes, i've checked that its correctly wired many times and it is. I put a light on it when i first fired the engine and noted that my initial timing was a bit higher than most peoples.

Get a timing light on it and find out for sure how much timing you have.... might not be as much as you think. Try to get around 16/36 as some ballpark timing figures. Those cams will idle decently. The idle and vacuum figures mentioned previously can be attained with a 800 cfm Edelbrock carb leaned down a couple of metering rod sizes with the timing in the range previously mentioned... I JUST did that for a customer a short while ago.... and he was running PORTED vacuum. Your tune up is there.... you just need to take your time and find it. We're talking a screw driver, a 9/16 wrench and a half hour's time.

BTW... To get enough travel on the HEI, you may need to trim the vacuum port on the canister slightly.

[Skip Fix]

Get a Holley 4781 850 DP buy or borrow a wide band. Bet a step leaner jetting will get you close. Bet the fuel curve is pretty close also. Maybe swap the 50cc pump to the front and the 30 to the rear.

My 455 pump gas has a close cam but an UD 239/247 @0.050 and pulls 14" in park and 10" in drive(tight convertor pulls it down. 38 degrees timing total, direct vacuum, fairly fast mechanical advance.

[scott-462]

I'm under the assumption this stock remanufactured Qjet is going to need more than a jet change to work correctly? Its going to be atleast a few days until i can put a timing light on it again. And i noticed someone said going leaner would probably get me close.. I figured with this larger cube higher horse motor i would need to jet richer??

[paul s.]

A larger engine under a given size carburetor INCREASES velocity through the carburetor... pulling more fuel (VELOCITY--- not engine vacuum--- draws fuel. Contrary to what some people think).

More overlap and the addition of a single plane intake (for example) causes more "interference" in the carb signal as there will be REVERSE pulses that draw fuel and disrupts a clean metering signal.

How YOUR carb responds to your cam depends on how your engine is built; cam phasing, ACTUAL compression ratio, squish/quench, type of manifold, spacers/adapters (lol), ignition timing, etc.

Mild 461's with E-heads (10.75:1), mild flat tappet, and 750 Q-jets can exhibit low to mid 4's in bsfc and put out over 550hp. Not necessarily the best for hp output, but it shows a trend.

So I can say with the utmost certainty: It depends. Without getting into the whole wideband thing (which IS useful)... lean it down a couple of steps and drive it. Do A LOT of gentle "tip-in" driving and FEEL how it responds. See how it responds to progressively more throttle opening. Keep going leaner. Sooner or later it will want to start surging, at that point you can richen it back up. An adjustable part throttle (APT) modification will come into play at this point so you can really fine tune it but jetting and metering rods will get you pretty darn close, again, depending on the build ... been that way for 40 some years.

After you have the drivability on the front 2 barrels figured out, then I would look at the secondaries by going FULL WIDE OPEN THROTTLE! THE FUN STUFF!

I believe you will be good to go with a little more base timing and jetting/metering rods as long as your carb is in good condition.

On a side note: Low compression and long cams need more tuning because the carbs weren't calibrated for that so the worse the condition, the more tuning and then the need for recalibrated circuits. That's why Holleys became so popular back in the day. That and the fact a lot of guys didn't know how to set the tension on the secondary air valves!

It's a shame; so many Q-jets were thrown away because of that.

[Cliff R]

Factory Q-jets, even emission calibrated units aren't going to be off that much for fuel delivery past idle. A well thought out "high performance" engine will often enjoy LESS fuel as it is more efficient, not MORE fuel to support the additional power level. Most folks don't understand this concept.

Where factory carburetors fall WAY short in fuel delivery is idle and at often at very light throttle openings (cruise). As soon as you give them more fuel thru the idle system, they work perfectly on pretty heavily cammed engines having low or poor signal to the carburetor at idle speed.

Modifications to the idle system also help off idle, transition to the main system, and feed some fuel to the main system at low throttle openings. Aftermarket carburetors are EXTREMELY generous in this regard, and why they work better than factory carburetors right out of the box.

If you want to use a Q-jet for high performance use, it is going to REQUIRE some minor modifications to the idle system for this particular application. I've built hundreds of them for very similar engine combo's, making at or even less than 10" vacuum at "normal" idle speed.

You will find that once the carb is up to par for what you are doing, the engine will not want or need a lot of timing at idle speed to be "happy". Custom tuning these engines typically takes modifications to both items, carb and distributor, to get the very best performance from the engine in all areas.

It's one of the most interesting parts of this hobby, and not beyond the scope of capabilities of most folks if they are armed with useable/accurate information, and access to the parts needed to get a good end result......Cliff

[paul s.]

Yes, a high performance engine SHOULD be more efficient, but many are not. Especially down low where we are doing most of our driving. That is evidenced by the need for more fuel.

That gets back to camming, compression, cylinder head port design, intake manifold porting, etc... for the guys that may not know.

[Tom Vaught]

This article is actually a pretty good one on Booster function and design.

http://www.popularhotrodding.com/tec...boosters_tech/

Take a piece of tubing and add some colored water to it and leave both ends of the tubing open and you have no movement of the fluid in the tubing. (same deal works with a soda straw and a coke in a paper cup).

If you create a pressure differential across a straw, a piece of tubing, or a booster you will get a signal and fluid movement.

For years Holley, Carter, and Rochester Engineers assumed that a typical large High Performance engine (be it aircraft or vehicle) could generate about 1.5" of mercury pressure differential at Wide Open Throttle. That works out to 20.4" of water test pressure on a flow bench. So that 20.4" H20 test pressure was used to evaluate carb and booster design on their products.

Without a Test Pressure (or some would call it a vacuum) you will not have any flow or velocity of a specific molecule of air.

Big Engines, Small Engines, and Lawn Mowers all create a Pressure Differential across a Carb Venturi. Installing a "Booster" will BOOST that signal to improve the flow between the carb fuel bowl and the intake manifold. The Booster can also atomize the fuel better to mix with the appropriate air mass.

That being said, there is always liquid fuel in the intake manifold (even on EFI applications). Adding a heat source possible keeps more fuel in suspension with the air during some modes but there is always liquid fuel.

The one time when the fuel disappears from the manifold is when you go from a cruise condition to transition to another flow point. The carb fuel lags behind air mass flow so the engine would run lean without an enrichment device to replace the fuel sucked from the walls of the manifold and also add extra fuel for the additional air mass flow. So you need an accelerator pump and squirter for that deal. That was why it was designed. EFI applications add extra fuel when they sense a pedal change on modern efi engines.

So back to the original comment about velocity vs vacuum. Need to look at the big picture of what is happening vs a specific term in explaining how stuff works.

Tom Vaught