Hi all,
I'm rebuilding Rochester qjet to my -70 350SS 4-speed car. I try to get quite near of original setup for jets and rods. Now the problem is secondary hanger: What was originally used on 7040201 (or ..203 ...205) carbs ?
Is is same S than in automatic tranny cars ??

Primaries are now 76 and 44, secondaries are BA (or CH, they looks to be same) but hanger information is missing....?
br;
/jarit

http://peterpam.customer.netspace.net.au/hanger.jpg


The BA rod will be a little richer than the CH in the low and mid positions of the air valve opening.
Here's a table on hanger dimensions. Most performance apps use J or lower.

You'll never go far wrong with a "G" hanger, regardless of what was used in the carb originally. Hangers only make a very small difference in the calibration. The rods do 90% of the work.



[This message has been edited by Damon (edited July 22, 2005).]

Okey, I was a bit confused what is really effect of hanger compared to effect of rods, so which kind of role the hanger really plays :?

Yesterday I made few test runs with K hanger, AK rods and power piston spring #305 (6-10 HgIn, anyways quite tough & long but engine makes 22HgIn vacuum in idle 650 rpm).

Primary side feels fine, 800-2000rpm range is really great but when "stepping the gas pedal" it was somehow....not lazy but hesiting too much. Power piston spring is the main reason for this I believe ?? Also in 3000rpm and above it did not respond to gas pedal I was expecting. For that I decided to go to original stock configuration as a first. AK rods are bit leaner and shorter tip than BA's.

Naturally, nowadays european 98 octane gas is not same than good old days US premium http://www.nastyz28.com/ubb/smile.gif so 100% original setup might not be most optimal one.....just have to try different combinations. Selection of springs needs to be purchased as a first, then try K with BA, CK, CH, AY and then P with same rods. Next question is that can I really feel some difference http://www.nastyz28.com/ubb/wink.gif



[This message has been edited by jarit (edited July 22, 2005).]

[This message has been edited by jarit (edited July 22, 2005).]

Ah, now you're asking for PERFORMANCE tuning information. That's different.

Try a set of DR or CP secondary rods. They are MUCH richer on the taper than a set of AK secondary rods. THAT'S where you "feel" the difference on a QJet. That first 20* of secondary air valve opening. The difference between a set of AK rods that start out at .1350" on the "fat" part of the taper and a set of DRs that are .1320" is like night and day. .003" may not sound like much, but when you're shoving it into a fixted jet size of .1360" it's like a 300% difference on initial opening!

Try it. You'll like it.

Also make sure you have no more than about 3/4 of a turn of tension on the secondary air valve tension spring vs. the zero tension point. If you're not sure what I'm talking about or how to adjust it correctly, shoot me an email.

General tuning information for QJets: It's almost impossible to make them too rich. They were ALL emissions carbs from day one. Richer everywhere (sometimes by a LOT) is what's usually called for in a performance application. Your #76 primary jets and 44("K"???) rods are fine. You need to fatten up the secondaries, especially on initial opening. Don't worry- it's practially impossible to go too far.


[This message has been edited by Damon (edited July 22, 2005).]

I love these strings. Especially when Damon answers them. I've considered a lot of these Q-Jet posts and I like what I'm learning. This is the first time I've heard the statement "It's almost impossible to start out rich with a Q-Jet". I do know they are emissions driven in metering from the early 70s on. I have been aware of and have experianced the WOT performance when they been set up pretty good and they can run a chevy just fine thanks.

I think what I hope to learn more about is the myriad little ideas concerning off - idle performance and low speed, low load behavior. I have a good collection of jets, rods and hangers and will use Damon's oft suggested G hanger ( I have 2 ) and I have both DR and CP rods and oddly I came accross a set marked with both numbers! I am only concerned about the altitude here. I don't think I'll start at 77 right away on my 406. I try 76 to start and I have some 44k's ( sorry, haven't found any 46k's yet ).

Performance ...LOL http://www.nastyz28.com/ubb/biggrin.gif

Great!!! Thanks Damon!!!
Now I understand (hopefully). I've been just watching the tip: length and diameter but the initial opening is also important. I've got AV's in garage which are a bit better in 20* than AK's but have talk my friend who has several DR's.

Air valve tension spring: I think I understand the thing....tomorrow I'm gonna check how it has been adjusted.

I've been running a Qjet for a few years now. It smokes any Holley or Eddy I've tried so far! Keep tuning, you'll be very pleased, and the extra gas mileage is great in this day and age http://www.nastyz28.com/ubb/wink.gif

Yesteday I finally had a possibility to change DR -secondaries and they are great !!!
I also changed primaries a bit leaner,74 and 43 and did not feel difference to 76-44.
Hanger is now K.

Question: how to select power piston spring ?? What are the things to be measured and how to decide which is most suitable one ?
This issue is a bit mystery to me, so a "for dummies" -type answer and instructions are just fine http://www.nastyz28.com/ubb/wink.gif
thanks;
/jarit

That spring does basically the same job as selecting the proper power valve for a Holley. It determines at what level of manifold vacuum the primary rods get down onto the skinny part of the metering rods (the power tip) to give you the richer mixture you need for heavy load situations. Most of the time the stock spring works just fine.

If you want you can buy a selection of different power piston springs from Edlebrock for their Performer RPM QJet clones.

Here's what vacuum levels they get down onto the power tip BY MY OWN MEASUREMENTS. Don't listen to what they say on the package- it's absolutely DEAD WRONG. Here's the truth:

Yellow: 3" vacuum
Orange: 5" vacuum
Black: 7" vacuum
Gold/green: 8" vacuum

For most modest street engines (95% of everything on the street) the Orange spring will work just fine. If you have a big cam and low manifold vacuum you might want to use the Yellow. If you have a very economy oriented combo with a small cam and tons of manifold vacuum you might consider the Black or Gold/green spring.

THANK YOU DAMON!!!!!!
That is very practical instruction.

I had a very small doubt when looking the Edelbrock specs for the springs. Then I read some theory about springs from the books but could not figure out how to test and decide.

Anyways the spring kit is already available and some stock (GM part#) springs also. Now there is GM *305 in place that corresponds black Edelbrock. I'll change the Edelbrock orange or GM orange to there and will test it how it works. In my understanding, the GM used is orange for this engine.

Cam is stock cam with 1.6:1 rockers and engine makes 22"Hg vacuum (I had to doublecheck this with other gauge, I did not believe this as a first time).
So the truth is orange or black and it has to be just tested how it feels.
Thanks again;
/jarit

Hey Damon!

My compliments to you and thanks for providing all of us with your knowledge & experience of Q-Jets! It's great to see someone else who appreciates that these carbs are much more that stock fuel mixers!

jarit & Camertom, another thing to consider with the declining quallty of fuel these days is to change the amount of timing advance in your distributor.

The original cars still had advance curves that had to allow the vehicles to pass emissions tests but the fuel was much better in the 60's- 70's than it is today!

I've had more enjoyment driving a car with a crisp-snappy throttle response and increased mileage these days as a result of looking into the distributor and adjusting the vaccum and mechanical advance to where the vehicle runs best. This is due to the performance of a good tuned Q-jet and the optimization of the timing and spark.

In 4-speed cars, they will take more timing and quicker advance curves but you have to back the initial timing off to prevent detonation and hard starting.

Automatics are usually take a little less but still respond to a faster advance curve than stock.

If you can have someone who has an old Sun distributor machine and knows how to use it, dial in an advance curve for you vehicle, I suggest going to them with all the info on you vehicle and let them work their magic! If not, you'll need to get a timing light with and advance feature on it and do it yourself.

You'll need to make a graph of your advance curve to see where you can or need to adjust it, Then you can use an advance curve kit and an adjustable vaccum advance can to "speed-up" the advance curve.

It takes awhile to find the right combination but once you do, I guarantee you'll love driving your cars!

YES, one thing that makes a very fun difference in the idle to midrange performance is ignition advance. Total of 34-36 all in by 2800-3200 or so is very snappy.