Gross polluting Q-jet - Team Camaro Tech
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post #1 of 10 (permalink) Old Jun 2nd, 00, 03:15 PM Thread Starter
Join Date: Jun 2000
Location: Adelanto,cal,us
Posts: 4
I recently took my 76 in to get smogged. The 350 in it came out of a 76 p/u. It has a Q-jet 4bbl.serial #7042240. The nearest match I can find in my book is from 1972. The smog guy says that at high idle my hc's are off the chart. Anybody know if this carb is for this year? what size it is, and what the problem could be? I rebuilt it before taking it in, it runs great until ya punch it, then it bogs. I don't see how the secondaries ( according to smog tech) could be dumping too much fuel at 2200rpm, when it bogs at opening of secondaries
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post #2 of 10 (permalink) Old Jun 3rd, 00, 06:13 AM
Join Date: Apr 2000
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Have somebody who knows Q-jet do these things:

Seal all 4 fuel passage well plugs on the bottom of the body (these are the key reason Q-jets run rich and make too much emissions).

Install brass float, available from places like NAPA, etc., most Q-jets have black Nitrophyl float material, can saturate with float, sink and make the mixture rich.

A good carb person will just know all this already, these problems are common to Q-jet, are probably the key reasons they are removed, can be fixed quite easily. Carb isn't bad performer, just needs a little maintenance and problem resolution.

I said good carb person, not Pep Boys, etc.

Most rebuilds from places like Pep Boys, Kragen, Auto-ZZZZZZZoned-Out aren't worth the effort, don't. Find a good carb person fairly local to you.
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post #3 of 10 (permalink) Old Jun 3rd, 00, 10:39 AM
David Pozzi
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Join Date: Nov 1999
Location: Central California, USA
Posts: 10,080
With the engine off.
Stick a 1/4" punch down the fuel vent, you should feel a light spring resistance of the Power valve which operates the metering rods. It's really easy to have the metering rods pop out of the jets when re assembling the carb top, and this will make it go rich.
At high idle no load the metering rods should be pulled down to lean out the jets.
Make shure the float is set right.
Ignitionman said the rest...

The older I get, the faster I was!
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post #4 of 10 (permalink) Old Jun 3rd, 00, 03:38 PM
Join Date: Aug 1998
Location: Waldorf Md. USA
Posts: 3,283
7042240 is a 1972 Buick 455 carb. Probably jetted a bit rich for your 350, but it sounds like there are other problems with the carb. Are you sure you have the carb number correct ??? If the fuel line goes straight in the front then it is a Buick/Olds/Caddy/Pontiac carb. If it goes in the side it is a Chevy carb and you might have the number wrong.
Hope this helps,

Bill Koustenis
Advanced Automotive Machine
Waldorf Md
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post #5 of 10 (permalink) Old Jun 5th, 00, 08:57 AM Thread Starter
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Location: Adelanto,cal,us
Posts: 4
Thanks for the reply. BILLK my fuel line does come straight out so its obviously not the original carb. (# 7042240) I did however come across another qjet (# 17058504....0528...8jt) The fuel line comes out and "L" to the left. The closest thing my book shows is 17057504 which is a 77 model. It has electric choke as the other is vac. I pulled the metering rod on the 7042240 model and the end of the rod appears to have been snipped off with dikes or something. the end should be rounded or fine point? Instead of snip marks? (^) like that.
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post #6 of 10 (permalink) Old Jun 6th, 00, 12:11 AM
Join Date: May 2000
Location: AZ
Posts: 16
This message has two parts:
Part 1 - Emissions failure.
Part 2 - Rochester carbs.

Part #1:
<BLOCKQUOTE>quote:</font><HR>Originally posted by powerles:
The smog guy says that at high idle my hc's are off the chart.<HR></BLOCKQUOTE>
Lets first clarify what we're talking about.
- Hydrocarbons (HC): Unburnt fuel exiting the tailpipe caused by an incomplete combustion process.
- Carbon monoxide (CO): Directly related to the air/fuel mixture, i.e. the richer the mixture, the higher the CO concentration.
- Oxides of nitrogen (NOx): Nitrogen present in the A/F mixture combines with oxygen under high temperature & pressure.

Smog is caused by CO, NOx & HC pollutants combining in the atmosphere then being heated by the sun.

If the HC are off the chart, this normally indicates an ignition component, timing, vacuum leak, misadjusted carb and/or internal
engine problem. Are you sure your ignition components (plugs, wires, cap, rotor, etc.), timing (over advanced) and any emissions equipment (PCV, EGR, air injection, etc.), are working properly? I would start there.

You didn't mention what the CO reading was? Also some states (I'm sure Calif. may be one of them) also look at NOx (oxides of
nitrogen) when testing a vehicle for emissions.

If the CO value (sometimes measured in percentage) is high, say over 4.0%, I would agree, an internal carb problem may be the culprit. BUT if the number is low (&lt;1.0%) or normal (1.0 - 3.0%), the carb is okay or a bit lean, again depending on the number.

Unless your carb is running so rich where the plugs are fouling and/or you have black smoke coming out of the tail pipe, I wouldn't tear into the carb. The four main wells mentioned in previous posts allow the introduction of unmetered fuel into the engine causing it to run rich thus higher CO reading.
If on the other hand, the engine is running extremely lean, the HC reading will be high and the CO very low. This will cause a miss. The engine will also seem to hesitate until the power enrichment systems (rpm, power valve & secondaries are open enough to pull additional fuel) kick in.

If your vehicle has a catalytic converter in good working condition, it will generally reduce your emissions readings. Cats. are
designed to burn any emissions the engine, for what ever reason, wasn't able to.

Part #2:
In 1975, Rochester changed the design of their metering rods in an effort to reduce emissions. You mentioned you had two carbs, 7042240 & 17058504 available to you. As mentioned the 7042240 is off a '72 whatever. This carb uses a different and IMO better metering rod. This carb was built AS the Government starting cracking down on emissions so it shares a lot of similarities (and parts) with the late-'60's quads. The 17058504 on the other hand is an "emissions" carb which uses as you put it, " the end of the rod appears to have been snipped off with dikes or something", in addition to a modified idle, off-idle & main circuits.
They also introduced all sorts of gizmos like adj. part throttle sys., altitude compensaters, etc. in an effort to help clean up the exhaust. Now add a nice HOT catalytic converter and that boys and girls, is why emissions is supposed to be lower on post-'75 carbs. Primary metering rods from a 70-----
and a 170----- ARE NOT INTERCHANGEABLE!! The reason being Q-jets beginning with 170 have metering rod that are .080" SHORTER and also have a different taper.
To throw another variable into this mix, as you may be aware, there are two CFM ratings available. A 800 cfm with a 1-7/32" venturi and a 750 cfm with a 1-3/32 venturi. Unless you engine is really hopped-up, I wouldn't use the 7042240 since it may allow too much air thus causing a hesitation. But your alternative, the 1705---- carb, isn't what I would call a great performance carb.

I would suggest thoroughly rebuilding the 1705---- carb, get your car through emissions, then look for a Q-jet off a '67-71 small block (350 cid) Camaro, Nova or Chevelle and play around with it. It's like having cake and eating it to, meaning with the right mods., you'll have performance and it will pass emissions.

Good luck and Have fun,

Robert Yates for President!!
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post #7 of 10 (permalink) Old Jun 6th, 00, 11:45 PM Thread Starter
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Location: Adelanto,cal,us
Posts: 4
Thanks for replying yates.I DID mistake the hc and co. Here is what my readings were when tested:RPM;700...HC:285 PPM...CO:6.25%...CO2:10.7%...O2:0.7% timing:0.0tdc.-----RPM: 2246...HC: 168PPM...CO: 7.81%...CO2:9.8%...O2: 0.4% ...timing .otdc At low idle the HC have to be below 250 PPM and the CO below 2.5% At high Idle the HC below 200 PPM and the CO below 3.0% the motor runs awesome. Doesn't stumble miss or anything. It starts right up when cold or hot, and idles smooth.There is no cat converter, the plugs are clean, no black exhaust. And no vac or air leaks. I'm in the process of rebuilding both carbs, and will probably take both with me to the smog check next time!!!!
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post #8 of 10 (permalink) Old Jun 7th, 00, 02:12 AM
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Yates, You aint one of those AZ Smog Cops are ya?? I just saved and Emailed this whole thread to use as future reference just in case the powers that be decide to sniff the older vehicles.
I think this subject belongs in tech reference.
Thanks again

Have you checked your SS's lately
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post #9 of 10 (permalink) Old Jun 7th, 00, 07:55 PM
Join Date: May 2000
Location: AZ
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No!!! I'm not a smog cop!!...couldn't pay me enough!!!LOL Before I decided to get a "clean hands" job, I was certifed by Arizona to repair vehicles that failed emmissions.

It certainly sounds like the previous posts had you going in the right direction...bad carb. By the way, did you happen to notice if fuel was dripping from the discharge ports while the enigine is running? Also as you may know, a good starting spot for your idle mixture screws is 2 to 2-1/2 turns out from seated. This should lean your idle CO's out to passing and may drop your off idle CO's a little. Another suggestion is to change your oil before getting your car retested. With as rich as your numbers indicate (6-7% CO), you oil is contaminated with gas which will not help you pass.

Good luck,

'70 SS 350 w/383,4spd.

Robert Yates for President!!
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post #10 of 10 (permalink) Old Jun 7th, 00, 10:55 PM Thread Starter
Join Date: Jun 2000
Location: Adelanto,cal,us
Posts: 4
Thanks alot yates, I will try to get it back to be tested mon. I know where to go for advice if there's anymore problems.
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