This message has two parts:
Part 1 - Emissions failure.
Part 2 - Rochester carbs.
<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 (<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.
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!!