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Old 10-01-2011, 11:06 PM
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Quadrajet Questions-Carb Not Original

Doing some tuning and decided to check the numbers on the carb and see what I could find out. This is a late 78 3/4 ton Chevy that came with the California emissions option and manual trans. The carb checks out (per Cliff Rugles site) to be a 76 Chevy with Federal emissions and auto trans.

My cousin owned this truck since it was 4 years old, so I am familiar with its history, but not as much as I thought.

My questions are:

What did California emissions consist of back in the 70's (lean settings and blocked screws?).

Does running a carb like I am now with 4 or 5 vaccum ports blocked off affect the fuel curve and how well it may or may not run?

As best as I can tell, the charcoal canister is broken and not hooked up, EGR is gone and a plate in its place, warm air flap in breather not hooked up.

What effect do these have, if any, on how a carb is calibrated?

I'm thinking these things might need to be taken into account as I am tuning.

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Old 10-01-2011, 11:52 PM
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Quote:
Originally Posted by Hoyt71
Doing some tuning and decided to check the numbers on the carb and see what I could find out. This is a late 78 3/4 ton Chevy that came with the California emissions option and manual trans. The carb checks out (per Cliff Rugles site) to be a 76 Chevy with Federal emissions and auto trans.

My cousin owned this truck since it was 4 years old, so I am familiar with its history, but not as much as I thought.

My questions are:

What did California emissions consist of back in the 70's (lean settings and blocked screws?).

Does running a carb like I am now with 4 or 5 vaccum ports blocked off affect the fuel curve and how well it may or may not run?

As best as I can tell, the charcoal canister is broken and not hooked up, EGR is gone and a plate in its place, warm air flap in breather not hooked up.

What effect do these have, if any, on how a carb is calibrated?

I'm thinking these things might need to be taken into account as I am tuning.
I'm not that familiar w/CA emissions.

Not having an EGR can negatively effect emissions and can increase the chance of detonation on a stock calibrated engine. But the detonation effect is less pronounced the further from stock you go.

You will want to use a PCV system, but the other things you mention will not have an effect on how the engine runs- it WILL effect the emissions, though. Even if the vacuum ports were all hooked up, most vacuum accessories didn't allow air to be pulled into the carb- so in effect they were "blocked" even when they were in use, unless a vacuum operated accessory failed- like if the vacuum advance can's diaphragm was torn and allowed a vacuum leak to occur.

The PCV is accounted for in the carb calibration so it can be used w/o a negative effect on how the engine runs and using it will extend oil change intervals as well as help prevent sludge buildup, etc.

Bottom line is I would say to rebuild the carb using the parts that are in it now. It's very hard to say what you might find- many carbs found nowadays have been rebuilt to a "universal" spec. This can mean there are lead shot blocking some of the idle air bypass channels, for example. This type of mod allows a standard needle and jet combo to be used in a carb that may have had much different calibrations to work best on the factory engine it was used on.

If you know ahead of time what mods you're going to make to the engine, you can take a guess at what might be needed- like a weaker power piston spring for a more radical cam, or different jetting, etc. Ruggles is good about answering questions either over the phone or on his forum. I'd suggest you ask him on specifics if you don't find a good answer here or elsewhere.

Setting up the distributor w/a good timing curve will help almost as much as any bolt-on/in part you can come up with, and is especially needed if any changes are made to the cam or engine in general- even if bone stock this will help, often a LOT. There are many threads on doing this if you do a search.
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Old 10-02-2011, 03:07 AM
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Cool. Didn't know if there was a lot of calibration differences depending on what the carb originally had hooked to it. Emissions make no difference where I'm at (yet). Should I put an EGR back on it?
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Old 10-02-2011, 07:54 AM
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Wow. This is weird.. I JUST stripped a 78 3/4 ton truck for the axles/drivetrain going in a '67 M-715 Five-Quarter ton Jeep..

Do you have a link to this carb site? I kept the carb that was on it, but as he said above, might not be the 'right' one or may have been rebuilt several times.
The engine looks to be a 74 (3970010- 4bolt) block, so I know that's been changed and even broke the crank in half.
It came with the 4.11 gearing and a locker 14 bolt (don't know what it might be in CA) so if you are needing a 'correct' Rochester I can shoot ya the #'s off of it and we can talk.. I'm going with a Holley just because I got it almost new for 20.00!!

They can be kind of a pain to mess with, getting all the parts to 'play nice' with eachother, and just a little trash can drive you nuts.
An Oxygen sensor would probably be the best tuning tool but can be expensive. Or if you can find a pyrometer to measure the exhaust manifolds heat.

Another thing kinda off base would be to have your dizzy recurved and 'checked out' making sure the gear driven off the cam isn't gouged, and getting the advance 'all in' by about 3000rpms.
If it's the orig motor for that truck, the timing gears SHOULD be metal instead of the plastic covered ones. (if the old plastic starts to chip off, it can give erratic timing thus goofy symptoms that some might attribute to the carb)

Besides that. see if the local library has any vids on "How to rebuild your Rockester Carb" just to give you a better idea on what's going on or what you have to work with.

Just my .02.. E
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Old 10-02-2011, 08:37 PM
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I've got a book by Doug Roe, but I hadn't found the answers I was looking for. I have always had trouble getting a q-jet right. Thought I might be overlooking something.
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Old 10-02-2011, 10:42 PM
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The website was www.cliffshighperformance.com .
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Old 10-02-2011, 10:51 PM
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Quote:
Originally Posted by Hoyt71
Cool. Didn't know if there was a lot of calibration differences depending on what the carb originally had hooked to it. Emissions make no difference where I'm at (yet). Should I put an EGR back on it?
At this point I would not reinstall the EGR system. It operates best on a bone stock engine w/OEM calibrations throughout. If you plan on headers, a cam swap, different heads or compression, etc. the EGR will be more of a hindrance than a help.

Do the distributor recurve I mentioned above and rebuild the carb reusing the parts it contains w/the possible exception of the power piston spring if you use a cam that lowers the idle vacuum and possibly the secondary rods and secondary air valve stop to allow full opening of the air valve and reset the air valve spring tension to suit the vehicle- all this is SOP for any Q-jet rebuild. Leave the primary jets and rods as-is, unless you see something that is obviously wrong.
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