Hello slant sixers, my 1979 super six barely runs for the first 3 minutes or so, then runs perfect. It seems like a fuel or vacuum problem. I've rebuilt the carburetor (carter bbd), made all the adjustments per specifications and even tried a different carter bbd carburetor with the same results. I've also checked for vacuum leaks, timing, etc etc. Idle speed is at factory spec (700-750) and I think the idle mixture is good (I've tried different variations of enrichment)

One thing I noticed if I unplug the vacuum line from the air temperature sensor for the hot air inlet system (on the air cleaner), the problem goes away. Of course this disables the hot air inlet system and creates an unwanted vacuum leak. Could it be a bad air temp sensor?

Thanks in advance.

Sounds very much like a choke problem. Specifically, it sounds like the choke pull-off isn't working or adjusted correctly, or its linkage is bent or missing. When you create the vacuum leak by disconnecting the hose, you serve the pull-off's function, kinda, by introducing extra air into the mix. Try starting up the car with the air cleaner lid off. While it's running poorly, use a pencil or your finger to push the choke plate open more than it is. If the engine speeds up and smoothes out, you've got some choke work in your future.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks, Dan. I forgot to mention, I installed a new choke pull off. It does the same thing without regard to position of the choke plate or temperature of the engine. If I run the engine for 10 minutes, then turn it off and restart it, it stumbles the same as if it were sitting cold (for about 3 minutes, then runs fine).

I'm kind of stumped. I agree it acts like its not getting enough air (hence the improvement when air temp sensor vac hose is pulled), but has the same problem even if engine is warm and choke plate is wide open. I'm wondering if it could be something clogged up in the air temp sensor, etc? I think the air temp sensor is supposed to get a small amount of air from the air cleaner? Thanks, awesome website.

Like swapping out for an electric choke, am I right?

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Interesting. OK, time for more questions: Which emissions system has this car got (Federal, High Altitude, California, or Canada)? Check the underhood emissions label if you're not sure. Are all the emission control devices and systems still in place, or have any of them been removed, disconnected, or modified? When the engine's running poorly, are there any additional symptoms like black smoke out the tailpipe? What happens if you shoot a bit of spray carb cleaner into the carb throat while the engine's running poorly?

I'm beginning to suspect a malfunctioning emission control device, and pending answers to these questions, my initial guess is related to the EGR system. I'd be tempted to pull and plug the hose from the EGR valve and see if the condition improves or changes.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If not running well you may also need to check to make sure the distributor is still functioning as well as being set in the right place (the 1979+ models use the 'claw' style of distributor hold down so if it was loose, it could be out of time...)...

-D.Idiot

Thanks, folks. I just noticed that if I pull the vac hose from the EGR valve (with engine in "stumbling" condition) the engine runs properly. Continues to run properly if I put my thumb over the end of vac hose. When I hook the vac hose back up to the EGR, it either goes dead with a white puff of smoke out the carb, or continues to run crummy.

I also noticed with the vac hose hooked up to the EGR, the shaft going from the exhaust to the EGR valve moves back in forth in unison with the engine "stumbles". Bad EGR valve??

I believe the emissions to be federal, not California, although I was unable to confirm despite label searching-to the best of my knowledge it is an original Nevada car.

The emission system seems intact with one exception, and hopefully this won't confuse things:

There is an open hose dangling from the carb. It goes to the EGR tube, just above the hose going to the choke pulloff. I've owned the car for about 3 months, and haven't been able to figure out where this hose connects. All other fittings, tees, lines, hookups etc. are all present and or accounted for. I was unable to discern from my Chilton's or from my underhood label where this hose goes. Of course an unhooked hose raises an eyebrow, but one item of note is the vehicle ran properly for several weeks in spite of this unmated hose.

No noticeable change by shooting carb cleaner.

No black smoke that I could tell.

Don't think it's an ignition problem--entire system is almost new.

I'm thinking emission system, possibly EGR??

Thanks for the inputs, this is fun.

I thought so!!!
No. The EGR valve is working correctly, for when vacuum is applied it opens and admits exhaust into the intake tract, and when vacuum is removed it closes (all the way closed). There's a problem with the EGR control system. There should never be vacuum applied to the EGR valve with the engine idling and/or cold. The EGR valve should only be activated (open/vacuum applied) during part-throttle operation with the engine warmed up to operating temperature. The fact that your EGR valve is being opened at idle means either there are problems with some of the EGR control widgets, or the system is not hooked up properly. Do you have a factory service manual for your car?
I posted a big Carter BBD operation and repair manual (pdf) ; it should have at least one photo showing what all the carb's vacuum nipples are for. Get rid of that Chilton book; they're junk. You need the factory service manual, which will contain a clear "road map" for where all the vacuum hoses go.

In the meantime, you may want to try disconnecting the hose from the EGR valve and plugging it with a bearing ball or properly-sized screw (or even a golf pencil). If the car doesn't ping when you drive it, you'll do no harm by leaving it disconnected, and will likely find your driveability and fuel mileage improved.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the interim fix idea--that's what I'll do until I get it figured out

It was the EGR timer, purchased at local junkyard for $5. Finding out the line wasn't supposed to get vacuum during idle saved me from going in circles around the air temp control and EGR valve itself--thanks for the help.

Easy/cheap fix — onward and upward! (But you should really figure out what that dangling hose is).

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.