Does the 7040202/203 have a provision for ported vacuum port ?

 

(Lars stepped in the the correct answer)
M

 

No. The 7040202 carb does not have a ported vacuum nipple. The carb should look like this with no nipple in the forward, driver's side of the float bowl:


Most other 4MV Q-Jets have a nipple in the forward driver's side of the bowl, and this nipple will be ported vacuum. Here is the ported nipple in a 7041201 (1971) carb - the ported nipple is to the right in the forward side of the float bowl:


Be aware that most M4M Q-Jets ("integral choke" carbs produced after 1975) have this same nipple location, but it's manifold vacuum. The ported nipple, on most M4M carbs (not all...), was moved to the forward passenger side corner of the lower throttle plate...
Lars

 

Lars...

If there was no ported outlet on the 1970 C3 Q-jet, what was used to trigger the vapor canister to purge collected vapors? [I believe that '71 and later used ported vacuum to signal the canister to 'purge'.]

 

If my memory serves me correctly... a 1970 does not have a charcoal canister. I cannot remember seeing one under the vacuum storage tank on a 1970 I restored not to long ago.

DUB

 

That's correct. The canister was first used in '71 on Federal cars.

 

Thanks all.
My 70 , which I have owned for nearly 40 years, does not have a carbon canister.
I have vacuum available at idle for the distributor from the TCS solenoid. Vacuum to the distributor should only be available when the trans is in 3rd or 4th gear. Either that is defective or the switch at the trans.
I have connected the hoses per the AIM and the 70 Corvette service manual.
That is why I was looking for a ported vacuum port.

 

You don't want to run your distributor off ported vacuum, so good thing the port is not there...

Running through the TCS is good for a car show and NCRS judging points, but if you want to enjoy the car, hook the distributor up directly to the manifold vacuum source on the carb and run the heck out of it.

Lars

 

While I have Lars ears, I want to thank him very much for a 100 percent diagnosis of my 7040202 carb. It was junk just like he said.
I went up to see Lars about rebuilding my carb which I have owned since 1980-81. A quick look by him was all it took to diagnose has junk.
When I got home, I took the carb apart and found the secondary rods JB welded to the body. So, I had a 2 barrel carb in a 4 barrel body.
My belief was that since this carb was rebuilt in 1980, that they would at least not do a butcher job on it.
I searched Craigslist and found another 7040202 carb that had not been rebuilt but was missing the top 2 flaps on the horn. I figured that I should be able to make one good one out of two.
I took the second carb apart and all was well, no JB welded secondary rods.. I used the base from my original carb since it was in better condition than the second carb. the body and horn from the second carb. Took the flaps from my original horn and installed in the second carb.
Purchased the book from Cliff Ruggles and a rebuild kit and put it all together.
Once again, thanks very much Lars..

 

Excellent job - good work!
Lars

 

 

Thanks lars,

I try to remember stuff so I do not have to look through countess photos to show it...and sometimes my memory has hiccups.

DUB

 

On a 4MV (divorced choke carb), all ports are manifold vacuum except the forward, driver's side nipple in the front of the float bowl (if equipped) as shown in my photo of the '71 carb above.

Lars

 

Note the post dates too!

 

You need to do a little research before you comment on things you appear to not know much about. If you can humble yourself enough to request the timing and vacuum advance papers that me and several other GM engineers have put together, you might learn a little about the topic.

And note that you're commenting on a 5 year old dead topic thread.

Timing and vacuum advance tech papers available by e-mail request:
V8FastCars@msn.com

 

The only idiotic response is yours. Ported vacuum advance is a emission control technique to heat up the exhaust gas to help oxidize HC and CO with injected air back in the early days of emission control and is used to this day to keep the catalyst bed hot in modern cars at idle and low speed stop and go traffic.

The first wide use of ported vacuum advance were engines in cars sold in California beginning in 1966, which had to be certified to pass California's tail pipe emission test, and ported vacuum advance continued in all '68 model cars to meet the new Federal emission standards and is universal to this day on emission controlled cars.

Prior to emission controls car engine with vacuum advance were full time with a couple of exceptions one of which is the '63 Corvette L-84, but the reason is inexplicable, and full time vacuum advance was implemented on the '64 fuel injection engine.

Chevrolet engines require different spark advance maps due to different states of tune. For example SHP type engines need and can tolerate more aggressive centrifugal advance due to later closing inlet valves that reduce dynamic compression for any given static CR compared to a base engine. They also need more low speed, low load advance because high overlap SHP type cams cause high exhaust gas dilution that slows down combustion propagation. This is accomplished with the combination of aggressive centrifugal advance and aggressive VACs that must provide full advance at less than idle vacuum to maintain idle stability. (Search Two-Inch Rule.) In addition to more aggressive centrifugal curves there is usually less total centrifugal and more initial to maintain higher total advance in low speed, load load situations, which is most common in city driving.

So how do I know all this? In addition to working on cars since I was in my early teens I spent six years in name brand universities studying the design and emission control of IC engines earning both undergraduate and graduate degrees and from there went into the automotive, aerospace, and computer industries.

Lars is a similarly qualified wrench and automotive/aerospace engineer.

So what are your credentials, bubba?

Duke

 

This is why i cut my posts down to a minmum....too many facebook ********