63, 327.340 idle vacuum

[Peter L.]

I have a problem with the choke not opening wide enough after the engine is started. It idles rough, and unles I manually open the choke (or rev engine for 5 to 10 seconds at about 1500 to 2000 rpm), it wil idle down and stall. I think the idle vacuum is too low, and doesn't open the choke enough. I have no idea what the vacuum should be. I put a vacuum gage on the vacuum line, and after starting, while idling (either rough or smooth after warm up) it reads between 12 and 13 in hg. What should it read????
Thanks.

[Richard G.]

Re: 63, 327.340 idle vacuum

Peter;
Have you pulled the preheat line from the carburetor and confirmed you have vacuum at the port?
I would turn off the choke, by rotating the choke cover, and confirm the motor operation without the choke operational.
There are known issues with the vacuum advance on the 340hp motors. I am assuming you know about them?
Rick

[Duke W.]

Re: 63, 327.340 idle vacuum

The number one AFB problem is the choke vacuum break piston getting sticky and not working properly. This often manifests as the choke not fully closing on the cold start procedure, but your problem can be caused by the same issue. You can remove the choke housing for cleaning without removing the manifold, but get a zip kit. There is a very small O-ring that seals the vacuum passage between the choke housing and main body that must be replaced to avoid a vacuum leak.

Also check the choke clean air tubes for leaks. The internal tube in the manifold will eventually rust out and cause the choke vacuum break piston to rapidly foul.

The choke should not open wide upon starting, but open only partially upon initial cold start, the slowly open as the thermostatic spring unwinds. This adjustment is specified in the AFB section of the 1963 Corvette Shop Manual. Study that section and the AFB specs at the back of the manual as if you are preparing to take final exam on it!

Normal idle behavior of the Duntov cam is about 12-13" Hg in the 800-900 RPM range with around 30 degree total idle advance. This creates a problem because the OE 15" 201 15 VAC does not pass the Two-Inch Rule.

When new my SWC's 340 HP engine would not idle stably and would stall after about a minute unless I blipped the throttle. The dealer and all the "experts" I talked to were clueless. I finally figured out the problem and in 1965 installed the 8" 236 16 VAC used on the 365/375 HP engines, and the idle stability issue was solved since the 8" VAC keeps the VAC locked at full advance at idle. The OE 15" VAC would cause loss of idle advance, which reduced idle speed/vacuum, which further reduced advance/idle speed/vacuum until the engine stalled.

In addition I installed the centrifugal weights/springs from the 365/375 HP engines that brought full centrifugal in at 2350 instead of 4600, and the engine exhibited newly found low end torque.

To the best of my knowledge Chevrolet never issued a TSB on the '63 340 HP idle stability problem even though it effected every one built and still does so, today.

The modern replacement 236 16 VAC is stamped B28, is made by Standard Motor Products and available in several brands, however, my understanding is that the only currently available brand is Airtex, part number 4V1053.

Duke

[Richard G.]

Re: 63, 327.340 idle vacuum

Thanks Duke for the 340hp informational. Always a pleasure to get your take on things.

When my 340hp was judged the judges noted the incorrect vacuum advance.
It wasn't much of a point deduction and a good trade off for run-ability.
My 340Hp has a rock solid idle at 850 rpm.
Can you address how the vacuum cans are judges and how the judges can tell if the vacuum can has been replaced?
Thanks
Rick

[Duke W.]

Re: 63, 327.340 idle vacuum

The OE '63 340HP VAC has MS 201 15 stamped on the bracket, as do ALL '63 Corvette engines. Sometimes only part or none is visible with the cap installed, but I believe there are some slight detailed differences in the construction of modern SMP-manufactured replacements relative to originals, and I don't think the plating type is exactly the same... like modern might be a little shinier, even with some patina with time.

Total VAC points for the VAC is three - 2 for originality and one for condition. When I used to judge at chapter events I might take one originality point if I could a B-number on the bracket, but if not, I gave the owner the benefit of the doubt. Most of the time the VACs I saw either appeared original or had a functionally incorrecty replacement that I brief the owner on.


What's installed on your engine? If you can't see the ID, loosen the cap and move it aside, and it should be visible.

Duke

[Peter L.]

Re: 63, 327.340 idle vacuum

I have confirmed vacuum at the choke port. I've aready tried to start with the choke adjusted partially and fully open, and the motor will not start under those conditions. Not aware of vacuum issues with the 340 hp engine.

[Peter L.]

Re: 63, 327.340 idle vacuum

I've looked for the Airtex number on several sites, and none seem to carry it. I did find one site (https://www.yoyopart.com/oem/11026066/airtex-4v1053.html) that listed 5 or 6 "direct replacements" but couldn't find any of them either. Any suggestions on where I might find a Airtex unit?

[Peter L.]

Re: 63, 327.340 idle vacuum

Duke,
I've done some more looking, and found three suppliers listed that carry this on ebay (https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Airtex+4V 1053&_sacat=6028) , but all say it's for 1973 models only. Any thoughts on that? Do you see any reason to buy one over the other two?
Thanks,

[Duke W.]

Re: 63, 327.340 idle vacuum

I have confirmed vacuum at the choke port. I've aready tried to start with the choke adjusted partially and fully open, and the motor will not start under those conditions. Not aware of vacuum issues with the 340 hp engine.

I have no idea what you're talking about. What is the "choke port"?

Idle vacuum should be measured by teeing a vacuum gage into the VAC signal line.

When you pump the throttle prior to a cold start the choke valve should fully close. Once the engine fires the "choke vacuum break" should open it slightly. The spec is in your '63 Corvette Shop Manual. You should thoroughly read the AFB section thoroughly so we are talking the same language.

Duke

[Duke W.]

Re: 63, 327.340 idle vacuum

I've looked for the Airtex number on several sites, and none seem to carry it. I did find one site (https://www.yoyopart.com/oem/11026066/airtex-4v1053.html) that listed 5 or 6 "direct replacements" but couldn't find any of them either. Any suggestions on where I might find a Airtex unit?

Before you buy an Airtex 4V1053 report the ID stamped on the currently installed VAC. The parts books are meaningless when looking for a VAC for many engines due to ill-conceived parts consolidations by GM decades ago and the aftermakert followed suit.

In the case of the '63 327/340 the original VAC was functionally incorrect.

Duke

[Peter L.]

Re: 63, 327.340 idle vacuum

The vacuum advance currently on my 63, 340 hp, is stamped 201.

[Peter L.]

Re: 63, 327.340 idle vacuum

I misspoke when I said choke port. I measured the vacuum by installing a tee in the line going to the vacuum advance, and installing the vacuum gage there. I have the choke adjusted so the when the gas pedal is depressed, the choke closes fully (this is at approx. 50 degrees F), with very slight pressure being applied by the choke spring to hold it closed. If I do nothing, the engine starts almost instantly, the choke opens slightly (my guess is about 1/8", then the engine runs rough at about 850 to 900 rpm, and then will slowly idle down to stall. If I repeat the process, and open the choke manually after starting, the engine smooths out, and maintains about 900 rpm for about 20 to 30 seconds, then climbs up to high idle of about 1400 rpm, and stays there. Or, I can just increase throttle to get the engine to about 2000 rpm for 5 to 10 seconds, and when I let up on the throttle it will smooth out and drop to high idle (1400 rpm). In any case, after running at 1400 rpm for about a minute, if I tap the gas pedal the engine drops to about 1200 rpm, then if I tap it again after another minute, it drops to 900 rpm, my normal idle speed adjustment.

[Duke W.]

Re: 63, 327.340 idle vacuum

You need to replace the OE 201 15 VAC with a B28 (Airtex 4V1053). Then you may need to do some adjustments like the choke vacuum break and fast idle cam/speed. With the cam properly adjusted the fast idle speed should be set at 1750 with then engine fully warmed up. Upon initial cold start it should attain about 1500 within a couple of seconds.

I assume you have a 1963 Corvette Shop Manual. If not you need to procure one and study the AFB section including the specs section at the back of the manual before you proceed.

To change the VAC pump it down until the link is pulled to the limit. Then remove the two screws and carefully wiggle the pin out of the breaker plate. Pump down the new VAC and install in the reverse procedure.

Duke

[Carl S.]

Re: 63, 327.340 idle vacuum

Duke,
I've done some more looking, and found three suppliers listed that carry this on ebay (https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Airtex+4V 1053&_sacat=6028) , but all say it's for 1973 models only. Any thoughts on that? Do you see any reason to buy one over the other two?
Thanks,

Peter,

I just checked RockAuto.com and they have the B28 vacuum cannister in stock for $11.33 Although the pictures don't show it, the vacuum cannister will have B28 engraved into the arm that extends back from the vacuum pod into the distributor. As Duke said, the two most common part numbers for the B28 are WVE 4V1053 and DV1810 If you are worried about whether or not it is the correct vacuum cannister for your motor, you can always test it with a mighty vacuum pump after it is installed in the distributor. You can then measure and see exactly when it starts to open and when it is fully open. For $11.33 you can't go wrong.

Duke W. and Lars G. and John H. have written extensively on the advantages of using the correct (not necessarily from the factory) vacuum cannister for your particular motor along with the appropriate adjustments (and the two inch rule) and it is by far the cheapest single-most, effective, improvement that you can do to your corvette to make it run better, cooler and more efficiently. The ultimate bang for the buck.

Good Luck. C.J.

[Peter L.]

Re: 63, 327.340 idle vacuum

Rick,
I have ordered the B28 vacuum advance from Rock Auto (actually a 4V1053 per Dukes' instructions) and will compare and try it against the "standard" 201 that's one the car now and that every supplier says is correct, and let you all know what I find. To answer someone else's comments, I currently have the choke adjustment set to fully close when cold and the gas pedal is pumped to set it. It starts fine, runs around 900 rpm, I can see the choke pull open approx. 1/8", and if I do nothing, it runs rough, then slowly idles down to stall over about 20 seconds. If I manually push the choke butterfly valve open, is smooths out, and slowly (over about 30 seconds starts to run smooth, and increase to fast idle (approx. 1500 rpm). Or I can give it some gas to about 1800 rpm for about 10 to 15 seconds, and when I let off the gas, it run up to 1500 rpm smoothly. Choke works fine after either of the two procedures above.

Rick, in your first response you mentioned "

There are known issues with the vacuum advance on the 340hp motors. I am assuming you know about them?" Not sure about what you mean. is it that the vacuum advance should be a B28, or is there more to the story?
Thanks,
Pete