Should I connect my vacuum advance?

[David B.]

First-time poster here, guys.

I need your opinion on whether or not I should hook up the vacuum advance on my 4-speed '58 2x4 270 283. As it stands now, there is no connection between the vacuum can and the vacuum port on the front carb. They're both capped. I'm a dunce when it comes to things like this, but several guys pointed out the missing connection at a shine and show gathering last night, telling me that I should definitely make the connection.

The vacuum can on my distributor has these numbers on it: MS 148_15. According to ST-12, this looks like a fuelie distributor, since it has the can on it. The car was originally a fuelie, but was converted to 2x4. My questions:

- What's involved in connecting the vacuum advance? Is it as simple as running a rubber hose from the distributor vacuum can to the front carb port? I imagine that the timing may have to be changed, too. (This is where I would get nervous.) There's no vacuum port on the rear carb, right?

- Should I leave well enough alone? (This is my gut instinct.) It starts right up, idles fine (no stalling), doesn't overheat, and has plenty of power. I get 14 mpg around town with plenty stop and go. Since it ain't broke, should I fix it?

Thanks, guys.

[Timothy B.]

Re: Should I connect my vacuum advance?

David,

If you drive your car on the street the vacuum advance has many advantages. The engine will run cooler both at idle and cruise, and your fuel milage will increase. The important thing to remember is to make sure the vacuum advance control pulls to it's limit at 2" vacuum less than the engine makes at idle.

You need a vacuum gauge to check engine idle vacuum and a mity-vac hand held pump to check the vacuum advance control. Someone may be able to post the specs for the #'s you listed on the vac control but I don't have them. Don't be afraid to try it, I think you will be satisfied. Lots of info in the archives for you to read about vacuum advance.

[Duke W.]

Re: Should I connect my vacuum advance?

The 283/245 and 270 HP 2 x 4 carb. engines were never originally equipped with a vacuum advance distributor. Neither was the Duntov-camed FI engine. All originally had the dual-point non-vacuum advance distributor.

The 250 HP base cam FI engine did have a vacuum advance distributor, so your post of what your engine was and is now is confusing. If you replaced the FI system on a 250 HP engine with the dual four-barrel setup, you would essentially have a 245 HP engine, not a 270. Posting the distributor part number from the band if it's still there might help determine what distributor you actually have.

Reports from other 2x4 (245 or 270 HP) owners who have installed vacuum advance single point distributors indicate that the dimple in the drive gear must be rotated 180 degrees to point in the opposite direction of the rotor tip in order to install a vacuum advance distributor and get the initial timing in the proper range without the VAC interfering with something solid.

Vacuum advance does offer advantages - better fuel economy and less load on the cooling system at idle, but, if your engine has the OE type Duntov cam that pulls about 12" Hg. manifold vacuum at 900 idle, it needs a B28 VAC connected directly to a source of full time manifold vacuum, including at idle speed.

The first thing you need to do is measure manifold vacuum at idle speed since you may not be able to swear on a bible what cam the engine has.

Your MS 148-15 VAC has 15 deg. max vacuum advance, but I don't know at what vacuum it's all in, so you should test the start and stop points with a Mightly Vac and timing light.

Then you can use this information and the Two-Inch Rule to select a VAC. The archives has lots of information on this simple process.

Duke

[David B.]

Re: Should I connect my vacuum advance?

Timothy and Duke, thank you for all your insights. I'm going to try to post a pic of my distributor and front-carb vac port, which might help you all to understand my situation better. I'll do that at the end of my new comments here.

My two fours have progressive linkage. The two Carters aren't the correct carbs, though. The front one is a 6-1271, which I understand is from a '57. The rear carb is a 6-1672, which is from a '63 or later. The car is a cobble of various years' components.

On page 6Y-2 of ST-12, you can see a pic of my distributor. It's clearly the FI, dual-point unit on the right. It shows the exact vac can that mine has, including the access plate in the front. I couldn't find the part number tag on my distributor but will try to post a pic below.

Here are those two pics. If they don't appear, please click the direct URLs. Other times, kind souls have posted and sized my pics for me. I really appreciate that.

First, my distributor:



URL: http://i1212.photobucket.com/albums/...s/IMG00039.jpg

Next, my front carb's vac port:



URL: http://i1212.photobucket.com/albums/...s/IMG00040.jpg

Thanks, everyone, as always.

[Duke W.]

Re: Should I connect my vacuum advance?

The distributor in your picture appears to be a Delco single point, not the old Dual Point type used on some 283s. The single point has the centrifugal advance mechanism under the rotor. On the Dual Point, it's under the breaker plate.

The distributor appears to be at its rotational freedom limit - bumped up against the coil bracket. What is the initial timing?

Use a vacuum gage to check vacuum at the carburator spark advance port. Report "inches HG @ idle speed".

The nipple on the carb probably sees full manifold vacuum at idle, but the above test will prove one way or another.

You still need to get a vacuum pump and determine the vacuum levels required to start the VAC and the vacuum required to pull it to the limit. Maybe someone has the specs for this MS 148 15 VAC, so we can compare your readings to OE specs. It's looks old, so it may be out of spec or frozen, and depending on your typical idle vacuum it may not be well matched to the engine.

What camshaft do you think this engine has? Mechanical or hydraulic lifters? Does it have a lumpy idle at 800-900 or a smooth idle at 500-600?

Duke

[David B.]

Re: Should I connect my vacuum advance?

Duke, thanks again for your cogent insights. I just entered the vintage 'Vette world last August with my '58 and have been slowly learning the ropes (thanks to kind folks like you) about what I have and what need to do. As I mentioned before, this car appears to have been put together using a number of components from different years. It would be a real knee-slapper for NCRS judges.

I don't yet have all the tools and equipment necessary to assess such aspects as vacuum, but I'm getting to know some local C1 guys who do have the equipment, so I'll eventually be able to answer questions like the ones you ask.

As for my cam and other engine specs, here's the description I got from the guy who sold it to me:

"Original Solid Lifter 283/270hp with proper dual 4's and 4 Speed Transmission ..."

So, even though the carbs aren't "proper," it does sound and feel like a true solid-lifter, high-lift-cam engine. At 900 rpm idle, it's definitely "lumpy" (good word) and I can hear the ticking of the lifters.

Here are a few pics of the car, FYI:









I'll try to find out more and pass it along, as I discover it.

Thanks again for your time and help.

Dave

[Duke W.]

Re: Should I connect my vacuum advance?

So, it sounds like you have a Duntov cam - or something close - so your engine is effectively a 283/270.

Even though OE 270s had a dual point distributor with no vacuum advance, I think installing a single point with vacuum advance is a good idea, and as long as it has the wrong carbs, so what! The trouble is getting the dist. oriented so you can get the proper range of initial timing.

A Mighty Vac and dial-back timing light is what you need to run the necessary tests. You'll also need a few feet of 1/8" vacuum hose, a 1/8" tee, and a golf tee to temporily plug vacuum hoses.

Suggest you tie up the centrifugal and test the initial advance limits with the VAC against the coil bracket and manifold travel limits. Hopefully its in the normal range. Then test the centrifugal advance curve - start and stop points and total advance.

You want the sum of initial and full centrifugal to be in the range of about 34-38 degrees. Once that's dialed in you can try some lighter centrifugal springs to get the centrifugal in faster. (see the nearby thread on air fuel ratio.)

It's likely the installed VAC is not well matched to the idle characterisitcs. The Duntov pulls about 12" @ 900 and needs a B28 VAC.

Once you have the spark advance map dialed in, any driveability problems will likely be carb related.

Duke

[Timothy B.]

Re: Should I connect my vacuum advance?

David,


Run a new vacuum hose from the V/A up to the carburetor nipple and plug it in and report back if the idle speeds up. I am sure the engine will run better but you should do all the engine and vacuum control tests so you know how the engine is timed.

A vacuum gauge and miti-vac are good investments and don't cost much$$, they are available at Sears.

[Dan D.]

Re: Should I connect my vacuum advance?

Miti-Vac at Harbor Freight too. $39. Just bought one. -Dan-

[David B.]

Re: Should I connect my vacuum advance?

Guys, just for kicks (being the moron that I am about tech stuff), what would happen if I DID connect my vac advance line (assuming that my vac can is working) but DIDN'T change the timing? Consequences? Symptoms?

Dave

[John H.]

Re: Should I connect my vacuum advance?

Guys, just for kicks (being the moron that I am about tech stuff), what would happen if I DID connect my vac advance line (assuming that my vac can is working) but DIDN'T change the timing? Consequences? Symptoms?

Dave

Dave -

In order to give you a good answer, we need to know what your base/initial timing is set at now, and how much advance is added to that when you connect the vacuum advance line. Would also be helpful to know what your idle rpm is now, and your manifold vacuum reading at idle now.

[Tom B.]

Re: Should I connect my vacuum advance?

Assuming you have vacuum to the can and the can is good rpm should increase by just connecting the vac line.

Tom

[Duke W.]

Re: Should I connect my vacuum advance?

You can try it - no harm will be done, but unless you know the VAC is functional the results may be difficult to interpret.

If the VAC is functional, idle revs will increase as will spark advance, but the idle could also increase if it's leaking, but the idle will become very rough and the spark advance won't change.

Also, I don't think your engine pulls enough idle vacuum to pull the plunger to the limit, so it might "dither", resulting in an unstable idle. That's why you need to test manifold vacuum and how much vacuum is required to pull the VAC to the limit, and it probably won't meet the Two-Inch-Rule, so you will have to buy a functionally correct VAC and install it before proceeding with final hookup.

If you just can't control your curiosity, go ahead and do it, but don't make any conclusions until your run the diagnostics and tests with a Mity-Vac and dial back timing light to determine what you have and what needs to be changed to achieve optimum performance.

Duke

[David B.]

Re: Should I connect my vacuum advance?

Okay, guys. I'm gonna get a timing light and see where my timing is set now. I'm also gonna get some vac hose and hook up my vac can to the manifold port (without changing timing). I'll report the results and y'all can coach me from there.

What a super resource your collective knowledge is! Thanks again, to all of you. Stay tuned.

[Dan D.]

Re: Should I connect my vacuum advance?

If you find a good source for a dial back timing light, please post it if you would. I'm having somewhat of a hard time finding one here in good-ole upstate NY. But I admit, I hare not tried NAPA yet. All other local sources come up negative. -Dan-