After installing new plugs, wires, and distributor, good ole basic tune up. I went to set timing, plugged the ported vacuum advance, set idle at 700 RPM, set timing 6 degrees BTDC, well she idled real smooth, sounded great. Then I plugged my vacuum advance back in from the ported outlet on the carburetor and the RPMs jumped to 850 and idled rough. So I unplugged the vacuum advance idle went back to 700 RPM and ran smooth.

So I got to checking my vacuum line from the ported outlet and I have constant manifold vacuum and with the vacuum line plugged into the distributor I have full vacuum advance at idle.

The carburetor is the original Quadrajet, matter of fact all my engine components are original thats the reason for the new plugs, wires and distributor.

The throttle plates seem to be working normal, acceleration is good without being under load. So I'm lost as to why the ported vacuum is constant manifold vacuum.

What do you guys think?

Which port are you using on the carb?

I think the driver side is the vac adv and the pass side is/was the EGR. Mine was connected directly from carb to distributor.

I think mine also changed when the hose was reconnected. (I guess that's why they tell you to disconnect when setting timing?...) Maybe the ported part isn't always true.

With Thorley headers they suggest 12 degrees BTDC, I suspect even with stock manifolds you could run with 8-10.

It is the driver side port that I'm plugged into. Steve,

I did some research on the internet to make sure I had the right port, and all the diagrams and examples for vacuum advance porting is the port I'm using. From what I've read the ported vacuum port should be zero vacuum at idle then climb as the throttle plate opens. I can't find any blogs that would explain why I would have full manifold vacuum at idle.

I see what you're saying about unplugging the vacuum advance, if the ported port on the carb is at zero vacuum at idle than why unplug it. Or perhaps you could have a small amount of vacuum that could influence the timing.

However I do have a small amount of leakage around my carb gaskets I noticed the gaskets looked a little wet. So I'm thinking my carb is in need of a rebuild or replaced with one. Don't know if any of that would have anything to do with the ported vacuum.

Thanks

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From what I've read the ported vacuum port should be zero vacuum at idle then climb as the throttle plate opens.

I think you may be confusing "manifold vacuum" with "manifold pressure". Manifold vacuum (MV) will be the highest at idle while manifold pressure (MP) will be lowest. As the throttle plate opens, vacuum drops and pressure rises; these two measurements are directly related (inversely). MV is lowest (at 0) and MP (at atmospheric) is highest with the engine off.

"Vacuum advance" actually should be termed "manifold pressure advance"; as the throttle is opened, the vacuum diaphragm relaxes and the advance spring advances the timing. Many distributors also incorporate a centrifugal advance, which increases timing advance as the distributor RPM increases.

Check your vacuum advanced diaphragm for leakage. The purpose for the advance is to allow start up with less initial timing. This allows for starting, the advanced kicks in. AT W/O throttle vacuum drops and retards timing to prevent pinging.

Timing advance on RPM is controlled with weights and springs.

Some engines have a valve that switches from ported to manifold vacuum based on engine temp.

There are lots of YouTube videos of Q Jet vacuum diagrams and tutorials. However: Port vacuum is zero at idle and increases off idle - it does not continually rise as you accelerate. If the port has vacuum at idle you have the wrong port. There are many on the carb. Use a gauge and sequentially connect to the small tubes one at a time while gently moving the accelerator arm. The one with no vacuum at idle and some ( any) off idle is the distributor advance. This port applies vacuum to the diaphragm to advance the timing. Under full acceleration you will see vacuum drop, that's the mpg gauge you see on some cars that shows lower mpg under full fuel loads. This is where the mechanical or centrifugal weights take over and advance the timing to compensate for the reduced vacuum. You can have several problems that are related. All ports in the base plate must be either connected to a functioning assembly of some sort like charcoal canister, gauges, accessories etc or blocked with a rubber cap. While they don't affect timing they will affect mixture and performance. There are several vents on the carb body that are constant vacuum and they should also be capped, except for the one that leads to the float bowl. That one will not affect the carbs performance. I would suggest carefully checking each port connection and device to see if removing the line and capping the port smooths the engine rpm. Fix or delete what is not working. Find the proper port and double check the diaphragm on the distributor to make sure it is not broken or leaking. It is very common for those diaphragms to fail after lots of years and they are sold individually.

It may be a carb problem...

The ported vacuum source is just a hole in the side of the carb bore JUST ABOVE the throttle plate. When the the throttle plate is closed there is full manifold vacuum on the downside of the plate but the port is above the plate and sees no vacuum.

When the throttle is opened, the plate moves past the port and exposes it to manifold vacuum.

It possible under some idle conditions that the throttle plate may be open far enough to let the port see vacuum, ie throttle plate open a bit too much but leaned with mixture to still get the proper idle rpm.

You might try setting the throttle plate as low as possible just to see if the port ever closes. It may then be possible to fiddle with the mixture and make it happy. Be sure there are no other sources of air, i.e. vacuum leaks.

It may be the carb is worn. The throttle shafts wear, get wobbly, and leak air. The plates won't always return to normal idle.

My old carb had idle problems, the idle mixture screws did nothing, in or out. (plugged?) I was going to get it rebuilt but decided to get the fuel injection system instead. I sold it to Larry Kiteboardman to use for a possible rebuild, I am curious to hear what is wrong with it.

Good luck with project, let us know!

I went on Y-tube and watched a 15 minute video on identifying Vacuum ports on the late model Quadrajet Carbs. Excellent video very informative, great job of explaining the function of each vacuum port.

The ported vacuum port on his video is the same port I'm using, all other ports are either used or plugged. He does a great job of explaining how the ported vacuum port works. He says vacuum does not exist on this port at idle. Vacuum distributor advance is not needed at idle and as RPMs increase so will the vacuum at the distributor which creates the vacuum advance. The same scenario applies to the EGR valve (if used) he claims you don't want exhaust gas recirculated at idle so the EGR valve has to be ported as well. My EGR valve is not used.

However I can't find any info on why or what would cause the carb to output full vacuum on this port, so I'm leaning on what Steve is saying that I have a worn out carburetor.

There a several Carburetor rebuilders on the internet. Summit Racing also sells rebuilt Quadrajets. Price range $200-330.00

Has anyone had any experience with getting a quality rebuilt Carburetor?

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Check your vacuum advanced diaphragm for leakage.

I forgot to mention my new distributor came with a new vacuum diaphragm.

I appreciate everyone's input on this subject, I can proudly say I'm not too old to learn something new.

Perhaps this would be a great time to install TBFI. Damn Budget Constraints, Well Christmas is around the corner.

I was planning to get mine rebuilt by Oscar Heritz, a carb guru in Benton Harbor (not too far from you?) Several people on the site commented favorably on his work. Good luck with that.

Fuel injection is nice, not sure if it will ever pay for itself but really works a lot better at higher altitudes.

I got mine done by steve zepeda in pearland tx i shipped it to him he did it very fast and was a quality job his # is 281-484-6648 hope this helps would love to put fuel injection on maybe some day!!!!!

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Originally posted by Rusty:
[QUOTE]
"Vacuum advance" actually should be termed "manifold pressure advance"; as the throttle is opened, the vacuum diaphragm relaxes and the advance spring advances the timing.

As I recall when the the vacuum diaphragm relaxes the timing retards. When vacuum is applied to the diaphragm, it pulls against the spring and advances the timing. When relaxed there is no advance.

Well I finally got back to working on the ole Barth.

I ordered a remanufactured carburetor from Summit Racing $343.97 + $45.00 core charge. I did a lot of research on the internet before I bought from Summit. Prices vary greatly and the lead time is anywhere from 1 week to 6 months. One rebuilder that was highly recommended had a 12 week lead time. Tried to locate Oscar Hertz from Benton Harbor but no luck.

So I purchased from Summit a UREMCO #URM-3-3837 Remanufactured Quadrajet carburetor. UREMCO had excellent reviews, company out of Chicago, I emailed the company to make sure I was getting the right rebuild. You can send the carburetor directly to UREMCO if you want, however since I didn't want to wait I ordered from Summit. UREMCO assured me that that carb off the shelf is all set up to run on the motorhome.

So with that being said, I installed the carb which comes with the thick heat shielding gasket. The Barth fired right up went through cold start fast idle then settled down to 700 RPM. My main concern was the ported port for Vacuum advance the old carb had full vacuum at idle and with that I couldn't control my idle speed and the timing was way advanced at idle. The new remanufactured carbs ported port works perfectly, no vacuum at idle, the vacuum increases as the throttle plate opens. Idles at a smooth 700 RPM. Next test will be the open road, need to take the Barth off of the storage insurance to do that.

I kept the old carburetor for a rebuild for when I have the time, which my wife says you don't need another project. I could only agree, for now anyway just need to get to that GTG in May.

Great to hear you found a satisfactory carb replacement! Not cheap but when they work they're great.

The cost of the carb was one factor that pushed me for the fuel injection system. It was $1200 but I figured the first $350 was covered by the carb cost. The improvement at high altitudes was really worth it.

Looking forward to seeing you at Milford.

future project teaser: I just ordered a 454HO 425 hp crate motor...

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future project teaser: I just ordered a 454HO 425 hp crate motor...

No kidding, if so when you getting it? Can't wait to read about this one. SWEET!!