Gregory Bender

Intake manifold modification to support vacuum balancing

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models

Updated:

// //

Personally, I am not a fan of modifying the original intake manifolds to support vacuum balancing because I generally don't like to irreversibly alter original parts. That being said, here are a couple of ways to perform this modification.

Thanks to Patrick Hayes for posting this information on the Yahoo! Loopframe_Guzzi news group and on the Wild Guzzi forum. In Patrick's own words:

You can't use a vacuum measuring tool until you can access to the vacuum space of the intake manifold. While the manifolds with factory original ports are stronger and better, it is possible to carefully machine modify the older manifolds for ports as well. The only tricks are to first make a flat surface for the bolt head to seal against, and second to shorten the bolt so it does not protrude in and interfere with air flow. Careful, you only get a few turns of thread and easy to strip out.

Intake manifolds modified to support vacuum balancing.
Intake manifolds modified to support vacuum balancing.

Photo courtesy of Patrick Hayes.

Intake manifolds modified to support vacuum balancing.
Intake manifolds modified to support vacuum balancing.

Photo courtesy of Patrick Hayes.

Intake manifolds modified to support vacuum balancing.
Intake manifolds modified to support vacuum balancing.

Photo courtesy of Patrick Hayes.

Port adapters for attaching the tool tubing can be had from MotionPro.

More details from Patrick:

  1. First, don't try to do it on the bike.
  2. Second, note very carefully that the left and right manifolds are dimensionally different. The base of the casting will have a D for the right manifold (destro) and an S for the left manifold (sinistro).
  3. Third, yes, the wall is very thin and you will only get a few turns of thread. So, be VERY gentle with anything you screw in there.
  4. Fourth, do make sure that your fitting and/or your final sealing screw do not protrude far into the air stream.
  5. Fifth, if you use a final sealing screw, you should machine a small flat area on the surface to allow for adequate sealing with an aluminum or nylon sealing washer.
  6. Sixth, the later manifolds had a big boss welded onto their surface to allow for a much longer, deeper threading.
  7. Seventh, opinions vary but IMHO the vacuum balance is best for a smooth running loop motor.

Thanks to Daniel Howe for sending me this information via private e-mail. In Daniel's own words:

I spent most of the day in my cramp garage working on the carb intakes. I am real happy with the results and thought I would send you the info to put on your site. Problem with this old bike is you can't use a vacuum port to sync the carbs, so I made some. I use a Cycle Pro mercury carb stick, it has adapters that are threaded 5 mm × 0.8 mm thread pitch. I purchased a tap, die, one foot of 316 inch copper tubing, one foot of 14 inch hose and did made adapters and drilled and taped the intakes.

Instead of putting the vacuum adapters on the outside I put them on the top. I don't believe it will affect the use of the carb stick and will look cleaner when I'm done. The vacuum adapters or barbs will be permanent. Once they are balanced I will put a piece of hose between the two carbs so they will stay balanced all the time and give me a better idle. attached are photos showing how to make the hose barbs and drill the intake. Just a note you don't need a hose clamp on the hose barb, it is a tight enough fit that they will seal and with age they shrink so no problems there.

I forgot to mention another advantage to putting the ports on the top is the manifold is 0.232 inch thick at that point. Plenty of material there for your threads.

3⁄16 inch copper tubing.
316 inch copper tubing.

Photo courtesy of Daniel Howe.

Cut to length.
Cut to length.

Photo courtesy of Daniel Howe.

Use drill press to keep threads straight.
Use drill press to keep threads straight.

Photo courtesy of Daniel Howe.

Finished adapter for 1⁄4 inch ID rubber hose.
Finished adapter for 14 inch ID rubber hose.

Photo courtesy of Daniel Howe.

Tools used.
Tools used.

Photo courtesy of Daniel Howe.

Clamp the manifold in a drill press vice.
Clamp the manifold in a drill press vice.

Photo courtesy of Daniel Howe.

Tighten the nut and seal the threads with a gasoline resistant sealer.
Tighten the nut and seal the threads with a gasoline resistant sealer.

Photo courtesy of Daniel Howe.

Thanks to Charlie Mullendore of Antietam Classic Cycle for providing the following information on the Yahoo! Loopframe_Guzzi news group. In Charlie's own words:

Here's a photo that shows where I drill them:

Intake manifolds modified to support vacuum balancing.
Intake manifolds modified to support vacuum balancing.

Photo courtesy of Charlie Mullendore of Antietam Classic Cycle.

The casting of the manifold is thickest there, so you'll have an adequate number of threads to hold. I drill and tap for 6 mm × 1 mm (can't remember the drill size right off - 732 inch? , it says what size to use on many taps). I use an 6 mm × 10 mm stainless steel cheese-head cap screw (shortened) with fiber or aluminum crush washer to plug the hole when not in use, my ancient Carb Stix came with 6 mm × 1 mm adapters.

I had to do one customer's on the bike due to a seized manifold bolt, but it's very tricky to do that way. Best to remove them and utilize a drill press.