Carburetor swap - replace Dellorto VHB with Mikuni VM

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

Created: 2020 November 29

Updated: 2020 November 29

Thanks to Mark Johnson for sending me this information via email. In Mark's own words and photos.

When I purchased my '73 Eldorado the Dellorto carbs were pretty much shot. Even after a full rebuild I never got them running right, so after stumbling upon an article here by Mike Tiberio called Mikuni Round Side Tuning Baseline, I thought about trying to adapt the VM32's to my bike. There are many variations of the popular carbs but I'd caution against buying a knock-off when the genuine article is available for just over a hundred bucks. In my case, I went with the round-side VM32-93 (left) and VM32-193 (right) models because, 1) I wanted the idle adjustment screws to be easily accessible, and 2) I wanted the larger bore of the 32 mm to better feed my 850's big bore conversion. Both carbs feature a manual choke level with a click-on, click-off setting. My units came from Niche cycle as a matched pair costing USD $225.00. (Map Cycle is another good resource for the genuine thing.)

Mikuni carbs are a terrific choice for several reasons: First, they are available new from distributors the world over; you'll always be able to find one that fits your need. There are more than 12 different VM models to choose from in their round-slide series. Flat-side pumper carbs in a variety of sizes are also available (but at a higher price). Second, spare and optional parts exist in abundance with everything from jets, needles, and seats to rebuild kits, manifold boots, air filters and velocity stacks. And third, the aftermarket support for this carb is massive - and almost every part is available as a knock-off if you want to go that way (and save some cash).

Buying the carbs is the simple part; mounting them is where the project really begins. You have a number of options as I'll explain, but the main issue is that the Mikuni's float bowl is wider (75 mm vs. 70 mm) compared to the Dellorto's. When using the stock manifold, this extra width clashes with the mounting bolt runner on the gearbox - especially on the left side. However, by making a few modifications and using the correct carb boot you can overcome this issue. That brings us to mounting boots.

Mikuni carbs typically attach to the manifold by using a unique rubber boot. I say unique because the boot not only matches up with the outside diameter of the carb and manifold, but features a rib that rides in the channel of both the carb housing and (hopefully) the manifold. When cinched down using the supplied clamps, the rib/channel connection locks the carb and manifold securely. As of this writing, I've found the best source for rubber boots, sleeves and tuning parts is a company in Australia called Mikunioz. They also feature a list of great videos that help you understand, adjust and tune their carbs. I learned a lot. (Note: shipping from Mikunioz is expensive, but quick and reliable).

Not really sure how I wanted to proceed, I ordered three different models: 1) 40 mm-to-35 mm ID reducing boot, 2) 40 mm-to-40 mm ID both ends boot, and 3) 13 mm offset boot with 40 mm IDs.

OPTION 1: 40 mm-to-35 mm reducing boot.

This is a simple solution that involves minimal modification to the stock manifolds. Per Mike's comments, I located a right-side manifold and fitted it to the left-hand bank. As most know, the stock left-side manifold has a sharper angle than the right-side, which causes the Mikuni's float bowl to strike the gearbox case. I believe I also needed to rotate the whole carb to add a little cushion for the float bowl. (The Mikuni manual states the mounting angle fore and aft inclination should not exceed 20 degrees from horizontal - so minor adjustments are anticipated.)

• Pros: Simple to implement; requires only 3 parts (boots and right-side manifold) and no machine work.
• Cons: No way to lock the boot rib to the manifold. Requires you tilt the carb to avoid bowl contact with the gearbox. If you go this way I would recommend carefully cutting off the rib to increase the contact area (otherwise the narrow rib has to bear the weight of the carb alone).

OPTION 2. 40 mm-to-40 mm boot with a 40 mm aluminum sleeve.

I didn't seem to have any scrap aluminum tubing with close dimensions, so I started by purchasing a piece of aluminum tubing from Speedy Metals that was 1-¹⁄₂ inch OD × 1.37 inch ID × 0.065 inch wall - an almost perfect size. I next chucked one end into my lathe and cut a groove where the rib in the boot would sit. Note: A lathe is useful but not required. You could spin the tube in a drill press and cut the groove with a file or hack-saw blade, or just file the groove by hand. (You only need a millimeter or so depth.)

• Pros: Works well with only minor issues. The carb is locked to the boot with the first rib and the boot is locked to the manifold with the second rib. Nothing is going to slip off with this arrangement. I still had to use the cut-down right-side manifold on the left bank to avoid float-bowl to gearbox contact, and also angle the carb slightly (a few roll degrees).
• Cons: You have to fabricate the sleeve unless you have a lathe and then you could cut a groove directly in the manifolds. (I liked sleeves because if you make a mistake you can just throw the thing away and make another one.)

OPTION 3: 40 mm ID/13 mm offset boot with 40 mm aluminum sleeve.

Still using the modified manifolds with their aluminum sleeves and cut channels, the offset boot locks both carb and manifold but additionally lets you position the assembly however you want, and totally avoid contact with the gearbox case and having to tilt the carb.

• Pros: Lots of flexibility for mounting carbs in a variety of positions, with no fitment issues.
• Cons: Still requires right-side manifold on left side. In theory, an offset boot is not as efficient (fuel/air flow wise) as straight boot, but seems to work just fine in practice.

Various options I tried with real-world results

Manifold configuration	Left side manifold	Rigth side manifold	Right side manifold relocated on left side
Stock intake with 40 mm to 35 mm reducing boot	Clearance issues between bottom of float bowl and gearbox	Clearance issues between bottom of float bowl and gearbox	Clearance issues between bottom of float bowl and gearbox
Cut-down intake with 40 mm to 35 mm reducing boot		Fits with minimal clearance between bottom of float bowl and gearbox*	Fits with minimal clearance between bottom of float bowl and gearbox*
Sleeved intake with 40 mm to 40 mm connector boot		Fits with minimal clearance between bottom of float bowl and gearbox*	Fits with minimal clearance between bottom of float bowl and gearbox*
Sleeved intake with 40 mm to 40 mm offset boot		No clearance issues	No clearance issues

* may require carb tilt to avoid clearance issues

• Stock intake = no modifications made to stock manifold (34 mm OD).
• Sleeved intake = ¹⁄₄ inch shortened manifold with aluminum sleeve (40 mm OD).
• Cut-down intake = ¹⁄₄ inch shortened manifold only (34 mm OD).

Supplemental notes:

1. My Tommaselli Daytona 2C throttle cables were an inch too short to connect to the Mikuni carbs. The cable housing was the correct length (45 inch) but I needed 1 inch additional exposed wire. To solve this problem I learned how to make my own cables after becoming familiar with a bird cage tool and how to use a solder-pot (good things to know.) Attaching the new cable ends to the slides is so easy - just unscrew the caps and poke the cable end through the slide hole. It takes only seconds.
2. When it comes to air filters you have lots to choose from, including open velocity stacks with mesh screens if that's your thing. Almost any clamp-on style air cleaner with a 58 mm-62 mm ID will work. Mikunioz offers a nice little oval POD filter, but I ended up going for the 1 inch thick pancake units for stylish reasons. Mapcycle offers a universal high performance air filter with a 62 mm inlet for around USD $25.00.
3. Tuning and jetting Mikuni carbs is half art, half science, but still a lot easier than expected. An excellent article on the subject can be found at Mikuni tuning and jetting guide.
4. The VM models feature three overflow tubes and you'll need to route these so they can drain safely if the need arises. Installing longer tubes is another option.
5. I cut off about a ¹⁄₄ inch from the manifold tips since the boots only swallow about a ¹⁄₂ inch of the boot.
6. Because the left side carb now sticks out a bit further (result of using the right side manifold on left side), I needed to trim off about ¹⁄₄ inch of the battery cover plate adjacent to the new carb.