Honda XLV750R Carb Tuning

Visual Method

This is the method briefly mentioned in the Honda factory manual, I've added some detail and provided these photos so you can see how it's done. This procedure will not work at all if your carbs have other problems, so while they're out, inspect the diaphragms, jets, etc.
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So, here's the XLV carbs, as seen from above as if the rest of your bike had suddenly been turned invisible.

The one on the LEFT I'm calling the LEFT carb. It provides fuel to the REAR cylinder, and has the idle set screw on the side of it.

The one on the RIGHT I'm calling the RIGHT carb. It provides fuel to the FRONT cylinder.

There's an adjustment screw between them which sets the RIGHT carb relative to the LEFT one.

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If you turn the carbs over and look up underneath, you can see four tiny holes right where the edge of the butterfly sits. If they're hard to see, turn the carb wheel to open the butterfly and wipe the carb clean.

We're going to use the first of these holes to align the butterflies. To do so, we'll peer over the opposite rim of the carb outlet, and align the edge of the butterfly to the CENTER of the hole. I find this is easier to do consistently than the to edge of the hole, because you can observe the reflection of the half-a-hole in the butterfly.

Align the LEFT carb first, because it is the "master" ... the RIGHT carb is set relative to it.

[ missing image ]	Too far open ... the hole + reflection is "8-shaped"
[ missing image ]	Too far closed ... the hole + reflection is "football-shaped"
	Just right ... the hole + reflection makes a circle.

Now you've done the LEFT one, do the RIGHT one, turning the adjusting screw between the carbs.

Then go back and forth a few times, making sure that the holes look identical.

The only important thing is their RELATIVE position, but by lining them up on that first hole its much easier to comapre, and the idle speed comes out about right anyway.

Manometer Method

The other way, not mentioned in the manual, is a bit of a pain but probably worth it if your carbs are on the bike already. You'll need somewhere to put the tank so you can get to the carbs and still have fuel coming in, and a very fine 7mm combi spanner to turn the balance screw ...
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These are MIG welder tips, from a small "hobby" MIG. They're M5 threads on one end, a 5mm cylinder on the other, and a 0.6mm hole through the middle. They're also available with 0.8mm holes if you like a faster response. You should be able to buy a packet of these for a few bucks at any decent welding supply / hardware place.

As it happens, the threaded end fits nicely into the intake pressure takeoff, and the cylinder end fits nicely into a bit of 5mm (3/16") PVC hose, as used in this Redneck Manometer.

It works pretty nicely:

Fit the LEFT pipe to the LEFT takeoff, and the RIGHT one to the RIGHT one.
Warm the engine a little, up the idle slightly, turn the choke off completely.
The columns of oil should be about the same.
If the level in the RIGHT one is HIGHER:
- The RIGHT intake is sucking MORE than the LEFT
- So you need to OPEN the right butterfly a little
- So turn the adjuster ANTICLOCKWISE.
If the level in the RIGHT one is LOWER:
- The RIGHT intake is sucking LESS than the LEFT
- So you need to CLOSE the right butterfly a little
- So turn the adjuster CLOCKWISE.

I think I got that right. It's awfully tricky to get that spanner in there, but you only need to turn it a tiny bit at a time.

The only problem is that copper MIG tip is screwed straight into the alloy head. The PVC gets a little hot and melty as a result. Stand by for a slightly cleverer plan involving a length of brass pipe on the MIG tip, and the hose on that.

Copyright (C) 2001-2004, Nick 'Sharkey' Moore. All Rights Reserved. These methods have been stumbled upon by trial and error, or heard of from other enthusiasts. I am not a professional mechanic and disclaim all responsibility for the accuracy, safety, legality and/or usefulness of this information. Don't you hate this disclaimer stuff?