The Le Mans 1000 uses a Dell’Orto PHM40 ND carburettor for the right hand cylinder and a similar PHM40 NS for the left cylinder. For engine-starting purposes, the fuel/air mixture is enriched not by "choking" the air supply but by providing extra fuel via a valve. On some versions of the bike, a cable connected to a handlebar lever operates the valve. On others, the lever is adjacent to the left-hand carburettor.
Carburettor adjustment tools
While not absolutely essential, using a carburettor-balancing gauge gives better results than empirical judgement, more commonly described as guesswork. Guzzi call the gauge a vacuometer while others might call it a synchroniser, a vacuum gauge or a manometer. A basic two-cylinder version of the gauge consists of a pair of flexible tubes connected at one end to dial gauges or to transparent rods containing moveable slugs. The other ends of the tubes fit onto the inlet manifolds, and as pressure varies, so do the dials or the slugs. Whatever instrument you buy, make sure it has adaptors with M6 threads.
The only other tools required are a flat-bladed screwdriver and an open-ended spanner, typically 8mm, for cable adjustment.
Preparation
Fairly obviously, it is important to complete other carburettor-related work before starting on adjustment. If necessary, replace the air filter. Check that the air-intake paths through the carburettors to the cylinder heads are leak-free. Make sure the cables from the handlebar throttle control run smoothly to each carburettor. Likewise check the starter cables from the lever to the carburettor.
Not quite so obvious is ignition timing. If you are going to adjust this, do it before carburettor set-up, because ignition timing can have an effect on engine tick-over speed.
Initial cable adjustment
Using the adjusters at the handlebar throttle-control end, ensure there is 1 – 1.5mm free play for the cables. The starter-lever cables, adjusted at the carburettor end, need 3mm.
Identification and initial setting of the mixture and idling screws
Turn the mixture screw (see Photo 1) of each carburettor fully in, then back off by one-and-a-half turns. There is no need to move the idling screws at this stage.
Connecting the gauge
Remove the screw on top of each inlet manifold and fit the 6mm adaptors supplied with the gauge. Push on the gauge tubes (see Photo 2).
Adjustments
- Start the engine and allow it to warm up. Ensure the start lever is returned to the "run" position and the handlebar throttle control is fully closed.
- Adjust the idling screws so that both indicators on the gauge are equal and the rev counter needle is between 1000 - 1100 rpm.
- Turn the mixture screws in or out slowly, aiming for a modest increase in engine speed. Stop when the engine revs no longer increase. The gauge indicators may not be equal at this point; what is important is to get the highest engine speed.
- Re-adjust the idling screws for equal gauge readings at 1000 – 1100 rpm.
- Check the settings by repeating operations 3 and 4.
- Slowly open the throttle while checking that the gauge indicators start to move in unison. They are unlikely to remain in sync all they way up the rev range, so aim for balance at lower revs. If they are not moving equally, carefully adjust the throttle cables until they do. Ensure that the cables still have a little free play at both sides of maximum steering travel.
- Stop the engine and allow it to cool before removing the gauge and refitting the screws.
Accelerator pumps
When the throttle is opened suddenly, airflow immediately increases but fuel flow is slower to respond, leading to a momentary leaner mixture. An accelerator pump overcomes this by squirting extra fuel into the carburettor. Gentle throttle opening does not actuate the pump. The amount of fuel injected can be adjusted, but Moto Guzzi manuals do not explain how. Apparently, you have to capture and measure the amount of fuel squirted out. My advice is to check that it works by looking into the air intake end of the carburettor while quickly opening the throttle, then leave well alone.
More on the Guzzi Le Mans
Sources
- Moto Guzzi V1000 G5, 850 Le Mans II, 1000 SP, 850-T3 Workshop Manual. Part No. 17920161
- Moto Guzzi Le Mans 1000 Workshop Manual Supplement. Part No. 28920161
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