Vespa Maintenance Guide

Tuning Your Carb

The Carburettor has to mix the gas, oil and air very carefully to give the best power output. The jets are precisely tuned to put out a specific amount of gas on demand; however, this system needs fine tuning from time to time. When the scooter is subjected to high altitudes, or the air becomes thin, a bit of tuning may be required. You should also tune your carb after a carburettor rebuild. Some symptoms of bad carb tuning include: loss of power at high throttle (flooding), bogging even when the engine is warm, stalling, running rich or lean (too much or too little oil signified by the fouling or destruction of spark plugs) and bad acceleration. Before attempting to tune the carburettor make sure that the spark plug is in good condition (see electrical section) and that the carb is not leaking or plugged. It is also advisable to inspect your jets before taking on this task, so all the work you do will not go to waste. This is really not an art form. Listen to the engine, you can't do much harm if you turn the screws the wrong way. If you cannot get your bike running properly by changing the screw settings, you should rebuild the carb and check the timing - especially if it backfires or it bogs terribly at 3/4 throttle. The mixture screw generally effects only the lower RPM and Idle settings (off the line acceleration). If you find your plug is very oily or heat damaged, you may need to alter the jet sizes in addition to the mixture screw setting. On a stock bike, this should NEVER be done unless you have a radical change in altitude. Instead, inspect the carb for damage, warping or plugged passages and jets. Air leaks in the engine will make the engine overheat. If the engine revs extremely high on its own this is an indicator for a air leak or a failed engine seal. If you add performance gear, ask around to find the best jet sizes for the respective part. Always consult a knowledgeable mechanic when changing the jets as they can affect the entire rev range of your engine.

Tools & Parts Needed
Flat-Head Screwdriver
Spark Plug Wrench

Part Identification
The Carb Box

Idle Set Screw - This screw sets how fast your engine idles. The screw pushes on a slanted surface on the center of the slide. If you tighten the screw, the idle rate will speed up (it will push the slide back opening the neck of the carb) if you loosen the screw the idle rate will slow down (it will let the slide close off the neck of the carb). This screw will determine your idle speed so you should set it low to preserve fuel at stop lights. If you notice that your bike stalls often, this screw may be set too low to maintain a reliable idle speed. The result is the engine will die out.

Mixture Screw - This screw is hidden behind a rubber grommet in the carb box. Prise the grommet out with your fingers and look for a flat head screw just a few mm inside the box. This screw is responsible for fine-tuning the mixing of the air fuel mix. Tightening the screw will reduce the mix, and loosening it will increase the mix. Pay close attention to this screw if you are fouling or damaging plugs or bogging at low and high throttle rates.


Step 1 - Remove the grommet that covers the mixture screw if you haven't already done so. If possible, run your bike around the block to get the engine warm. If the engine will not start, tighten both of the screws until they resist and stop. Then loosen the mixture screw 1 an 1/2 turns and the idle set screw out 2 1/2 turns. this is usually close the default setting. Try kicking the engine over. If it still refuses to start or quickly dies, pull the choke out half way, tighten the idle set screw and loosen the mixture screw using 1/4 turns. Eventually the engine should run, but badly. Once it is running, let it chug away for a minute or two so you can keep it running it without choking or throttling it.

Step 2 - Leave the engine running for this procedure; if it dies, modify the settings and kick it again and keep it running. Pull the throttle handle hard to about half way and release. The engine should not hesitate when you do this. If it "bogs", or temporarily sputters and then revs, adjust the mixture screw until there is no hesitation. I use 1/8 turns in either direction as the mixture screw is very sensitive. If you get really close, just tweak the screw a tiny bit. Once you achieve the least amount of hesitation, lower the idle rate (loosen the screw). Keep repeating these steps until the hesitation is negligible and the idle rate is low, but reliable (no sputtering or sudden changes in the flywheel speed).

Step 3 - Once you are done, replace the grommet and take a good long run on the bike. Try some challenging roads and return home. Let your bike cool a bit, then pull the spark plug. It should be a light brown color. You should still have no hesitation when you pull the throttle. If the plug and reaction time are good after a long ride, you're finished. If the problems remain, see if other people have had the same problem on the scooter BBS and get it sorted out. This procedure will take about 30 minutes the first time, so don't do it before the sun sets.

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All Material Copyright 2001 Richard Hoar. Use at your own risk.