Tech Torque: Fuel’s Gold
Our long-term test PX Polini 210 is getting ready for the new season so it’s a perfect time for Ron Daley’s to sort out the carburettor.
Our long-term test PX Polini 210 is getting ready for the new season so it's a perfect time for Ron Daley's to sort out the carburettor.
It's been a while since Stan's PX200 has seen the light of day, but it's finally off the naughty step. During its exile Dan's helped it to grow both a refurbished r autolube motor and a new Polini 210 2 kit so it should be an exciting season. More cubic capacity brings the demand for f more fuel and experience shows that running r a standard carb with a 210 kit can cause c pinking at high revs. A BGM 26mm carb c looks like the perfect solution and who better b to fix it than PX specialists Ron Daley?
In the workshop Michael and Martin Daley were w responsible for wielding the spanners, so s we'll let them talk carbs… “We hear on a regular r basis how ‘easy' a Vespa is to work on” o Martin began, “but one thing that seems to t cause the average owner lots of problems is the carb. Like all critical components the carburettor c needs to be maintained properly so s it's probably best to start with the basics and a describe how we'd perform a full carb c service...”
Getting access
The airbox cover is removed by undoing the two long screws which are found at either end of the airbox lid. Having lifted the lid away you should see the air filter with two mounting screws either end. In the middle sits the idle screw with its ‘top hat' seal. To remove the air filter, remove the idle screw seal together with both air filter mounting bolts and lift the air filter away from the carburettor. Note that on a T5 the idle screw must also be removed before the filter will lift away. Once the filter's removed you will see the carburettor exposed on the inside of the airbox.
Removing the carburettor
Carefully remove the petrol pipe clip (this holds the petrol pipe to the carburettor spout). This is much easier using long-nose pliers to pull the clip together, loosen it, then push it down the pipe away from the spout.
Now disconnect the throttle and choke cables.
TOP TIP: Sounds obvious but when removing the petrol pipe DO NOT FORGET to turn the petrol off!
Once the fuel pipe is removed, turn your attention to disconnecting both mounting bolts on the carburettor (The mounting bolts have an Allen key fitting). These have one flat washer and one spring washer. Take care with the washers as it can be quite easy to get them stuck inside the airbox and subsequently risk them falling into the engine. Once the carb has been removed, inspect the airbox for any leaks. Don't worry if there's a coating of two-stroke oil, that's pretty normal.
TOP TIP: Whenever you remove the carburettor for servicing, we recommend changing the carburettor base gasket. This will ensure the refitted carburettor has a clean surface to seal against.
Dismantling the carburettor
TOP TIP: Cleanliness is important when dismantling the carburettor. We work over a clean piece of paper to ensure the parts aren't contaminated with workshop grime.
Start by removing the throttle slide cover. This is kept under tension by the spring inside. As you undo the screws, carefully apply pressure to the cover. This will make the screws much easier to undo. Once the screws are removed, slowly ease this pressure so the throttle spring and slide inside can reveal themselves. With the slide removed, undo all the outer parts including the idle, air and mixture screws. Next you can turn your attention to dismantling the float chamber. Firstly remove the fuel spout and then the float chamber cover. When the cover is removed it will expose the float chamber filter, including the float chamber seal.
TOP TIP: It's much easier to remove the fuel spout using a 10mm spanner rather than a screwdriver.
Dirt from the petrol tank can pass through to the carburettor, causing blockages. Although it's possible to clean the filter, this is an inexpensive part and we'd recommend changing it as part of a full carb service. While inside the float chamber it's important that you check the float needle and float. Over time the rubber on the needle can perish; this is a particular problem with older ‘black' needles. These should always be replaced with a ‘red' needle, the rubber on which is more resistant to the additives in modern fuels. It's also possible for the float to split, which allows fuel to get inside it and reduce buoyancy. Either of these problems will make it all but impossible to set up a carb properly. The float chamber gasket should also be inspected and replaced if necessary.
Removing the jets
TOP TIP: Always ensure any dirt has been removed from the jets before refitting. This can be done with clean petrol or better still use carb cleaner, which can be purchased from any auto factor.
One of the major parts of the carburettor to look at is the main jet stack, which includes the main air correction jet, mixer tube and main jet. These are critical in the carburettor set-up and for smooth running of the scooter. Many people don't realise that the stack can be taken apart and its components changed to different sizes. Once the carburettor has been dismantled all its component parts should be thoroughly cleaned. We would recommend using a specialist carb cleaner spray which can be obtained from any motor factor.
TOP TIP: Setting the air mixture screw is critical to ensure smooth running. Before removing one from a working engine, first tighten it fully and make a note of the number of turns this takes. When refitting the screw, tighten it fully and count out the same number of turns. If you lose track, screw the mixture screw in fully and then wind it out by around 1.5 to 2 turns (standard 200 setting) or 2.5 to 3 turns on a 125). From there the mixture can be fine-tuned by screwing in to make it weaker and screwing out to make it richer. There's a trade-off between idle speed and throttle response. To get a strong tickover you screw in, but you might find you have a bit of a flat spot, to get rid of that you have to screw that out but this may lead to a ‘lumpy' idle speed. Unfortunately increasing the idle screw will be counterproductive and will cause issues when changing gear.
Fitting the carburettor
Whether it's a new or used carburettor we always strip it fully to make sure all jets and bolts are tight and fitted correctly. To replace the carb it's simply a question of reversing the dismantling process.
TOP TIP: When refitting the carb to the engine body it's best to take things slowly, alternating between front and rear bolts to ensure the carb doesn't warp.
Final word
“Because there are so many small components this is a job that looks harder than it actually is,” Martin said. “Take it slow, keep things clean and it's a fairly straightforward job for the home mechanic.”
www.rondaleyscooters.co.uk