Welcome back folks,

Like everyone during the festive period, I overindulged and I am now trying to remedy the effects of eating a plate of cheese after every meal and scoffing desserts every night… I mean technically the gorging started in August but we will just blame the additional stone in body weight to the additional carbs over Christmas…This post is not about how much of a podgy mess I am currently, in fact, it’s about the rebuilding of one of the fundamental parts of the fuel system, the carburettors.

Carburettors allow fuel and air to be dosed in the correct ratio to allow effective combustion in the cylinders of the engine. In its most simplistic form, a carburettor works by having two sections, an air chamber and fuel (float) chamber. The airflow through the main chamber (see the crude drawing by le’Struan below) is controlled via a butterfly valve (connected to your throttle), running off the side of the main chamber is the fuel chamber.

 

Carb Sketch
Image courtesy of my funky new work laptop with sketch capability

 

The air chamber has a restriction in its profile, meaning its function uses the venturi principle (quickly trying to remember my Mechanical Engineering degree here). The venturi principle is a fluid dynamics law that states should a fixed rate of airflow meet a restriction, the pressure will decrease across this area. In essence, this decrease in pressure coincides with the entrance to the fuel system. The pressure drop at this point causes the fuel to be pulled into the system (via the differential pressure) allowing for the fuel/air to be mixed (still with me? good because I got lost several times). The connection between the fuel and air is regulated in the form of a jet. The jet is a small orificed (hehe) component, which is set to the correct size from the factory for a standard engine set up. this will only allow a certain amount of fuel pass through the orifice.  Fuel is kept at a constant level in the fuel chamber by means of a float. The float will start to drop when fuel is used, which opens a valve that allows more fuel in the chamber. This is fed from the fuel tank. This fuel and air mixture feeds into the inlet manifold of the engine. Magic happens (combustion) forcing the piston downwards, causing the crankshaft to move, which makes its way through your gearbox and chain/shaft and causes the wheels to move…hopefully…ahhhh simple.

 

The carb has other components to aid engine to function correctly. The choke allows more fuel to be put into the chamber, allowing the engine to run on a richer fuel mixture during cold starts. An idle circuit allows the fuel system to operate when the throttle is not touched by the user. The XS750 carburettors also have a diaphragm, which works on a vacuum principle to help pull fuel into the air chamber through means of a needle valve. As wonderful as my sketches are, this video does a shit-hot job of animating the process.

Carburettors in modern day engines have virtually been surpassed by Electronic Fuel Injection (EFI). This is due to various reasons including efficiency, drive for lower emissions,  ease of tuning and the need for everything to be connected in a form of a feedback loop to the engine control module. If anyone has worked (or tried to) on a modern-day engine they can relate….there are so many god-damn computers in there…My Harley developed a misfiring issue last year….funnily enough it was difficult to diagnose because it always sounds like it is misfiring…(potatopotatopotatopotato) which led to a £1300 bill to fit a new brain to my Harley….thank you for giving me the D in Harley Davidson.

Anyway, if you managed to understand my crude, ham-fisted description we can get onto the fun stuff, the rebuild! There are three carbs on the XS750….one for each cylinder. As you can imagine, on a bike that has been sat out for the best part of 20 years, there was a lot of crud attached to them…and that was just the outside. The rebuild of the carbs actually started last spring time….(so productive). With the carbs off the bike, I started to strip them down. As to be expected, the inside was in a bad way…apparently 20 year old fuel does not age well…see photo album below.

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Initially, I looked for concoctions to soak the carbs in – I tried soaking in Pepsi overnight….nope a waste of good Pepsi was all that was achieved. Vinegar mix….nope.The really dangerous chemical that you can only order through the dark web….nope.  Eventually, I succumbed and purchased an ultrasonic bath from eBay. I figured I plan on doing a few bikes over the years (once I finish this one in 2029) so this would come in handy for future projects. I bought a specialist cleaner that goes with the ultrasonic bath. This was a great purchase and highly recommend it to anyone who is going to do a few projects. £40 well spent…The cleaner carried out the majority of the grunt work that had taken me the best part of a year to get started…There are plenty other parts of the bike I will throw in the bath to get them started and cleaned.

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Now I had all the parts cleaned and laid out, it was time to rebuild. Rebuild kits can be purchased for most carburettors. Yambits has an unbelievable range and sorted me out quickly with 3 rebuild kits. I also purchased an engine gasket kit (committed now…see a few posts down the line…)

For the most part, rebuilding was straightforward. anything that didn’t come in the service kit I tended to leave in place and not mess with the settings too much, whether this is right or wrong I am unsure. I did run into difficulties trying to remove the (something something) jet….this was well and truly glued in place. I ended up damaging the screw head trying to remove. I checked it was clear with some jet cleaners dad had in his garage (picture) and left it in place….hopefully this doesn’t come back to bite me…”yeah that’s the reason it doesn’t start….not the fact you tore the engine to bits not having the faintest clue what you are doing”.

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I am now in split minds what I want to do next. I know I could spend a week hand polishing every single part, and it would look good….but that’s not my style and not sure it would go with the overall look of the bike…Also a thing to note is the fact I am removing the air box and plan to run cone filters on my carbs (see below image) means i am altering the air amount being sucked into the engine and therefore will need to re-jet the carburettor to allow a fuel level to suit. Interestingly there is a lot of enthusiasts that change their carbs to Triumph models off of their 900cc triple bikes (Trophy, Thunderbird etc). So this may very well become a fall back plan if the originals don’t like my handy work…

On another note I ripped some of my garage inner roof down to reveal 3 huge holes in the abestos…source of the leak identified….after inhaling copious amounts of asbestos dust a plan needs to be put in place to fixxxxxxx (dies of coughing fit in Starbucks).

Will he survive? Who knows? Who cares!

Tune in next time to see how a minuscule task can be drawn out into a poorly written cure for insomnia.

Struan

 

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