Along with the mandatory noisy pipe, Dynojet kits are the de-facto tuning mod, unless your bike is fuel injected of course, when you have to go and buy yourself a rewritten rom chip or shell out shed loads of money for a Power Commander. This magical box allows you to download fuel and ignition maps from the Internet and upload them to your bike ... so what are you buggering about with carbs for!?

Typical bits you get in the kit

The basic carb kit consists of needles, circlips, shims and jets. For some bikes you will also get needle shroud extensions, replacement slide springs and drills to enlarge the slide lift holes. When comparing the original needles you'll notice the Dynojet ones have a different profile, designed to match the supplied jets to give the fuelling at different slide heights that Dynojet want. Although they probably look just like your old ones, the jets themselves are not the same and are matched to the needle design. Because of this you may find you are given a different size jet, perhaps smaller than your original one. Don't think this means less top speed! Better fuelling will enhance performance.

Main jets

A word about carbs if you're not sure how they work. Most production road bikes use CV or constant velocity carbs. These employ a vacuum slide to which the needle is attached. As you open the throttle you rotate a butterfly valve in the carb throat, allowing more air to flow into the engine. The increased airflow creates a low pressure area above the slide which rises to equalise the pressure, so lifting the needle and allowing more fuel to be drawn through the jet. This means the carb will not over-fuel as the needle will only rise in line with the airflow into the engine. When you close the throttle the airflow reduces and the slide spring pushes the slide back down, so lowering the needle.

The only downside to this design is that there is a butterfly valve across the inlet tract, disturbing the air flow. Flatslide carbs get round this by connecting the throttle directly to the slide, so when you twist the throttle you lift the needle without any intervening control. The problem here is over-fuelling - twist the grip too fast and there is insufficient airflow to atomise the fuel properly so the engine bogs down. This is a real pain on the road where the engine is often revving slowly, so the CV carb makes it all pretty foolproof.

Dynojet often supply different slide springs and drills to enlarge the lift holes. Their thinking is that with better airflow generally if you have fitted a K&N air filter and race exhaust, the slide lift speed can be increased to give better throttle response. Don't be tempted to make the lift holes bigger than recommended, or the carbs will over fuel and ruin throttle response.

Fixed standard needle is at the bottom

Occasionally Dynojet recommend increasing the size of the main and pilot air jets, and some kits include needle shroud extensions which are fitted over the existing shrouds to assist fuel atomisation. Other less common parts include replacement pilot air jets, air corrector jets and main jet holders.

One of the bonuses of the Dynojet needles is the fact that their height in the slide can easily be adjusted, as they have 5 grooves at the top with a circlip retainer. This allows you to lower the needles to obtain the best fuelling, aiming to run the mixture as lean as possible. You should also get small washers or shims which go under the circlip to give a half-notch drop. You may also get a range of jet sizes, again allowing you to experiment with the fuelling mixture.

Unfortunately setting up carbs is a black art and is most easily accomplished with copious amounts of dyno time, which gets expensive. The standard recommended set-up usually works well, and experimentation can be carried out by simply riding the bike and noting the changes in throttle response and performance. But be prepared to strip your carbs many times before you get it right! Often you will get midrange fuelling right and then have a weak top end, or get blistering top speed and a fluffy midrange. Most decent dyno operators will know the setup for each bike and can save you much time.

K&N filter kit

Most modern Japanese bikes have pretty good fuelling and a Dynojet kit fitted to a standard bike will not make much difference, unless your needles are worn. However, start adding free-flow air filters and race exhausts and a kit will allow you to change the fuelling to compensate for the better airflow. K&N filters are the usual choice and for around £40 you can forget about replacing paper elements every few thousand miles. The K&N relies on oil to trap particles in the weave and they just need cleaning and re-oiling to restore them to top condition. They also continue to work well when dirty, whereas paper elements start to block up and restrict airflow. Most useful with ram-air bikes, where the filter gets a hard time.

An alternative to Dynojet is Ivan's Performance Products. They reckon that their carb kit is the result of 8 months of combined testing. It is designed exclusively for use with a 4-1 (not 4-2-1) exhaust system. This means that it is designed for use with Akrapovic race exhaust pipes or any brand of slip-on can, including the stock one.

The needles have 4 angles ground on them, the final design chosen after testing. The test bike was tried with numerous airbox modifications, different air filters and ignition advancers. Ivans reckon none of them would outperform their kit with the stock air filter, at any engine speed above 4000 rpm. Ivans do kits for the R1 and R6.

After 25000 miles the R1 has developed a bit of high frequency vibration. Doesn't seem as if anything important has failed but it's enough to make the hands and feet tingle and go a bit numb. It seemed to happen quite suddenly, but it may just be that it's reached a point where it's become a nuisance.

The problem is definitely engine related and not really dependant on engine revs, only disappearing when chugging along at 2000 rpm or so.

First logical idea was to check carburettor balance as the tickover was a bit uneven. Not at all sure imbalance of the carbs would cause such high frequency vibration but no harm in balancing them anyway. How to do it? The easiest bit of kit is the Carbtune by Morgans, a brilliant design employing four stainless steel rods in plastic tubes. Simple, robust and accurate. £52 delivered to your door.

A quick bit of fiddling needed to start with. Cut 10cm off each of the metre long connecting tubes, insert a damping restrictor cut from a supplied bit of narrower tube and push the bits of tube back together. So we wind up with four tubes again, but now with restrictors in. These stop the rods from oscillating wildly about when taking the pressure readings.

The gauge is marked in centimetres of mercury pressure. Your bike specs should tell you what pressure your carbs should be at tickover. The R1 is 22 cm/hg at 1100 rpm.

OK, the slightly tricky bit - getting to the blanking screws in the intake manifold, between carbs and head, so you can fit the balancer pipes. Lift the fuel tank and prop it up, remove the airbox and shove a torch down behind the engine so you can see what you are doing. Getting at the blanking screws is not too bad on the R1, but make sure you don't lose the screws or copper washers.

Then insert a supplied adapters into the end of each rubber tube and simply screw them hand tight into the manifold - rubber washers make sure the seal is good without having to overtighten. You can use the tube to guide the adaptor in as the chances are you won't be able to get your hand in there and it saves the extra work of having to pull the carbs off to gain access.

Start the engine, set tickover to around 1100 rpm and watch the rods bouncing gently up and down in the Carbtune gauge. As you can see from the pic, my number 1 cylinder was way out, which was encouraging. It took all of 5 minutes to adjust the stop screws to get the rods all lined up. Firstly adjust the screw between carbs 1 and 2 - this balances 1 to 2. Then adjust the screw between 3 and 4 and balance those two carbs and finally the centre screw which balances 1 and 2 with 3 and 4.

If you can't get to the stop screws easily simply stop the engine and twist the throttle to bring the screw into a more easily reached position. An eighth turn or so of the screw is usually enough, and then retest. If the reading's worse just turn the screw the other way.

Once everything is within one cm/hg then you're finished. Let the engine cool down and the adapters can easily be unscrewed by twisting the connecting tubes. Replace the blanking screws and do a final test. If you want to make life really easy next time you can get a kit of adapters and heatproof tubing which fits permanently in place, well worth the £10 it costs if your carbs are a bugger to get at.

Did it help? Yup, tickover is super smooth now.

You can use the Carbtune on fuel injection units as well to ensure that the butterflies are opening together.

The following setup is for a standard engined with race can (not full exhaust) fiitted.

As you may recall from the VTR feature I wrote many moons ago, I had all sorts of problems setting up my VTR, but that's because I'd buggered about with the engine internals and it's probably best to leave that alone, unless you have a larger budget than I did!

Last year, when Pete bought his Firestorm, he did me the honour of purchasing my Arrow cans, Dynojet kit and K&N filter as a job lot, but on the condition that I fit them and set it all up. This I agreed to and I took his money.

The Arrow cans were fitted the day he bought the bike, for obvious reasons, and the jet kit and filter have been fitted this week, (quite a few months later). The beauty of fitting a jet kit to a twin is that there are only two carbs - makes life much easier. Although everything is much bigger, this in turn helps too. The carbs fitted to a VTR are 48mm! I can only remember carbs this size being fitted to cars and usually in the shape of Weber and Dellorto, but now that size is on bikes, or at least one type of bike.

The carbs on a VTR are a piece of piss to get off (especially when you 've done it in excess of 50 times before). First unbolt the tank at the rear and lift. Using an 8mm open ended spanner turn off the fuel - you can't see the valve, so do it by feel, it only turns 180 degrees. Next pull off (ooer) 5 fuel and breather pipes, 3 from the fuel tap and 2 from the underside of the tank.

Remove the 2 bolts at the front of the tank and lift the whole thing off. Next remove airbox cover and element. Unscrew bellmouths (short one on front carb and long one on rear carb. Underneath are 4 (on each carb) smaller screws holding the air box down. Next remove the breather pipes from the rear and the front of the air box, (these come off with a gentle tug, ooer again). Hey presto, carbs revealed. Use a 17 mm spanner to loosen the choke, remove it and push it under the thermostat housing. Undo the carb retaining screws on the inlet tract, the throttle cables and the throttle sensor. Pull off the carbs, gently. Before removing them completely, you have to remove the 2 heating/cooling tubes .. pliers will do for the clips.

Next you have to pull the carbs apart, I'm not going to go into details about each screw on this, but below are the settings that I had suggested to me once, and are exactly the same as Dynojet advise - funny that. Oh, by the way, you'll have fun with the slide springs, you'll never find longer and more springy ones on any other motorcycle, but again these are no problem for me now …. first time, every time.

Settings
Main Jets - 180s
Pilot jets - 2 to 2.25 turns from closed.
Needle - use notch 4 from the top and retain the Honda supplied shim under the circlip.
Drill an extra hole in the needle slide.