So, you want to go faster. Bad boy ! Go to the back of the class!!
Its not economic advisable to tune your bike. It's value will not increase by nearly the amount of money you put into it. Actually its a cheaper solution to sell the bike and buy a faster one.
But all the talk about economics it no good when you don't want to part from your Moto Guzzi, but still want to go faster. So, max out your credit card and buy some fancy parts for your bike.
And besides, its great fun to work on your bike during the long and cold winter. Oily fingers, Thin Lizzy on the radio, and the mind completely locked into technical details like camshafts and valve guides. It doesn't get much better than that - especially not if your garage is heated :-)
I'll describe the most ordinary ways to tune your Moto Guzzi, modifications done with success by many other owners before you. I presume that the bike is to be used on the street, so I'll elegantly avoid the most extreme solutions. This means that you have to look elsewhere if you want to read about turbo and Nox equipment.
If your Moto Guzzi is born as a 850cc you have plenty of opportunities for more cc's, because Gillardoni makes 88mm = 944 cc cylinders to bolt right (this is what Moto Guzzi calls 1000). Gillardoni is OEM manufacturer for Moto Guzzi A.O., so quality is top note. They also made a kit with 90mm cylinders, but it was never very successful because you have to enlarge the hole for the cylinder base in the block. This makes a somewhat larger operation for the extra 48cc provided by the 90mm cylinders.
Not many people enlarges the 1000cc's but it can be done. German Dynotech makes a kit that can enlarge the engine to 1200cc, but only for square fins.
Large cylinders will only give a few extra HP's, but a lot of torque to make the bike a more relaxed ride on the road. Nothing is for free - the larger pistons will roughen the engine a little.
If you own a small valve model (T3, SP A.O.), carbs larger than 36mm wont benefit you, but a Le mans works beautifully with 40mm's. When you fit larger carbs, remember to open up the inlet duct to make a free flow to the valve. If you are uncertain about how to do it right, pay a professional to do the job, cylinder heads are expensive.
Also, allow a lot of time for jetting, you don't just bolt on bigger carbs and take the bike to the street. To gain the full benefit of the new carburetors, they must be jetted for your bike. The difference between a poorly and a correct jetted bike is tremendous, they are really worlds apart.
Some people fancy Mikuni's, but personally I prefer the Dell Orto's, they are easy to work on and parts and jets are quite cheap. If you scrap the air filter and install velocity stacks, you will gain more air flow (more power) and a nice intake roar, but also a greater degree of wear on carburetor and piston rings.
There's a lot of original and after market cams available for Big block Guzzi's, I'll describe the three most commonly used.
The first one is the ordinary one that is fitted by the factory to a great number of models. It has been the standard cam for Moto Guzzi's for many years and don't even have a name. Some refer to it as the "lawn mover cam".
The next one is called the B10 and is a tad faster. Its also a factory cam, and was included in the official tuning kit sold by Moto Guzzi years ago. It was also the standard cam for the Le Mans 4+5 models.
The last of the ordinary cams are named P3 and is somewhat higher than the B10 without being extreme. It's not a factory cam, but lots of people are using it and its still a nice street cam. If you want to install one of these you should also consider harder valve springs to make sure the valves are closing in due time.
There's more information about cams in Pete Roper's Cam Notes.
The original clutch/flywheel has almost the same weight as an oil tanker flywheel !
It makes the engine response rather sluggish which is untypical for modern motorcycles, fortunately lighter units are available. A lighter clutch will not increase power, but the gas response will be so much quicker that it feels like the bike has an extra 20 HP's. Most owners also experiences that the gear change gets better. An original clutch weighs 8,3 kg, where the RAM clutch (a popular exchange) is only approx. 4 kg's. this is a major reduction of the rotating masses. Believe me, the difference is just incredible.
With two big valves in the cylinder head there's no room for the spark plug in the middle, so it is installed off-center. This is not an ideal position because the gas/air mixture will not be ignited symmetrically and it takes longer time before all of the mixture is on fire. This may cause the mixture to detonate rather than burn, also known as "ignition knock"
The slow ignition also means that the spark has to occur earlier, which is also bad. The advanced timing makes the engine hotter and steals power. The solution is of course to install an extra spark plug in the other side of the cylinder head. Several companies can do this for you - it's not homework.
On a roundfin engine its quite simple, its just a question of removing a couple of fins, drill a hole, and cut the thread for the spark plug. That's it. On the squarefins there's unfortunately an oil channel exactly where you want the hole for the plug, so you'll have to move the oil channel to another location. Its possible, but not cheap.
But you are not done yet, you also need two dual ignition coils including cables and spark plug caps, and you'll have to limit the ignition advance from 34 to 28 degrees. This is not easy to do correctly, so many people buy a Digital ignition instead where ignition curves and maximum advance can be selected freely.
- Talk about tuning, and most people will think engine and horse power, but chassis tuning is just as important. If you cant get all that power transferred to the tarmac, and the bike don't steer properly, all the time and money you spend on the engine is worth nothing.
- The front fork is rigid enough for street use, but the original dampers inside it are worthless. FAC and Bitubo make proper ones. Or you can buy a complete 38mm Marzocchi fork that will bolt straight on.
- Rear dampers must be of good quality because they are challenged a lot by the short swing arm on a Guzzi. Unfortunately KONI are no longer in production, but there are other fine brands. Don't buy the cheapest ones.
- Tires are important. There's a lot of religion involved in opinions about tires, but start by picking a brand that is commonly known in the market. Don't buy some Chinese nylon tire to save a little money. Next thing is to have the correct pressure in the tire, surprisingly many people ride around on semi flat tires. It increases tire wear, and the bike behaves like shit. And one last thing: Buy a tire that fits the rim !!! What I mean is that you should not buy a wider tire than the instruction book tells you to. A wider tire will NOT increase your sexual performance and the bike acts like a pig on roller skates ! Get it ?
- End up by checking the steering head bearings and the swing arm bearings, and install a set of steel braided brake hoses (These are actually not allowed on the street in some countries - Crazy bureaucrats !) and you'll have a nice ride. It aint that hard.