Harley-Davidson's survival and success has been in promoting the modification of its bikes. Among those modifications is a kit that allows you to upgrade an 883cc Sportster to a 1200cc Sportster. While the kit can increase the performance of your Sportster, it is not without disadvantages.
No matter the manufacturer, the components and the job of installing the kit are similar. Essentially, you are removing the entire top end of your Sportster's engine and replacing it with a larger top end. This is not a minor mechanical task. While you may be familiar with motorcycle mechanics, this job can get overwhelming in a hurry. Before you start, ask yourself, "What if I follow the instructions but something doesn't work right?" Do you have the knowledge to get the project back on track? Do you have all the tools? Do you have the money to pay a professional shop to complete the project? If you can't answer "yes" to all those questions, your biggest drawback may be an unfinished motorcycle in your garage, indefinitely.
Reliability and Tuning
Harley-Davidson motorcycles are engineered to run properly as originally purchased from the manufacturer. The company has had generations of trial and error to refine its designs and manufacturing. Choosing to replace nearly half the engine with someone else's design doesn't mean it won't work well. But a risk does exist. It's no longer a Harley. It's a modified Harley. If you had a warranty, it will be void. And if you take it to another mechanic, it won't necessarily be familiar to them. Conceptually, it will be the same. But it won't be an 883 Sportster just like other 883 Sportsters. Once that change is made, if you lose reliability, especially when you install new carburettors, it can be very difficult and costly to get right. Jetting carburettors is part art and part science. You can do it. But don't expect it will be right out of the box. And don't expect it to be easy.
A 1200 upgrade kit is not an inexpensive proposition. Premium kits from well-known performance brands can cost upward of £780. You also may have to buy new tools and equipment to get the job done, or shell out the money to a pro to do it for you. Financially, you have to be prepared for a major operation such as this. Everything might go perfectly. You might assemble the bike, start it, and have no problems. To help you asses the odds of that happening, look in the classified ads of motorcycle market magazine. Plenty of people who start these projects, for one reason or another, are unable to finish them and try resell the kits or entire bikes. Also, look on a Harley-Davidson forum and ask people who have done this. You'll likely get some great tips and run into a few people who wouldn't do it again.
A 1200cc big-bore kit is an engine upgrade. It won't help your Sportster handle better or stop better. But it will make it accelerate quicker and go faster. Because so many conversion kits are available and so many possible combinations of modifications are available, it's impossible to tell exactly how much performance you will gain. A bare-bones 1200cc upgrade, installed by a professional with off-the-shelf pistons, re-jetted carburettor using stock exhaust, ignition and cams should produce between 65 and 70 horsepower and approximately 65 foot-pounds of torque.