The Yamaha FZR750 Engine Rebuild

Right from the first day I saw this bike I knew it was doomed, I mean it's a indescribable color and the engine makes sounds I never hear before.

It's another one of those bikes I just bored everyone about how good it was, and how if everyone bought one of these instead of being obsessed with style and getting an Yamaha YZF-R1 they can't control they'd all be better riders blah, blah blah.

But when all is said and done, it is still hand painted in the most ugly colors I ever seen, which is a bit like going our with a red headed bird, cos we all know the cuffs and collars match. Anyway, the Exup (Exhaust Ultimate Power valve) was so typically Yamaha I should have known it wasn't going to stand my imbecilic lofting techniques even after a decade of refinement. With a gear shift that feels like it's made out of dental amalgam, and a clutch basket that sounds like its full of broken Chinaware, the Exup and indeed the whole Yamaha range has always been the most frail of 4-stroke engines.

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Blueprinting and Building a Good Engine Case

Engine blueprinting, as in previous article, starts at the crankcase, where the builder confirms that the main bearing surfaces are within a few ten-thousandths of an centimeter of being concentric and in-line. He also checks that the cylinder decks are correctly machined, and the oil passages are clear, open, and go where they should.

From there, it progresses to the crankshaft assembly, where the flywheel, or -wheels, should be within 0.0254mm of being perfectly true (in twin parallel to each other, in single perfect to design). The connecting rods will also be fitted to the crankpin to allow the bearings their 0.0254mm of clearance.

Pinion bearings require slightly less clearance. To measure that, the builder measures the inner diameter (ID) of the bearing race with a dial bore gauge and the outer diameter (OD) of the pinion shaft with a micrometer. He then subtracts the shaft OD from the race ID and compares the difference to a chart to determine which size bearing to use.

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Dynamic Balancing and Beginning Blueprinting

Static of Dynamic balanced fly-wheel, the process we described before is know as static balancing because the flywheel is not in motion when balanced. Static balancing is still favored by many engine builders, but dynamic balancing has gained a large following.

Because it allows computer sensors to assess flywheel balance with the flywheel balance with the flywheel in motion, pinpoint any imbalance, and determine the magic number in matter of seconds, dynamic balancing is significantly faster. Proponents also claim that it is more accurate.

Although a flywheel may appear perfectly balanced on the stand, the story sometimes changes when the flywheels are assembled with the other crankshaft components and the assembly spun up to several hundred rpm. Shaft and flywheel flex can cause high-speed oscillations that throw balance off, as can minute imbalances that are undetectable by static-balancing methods but which increase dramatically with engine speed.

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A Well-Balanced Flywheel

A well-balanced flywheel, with all that, from the previous story, out of the way, let's plug the weight of some imaginary components into our formula and balance a hypothetical engine to 60 percent. Note that although some engine builders take pains to match piston weight exactly, others feel that a few grams 'difference is insignificant. To limit the story length, we'll ignore the controversy and work with combined weight rather than the weight of each component.

Rotating Weight: Rotating end of Rods: 865grams, Crankpin and keys: 565grams, Crankin nuts: 80grams, Bearing assemblies: 70grams, Lockplates and screws: 10grams, Total rotating weight: 1,590grams,

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Turning a Yamaha Fino into a Racer

Before we start, we need to be clear that the Yamaha Fino will never be one of the top choice for speed geeks. We found after our installment of the engine modification that the Yamaha Fino becomes hard to drive at higher speeds. Some moments the Yamaha Fino was outright dangerous to handle. After we examined the cylinder of the Yamaha Fino we came-up with the idea to do a bore-up to 63mm, the stock bore of the Yamaha Fino is 55mm. Changing the bore from 55mm to 63mm will change the Yamaha Fino from a 114cc to a 189cc motorcycle.

After contacting some of our sources for accessories we found a manufacturer who produces a complete set, which includes a 63mm Ceramic Laminated cylinder, a high performance 4-valve cylinder head (the Fino comes standard with only 2-valve head), 63mm Piston, high performance racing camshaft and the materials to make this all fit and assemble.

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Carburetor Tuning, to Rich or to Lean Condition

In previous stories I talked about carburetors Rich conditions and Lean Conditions, this is all fine of course if you know how to detect if your engine carburetor is set Rich or Lean.

To help with that, I will explain simple means to detect the setting of your bike. With a perfectly tuned engine you will get the power you need, it amazes me sometimes what people spent on kits to improve performance, while all they needed to do is get the settings right.

Okay that said what is a Rich Condition, while a black, scooty spark plug is a sure sign of richness, there are other indicators that are a bit subtler and better ways of dialing in the carburetor jetting.

If your engine responds crispy at low throttle when it is cold, chances are the main jet is one size larger than it needs to be. Assuming, of course, that the idle circuit is correctly tuned.

Poor fuel mileage is another sign of richness and because of the way most of us ride our motorcycles, that richness is unusually the result of a needle that is too small.

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Kidding Me I Not Have a Dynamometer

In some previous story, the tuning of a carburetor, I talked about using a Dyno, we got several reactions about this. A Dyno by motorcycle mechanics or dynamometer for English Professors, is a machine used to measure torque and rotational speed (rpm) from which power produced by an engine.

Okay this Dyno's are not something you would buy just to tune your carburetor, hence it is cheaper to visit multiple motorcycle tuning shops, until you have the right performance.

So if you don't have free access to a dynamometer, don't worry, I did years without also. With a stopwatch, tachometer, and some imagination you, too, can measure the power output of your engine accurately enough to get your carburetor dialed in correctly.

Consistency is the key to these types of performance tests. When doing any of these tests take into account air temperature and humidity.

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Modifying a Yamaha Fino Scooter

If we look at the basics of the Yamaha Fino, it is a 115cc 4-stroke SOHC, 2 Valve, Air Cooled engine. And officially the engine is a 113.5cc engine (but guess 114cc Fino did not sounded cool enough for Yamaha marketing).

We always start our tuning / modifying with a simple study of the motorcycle, in case of the Fino, the engine is a regular visitor to our modifying shop all is this the first Yamaha Fino, the also very popular Yamaha Nouvo uses the same engine.

After we had a quick look at what the Fino is made off, we decided to not overdo ourself, as this Yamaha Fino will be mostly used by family members and collages, we surely did not wanted to forget the safety futures.

The Yamaha Fino uses a Mikumi BS25 x 1 carburetor, or Carb for the Americans, some will suggest to replace the carburetor with a more sporty one. But as we have maybe a bit more know-how, we know that the Mikumi BS25x1 carburetor can be found in much larger bikes then this 115cc Yamaha Fino scooter.

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Motorcycle Lubricants what to select

All Motorcycle engines require different oils a 4-stroke engine needs different oil then a two-stroke, The 2-stroke engine needs oil mixed with the petrol (gasoline) before combustion. In case of a 4-stroke engine the oil is just used to lubricate the engine. Make sure that you are using the right oil, the wrong type of oil can cause serious damage to your motorcycle engine.

Consider using non-synthetic oil for the break-in period according to the owners manual, the break-in period is the first 1000 kilometers. In normal situations there is no need to replace the factory oil. During the break-in period, riding at no more than 80% of top speed is highly recommended.

In addition to this, many mechanics suggest that during the break-in period owners should use non-synthetic oil, and then switch to synthetic after the break-in period is over.

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Carburetor Tuning, Tuning the Main Jet Circuit

With the introduction of the new Yamaha motorcycle 2008 model line, it looks that the end of the good old carburetor is in sight. The carburetor as we know it today was invented by a Hungarian engineer back in 1892.

But enough about the sentimental part, and back to our task: tuning the main jet circuit of a regular carburetor.

Tuning or adjusting the main jet circuit is where the most confusion seems to be. The main jet really doesn't come into full play until the last quarter turn of the throttle. When the needle rises in the tube far enough that gap is equal to or greater than the size of the main jet, the main jet becomes the metering device for fuel flow.

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