Big ports mean big horsepower, but they also mean narrow power band. A power valve is just a piece of metal that moves down and covers part of exhaust port making it smaller. (If this has been useful, please give it a rating.Most if not all the two stroke race bikes sold now-a-days have a power valve in the exhaust port. An overly smoky exhaust is another indication that the mixture may be too rich.Īn example of 'Four Stroking', (thanks Al.Fisherman, for pointing it out):. This occurs when the fuel/air mixture is so rich that the engine only gets enough air to fire every second revolution, reducing power and making the engine sound like it is only doing half as many RPM.
One of the more common signs of a rich mixture is 'Four Stroking'. I had thought about including pics of plugs, but don't have any that aren't tan, right now. If the plug is grey or whitish the mixture is too lean. If black and/or wet, the mixture is too rich. If it's tan, the mixture in that range is OK. Now, without changing throttle opening, pull in the clutch, hit the kill switch at the same time and roll to a stop.Ĭheck the plug color. Run the engine at the throttle opening for the area of concern for a bit. In effect, the engine is first warmed up to operating temperature, then a new plug fitted. One of the best ways to check the mixture is by doing a 'plug chop'.
To adjust the mixture at ¾ to full throttle openings, the size of the main jet needs to be changed, smaller for a leaner mixture or larger to make it richer. When the needle is set lower, relative to the slide, less fuel is allowed into the mixture for a given amount of air, making the mix leaner and vice versa.
The slide needle has a series of notches near the top, (four on earlier carbs and five on the later ones), so that the attached 'C' clip can be removed and refitted to raise or lower the needle relative to the slide and slide needle valve. There is only one direct method of mixture adjustment on these carburetors. Before attempting any carby adjustments, ensure that the carb to inlet joint is well sealed, the inlet gasket is in good condition, the carb is clean, the fuel filter and air filter aren't blocked and that the inlet tube nuts are tight. See 'Four-Stroking', in 'Diagnostics' below. The second most common problem is a rich mixture, (usually after sealing the inlet leaks), resulting in four-stroking and reduced power output, often in both the mid-throttle openings and at full throttle. The most common problem is a lean mixture due to an air leak where the carburetor attaches to the inlet tube or where the inlet tube attaches to the cylinder.
The NT carb supplied with most typical HT engine kits is very simple and so should be easy to tune, but many have trouble getting it right, resulting in low power, low revs, smoky exhausts, fouled plugs and excessive buildup on heads/pistons if the mixture is over-rich or, in the other extreme, overheating, melted pistons, seized engines if the mixture is ( way) too lean. In the end, many seem to run well on about an 0.72 to 0.73mm main jet, with standard exhaust and porting. Specs vary with individual carbs and suppliers.įor a 66cc engine, I've heard of stock main jets sized at 0.6mm,(MBB Imports), 0.7mm,(Many US kits) and 0.79mm, (ZBox 66cc). The slide needle setting also controls the idle mixture. There is no idle circuit, pilot jet or idle mixture screw on these carburetors. To adjust ¾ to full throttle mixture, the size of the main jet must be changed, by replacement usually, although drilling or soldering and drilling is an alternative. The slide needle is adjustable, up/down, to increase, (enrich), or reduce, (lean out), the amount of fuel in the mid-range mixture. When the throttle is opened further, from ¾ to full throttle, the 'main jet', screwed into the bottom of the slide needle jet, regulates fuel flow. A slide needle extends below the slide into the slide needle jet to regulate fuel flow during low to about ¾ throttle openings. The slide regulates the flow of air into the engine. 'Venturi' action draws fuel from the float bowl upwards into the in-going airstream. The NT side-draught carburetor operates by drawing air into the air inlet past the slide needle jet. Also see srdavo's NT Carburetor Assembly Pics for a bit more info.