I was having a look around on TZ250racing.com and found a great article on power jets. The article is specfic to TZ's but gives an overview of how they work. A good read for those not 'in the know'.
[b] How power jets work - and how to diagnose them [/b]
[quote] I have not seen much written about power jets. Thought I'd share what I know, and hope others can augment it. Perhaps the most succinct definition of a power jet comes from A. Graham Bell's book Two-Stroke Performance Tuning.
"The Powerjet has the effect of enriching the mixture at full, and close to full throttle, and then only when air velocity is high enough to create a vacuum of sufficient intensity to discharge fuel. It is, in effect, a load sensitive enrichment device."
Kay Nissi (sp?) at Mikuni America calls the power jet a "Band-Aid". He says it allows you to run a leaner main jet (for better part-throttle response) while maintaining a safely rich mixture at high RPMs and WOT.
The electrically-operated power jets used on later-model TZs employ a solenoid to enable and disable the flow of fuel. I have observed the power jet solenoid's behavior with my data acquisition system. Under most operating conditions the solenoid is de-energized permitting demand-based flow of fuel through the power jet. However at very high RPMs, the solenoid is energized by the CDI computer, thus *stopping* the flow of fuel. This leans the mixture increasing the temperature of the exhaust gas. Note: This behavior ensures a fail-safe mode of operation. If the solenoid fails or should become unplugged, the mixture will remain rich.
This assessment is confirmed in SAE paper 983072, Measurement of Exhaust Gas Temperatures in a High-Performance Two-Stroke Engine: "Air/fuel ratio may also be managed to influence the exhaust gas temperature. On the test engine, the carburetor was fitted with a solenoid-controlled jet. Below 11700 rpm the jet is open and additional fuel flows to the engine. Above 11700 rpm, the control system actuates the solenoid which closes the jet. This results in a leaner air/fuel ratio which increases the combustion and exhaust gas temperatures, thereby improving the exhaust tuning."
The electrical part of a power jet (the solenoid) sometimes goes bad. A bad power jet solenoid is difficult to diagnose. The engine just runs a bit rich on one cylinder. Therefore, it is a good idea to test the solenoids occasionally.
There are several ways to test the power jet solenoid. On TZs which have a battery, all you need to do is switch the bike on. The computer in the CDI box performs it's power-on self test which includes cycling the power valve servo-motor and the power jet solenoids. Using a piece of radiator hose, you can listen for two distinct "clicks" emanating from each of the solenoids. The first click is produced when the solenoid is energized -- retracting the pintel (this would stop the flow of fuel). The second click is produced when the solenoid is de-energized -- extending the pintel (allowing the flow of fuel).
If you unscrew the solenoid from the fuel-handling part of the power jet (as if you were going to replace the jet itself), you can actually see the pintel retract and extend.
On TZs that don't have an internal battery, you can perform the same test by using Yamaha's test lead (P/N 3TC-82117-00) and an external battery.
To test a power jet solenoid that is off the bike, simply apply 12 volts DC to it. (The solenoid is not polarity sensitive.) You should see the pintle retract and extend each time you apply and remove power.
Because it is next to impossible to repair the solenoid if its electrical connection comes off internally, it is prudent to zip tie the cable to the body of the power jet. This preventive maintenance provides strain relief and should keep the cable from becoming detached in a crash or due to rough handling.
Although OEM power jets invariable come fitted with an opaque hose, I prefer to run a translucent hose. (I use an alcohol resistant fuel line manufactured by Bing and sold through Aircraft Spruce and Specialty.) This allows you to verify there is fuel in the float bowl and also lets you see the fuel level. [/quote]
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