Powerjets

Electrically-Operated TZ Powerjets

Electrically-operated powerjets used on later-model TZs employ a solenoid to enable and disable the flow of fuel.  I have observed the powerjet 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 powerjet.  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 powerjet (the solenoid) sometimes goes bad.  A bad powerjet 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 powerjet solenoid.  On TZs which have a battery, all you need to do is switch the bike on.  The computer in the CDI box performs its power-on self test which includes cycling the power valve servo-motor and the powerjet 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 powerjet (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 powerjet 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.

Although OEM powerjets 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.

TZ250 Powerjet Solenoid Repair

The photo below shows an original Yamaha 3XV-1410H-00 powerjet (top) and one I repaired (bottom).  The last time I saw a price (before being discontinued) it was ~$280.  Repair is not just be an economical option, it may be your only option.

My failure was caused by a broken wire below where it was molded into plastic (under the rubber top).  

The repaired unit is approximately 10 mm longer than the original.  This additional length did not cause any clearance issues with my bike.

The job involves cutting the body apart in a lathe, repairing the broken wires, silver-soldering a short brass extension onto the body, and potting the extension with epoxy.

I could think of no way to “unroll” the rolled edge of the steel body, so I started by cutting the body apart in a lathe.  I held the steel body in a 5C collet so as not to distort it.  I used a 1/8-inch parting tool to just cut through the rolled edge at the top. This method of cutting off the rolled edge just barely touched the steel top.

The coil assembly comes out of the steel body via the top.  I was surprised to find the coil was completely encased in plastic. (I had expected a mess of wire.)  That coil assembly is riveted to the steel top.

I was able to pare away the encasing plastic with an X-Acto knife to expose the wires.  This revealed several millimeters of stranded copper wire. (Which was still the connection wire to the outside world, as the solenoid coil itself would have been wound from enameled magnet wire.)  It was easy to solder new external connection wires onto the stubs that stuck out of the solenoid assembly.

I cut a short length of 5/8-inch brass tubing (available at large hobby stores) to fit onto the top of the steel body.  The ID of the brass tubing is just slightly larger than the powerjet's steel body (above the hexagonal wrenching area).

I “tinned” (applied a thin coat of solder) to both the ID of the brass tubing and the OD of the steel body.  I used a liquid acid flux to clean the steel part.  Brass solders very easily, but the dirty plated steel needs a lot more prep work.  I used an electric soldering iron to heat both parts.  While soldering the brass to the steel, I wrapped the pintel end of the assembly in a wet rag. (I had removed both O-rings, but the rubber seal for the pintel must remain.)  I used a 3% silver solder to tin the steel (not sure if that was necessary) and regular electrical solder for everything else.

After cleaning to remove any remnants of the acid flux (this is VERY important), I could drop the plastic solenoid assembly back into the steel body.  You need to make sure the solenoid assembly is “bottomed out” in the steel body.  At this point I tested the unit by applying 12 VDC to make sure the pintel would retract.

Then I filled the brass portion with my favorite epoxy (J-B Weld).  This is what provides the strain relief for the external wires as well as securing the solenoid assembly in the steel body.

In the photo, you can see that the repaired solenoid is longer than the stock powerjet solenoid.  I think I could have made the increase in length shorter than I did, but it does not interfere with anything.  

It is prudent to zip-tie the cable to the body of the powerjet.  This preventive maintenance provides strain relief and should keep the cable from becoming detached in a crash or due to rough handling.  You can also see the zip-tie strain relief on the stock powerjet solenoid.  This is essential to assure a long life.

In some cases it may be possible to avoid the lathe work and brass/steel soldering.  If you are able to get to unbroken bare wire with a knife, then just epoxy the brass tube in place to secure the connection and provide strain relief.  This could make a quick and effective repair.