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PTO Burn Downs

"Dear Ralph" December 11, 2006 0
Dear Ralph: I have a 2002 Polaris XC 800 SP with a single pipe, high flow intake, (2) flow-rites in the dash, boost bottle,...

Dear Ralph:
I have a 2002 Polaris XC 800 SP with a single pipe, high flow intake, (2) flow-rites in the dash, boost bottle, & clutching all of which is SLP. Their products are as advertised. My problem is I’m on my third piston/jug set in three years. It keeps burning down the PTO side while cruising 45-55 mph in temps in the mid to high 20s. Main jets are 620, clip position # 4 from the top.

I’ve checked timing (mechanical & timing light), crank phase, exhaust valves are updated & clean, Amsoil Interceptor oil, 92 or better octane, & all carb component part numbers have been verified. I realize the main jets are extreme for +10 to +40 degrees but I’m reaching for insurance at this point. Also have checked TPS (4.0 at WOT), air leaks (starting fluid), carb cleanliness, spark plug number and gap is correct.

The damage is always on the exhaust side of the piston and the jug is wiped out instantly. I’ve done the visual checks on the plugs at various rpm ranges & they’re almost black throughout. Snow conditions were good & water temps were fine & I monitor that constantly while cross checking readings with my brother’s same machine. I’m at the end of my tow rope with this jewel and you’re probably it’s last hope of not becoming a very expensive mailbox. Thanks for your help & keep making other mags bird cage liners.
Mike Vranich

OK, so the engine is going down on the PTO cylinder when running in the midrange. It sounds like it is detonating in this area. Detonation can be caused by several things; poor fuel quality, advanced ignition timing, poor or improper fuel delivery or an air leak in the intake tract are the most common.

Now, you mention running a larger main jet (620) for some extra insurance (recommend jetting is a 540 main at 10-40 degrees F). Problem is, increasing the main jet size does very little for the throttle range you’re seizing up at. The larger main jet will overload the engine with fuel when running 3/4 to full throttle. When you back off on the throttle and start cruising, the engine will be overloaded with fuel for a while until it dissipates and runs clean in the leaner jetted midrange area.

Ideally, to add some insurance, put a .020 needle shim washer under the needle e-clip in the number 4 position. This will put the needle in the effective #4.5 position, and this will increase the fuel delivery from 1/8 throttle to 3/4 throttle. Exactly where you are cruising and having seizure problems. You should also drop the main jets back to something closer to what is recommend to make the sled perform well. At a 620 main jet, the sled will be slow because it is always overloaded with fuel when you’re at 3/4 to full throttle.

But before we recommend that you just add fuel to the midrange, you need to verify that everything is correct in that engine. It sounds like you’ve checked the ignition timing both with a timing light as well as with a dial indicator and everything shows correct. If so, and if you are running the proper CDI box for this model, timing should be a non-issue. If you are running the wrong CDI that has high timing in the midrange, it could cause this problem and either changing to the proper CDI or adding some fuel in the midrange via the needle shim should correct it. Running in the regular-fuel switch mode may help as well. This would reduce the timing to provide more engine protection, but the negative to running in the regular switch mode would be a little less horsepower.

Next, you need to make sure the fuel is at least 91 octane non-oxygenated, ideally purchased from high traffic fuel stops to insure it has not lost any octane from sitting in the tank for an extended period of time. If you are buying his fuel in the middle of nowhere, it would be wise to either add some octane booster (NOS racing formula works well) or run in the regular switch mode. If the only fuel available is oxygenated fuel, you should consider dropping the e-clips (raising the needles) to the #5 position (oxygenated fuel runs leaner and requires larger jets) and, then the main jets should be increased by 2 sizes (580 mains).

As far as fuel delivery goes, disassemble the carbs and make sure there is nothing lodged in one of the orifices. The PTO side carb is the one that needs to be cleaned. Make sure to take the needle and seat and the main jet out, inspect and with compressed air blow through those orifices. If something such as a piece of fuel line or other debris was sitting above the needle and seat, it could block fuel flow into the carburetor. This would cause an intermittent lean condition that would be tough to troubleshoot (we have actually run into this type of problem before). Next, the carb and fuel tank vent lines need to be inspected to insure they are venting properly. A pinched or kinked carb vent line would cause this condition as well. A fuel tank vent line kink or blockage could also cause this problem, but it would be less intermittent than what you’re seeing. Also, a fuel tank pickup that was plugged with debris could cause this problem, but it would be a more remote possibility as well.

Finally, air leaks in the intake tract after the carburetor could cause this problem. You state you’ve checked the engine for an air leak with starting fluid and one did not show up. That’s good, we know that for now, there are no leaks. Did you make sure to check it with the engine at full operating temperature? Sometimes air leaks will not show up until the engine reaches its operating temperature. Also, if the base nuts came loose, it could have developed an air leak that you did not know about. The only one who would have known was the guy who disassembled the engine after it went down. In any event, you should be using the metal base gaskets rather than the paper ones. I believe that all 800s built after 2003 had this metal base gasket.

I hope this helps. The needle shim position should take care of the problem unless it is caused by something wrong in the fuel delivery system or an air leak.

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