Snowmobile Service Tips for Ski-Doo DPM diagnosis, REV 600 H.O. SDI Silencers, Cylinder Head Repair

Q: I have a 1999 Ski Doo MX Z 670 H.O. with DPM. At the beginning of last season it developed a lean bog condition when accelerating. I assumed the carburetors were dirty, so I cleaned them thoroughly and replaced the pilot jets. Still no success. I was told this model is known for carb boots that crack after a few years and can leak, but my sled’s boots are in perfect condition. The airbox is correctly seated on the carb bodies and I checked for leaks in the base gasket and crank bearing seal on the clutch side by spraying starting fluid around the seal. This did not reveal any problem, although I could not access the flywheel side crank bearing seal to check it. I am out of ideas. Could something be wrong with the DPM?
Jason Magnin
Oconto Falls, Wisconsin

A: First, confirm that there aren’t engine leaks by performing a leakdown test. This test pressurizes the inside of an engine and allows you to monitor for loss of air pressure through head gaskets, base gaskets or crank seals. Your dealer can perform this or you can do it yourself with a test kit. DPM systems are typically rugged and we have not heard of many failures. The system calibrates the air/fuel mixture for the best engine performance in varying conditions. Start simple. Check for cracks in the hoses that travel between the DPM manifold and the carburetors. There are four hoses that connect to the carb vent lines and two hoses to the carb venturi. If a component linked to the DPM system fails, it can cause a lean bog. Perhaps the engine temperature sensor failed and is causing the DPM to incorrectly compensate for a false hot engine coolant-temperature, which would cause the lean condition you’re facing now. The temperature sensor is screwed into the PTO side of the cylinder head and should have resistance of 2500 ohms +/- 300 ohms at room temperature. If it measures out of this specification, replace it. If the manifold hoses and temperature sensor checked out OK, the DPM manifold itself might be blame. There are tests that can be performed to check the solenoids inside the manifold, but this diagnosis is quite involved and should be performed by a competent mechanic. If you’re confident in your ability to perform these tests, buy a Ski-Doo service manual and follow the step-by-step testing procedures. The retail price of a new manifold is more than $300.

Stripped Threads
Q:I accidentally over-tightened the spark plug and stripped the threads in the head of my Polaris Indy Lite. The threads are torn up badly and I wasn’t able to cut new ones with a tap. This is just a beater of a sled and I don’t want to stick a lot of money in it. Is there anything I can do other than buy a new head or JB weld the spark plug in place?
Shane Veldhauzen
Green Bay, Wisonsin

A: Don’t worry, Shane. There is a good option for you to save your Indy. Heli-Coil Screw Thread Inserts are an inexpensive way to repair damaged threads. A Heli-Coil is a coil of stainless steel wire with a diamond-shaped cross section that can be threaded into clean threads, and the internal threads accept a fastener — in this case a spark plug. These things work so well they’re even approved by Boeing Aircraft. Basically, what you’ll do is drill out the spark plug hole to the size prescribed with the Heli-Coil, cut appropriate threads into the head and then thread the insert in place. Heli-Coils are usually available at auto parts stores and some well-stocked hardware stores. Be sure to remove the head for this modification so you don’t put metal shrapnel inside the engine while you drill out the spark plug hole. You can expect to spend about $10 for the part.

Aftermarket Silencers
Q: I have a 2007 Ski-Doo REV 600 SDI [and] I would like to install a trail-friendly silencer on it. Can you tell me which manufacturers make one that bolts on and doesn’t require clutch or mapping modifications?
Phil LaBine
Grand Forks, North Dakota

A: Clutch or fuel system modifications aren’t required when you install a silencer. Clutch calibration changes are only required for modifications that increase horsepower, but since the cans only add 3 to 4 hp the pulleys can run stock. Fuel system mods aren’t necessary either because, while silencers increase airflow through an engine, they don’t increase it enough to necessitate more fuel. There might be a silencer out there that calls for transmission or fuel mods, but generally speaking they’re a bolt-on and go part. More importantly, though, is that aftermarket silencers aren’t typically “trail-friendly.” They do cut weight, but make sure you buy one with a stock or near-stock sound level. Whether it’s a dirtbike, ATV, boat, motorcycle or snowmobile, making more noise than what is reasonably necessary is bad. Loud sleds annoy landowners, neighbors and other snowmobilers, and that can lead to trail closures.

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