BILLINGS, Mont. - Most of the newer model snowmobiles certified to operate in Yellowstone National Park bump up against noise and carbon monoxide limits set by park officials.

And for eight that exceed those limits, the National Park Service is requiring special kits that stop the throttle from opening all the way.

Earlier this week, the agency released the list of snowmobiles that will be allowed into Yellowstone and Grand Teton national parks this winter.

More than half of the 41 approved models dating back to 2002 are louder than 73 decibels at full throttle, which is the limit set as part of the park's "best available technology" requirements intended to reduce noise and pollution.

All, though, are within a 2-decibel variance allowed by the Society of Automotive Engineers to account for differences in testing conditions.

Several of the newest snowmobiles are also pushing the limit for carbon monoxide emissions.

The Park Service allows machines that emit up to 120 grams per kilowatt-hour. Of 24 certified models from 2006 to 2008, 13 emit 120 grams per kilowatt-hour.

Most of the machines don't approach the limit for hydrocarbons, another pollutant tracked in the "best available technology" requirements.

John Sacklin, a Yellowstone planner who has worked on the snowmobile issue for years, said he's pleased that manufacturers are producing machines that fit the agency's standards.

"The important thing is that all the snowmobiles being certified do meet 'best available technology' requirements," he said.

Those requirements are part of a temporary plan in place since 2004 allowing up to 720 snowmobiles a day into Yellowstone. A new long-term plan that goes into effect the winter of 2008-09 will allow up to 540 snowmobiles a day.

All riders will have to be with commercial guides, and all snowmobiles will have to meet those noise and pollution requirements.

Years ago, as plans were being drawn up for the future of snowmobiling, Park Service officials expressed hope that newer models would be quieter and less polluting. But as the number of snowmobilers in Yellowstone dropped, so did the market for producing machines that met the Park Service's requirements.

Snowmobile manufacturers "make a minimal number just to give them the opportunity to sell a few vehicles to the dealers in that market," said Ed Klim, director of the International Snowmobile Manufacturers Association. "It's not a money-maker."

Most of the people visiting Yellowstone these days aren't snowmobile enthusiasts, but people who simply want to tour the park, Klim said.

Manufacturers make a few machines for those customers, but most of their focus is on higher-performing snowmobiles used somewhere outside Yellowstone and Grand Teton, he said.

Still, Sacklin pointed out, those machines are far quieter and far cleaner than the older two-stroke snowmobiles that dominated Yellowstone. Since then, about $10 million has been spent over 10 years coming up with a new plan for snowmobiles and snowcoaches in the park.