BAR HARBOR – When the Island Explorer bus system hits the road later this month, its fleet will not include the five to nine new buses it had hoped to procure this year.

Downeast Transportation Inc. of Ellsworth, which runs the Mount Desert Island bus service, has leased three diesel buses as backup vehicles for its fleet of 17 propane buses, according to Paul Murphy, Downeast Transportation general manager.

Downeast Transportation also will secure a school bus “as a last ditch backup” should it be necessary, Murphy said Friday.

The new buses Murphy expected to get this year are still on hold until the complex purchasing processes are complete with both the federal and state transportation departments.

As Murphy put it, “It’s just that the wheels move slowly.”

The Island Explorer was launched in 1999 as a free transportation service to MDI residents and visitors, which number in the millions when Acadia National Park opens each year for a new season.

The service was meant to relieve traffic congestion and air pollution on MDI, Maine’s premier tourism destination, both by operating on clean-burning propane and getting people to park their cars.

Last year, Island Explorer buses carried 340,000 riders, a 7 percent increase over 2002 despite a sharp drop in Acadia visitors.

Murphy said he could not find any propane buses to use as backups this summer. Two of the diesel buses will be used only as backup vehicles to Island Explorer buses and the third will be used to increase service on the busiest routes.

The national park continues to be the major financial sponsor of the bus service, contributing $403,000 this year, or nearly half the bus service’s operating budget, Murphy said.

Beginning this summer, Acadia will charge $20 for a weekly pass, up from $10, and double the annual pass to $40 to raise more money for the bus service.

Len Bobinchock, Acadia’s deputy superintendent, said Friday the new revenue, which is being called a “transit fee,” will be dispersed a year behind collections, so this year’s revenue will be used next year.

L.L. Bean, the legendary Freeport outfitter, has provided $1 million over four years to the Island Explorer system, making it the second-largest contributor. The towns and businesses on MDI also help fund the service and together will donate nearly $100,000 to the effort.

According to Murphy, the only notable change for the new season, which starts June 21, is the addition of a midday campground run on peak visitor days.

He said the noontime run from the campgrounds is the most popular, so much so that many visitors last year had to wait for a later bus.

This summer, the bus service will run two buses at midday on the campground route, Murphy said.

Ken Olson, president of Friends of Acadia, another founding partner of the Island Explorer, said Friday he hopes MDI businesses are promoting the bus service in their advertising campaigns to further encourage ridership.

Record high gas prices also may entice more visitors to park their cars and take the bus, he said.

Information on the bus service, including routes and timetables, can be found at the Web site, www.exploreacadia.com.