Dozens of tourists set off through Indonesia's malaria-infested jungles and potholed roads on three-wheelers as part of a rally to raise funds for forest conservation, organisers said Monday.

The 26 three-wheelers -- with wimpy 250cc engines, five-litre fuel tanks and little capacity to climb slopes -- began a two-week adventure Sunday over some 2,000 miles of mostly unlit roads between Sumatra and Bali islands.

"This is only for those with a spirit of adventure. We've told them of the difficulties," said Julian Ananda, a local organiser in Medan, a city in northern Sumatra from where the rickety vehicles took off.

"Most of them are prepared for the worst -- they have jerry cans, tyre pumps, and if we search their bags, I'm sure they'll have stashed malaria tablets," Ananda said.

Seventy-five foreign tourists -- from Britain, United States, Australia and New Zealand among other nations -- are taking part in the rally, named the "Bajai rally" after the Indian auto rickshaw brand Bajaj.

Bajaj has a presence in the Indonesian capital Jakarta, where it is pronounced "Bajai".

The rally was organised by The Adventurists, a Bristol-based group aiming to raise funds and awareness to promote forest conservation, with the participants hoping to make it to the resort island of Bali by January 26.

As Southeast Asia's largest economy grows rapidly, swathes of biodiverse forests across the archipelago of 17,000 islands have been cleared to make way for paper and palm oil plantations, as well as for mining and agriculture.

Since taking off on Sunday, some of the tourists made it to the idyllic Lake Toba, the world's biggest volcanic lake, while some teams were left stranded at workshops after their three-wheelers broke down.

Mike Margrain, 27, in a team of four from New Zealand, waited at a Medan garage Monday, hoping their third time's a charm -- their first two vehicles sputtered to a halt.

"I think our Bajaj is going to fall apart in the middle of the road and I think we'll have overheating and electrical problems," Margrain said.

"But we're four young guys and we're keen to get into it."

Others described their disastrous travels on social media.

"Endured some monsoon rain behind the wheel, couldn't see (anything)," team N-Zone from New Zealand wrote on their team's Facebook page.

"Bayly drove through the night with Woody hanging out the side holding a head torch for light. Scary as hell on the road."

British team It's a Shaw thing wrote on Facebook that their vehicle ran briefly Monday before it began belching out smoke.

"But with a little rewiring we'll be ready to start tomorrow... maybe."