Crazy erratic idle. Any ideas?

[Sandman251]

So I'm considering the purchase of a 2000 Challenger 20 with a 240 HP. Motor was rebuilt by a reasonably high-volume shop about a year-and-a-half ago. Premix block off kit was installed . Owner claims to have pulled and reinstalled the motor "himself". Compression is 130 lb on 5 cylinders, 134 on 6th. Boat turns over and starts fine then starts this cycle where are the RPMs go up and down from idle to around 2500 RPMs and back over and over again. Throttle inputs apparently have little or no effect. Owner claims he took it to a Merc shop and they were certain it was a vacuum irregularity but wanted $125 an hour to track it down so he passed and brought it home. Wondering if there is a range of potential fixes between easy and Doomsday. Thanks a lot.

 

[henryb]

Sounds like a job for [MENTION=77252]DRH[/MENTION]onda

 

[Tim75]

vacuum hoses suck!

So I'm considering the purchase of a 2000 Challenger 20 with a 240 HP. Motor was rebuilt by a reasonably high-volume shop about a year-and-a-half ago. Premix block off kit was installed . Owner claims to have pulled and reinstalled the motor "himself". Compression is 130 lb on 5 cylinders, 134 on 6th. Boat turns over and starts fine then starts this cycle where are the RPMs go up and down from idle to around 2500 RPMs and back over and over again. Throttle inputs apparently have little or no effect. Owner claims he took it to a Merc shop and they were certain it was a vacuum irregularity but wanted $125 an hour to track it down so he passed and brought it home. Wondering if there is a range of potential fixes between easy and Doomsday. Thanks a lot.

The good news--it does sound like vacuum problems. The suck part is the number of hoses and quality of hose that Merc used. You need a service book avail. here that shows the vac. line layout. The hoses are prone to breaking at the nipple they push onto. I used weedeater fuel line that was the perfect size. The air leaks mess with the computer just like on a car.
Careful--don't break a nipple! They are just brass.