Low compression #5, 115hp inline 6

[MNhunter1]

1982 115hp Mercury inline 6, serial number identifies this motor as prop rated. This motor was picked up as a “parts motor” as it was not running after a rebuild. Apparently had a piston issue, cylinder was honed, piston replaced, and the original owner could never get it to start again. Has good spark and my compression test shows 125-130 psi on all cylinders except for #5, which shows 105 psi.

So low compression in #5, but still over 100 psi. First thought would be possibly stuck ring, but it also could have been a result of improper rebuild/assembly. I wouldn’t expect the lower compression in one cylinder to be a contributor to the no start issue, but I haven’t dug into any of that yet.

Is this motor worth digging into, or do I just leave it as a parts motor as originally intended? Would it still run okay with the lower compression in #5?

 

[Chris1956]

Pull the three transfer covers on the starboard side and have a look at the cylinders, pistons and rings. if they look good, I would run it.

That one cylinder's compression is a bit low, but It is worth a try.

Does it have spark on all 6 cylinders?

 

[racerone]

These inline 6 cylinder motors run really nice in in good condition.

 

[MNhunter1]

Thanks everyone. I have a great condition 1984 115hp with 125 psi compression in all cylinders as my primary motor, so this is an extra parts or backup motor. Was just trying to determine whether I should:

1) Keep it as is for a functional back-up, even with the low compression in #5
2) Use it for parts (pretty much everything interchanges with my '84)
3) Dig into it further and address the low compression

I've never tore into a powerhead, but have a local contact that can hone or bore these cylinders if needed(not many around anymore), and another local contact that has a set of ring compression tools. I'd honestly prefer not to have to dive that deep, but it might be a fun undertaking and teach me a thing or two. As noted, it's a backup/parts motor, so nothing I am depending on.

 

[racerone]

Considering the cost of a new 4 stroke motor.-----And the required dealer support at $100+ / HR for these electronic / plastic marvels.-----An older inline 6 is a good deal.----Tear it apart.----Inspect everything and with new pistons / rings it will run for years to come.

 

[Chris1956]

So your not going to look at the pistons and cylinders, before deciding to make it a parts motor or tearing it apart to overhaul it?

Inline six motors are too old to invest an significant $ into, IMO. The newest ones are 35 years old. If you can get them to run with some carb kits and a waterpump kit, great.

Get an early 2000's V6 instead. Much better motors, IMO. The only advantage of an IL6 is it's light weight (~50%). The V6 motors have much better low end power and have a much flatter powerband than the IL6.

JMO

 

[MNhunter1]

So your not going to look at the pistons and cylinders, before deciding to make it a parts motor or tearing it apart to overhaul it?

Inline six motors are too old to invest an significant $ into, IMO. The newest ones are 35 years old. If you can get them to run with some carb kits and a waterpump kit, great.

Get an early 2000's V6 instead. Much better motors, IMO. The only advantage of an IL6 is it's light weight (~50%). The V6 motors have much better low end power and have a much flatter powerband than the IL6.

JMO

I’d love me an early 2000’s V6, but I already have an ‘84 Merc 115hp and this ‘82 came with it, so it’s essential a spare. Hull is an ‘84 Starcraft center console weighing in at 705lbs, so the lightweight 115 makes for a nice retro package