Any old school Merc guys out there shed some light on this 175 xri black max ?

Serial number OG282625

Auto oiler deleted ( prior to me )
Compression is 105 on all 6 little to zero variance hot or cold. All 6 plugs in or out stays the same.
New low pressure fuel pump.
new high pressure pump
new stator , trigger and switch boxes
round filter before the VST tank clean
VST tank inspected and cleaned
New spark plugs NGK BU8H
Fuel water separator new also.
Have 36-37 lbs of fuel pressure at the valve while running ( @idle and or revved up on the hose pipe) Have about the same +/- 1-2 lbs when key is turned on.
Primer bulb and line to the engine new
No signs of fuel leaks or anything of that matter
hooked it up to my “ smartech scanner “ to see if anything looked abnormal or out of parameters. I don’t see signs of anything going on on it


Revs ok on the hose pipe

Same in gear on the hose

Took it to the water today to check things out ( just bought this rig for a winter project )

running in the water literally mashed to the floor it might have picked up 30-80 rpms?

When in neutral on the water it revs but not like it should.

on the pipe at home it’s snappy and crisp it seems to me anyway.


thanks!



What’s yalls diagnosis on where to start looking?



Thanks!

I personally think 105 compression is way low....been awhile since I messed with a 2.5 but I think they need to be up above 150 to run...hate to be the bad news bear but IMO she's about wore out...

To try and make her run...make sure your timing is moving when you get in the throttle....have seen linkages come off before and it will fuel but not advance timing and won't rev up like you're sayin

yep! I’m leaning in the same direction she is due for a refresh unfortunately! Before I put any more time , effort and parts into it!

A leak down test would really tell the story if you have access to one.

Did you remove the black boxes? Or did you keep those? Did you test spark to each plug? I have a bunch of 2.5s, they arent the most difficult to get running. They will run with 105psi but will feel weak but you will want to leakdown for sure

This is most likely electrical. Can you also put a small piece of clear hose to see if you are sucking air?

You will most likely get more eyes and help from screamandfly

Does it have the 2 switch boxes that stack on top of each other? If so, I had a similar experience a few years ago with them. I had one that went bad only 3 coils were firing. I ordered a pair of ebay specials and installed them, one of which was bad as well. I ended using one of the new ones and one of my originals and all was good. Check for spark on all cylinders.

Sounds like you’ve covered a lot already. If it revs fine on the hose but falls flat in the water under load, could be fuel delivery under load or timing. Even with good pressure at idle, the injectors might not be flowing properly, worth pulling and cleaning them.

You need to troubleshoot this in a systematic, structured way.

First thing to figure out is if the engine lacks spark or fuel.

Best way to do that is to switch off ignition when the engine acts up and take a look at the spark plugs. (Plug chop)

Are any plugs too wet or too dry?

I have the same motor on a Glastron Carlson CVX 16. I think my compression is around 125-130 per cylinder. There are two really good groups on FB to join, Keep the 2.5 Alive! and Mercury Black Max Owners.