502 MPI on a Blackhawk

Just had my Donzi out for the second run since I purchased it. Seems a bit limp.
The 1st run I had just warmed it up and basically got it on plane to get the fluids moving before I changed them out. Nothing over 30 MPH. Started well, idled well, no overheat, good oil pressure. Not really impressed w the power, but wasn't really pushing the stick very hard.
The only issue was a leak in the Water pump / fuel pump housing. Since replaced w new.

As the boat had been sitting for years - I had a Merc mechanic check it over - pronounced it healthy.

2nd run was about the same with no leaks. BUT - no performance. Max 3800 RPM, and Max 50 MPH. Also the alarm that sounds off when you turn the key to ON, makes a very quiet buzzing after the engine starts. (not the full on alarm sound) Also Temp gauge occasionally jumps around a bit, but I can hold my hand on the risers - so not overheated. Also not the drive oil bottle signal. I disconnected those wires and it didn't change.

1995 502 MPI, Blackhawk Donzi low hours - stored for a few years.
31 props
Full fuel ( 94 Chevron)
New Cap , Rotor, and Plugs
New primary fuel pump
New belts
New Impeller


Spoke to another Blackhawk owner w stock 502 - he's at 78 MPH and runs the 31 props. So I assume stock engine pulls them OK.
He suggested possibly temp senders due to gauge jumping around ( might be 2 on this engine), the engine harness plug in (his had issues),
or timing. I don't see a spec on the engine info sticker for timing - it says to see the manual - is it even adjustable?

The fuel filter and fuel are new. Engine runs and starts great. Just seems "soft".
I guess a compression check, throttle opening check, maybe new plugs wires

Any ideas out there?

Thanks in advance!

Could be a number of things. If it sat for a number of years send your injectors out for service.

Plus the compression check and a leak down test?

Thanks!
Did a compression test a while back - don't remember the numbers but it was good, and even. I've never done a leakdown on an inboard - only 2 stroke outboards - great care needed! Not looking forward to this learning curve. Going to reset the base timing, check throttle opening, check the harness, and have already installed the new sender and sensor. Not really looking forward to pulling injectors (or any more wrenches) right now after reassembling the Skater motor.
Probably take both boats to the lake this aft and spend the weekend testing tuning the Donzi, and putting some more break in hours on the Skater. Poker Run next weekend!

Let us know how it all goes.

Will do - turns out it was a one boat weekend. Holiday weekend here in BC, combined w wildfires in the interior = lots of boats locally. And no room at the ramp.
Got the Skater in on Saturday morning early and left it at a friend's dock. Hope to retrieve it today! I predict mayhem at the ramp. So no Donzi this weekend.
Also found a fried wire on the 502 - red w purple. It has an inline 20 amp fuse ( melted) and looked like it had been rubbing then grounded. No chance yet to even look at a wiring diagram. Skater ran good tho!!

if it was carb engine that sat you would not hesitate to clean it. Dont just pull them and clean them get them to an injector service shop and flow them as well. Fuel metering is the most important single function on the engine.

Good to know. I have a couple boats with injection to get going this fall that have been sitting.

Good advice, the injectors were the 1st thing I looked at when the #6 piston went all melty on the starboard 300X couple of months ago. So I have a good service company lined up. Haven't worked on any of these motors before - can I lift the plenum and throttle body off in 1 assembly? Not at the boat right now- and looking at Merc manual isn't mentioned that way, but would seem to save some valuable time.

There are a few different configurations. Should not be hard to get to injectors. Ill be doing a L29 type later this week. Once you see whats in the loop you'll know whether the entire loop needs to be cleaned. Its a good idea anyway. See what your circumstance dictates. If you have a vapor canister one top you be able to clean everything with out much hardship. If you have cool fuel and components under engine mount you'll have some blood sweat and tears involved.

Vapor canister on top, rear of plenum. Don't think it has cool fuel. You're right, I should clean the entire loop. Your advice is appreciated. Just trying to find the time.... Heading there now. Other boat ate my time and $ for a bit. Playing catch up.....



Attachment 84488Attachment 84488

Injectors out. Not too bad of a job. Plenum and throttle body taken off as one. Injectors came out fairly easy. Hardest part was the spring clips and bending over the rub rail instead of just walking up to an outboard and popping the cowl.

Anything else I should be doing while in there? I guess the Vapor canister should be opened up, cleaned, and new "O" rings installed on the supply and return lines to fuel rail.

Cool, let us know what they say when inspecting them. I'm real curious.....

There are filters in the canisters. Should change them. Noticed in pix you have stainless risers. They will not fail but, the gaskets under them do a lot of work and do fail. I suggest you add inspecting manifolds and changing risers gaskets to your short list.