HISTORY:

The best way I can describe the issue is synonymous with how a cold weed wacker or leaf blower bogs down and shuts off when you give it too much throttle when cold.

Serial # 0F886167 60 hrs on long block 227 hrs. on all other gear. Compression test within MerCruiser manual parameters.

Boat was delivered to me with 1/8 tank of fuel. I immediately added 5 gallons (non-ethanol) with the appropriate amount of Startron for that 5 gal. plus what was in the tank. Prior to the 1st run I topped off with 100 gallons of non-ethanol and appropriate amount of Startron. At that point the tank was 100% full @ 150 gallons.

Fuel smell in mechanical fuel pump lube prior to 1st trial so I drained and replaced the lube.

Zero fuel present in sight glass tube; however, I'm quite sure the pump is original with the hours and age listed above. I'm told all injectors were checked and flow balanced with the long block.

Fuel tank vent functioning properly. Both engines pull from the same tank and the 2nd runs to redline fine.

Fuel filter changed prior to 1st run. Pulled after 1st run and no water/fuel separation on sampled dumped out of filter.

Charging system functioning properly.

Much louder intake hissing sound which seems to be coming from the spark arrestor area. Or possibly directly under; however, I haven't checked for vacuum leaks yet.

I do not have a scan tool; however, no codes are present with the Renda service tool with led light and service mode switch.


ISSUE:

On my boat's 1st test run the engine would not rev. (in gear) above 1,200 rpm, would bog down and stall. Checked the supply line from the fuel tank to the fuel pump and it was original (20 yrs. old). Installed a new line and it would now rev. to 2,500 rpm then bog and and stall. When manipulating the throttle to save it and re-apply above 2,500 rpm it would pop through the intake. The engine was hard to re-start.

I replaced the fuel line barbs (check valve) at the tank with flow through units, inspected the pickups & screens (clean as a whistle) and inspected the 90* elbow from the fuel line at the fuel pump (also clean). I then took the boat out a 2nd time and it would then rev. to 3,000 rpm and do the same as the above. Noticed the throttles were off and while crawling around in the engine room a potential crack in the bad motor's distributor cap. Pulled the cap and saw corrosion on it and the rotor. Replaced all on both motors which instantly synched the throttles; however, did not improve the bogging issue.


Other than checking timing and for a vacuum leak, where might I go from here? Could the fuel pump and or wear tab (that rides on cam lobe) be bad while not showing fuel in the sight tube? Or is there something else much more obvious I may be missing?

Thanks for any insight you can lend.

check the filter attached to the pump in the vst tank.

Thanks, I'll dig around the manual to figure out what/where it is etc.

Double Post

Pulled and disassembled the VST yesterday. Pump tests fine and float doesn't have a hole in it. The diaphragm looks dry rotted (multiple tiny cracks) but there wasn't fuel in the sight glass, there is also tan flaky buildup between the diaphragm and the top of the housing. The pickup filter was 1/3 covered in silt and silt present in the bottom of the VST housing. Plan is to replace the diaphragm, clean the pickup and reassemble with all the other original parts, unless there's o-rings I have on hand that match sizes.

UPDATE:

I did all of the above in post #5 as well as pulled and cleaned each fuel injector filter screen, replaced all rubber on the injectors, cleaned all sensors and vac. ports on the plenum and reinstalled with new plenum gaskets.

That solved the bogging issue; however, I was still getting a backfire through the intake above 3k rpm. We swapped the ignition control module from the "good" motor and the backfiring went away.

Now though (I'm assuming because we've advanced past 3k rpm on both engines) I've picked up a detonation sound at 3500 rpm on the rear of on of the banks on the "good" engine. I'm going to pull plugs and check plug wires to start, hoping the mechanic who did the compression test just cracked porcelain on a plug but the way its been going I don't think it will be that easy.

UPDATE:

Pulled plugs on "good" engine to find the rear most port side wet with fuel as well as the 2nd back from the front wet with oil. The 1st at the front on the SB side also wet with oil. Tested the spark plug wires and found 2 on the SB Side ohm rating spiking when testing on a volt meter and manipulating the wires and boots. Gong to replace all 8 plugs as well as the wires to see if that relieves the +3,500 RPM popping. If the fuel fouled hole continues to get wet I'll get the injectors cleaned and flowed.

UPDATE:

Went back out after the above and rechecking timing (had to be retarded from last trial) and now the popping event has moved down in the RPM band to 3k.
I've talked to a local mechanic who says it sounds like a broken valve spring on #5 cylinder due to the "event" moving in the RPM band with different timing settings. He says it's common on these engines. We're going to sea trial it with the scanner to rule another issues out first and try and determine which bank is the problem. I'm hoping not to have to pull a head to replace a valve spring.

Popping solved by mobile mechanic I hired to help. Crossed spark plug wires, probably done by mechanic who compression tested it for me prior to purchase. I wouldn't have found it anyway as the distributors are clocked wrong and #1 post isn't toward bow. So even if I would have checked against the manual I would've been wrong. Evidently if their close enough in the firing cycle you won't have a problem until higher rpm's and probably explains why I had a fuel fouled spark plug.