Mag 350 MPI won't reach full rpm

CHEESEHEAD said:
i feel your frustration, i have twins also v drives, one engine will run 4600-4800, the other wont get over 4000. diacom software clearly shows my timing is being retarded TO 24 DEGREES. but i cant find the cause, im not getting any alarms, fuel economy is bad. been struggling with this for more than a year, swapping parts, compression tests, swapping fuel systems, caps rotors plugs wires, ECM's blah blah blah. starting to think vibrations in the tranny are affecting my knock sensor thus pulling the timing down. although the knock sensor will save me from engine damage, it is a troubleshooting hurdle hard to get past. if only there was a way to bypass the knock sensor to see if my rpms will pick back up, then at least i have a clue. but no misfire, cough, sputter nothing. im thinking about putting a rubber oring under the KS to make it not so sensitive. in the two diagrams, you can see the healthy engine's spark advance is hunting and the week engine is flat lined at 24 degrees. i have to manipulate my throttles to make my sync gauge center up. i guess i will continue to peck at it here and there. it really only bothers me when i have a loaded boat and struggle to get on plane, the stronger engine is doing most of the work.View attachment 90506 View attachment 90507
Click to expand...


360/24=15.

Are there 15 teeth on the distributor drive gear?

Could the distributor be off by one tooth?
 
Have you tried another KS?. You could unscrew it keep it plugged in and insulate it so it can't detect anything
 
This problem is similar to what I am experiencing. Hope you resolve it. Please let us know if you did. I am about 6-800 RPM short.
 
CHEESEHEAD said:
i feel your frustration, i have twins also v drives, one engine will run 4600-4800, the other wont get over 4000. diacom software clearly shows my timing is being retarded TO 24 DEGREES. but i cant find the cause, im not getting any alarms, fuel economy is bad. been struggling with this for more than a year, swapping parts, compression tests, swapping fuel systems, caps rotors plugs wires, ECM's blah blah blah. starting to think vibrations in the tranny are affecting my knock sensor thus pulling the timing down. although the knock sensor will save me from engine damage, it is a troubleshooting hurdle hard to get past. if only there was a way to bypass the knock sensor to see if my rpms will pick back up, then at least i have a clue. but no misfire, cough, sputter nothing. im thinking about putting a rubber oring under the KS to make it not so sensitive. in the two diagrams, you can see the healthy engine's spark advance is hunting and the week engine is flat lined at 24 degrees. i have to manipulate my throttles to make my sync gauge center up. i guess i will continue to peck at it here and there. it really only bothers me when i have a loaded boat and struggle to get on plane, the stronger engine is doing most of the work.View attachment 90506 View attachment 90507
Click to expand...
I haven't played with these in a while so this could be outdated information. But unless they've changed them, the knock sensor is piezoelectric. Just unplug the connector and the engine won't know the difference. Obviously it may be retarding the timing for a reason so this is a diagnostic step, not to be left off long term.
 
i did remove it from the block but left it connected. i grounded it with an alligator clip just in case, i got an alarm sound by doing this.
 
scoflaw said:
Have you tried another KS?. You could unscrew it keep it plugged in and insulate it so it can't detect anything
Click to expand...
i swapped KS from the strong engine to the week engine, no change, i have a HVS distributor (flat cap) this cap only controls phasing, the crankshaft sensor controls the timing i think, not an expert; thats why im asking
 
Last edited:
CHEESEHEAD said:
i swapped KS from the strong engine to the week engine, no change, i have a HVS distributor (flat cap) this cap only controls phasing, the crankshaft sensor controls the timing i think, not an expert; thats why im asking
Click to expand...

This topic came up not too long ago about distributor position and phasing. There is a procedure to ensure that the distributor is positioned in the proper location. It involves a special LED test light And some fiddling with the position of the distributor.

The original thinking was that dropping the distributor in and getting it “close” was all that’s needed as the ECM took care of the rest. However there are benefits to using the test light to get it perfect.
 
Espos4 said:
This topic came up not too long ago about distributor position and phasing. There is a procedure to ensure that the distributor is positioned in the proper location. It involves a special LED test light And some fiddling with the position of the distributor.

The original thinking was that dropping the distributor in and getting it “close” was all that’s needed as the ECM took care of the rest. However there are benefits to using the test light to get it perfect.
Click to expand...

I built the test light and used it to set the position of the cap. It indicated close to where my initial position (where engine would pop in the intake under load only) but still approx 1/8” off. So, I moved it based on the light fixture, ran it in the driveway and it sounded great.

Under load at the lake, however, it was worse than previous. So I got frustrated and started adjusting the cap manually by hand, then testing under load, then adjust, test, etc. It took 15 mins and I fixed the problem. Final adjustment was approximately 1/16” from the original position. Having a distributor wrench made this process very easy...best $8 I’ve ever spent. Engine ran great.

This was June 6th. We were elated we had fixed a problem that had plagued this motor for the last season and a half. Elation turned to horror as the boat was broadsided by a distracted driver on the way home from the lake. Geico totaled the boat last week :-(.
 
Espos4 said:
This topic came up not too long ago about distributor position and phasing. There is a procedure to ensure that the distributor is positioned in the proper location. It involves a special LED test light And some fiddling with the position of the distributor.

The original thinking was that dropping the distributor in and getting it “close” was all that’s needed as the ECM took care of the rest. However there are benefits to using the test light to get it perfect.
Click to expand...


I thought that procedure was for boats with catalytic converters that are throwing misfire codes?
 

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  • Diagnostic Supplement for MerCruiser Engines with Emission.PDF
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CHEESEHEAD said:
I thought that procedure was for boats with catalytic converters that are throwing misfire codes?
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Originally, that's what Mercruiser said. I think it was later understood that, in these HVS-based engines, timing chain stretch can result in similar ignition-related issues simply due to the HVS (aka distributor, but not actually a distributor) being far enough behind that occasional misfires occur. Adjusting the position of the cap is the only way to compensate for this "slop" introduced due to chain stretch.

Using the led test light is merely a fancy way to indicate where the cap is/should be when the timing chain is tight and the engine is at a known prescribed point. Looking back, my takeaway from the experience was the light gave me confidence that I was doing something in an "instrumented" fashion. In reality, it convinced me the "non adjustable timing" that these engines are described as having is only sort of accurate. There is indeed a fine-tuning that can be done (when necessary).
 
well after a busy few weekends finally got to work on the boat. regapped the plugs to .038ish, double checked distributor rotor location, it looked off a tooth so i pulled it and reinstalled, i felt confident it was fixed, but no. had a mechanic tweak the cap with underway to see if rpms came up, no dice. timing wont go past 24 degrees. gonna measure exhaust pressure next. got some gauges to put in the exhaust manifolds. anyone hear of those mufflers getting stopped up on inboards?
 
exhaust manifolds showed no visible pressure on a 0-15 psi glycerin filled gauge. so either i have live with this weaker engine or pay someone to redo everything i already tried.
 
CHEESEHEAD said:
exhaust manifolds showed no visible pressure on a 0-15 psi glycerin filled gauge. so either i have live with this weaker engine or pay someone to redo everything i already tried.
Click to expand...
Thanks for following up. Sorry you have not found your answer.
 
Updating this thread since I’m the op. Problem solved! Pulled drives off today and discovered the code on the driveshafts were different. Slow drive is stamped B which is 2.0 per service manual. Fast drive is K which is 2.2. I swapped drives and the problem followed.
 
Coz said:
Updating this thread since I’m the op. Problem solved! Pulled drives off today and discovered the code on the driveshafts were different. Slow drive is stamped B which is 2.0 per service manual. Fast drive is K which is 2.2. I swapped drives and the problem followed.
Click to expand...
Good detective work. Looks like this thread got jacked
 
Coz said:
Updating this thread since I’m the op. Problem solved! Pulled drives off today and discovered the code on the driveshafts were different. Slow drive is stamped B which is 2.0 per service manual. Fast drive is K which is 2.2. I swapped drives and the problem followed.
Click to expand...

So it seems at least one drive is not original to the boat?
 
Coz said:
Updating this thread since I’m the op. Problem solved! Pulled drives off today and discovered the code on the driveshafts were different. Slow drive is stamped B which is 2.0 per service manual. Fast drive is K which is 2.2. I swapped drives and the problem followed.
Click to expand...
So it was the gear ratio of the drives that was causing the rpm issues? That’s great, but upsetting that someone didn’t match up the right drive
 
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