454 TBI XL Recommended Spark Plugs and Timing

[390Express]

Changed the plugs at the beginning of the year, but the mechanic thinks I may have fouled them out running that bad gas through the motor. I installed RV8C's, copper plus, marine plugs, mostly because they came with the boat. I'll probably use an NGK plug next year, after I get my issues worked out. Pulling a plug from either motor shows that they're both running a bit rich. I have them gapped at 0.040. Recommendations online range from RV8C to RV12C for my motors, 0.035 gap. Wondering if I should go to a hotter plug, or leave it a little rich. Plug isn't wet, just a little black and sooty.

Fairly confident that my issue isn't fuel related at this point, unless its in the throttle body (possibly a plugged injector). The left injector has a slightly different spray pattern and an occasional "drip" vs mist, but it seems to be getting enough fuel, if not a bit rich.

I checked the timing, the port motor reads 14 degrees at idle (around 750), the STBD motor reads 18. When you swap the timing plug to set timing the boat idles slower (around 500) and timing goes down to around 10-12 for the port motor, and is around 14-16 for the STBD motor. The STBD motor is the motor that's giving me trouble (popping through the carb under load, and at random). The rotator/top side of the distributor (forgot the name of the gear looking piece at the top that usually wears out) does not have any play. The dist. gear and/or thrust washer at the bottom of the distributor has a small amount of play. As a result timing varies +/- 2 degrees on the starboard motor, but mechanic says it shouldn't be enough to give us the issue we're having. The advance for the Port motor goes up to around 36 degrees full advance, but the Starboard motor goes up to around 45. Seems way high. (changed the cap and rotor as well, rotor had a mild amount of corrosion)

Any idea as to why, or how to correct it?

[390Express]

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[bud37]

Two questions, do these engines have ECU 's controlling things, what is the idle in gear advance timing spec. The second one , how are you getting these total advance numbers ( procedure ), cause those numbers seem a bit odd, but may be seen if you just blip the throttle.
Ok a third question, do you have knock sensors ?

[km1125]

Is there a mechanical advance on the distributors or is the advance electronic?

The starboard sounds way too advanced, especially at full advance. If it's using mechanical advance, it's solely dependent on engine RPM and should be limited by the mechanical mechanism. That can get bent or worn and springs rust. If it's mechanical I would thoroughly inspect it to make sure it's operating correctly. I don't know anyone local who still has a distributor machine, but that would be cool if you could find one to validate/calibrate the distributors. The amount of mechanical advance is the exact same regardless of initial timing.

If it's electronic advance, then the pickup could be an issue. As the parts rotate faster there's a sharper 'pulse' to detect the cylinder which gives and earlier indication of timing, which would trigger a slightly earlier (more advanced) spark. ALSO, it can differ based on initial timing. As the electronic system is based on TIME instead of actual RPM, it depends on whether it's using a crank based sensor or a cam (distributor) based sensor. Having more base timing causes more "mechanical" advance, as it's based on a percentage of the base timing.

Gapping the plugs too much (not sure if 14% is "too much" though) would cause some misfires especially at high load. This unburnt gas could then be giving the impression of the plugs being "too rich".

[bud37]

While we are getting updated, something else occurs to me.....how are the plug wires, are they new..... sometimes wire routing can create odd firing issues, try to make sure they are kept away from engine ground posts, not pinched in looms etc. Worth a look, its free.

[390Express]

It’s my understanding that my distributor is the AC Delco 1104060, and it’s the same distributor as most of the TBI/MPI distributors for Merc and Crusader, ECU controlled timing).

Plug wires are new this year, routed the same as factory. I was checking the timing in neutral, looking at the time at idle (750), off idle around 1500, then near the full advance mark of 2800. Engine was slowly brought up to RPM and monitored through acceleration to see if it was jumping around a ton. Nothing out of the ordinary, just seemed unusually high at full advance.

[mjk1040]

It may just be fouled plugs from the shitty gas issue you had earlier. My Crusaders call for AC Delco MR43lts plugs? Its stamped on the MP intake housing.

[SplashyLady]

Good info on TBI troubleshooting here: http://tbichips.com/?page_id=351

[390Express]

It very well could be MK. It’s the solution that makes the most sense to me right now. If the plugs were filed out, an incomplete burn would make the cylinder run rich and make the motor sputter. Once it’s rich and supporting, it can’t recover, stalls. It runs great for 2-3 min, then the check engine light comes on, then it runs like hell, then stalls.


Side note - what does the knock sensor look like, where is it located, and how do I determine if it’s bad?

[bud37]

QR_BBPOST It’s my understanding that my distributor is the AC Delco 1104060, and it’s the same distributor as most of the TBI/MPI distributors for Merc and Crusader, ECU controlled timing).

Plug wires are new this year, routed the same as factory. I was checking the timing in neutral, looking at the time at idle (750), off idle around 1500, then near the full advance mark of 2800. Engine was slowly brought up to RPM and monitored through acceleration to see if it was jumping around a ton. Nothing out of the ordinary, just seemed unusually high at full advance.

Just cause wires are routed factory doesn't mean all is ok,...... I still cant figure how you are getting such high timing numbers on both engines......at idle maybe around 8 btdc and maybe full in around 32, I say maybe because I dont know what the spec actually is. It is possible to get higher numbers with a knock sensor to help control things under acceleration. Something is fishy here.
Do these engines run hot ? What does your mechanic say about the timing , he must have some opinion on those numbers....