Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
marine quadrajet 17082515 on gm 4.3
Zyen:
mudsport, your's is a bit easier, so i'll respond to it first. yeah, most 4.3 stock is 8° base to keep the timing peak around 1600 under control while giving the most power/torque to get on plane.
you're right...it's not a race boat, i don't time it, i don't run anyone in it, and im not trying to get a street curve in the ignition...that's a sure-fire recipe for disaster. the curves are lazy to account for the multiple variables in boats and their respective owner/operator. since i can control some of those variables, i can account for them and reset accordingly. i dont have room for a larger engine, nor am i looking for an outrageous output from this one. the mileage is negligible. dont' really care, but want it reasonably efficient and somewhat close to the 205hp it's rated for.
Zyen:
cadman, no marine engine should have a vacuum advance. that's way too much timing for marine loads. LPCP calc's are the same for any gas engine. the advance doesn't really matter, cause advance is advance in the math. im about to head to bed, but will give a breakdown on how im figuring it tomorrow. and, i may be wrong.
Zyen:
--- Quote from: Cadman-iac on May 18, 2026, 08:35:34 AM --- How did you calculate the LPCP for your engine?
Rick
--- End quote ---
Here's how i did it, using my current wot and ignition timing as example:
crank rotation: 60,000 ms / (RPM * 360°) = ms/deg
4600 rpm = 0.0362 ms/deg
~12.8 afr using 87/E0 and ~90% VE (~0.5" hg) = ~1.3ms burn time to peak pressure
~0.3 ms from spark to flame propagation. = 1.6ms
Total burn from start to peak pressure = ~44.2° of rotation.
timing is set to 8° base and hard stop of 10.5° advance @ 3200 = 18.5° TA > 3200 rpm
burn time from ignition to peak pressure - spark advance = lpcp
44.2° burn - 18.5° btdc start = 25.7 atdc lpcp
-Allan
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