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Thread: One engine 300 RPM down at WOT

  1. #1
    If anyone can help me, you guys can. I have a Sea Ray 340 Sundancer (yeah, I know, not a hot boat) with stock 7.4L carburated engines. The boat is an '89 with ~1100 hours on the big blocks. I'm not ready to replace them or dig in with big money, so I'm hoping to try all the "inexpensive" things before digging deep? Can you help?
    The starboard engine is running roughly 3925 RPM, while the Port runs 4200 (smack dab in the middle of the power range for the out of the box motor). Timing is per the book 8 degrees at idle and 31 at WOT. Fuel pressure is 5lbs at WOT (within spec) and adding fuel to the carburator throat causes a drop in RPM, so it does not appear to be getting starved. Compression runs between 125 and 110 in all cylinders. Surprisingly, although the "good" motor has a top compression pressure of 145, the cylinder right next to it reads 105!
    Of course, I know I can do a leak down test and find out if it's worth a valve grind or something more exotic (new rectangular port heads, intake & exhaust manifolds, etc), but I'm looking for the cheap potential culprits. I have already changed plugs, cap & rotor and fuel filter (not the one in the carb yet).
    It's driving me nuts (it's a short trip). I've thought of just swapping carbs between the motors to eliminate the whole carb thing. I can even draw fuel from the opposite tank for more giggles.
    Ok, any help appreciated!

  2. #2
    Compression tests are kind of misleading. Squirt some oil into each cylinder and repeat the test. 145 vs 105 indicates a bad cylinder. If your readings even out with the "wet" test then your rings are bad. If a valve leaks, it will burn. That could also be your problem. If, on the wet test, you still have one or two cyl way off, then you valves are shot. Hope this helps.

  3. #3
    Originally posted by mister460:
    If your readings even out with the "wet" test then your rings are bad. If a valve leaks, it will burn.
    Thanks, and of course, I will repeat the test w/oil to isolate rings vs. valves, but I'm still looking for other possibilities other than these two. Something that doesn't require $2K+ to repair. Call me hopeful, or stubborn. I just want to rule out anything we might have missed so far.
    BTW: What do you mean by the valve will burn? It will continue to degrade as it gets burnt by hot exhaust gasses? Just curious.

  4. #4
    Originally posted by mister460:
    145 vs 105 indicates a bad cylinder.
    I neglected to mention, the 145 vs. 105 is on the GOOD engine! All the cylinders on the "bad" engine are between 125 and 110. http://free.***

  5. #5
    Exactly. Exhaust gasses blowing past valves while the fuel is still burning will cut into the valves just like a torch. It doesn't take long for a valve to burn either. Oh yeah, if it gets really bad it will cut the seat as well.

  6. #6
    The most basic thing to check are the props. If one is slightly dinged or bent, it can cause the RPM to be down on that engine.

  7. #7
    Originally posted by ROBIII:
    The most basic thing to check are the props.
    Oh yeah, good call. The props on there just came back from Bay Propeller and were measured as well as getting tuned up and balanced. They measure true 17" medium cup -- exactly what Sea Ray put on at the factory.

  8. #8
    You Should have the tachometers checked out to make sure they are ok.

  9. #9
    Originally posted by DEL51:
    You Should have the tachometers checked out to make sure they are ok.
    Oh yeah. Did that too. Also, it's clear from the synchronizer and from the sound that they are not turning at the same speed. After I sync, I can push the port engine ahead. My tachs agree with the tachs that my mechanic brought out, sadly.
    I think as time allows, I will swap components (carb, ignition module, coil) from one engine to the other, just to rule out these inexpensive parts.
    Thanks again!

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