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  1. #1
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    Default Not hitting WOT/max RPM's both engines

    I maxed out at about 3,500 RPM's and 19knots which is about 5-6k lower than normal. Something is up so here is what I've done so far. It seems to be the exact same issue on both engines.

    Engines are carb'd 454's, 1988 with about 1,400 hours. Neither engine uses any oil, no smoke and run like a top. I have owned the boat for eight seasons so good year to year history to look back on.

    1) TA timing is 32 degrees at 3,200
    2) installed new HEI distributors, plugs, cap/rotor and wires, no change
    3) changed my pre filter screens, Racor and carb filters. Also, just rebuilt stb carb last week (didn't really need it).
    4) chokes are wired open on both engines
    5) verified that the engine will exceed 4,000 RPM's in neutral and that tach will read that high. Got to around 4,400 and pulled throttle back because I hate to run it that high with no load
    6) flame arresstors were just cleaned
    9) verified that the throttle cable aligns to the carb at WOT
    10) did WOT test with gas caps removed to verify no vent issue
    11) removed anti-siphon valve, perfectly clean
    12) Temps, oil pressure spot on, oil is clean
    13) shafts/rudders/props all clean (dove under to verify this weekend). Diver is checking bottom tmrw but not expecting anything but would be thrilled if this was the issue.
    14) hull is not cored so no weight gain
    15) on my 3rd tank of fuel now, no changes throughout

    Start up, idle and throttle up all have been and continue to be perfect. Not even the slightest feeling of an issue until you open it up.

    When I put the boat away last year it would get to around 4,200.

    What do you guys think, I'm a little stumped.

    Thanks,
    Bob

  2. #2

    Default Re: Not hitting WOT/max RPM's both engines

    Sounds like you've thought of most of the things that you need to verify. So when you open it up now, what does it do? Does it choke out, lean pop, bog down etc. Did you strike anything that your aware of?

    Duke1

  3. #3
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    Default Re: Not hitting WOT/max RPM's both engines

    Bob; did you verify the secondaries open fully? Try this engine off, make sure both the top butterfly opens fully and the weighted butterfly opens easily by hand and you can see all the way thru. Hope I'm not mixing this up with the carters. Don't know if you tied the chokes open recently, but it may be possible to have some linkages interfere at certain throttle settings.

  4. #4
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    Default Re: Not hitting WOT/max RPM's both engines

    Most unlikely that both engines are showing the same weakness suddenly when the only possible common element (bad batch of fuel) has been eliminated. More likely that only one is weak and your observations were not careful enough. If one is weak then the other does most of the work (limiting its rpm) while the weak one can coast along making almost the same rpm (but not quite). Do the throttle positions match up when both are at the same rpm? Or try adjusting the throttles so one runs at 2500 rpm and the other is WOT. Then reverse the process. Are both WOT engines really the same? If not, run a compression check on the weak engine. If so, then you can almost bet the problem is not the engines but the boat (dirty hull, damaged hull, taking on weight, etc.)

  5. #5
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    Default Re: Not hitting WOT/max RPM's both engines

    I'm thinking like diver_dave but think his description fits the carters....for the qjets, engines off, push both throttles to WOT. the air valves, over the secondaries should open with a finger push. make sure the throttle valves are open. if that's good, next time you are out, with appropriate help, inspect the air valves at WOT. they should be pretty close to fully open and atomized fuel should be flowing from the nozzle. CJ's test results would be interesting as well...

  6. #6
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    Default Re: Not hitting WOT/max RPM's both engines

    Guys, thanks for the replies. I have a couple of updates but no breakthrough. Dave's question prompted me to check the secondaries and there was a small hole in one of the vac lines that connects on the base of the carb to the diaphragm. I changed both side and then took it out for a run. I have video of both secondaries partially opened at around 3,000 and then snapping all the way open as the throttles are pushed to the WOT position. (I can post a youtube link if anyone is interested). No change...

    I also checked again that the linkage on both engines lines up with the cables so it is definitely not the shifters themselves as the issue.

    I spoke to a guy on my dock who owns an engine machine shop and has rebuilt over 50 454's. I walked him through everything I've done to date and he's stumped. I then caught a really good mechanic who happened to be on my dock and he was equally stumped. He did ask me if my pick up tubes had screens and I don't recall. I'm checking with someone now who has the exact same boat and just replaced one of his tanks (he just texted me, no screen).

    Finally, I was there when the diver checked the bottom and was hoping he was going to surface with a "you're not going to believe what I found", unfortunately his response was "not a barnicle to be found".

    CaboJohn, you raise a good point. Is my issue really one side and it's dragging the other down? I keep thinking of this as a collective problem between both engines which pretty much eliminates an internal mechanical issue. If I had a compression issue,for example on one engine which limited RPM's wouldn't the other engine still hit it's normal WOT RPM's but the overall speed would be down?
    Last edited by bobct; 07-30-2017 at 02:53 PM.

  7. #7
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    Default Re: Not hitting WOT/max RPM's both engines

    how much vacuum shows on your 'new' gauges....in the fuel line?

    what made you change the distributors?

  8. #8
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    Default Re: Not hitting WOT/max RPM's both engines

    Mark,

    I don't have an accurate reading yet, I still have them installed on the Inlet side. I need to find a 1/4" NPT tee and will try and do that today.

    This issue prompted me to change the distributors. I was going to change the plugs/wires as part of my normal maintenance. But, the plugs wires and plug gap are different on the HEI's so I accelerated this upgrade which I was thinking about anyway. I'm not seeing any difference but obviously have something else going on right now. It does start instantly versus a crank or two before but who knows if that's directly HEI related.

    I will post some pics of my plugs, I thought they all looked pretty good.

    Bob

  9. #9
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    Default Re: Not hitting WOT/max RPM's both engines

    This is interesting. I moved the vac gauges from the Inlet positions on my Racor to the Outlet positions. The gauge is now connected using a tee on each Racor along with the fuel lines going out the engines.

    The readings are about the same, maybe a hair bit higher... closer to 8 now. As a test of the gauge, I closed each shutoff valve while the engine was running and the gauge would immediately begin to climb. I stopped when they got to around 10 or 11. Should I be literal when interpreting the numbers? When I spoke to Racor about their gauge, their advise was to take note of the initial reading (whatvever it is) and be concerned when it jumps 5hg or more over the baseline.

    What does that tell me? I already did a test with the gas cap removed and checked the anti-siphon valves.

    thanks
    Bob
    Last edited by bobct; 07-31-2017 at 04:53 PM. Reason: I'm stupid

  10. #10
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    Default Re: Not hitting WOT/max RPM's both engines

    The readings should show more vacuum, with respect to the atmosphere, as you have 'added' the filters' "resistance" (pressure drop) to he measuremt point.

    Agree with racor, to a point, in that you want to establish a baseline and then look for an additional 'pressure drop' as the filter collects the crud.

    That does seem like a lot of resistance (pressure drop)....the 'newer' installation manual calls for less than 2" Hg of 'fuel inlet resistance' - what size fuel hose is used from the tank to the filter head? (assuming the lines are still good, this shouldn't be an issue as it hasn't been one to date)....

  11. #11
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    Default Re: Not hitting WOT/max RPM's both engines

    Mark,

    I believe it is 5/8" line but doing that from memory. It's the same aeroquip hose that was factory provided but I changed every bit of it about seven years ago. This problem just popped on both engines and never an issue before.

    The guy I know who has the exact same boat and engines just called me test he did for me. That was, prove that an underperforming engine cannot drag down the RPM's of the other side. He was able to hold 4,200 on one engine while the other was much lower.

    In the meantime, I did a quick vac test on each engine just now. Video links below, please let me know if anything jumps out.

    http://www.youtube.com/watch?v=HoFu9k4jevo


    http://www.youtube.com/watch?v=VhAGlTwXeRA







  12. #12
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    Default Re: Not hitting WOT/max RPM's both engines

    Looks textbook to me. One more thing. Take it to WOT under load and look at vacuum. Tells how much restriction is taking place from atmosphere to intake manifold. Should be quite low, a few inches or so.

  13. #13
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    Default Re: Not hitting WOT/max RPM's both engines

    Given the hours, I'd agree with Dave.....

    I'm surprised a line that big has that much restriction.....our 'vintage' is usually happy with 3/8" ID running thru a few 1/4" NPT fitting.....must be some extra plumbing in your supply side.

    An this happens whether the engine access is in place or not???

    One thing I didn't see on the list in the first post was fuel pressure.....its a long shot due to both engines being impacted but with 7"Hg on the supply side, it wouldn't be a bad thing to rule out.....also, was the 7" Hg on the fuel line at idle or under WOT?

  14. #14
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    Default Re: Not hitting WOT/max RPM's both engines

    Doesn't the 7" take into account the depth of the fuel in the tank? More air space, more vacuum rqrd? That's why I was thinking a differential gauge is best. Fuel doesn't weigh near as much as mercury, but still has weight.

  15. #15
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    Default Re: Not hitting WOT/max RPM's both engines

    Just a thought but in your 1st post you list 10) did WOT test with gas caps removed to verify no vent issue. So this tells me that you have 2 fuel tanks, with each one normally supplying fuel to only one engine. Does your boat have valves that would allow running both engines off of only one tank? If so that is a test you could try but my thinking is that if all the valves are open, including the crossover valve, then each engine is trying to pull fuel from each tank at the same time. A little competition there! The crossover valve needs to be in the OFF or CLOSED position when the other two valves are open.



  16. #16
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    Default Re: Not hitting WOT/max RPM's both engines

    To DD's point- yes, the reading reflects the environment....but the more fuel in the tank, the more the atmosphere will help to move it.....which is why most boats should have those finicky anti-syphon valves.... agreed a differential gauge would be optimal but not cost effective.

    Erich's thought is also worth checking though I'd be more inclined to think something isn't fully open vs the competition view.....guess it really depends upon how that's plumbed up...

    It may be easier to rule out the fuel delivery system by temporarily using an outboard tank to feed the fuel pump....at least on one engine...

  17. #17
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    Default Re: Not hitting WOT/max RPM's both engines

    It sounds like everyone is leaning towards a fuel/pressure/delivery issue. On the fuel line hose, it is Aeroquip 1/2" ID hose, not 5/8". The fittings are 1/4" NPT and none of that changed. The one thing I have not checked are my Flo-scan sensors. And, I am getting some funky readings and maybe have found the smoking gun. The sensors are between my 30m pre-screens and my 10m Racors.

    Maybe (hopefully), I have a clog in both sensors and will check this afternoon. The 7hg on my gauges is at idle. Update to follow. I will do the 5 gallon tank test next.....

  18. #18
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    Default Re: Not hitting WOT/max RPM's both engines

    Quote Originally Posted by bobct View Post
    It sounds like everyone is leaning towards a fuel/pressure/delivery issue. On the fuel line hose, it is Aeroquip 1/2" ID hose, not 5/8". The fittings are 1/4" NPT and none of that changed. The one thing I have not checked are my Flo-scan sensors. And, I am getting some funky readings and maybe have found the smoking gun. The sensors are between my 30m pre-screens and my 10m Racors.

    Maybe (hopefully), I have a clog in both sensors and will check this afternoon. The 7hg on my gauges is at idle. Update to follow. I will do the 5 gallon tank test next.....
    7" of Hg = 93" of water = 3.x psi. hmmm, at idle. That must be mostly the cracking pressure across the ASV. If so, that likely won't change much with flow. Seems like a lot of cracking pressure, but I've never measured across the ASV.

    There are two definitive tests to do:
    A vacuum test at WOT.
    A fuel pressure test at WOT.

    These tests will keep from chasing around all the fuel related objects in the boat.
    Last edited by diver_dave; 08-02-2017 at 01:33 PM.

  19. #19
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    Default Re: Not hitting WOT/max RPM's both engines

    Dave, I think the problem might be narrowing. Nothing with the Flo-scans but the guy I spoke to had the same reaction everyone else did here, 7hg is just too high and there must be restriction on both sides. In the meantime, I did a comp test on the Port engine and was 125 as the low and 137 as my high. 6 of 8 cylinders were in the 130's. Test was done cold with the throttle kept open. Total engine hours are approx 1,400.

  20. #20
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    Default Re: Not hitting WOT/max RPM's both engines

    Ugh, sorry guys but I screwed up on the gauge. I just realized (after calling the company) that your supposed to manually zero out the gauge to start. I just did that and now I'm right around 2.5hg at idle on my port engine. Hopefully it returns to zero on its own now but that seems to be a much better number.

  21. #21
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    Default Re: Not hitting WOT/max RPM's both engines

    I realize my last post is not clear, with all the competitive vacuum and pressure readings.

    There are two definitive tests to do:
    A vacuum test at WOT. at the base of the carb.
    A fuel pressure test at WOT. at the output of the fuel pump

    I realize that both are a two person job!

  22. #22
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    Default Re: Not hitting WOT/max RPM's both engines

    Erich, I just caught your post. Yes, I do have two tanks and a crossover valve. I've never actually had to use it other than as a test. It's always in the closed position. I guess it's possible that it's "in the position" but not actually closed which would create the scenario you describe. I will bypass everything as a next step and see what happens. Unfortunately, I'm less convinced of a fuel issue based upon my new numbers on the vac gauge but that's just a hunch and not where my real issue is....

    Dave, yes I will do those two tests next. I'm pretty sure the engine itself is fine at least the one I tested today.

  23. #23
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    Default Re: Not hitting WOT/max RPM's both engines

    Much better vacuum number....

    Those are decent numbers and the test is valid when done cold....

    might as well take the IR gun and measure the fuel temp (@ pump).

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