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  1. #1
    Join Date
    Aug 2013
    Location
    St. Louis
    Posts
    172

    Default issue with top speed

    My 89 SupraSunsport 351 PCM has lost some on the top end. We spent the last part of the summer focused more on wakeboarding than skiing, because our top speed seems to be ~32. The engine purrs just fine at idle and will rev to 4500+ at idle. At WOT I think we are getting ~3500 rpm.

    The boat lives on the water. The bottom was painted with anti-scum paint ~5 years ago. It has some gup down there, but not bad. The transmission was leaking like a sieve, so we dropped it off at an inboard shop last winter. They pulled the tranny and we had Fantastic finish marine rebuild it. The transmission is back in and not leaking, so great. We were planning to get the boat re-painted, thinking that might be the issue with top speed.

    For various reasons, the shop has been slow on repainting and we had a repeat conversation about the speed issue and that being our interest in repainting. They looked at the gup on the bottom of the boat and think it is not too bad and maybe we are loosing 1-2 mph from that issue.

    I think I will pick up the boat without the paint and see how we run. I'd like to get back to skiing though.

    We had OJ rebuild our prop last summer and that didn't help.

    We had changed the cap, rotor, points, and wires ~3 years ago and they still look good. The dwell time is good. Plugs changed last year. Fuel filter changed.

    The carb (Holly) was rebuilt ~4 years ago. I've adjusted the idle mixture screws a little bit at idle and the engine feels good at 1600 rpm. There is no vacuum leak that I detect spraying carb cleaner around the carb.

    The fuel pump is original, and there is no fuel in the external hose. I am getting spark to each cylinder and the exhaust ports feel warm from each cylinder.

    If drag isn't the major issue, is this likely a fuel problem?

    So, things I can look at. Compression, but I don't think this is it. Fuel pressure at the carb. Fuel filter at the carb. Timing advance. Rebuild carb.

    What is most likely?

    Thanks.
    Last edited by scottschmitt; 04-25-2019 at 04:43 PM.

  2. #2
    Join Date
    Jun 2014
    Location
    Lexington, KY
    Posts
    185

    Default

    I had issues like this on my Nautique 2001 (same engine setup). It turned out to be that the mechanical advance on the distributor was sticking and never fully advancing. it wouldn't make any power in the top end. You could use a timing light to make sure that it advances at higher RPM. I don't believe that you should have vacuum advance, so load won't matter (unless I'm wrong about vacuum advance - is there a vacuum line running to the distributor?). I ended up replacing the distributor to fix this, but you could probably disassemble and clean it. If i recall correctly, mine was more worn out than dirty. That boat had like 2500 hours on it.

    Also, run the boat without the engine cover (if possible) and see if the secondaries are in fact opening at WOT. For this you will have to be under normal load - so running down the lake.
    Korey Morris -
    Sold to Arkansas: 1986 Supra Saltare
    Current: 2018 Moomba Craz

  3. #3
    Join Date
    Apr 2002
    Location
    Florence, AL
    Posts
    770

    Default

    Quote Originally Posted by korey View Post
    I had issues like this on my Nautique 2001 (same engine setup). It turned out to be that the mechanical advance on the distributor was sticking and never fully advancing. it wouldn't make any power in the top end. You could use a timing light to make sure that it advances at higher RPM. I don't believe that you should have vacuum advance, so load won't matter (unless I'm wrong about vacuum advance - is there a vacuum line running to the distributor?). I ended up replacing the distributor to fix this, but you could probably disassemble and clean it. If i recall correctly, mine was more worn out than dirty. That boat had like 2500 hours on it.

    Also, run the boat without the engine cover (if possible) and see if the secondaries are in fact opening at WOT. For this you will have to be under normal load - so running down the lake.
    THIS. Distirbutor mechanical advance sticking or vacuum secondaries on the carb not opening. Also, check to make sure all plug wires are seated on the plugs and distributor cap are seated fully. You'd be surprised how well one of these motors will run on 7 cylinders. Only symptom is a slightly rough idle and loss of power at speed. Ran half a season years ago before I figured that one out.
    1992 Supra Mariah - Red
    PCM 351HO 285hp - PCM 1.23:1 Transmission
    OJ XMP 4 Blade CNC 13x15.5RH

    "People do weird things to boats" -Unknown

  4. #4
    Join Date
    Aug 2013
    Location
    St. Louis
    Posts
    172

    Default

    Thanks guys. Good places to start.

    Doesn't sound like you all think this is a fuel pump problem. Also, plug wires seem seated. I added a dab of dielectric grease in there too, for what that's worth.

    I think it is the original mechanical distributor. I don't think there is vacuum advance.

    I can look at the timing advance while rev'ing in neutral, correct? Seems a lot safer than doing it at speed.

    Also, I found this paperclip trick to look at secondaries opening (http://www.correctcraftfan.com/forum...not-opening-up). I'll give that a look too.

  5. #5
    Join Date
    Jul 2007
    Location
    Flowery Branch Georgia
    Posts
    2,383

    Default

    ''We had changed the cap, rotor, points, and wires ~3 years ago and they still look good. The dwell time is good. Plugs changed last year. Fuel filter changed.''
    It has been 3 years on the same points? I would start with new points set dwell and check the timing. You can check the advance weights while you are in there.

  6. #6
    Join Date
    Apr 2002
    Location
    Florence, AL
    Posts
    770

    Default

    Other possibilities could be fuel flow related caused by a clogged tank vent or stuck anti-siphon valve. You can check the tank vent by running with the gas cap slightly unscrewed, but if these were the problems, I would expect it would run good for a short period of time up to WOT before fuel starvation slowed it down. Just wanted to throw it out there for consideration though.
    1992 Supra Mariah - Red
    PCM 351HO 285hp - PCM 1.23:1 Transmission
    OJ XMP 4 Blade CNC 13x15.5RH

    "People do weird things to boats" -Unknown

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