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RedSnapper 08-12-2010 06:31 PM

Low MPH/RPM - '89 ProStar 190
Hey all,

Buddy of mine has an '89 ProStar 190 and taking it out this year we notice that it's currently topping out around 29MPH (on GPS) and ~3000RPM's. He's a fairly new owner to the boat and isn't sure what it was running at before this year, so hard to say if this is a new issue that's cropped up or not.

Overall, the boat seems to run great. It doesn't sputter or bog down or anything, and takes off great out of the hull without issues. It does seem that once the throttle handle reaches about 70% wide open, is when it doesn't grab any additional power between then and 100% open. Again, doesn't seem to be bogging down after that or anything, it just seems to top out there and not gain any more speed/RPM's.

I'm going to head up to his place in a week or so and start taking a look at things, but wanted to get some advice on what I should be looking into when I get there. Things that I can think of to check are:
- Throttle cable actually opening up the carb all the way (i.e. see if at that 70-100% it's actually not opening anymore, when there is still more to go.)
- Flame arrestor (I seem to remember seeing this to be pretty dirty)
- Fuel filter replacement
- Check fuel screen at pump and carb
- Timing (make sure it's set to 10 degrees BTDC at 600RPM's)

Is there anything else that I should be looking at first, or something else that stands out to you? Also, as far as I know, I shouldn't waste my time with throwing the timing light on it, till it's in the water and in gear, yeah?

Thanks all!


MakeMineaS&S 08-12-2010 06:58 PM

Secondaries not opening could be a culprit. Check the diaphragm on the secondary linkage, could have a pinhole, or the linkage could be blocked/bent, whatever.

jconover 08-12-2010 07:20 PM

I second the vote on secondaries; should be a vacuum activated secondary; typically opens around 3200 RPM.

Good idea to check the timing too. Not to be a party pooper, but don't forget to bring a compression tester - certain cylinder failures can produce significantly less HP, though usually you'll get rough idle too.

I'd place bets that the secondary isn't opening, however, if it were my over-the-web diagnostic.

1redTA 08-12-2010 09:35 PM

sounds like the secondaries not opening or the distributor springs hanging up

RedSnapper 08-12-2010 10:50 PM

Thanks for the advice all! I hadn't even thought about the secondaries for some reason. Will check it out next weekend and report back my findings. Thanks again!


oldairboater 08-12-2010 10:51 PM

Like everyone else secondaries are not opening. Mine open up between 3000 to 3200 rpm at approximately 30 mph. If the secondaries open and the fuel isn't there boat falls off quick.

RedSnapper 08-13-2010 01:52 AM

What do ya guys think? Pick up a secondary spring kit and bring it out to the lake with us? Anything else to make a quick repair out there (assuming it's the secondaries not opening), so I don't have take it back out and home to work on, only to take it back out to test?

MakeMineaS&S 08-13-2010 02:34 AM

Since it's vacuum-operated, it could be the diaphragm too. I had some vacuum-operated CV carbs in an old motorbike and even a pinhole in the diaphragm would screw up the slide opening.

You can check the idle timing easy enough in the driveway, but the secondaries would really have to be checked under load, so if y'all want to take stuff to fix it out there, I would go with springs and a diaphragm, at least, (and assuming the diagnosis is correct.)

jconover 08-13-2010 09:35 AM

Yeah Red - you'll need basically a carb rebuild kit that includes the secondary diaphragm -
FYI many kits dont come with it, so you might just order it separate - here is what you are looking for

RedSnapper 08-13-2010 08:02 PM

Thanks again guys. I'll make sure he picks up these parts before we head out to the lake.


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