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  #1   IP: 192.186.122.174
Old 09-18-2018, 10:00 AM
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Vacuum Gauge Update

Not sure which is the best category for the topic - if wrong spot please let me know and I'll post it elsewhere.

Having recently installed the a vacuum gauge, the numbers seem a little off from "expected" though I realize from reading here that one's numbers are unique to their own engine.

That being said, I have also not done a thorough tune up yet; and I am guessing this can affect the reading on the gauge?

Was out on the weekend in calm weather.

Moving along in gear at idle (~800 rpm) the gauge was reading 15, at "cruise" of 1500 it was reading 5 and anything higher (2000 seems to be max) the gauge was reading 0-1.

So I'm asking the hive mind here - what's my next step or steps?

my initial thoughts are:

- idle adjust so its a little lower
- Adjust distributor while at cruise to max/sweet spot rpm?
- with new bypass set at "50%" 45min cruising and needle never went above 120F (lake is about 22C) - should allow more bypass to raise temps to around 140?

oh.. and a pinhole just developed near the bottom of the standpipe.. winter project

Thoughts? ideas? Buehler?
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  #2   IP: 137.200.32.54
Old 09-18-2018, 11:11 AM
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These readings are boat and prop specific.
I usually cruise at 5 inches which gives me around 2100 RPM. My max RPM at 0 inches is around 2300 or so.
In flat water, if these reading change for the worse, i.e. less RPM at a certain vacuum, I know the bottom is getting dirty. Right now I *really* need to clean the bottom and full throttle (0 inches) is only getting me 2000 RPM or so and 5 knots
I'll fix that as soon as the water looks safe to swim in.
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  #3   IP: 172.95.97.58
Old 09-18-2018, 06:34 PM
Dave Neptune Dave Neptune is online now
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Gregh, you are definitely loading the cylinders, probably to much prop.

If you have a timing light you can check for full advance of the c-advance mechanism. You should have full advance by 15~1700 RPM's so you may not be getting all of the adv at your RPM's if the weights are sticking. Try rotating the dist at full cruise or at a point that the c-adv is fully advanced.

The prop and hull play a big part too as far as Vac performance is concerned. My ole #35MkII with the Indigo prop would cruise at 2100 RPM's at between 6~7 inches of vac and max out at around 2600 RPM. My engine was old tired and low compression numbers however it was well tuned and the carb had been balanced by me.

Try the dist tweek and then record the numbers. A higher vac number at a "given" RPM means a gain in performance.

Most important is gathering the numbers when the tune is fresh and the prop and bottom are clean. Any changes can be easily identified as Joe pointed out.

Dave Neptune
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  #4   IP: 69.159.25.191
Old 10-22-2018, 03:27 PM
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The boat came out today and as per talk above about the prop, it's a 13 x 7

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  #5   IP: 137.200.32.22
Old 10-22-2018, 03:58 PM
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My boat had a 13x7 at one time.
I would, in this order:
1. Compression check.
2. Timing check.
3. Exhaust backpressure check - maybe??
If all that is good, you may want to go to a 12x6 or an Indigo prop. Over-propped A4s are very common. In flat water it is nice, you get near hull speed right off idle, but any wind and wave kill you.
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  #6   IP: 97.93.70.7
Old 10-22-2018, 04:45 PM
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Exclamation

It looks like the location of the prop may have been changed. There is good clearance on the bottom of the prop but none at the top.

Do you know if it has been "raised" by "the or a previous owner"?

That 13X7 looks like it would be centered and have plenty of clearance if it were "centered".

Dave Neptune
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  #7   IP: 69.159.25.191
Old 10-22-2018, 09:16 PM
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Quote:
Originally Posted by Dave Neptune View Post
It looks like the location of the prop may have been changed. There is good clearance on the bottom of the prop but none at the top.

Do you know if it has been "raised" by "the or a previous owner"?

That 13X7 looks like it would be centered and have plenty of clearance if it were "centered".

Dave Neptune

Sorry, part of that bad looking fit is the camera angle - I was more focused on the dimensional information. I'll get a better one on the weekend. The position is as it was since I purchased the boat.
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  #8   IP: 137.200.32.22
Old 10-24-2018, 11:30 AM
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Greg - you may want to look into the Indigo prop. I was very happy with the change from 13x7 to Indigo.
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  #9   IP: 24.152.132.140
Old 10-24-2018, 11:54 AM
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There are factors working against themselves here. I see the benefit of a large diameter prop for good flow exposure in an aperture mount but that means pitch must be reduced to avoid overloading the engine.

Only because they were produced in the thousands, the Catalina 30 may be a reasonable place to start. The factory fixed 2 blade was a 12X7RH which was a little much, produced max RPM of around 2100. Keeping your prop at 13" diameter for the reason mentioned, I suggest a 5" pitch may be really sweet. You might want to research the viability and cost of having a shop repitch your existing prop. For value comparison, Michigan wheel offers a 13X5RH 2 blade sailor prop for roughly $350.00.

Last year I swapped my 12X7 for a 10X8 that resulted in a significant improvement meaning the engine RPM is up in the power curve better and working a little easier.

edit:
I should have asked, direct drive or reduction gear? My comments apply to a direct drive engine.
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Last edited by ndutton; 10-24-2018 at 08:55 PM.
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  #10   IP: 72.69.36.126
Old 10-25-2018, 10:56 AM
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I think it's pretty hard to beat the drop-in ease of the Indigo prop - it makes a big difference, and was specifically designed to - but I don't know how its improvements fare in an aperture like that.

Assuming it's OK in an aperture, with it in place you can mess with the other stuff to your heart's content next season, in the water. Otherwise you'll always be wondering how much the prop being tested is contributing to the sub-optimal conditions (if indeed they are sub-optimal).
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  #11   IP: 207.118.20.35
Old 10-27-2018, 02:29 PM
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I changed to an Indigo from a 12x5 two-blade. It is in an aperture with much more clearance than your photos. I get better thrust in reverse. After a haul- out, I get 6 knots at 1900 rpm, with vacuum 10.
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  #12   IP: 192.186.122.174
Old 10-29-2018, 10:10 AM
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FYI I started a new thread in the Prop section as this was originally about vacuum gauge
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