Prop questions on a 16' 1976 StarCraft Montego (Merc 888)

[jimmbo]

If the Original Carb was getting Gummed Up, and not staying Clean, how is a Different Carb going to be any Different?
You choose a Prop by making sure it runs in the WOT Rpm Range stated for the Particular Engine, which in your case is 3800- 4200 rpm.
You DO NOT Prop for Targeted Speed at any RPM. Several Props will likely get in the WOT Rpm Range, and they may have different top speeds because of things like # of Blades, Blade Area, Pitch and Dia, Rake and Cupping. Maybe amongst the choices, you will find a Match that is closer to the Speed(s) you want, but what ever Prop you Pick, it has to run in that Rpm Range at WOT, if you expect to get any Engine Longevity, and reasonable Performance
As for the Electric Choke. It should be set to just fully Close at a Temp of 75F. If your Leaving the Key on too long before Starting, then don't do that. If the Choke was correctly Set Initially, and you change that, well somewhere along the line you will have created a New Problem

 

[ratdude747]

If the Original Carb was getting Gummed Up, and not staying Clean, how is a Different Carb going to be any Different?

Couldn't tell you. The old setup was scrapped decades ago so it's a moot point... The Edelbrock 289 and Holley marine 4160 is what I have and seems to work fine.

You choose a Prop by making sure it runs in the WOT Rpm Range stated for the Particular Engine, which in your case is 3800- 4200 rpm.
You DO NOT Prop for Targeted Speed at any RPM. Several Props will likely get in the WOT Rpm Range, and they may have different top speeds because of things like # of Blades, Blade Area, Pitch and Dia, Rake and Cupping. Maybe amongst the choices, you will find a Match that is closer to the Speed(s) you want, but what ever Prop you Pick, it has to run in that Rpm Range at WOT, if you expect to get any Engine Longevity, and reasonable Performance

I'm told that "RPM matched speed" during the run, so likely it was WOT'ing at 4000 RPM. So the prop is right?

As for the Electric Choke. It should be set to just fully Close at a Temp of 75F. If your Leaving the Key on too long before Starting, then don't do that. If the Choke was correctly Set Initially, and you change that, well somewhere along the line you will have created a New Problem

Makes me wish I had gone ahead and rewired the trim gauge for to use accessory power like I wired the GPS speedometer... Next time I have the console out I may do that... Problem solved. Or heck, all the gauges other than the hour meter and oil pressure light/buzzer... Added to the fall/winter upgrade list.

Edit: I'll recheck my pictures I took of the console upgrade... But I think I could temporarily fix this from the ignition switch access cutout since all gauge powers are on a dedicated wire. Only issue is that the more I move to accessory, the worse the voltage drop becomes in such a mode; part of my current design is to allow me to flip to accessory from off and be able to get a very accurate battery voltage reading on the speedometer display. Engine off, I see about 2V of drop in RUN compared to accessory... The only things getting power in RUN are the other gauges, the pertronix ignition, and the choke. Already determined that the choke isn't a huge part of the drop... and the ignition switch is brand new.


Initially I set the choke to just barely close at ambient temp, the same way I set the dual heat choke on my truck (1 barrel Carter electronic carb, but same idea). Unlike my truck though, there isn't a fast idle cam being used, so as fast as the chole heats up, i don't see how a richer setting will hurt once warm. It's also possible that this choke, despite the Holley manual (it was a used purchase after all), is like my truck in that it's designed to run off the 7V field voltage from the alternator, not the 12V of ignition power... Causing it to heat way too fast. Might not hurt to replace it with a known 12v choke?

 

[jimmbo]

Couldn't tell you. The old setup was scrapped decades ago so it's a moot point... The Edelbrock 289 and Holley marine 4160 is what I have and seems to work fine.

My Comment is basically, why was the Old Carb getting all Dirty, and unless the Reason wasn't corrected, a different Carb would suffer the same Fate.

I'm told that "RPM matched speed" during the run, so likely it was WOT'ing at 4000 RPM. So the prop is right?

It puts you right in the Middle of the Range, which is fine. I don't know if the Boat was lightly Loaded, Heavily Loaded, or loaded like you always run it.
If lightly loaded, then when heavily Loaded, or even Normally Loaded, it might be too much Prop. On the Opposite of the Range, if that was on a Heavy Load, then an Empty Boat could result in Over Revving, meaning not enough Prop. You only have a 400 rpm Range so keep an eye on it if you have a few hundred Pounds difference in load, either up or down. No not form an Opinion that if you are Underpropped and the Engine is Over Revving, to simply pull back on the Throttle, is the Answer, as that changes the Fuel Delivery Curve-Calibration in regards to Ignition Advance, which might lead to Detonation.
Marine Engines run under a Very Different set of Operation Conditions than a Similar Engine operated in a Car or Truck on the Road. A Boat running 35mph on a Lake is similar to driving a Car/Truck up a Hill at 80-90 mph, whilst towing a 40 ft Holiday Trailer

 

[ratdude747]

My Comment is basically, why was the Old Carb getting all Dirty, and unless the Reason wasn't corrected, a different Carb would suffer the same Fate.

No idea. Possibly a bad rebuild. The other comment was that it didn't ever run right, so dirt/gum may have been a scapegoat.

It puts you right in the Middle of the Range, which is fine. I don't know if the Boat was lightly Loaded, Heavily Loaded, or loaded like you always run it.
If lightly loaded, then when heavily Loaded, or even Normally Loaded, it might be too much Prop. On the Opposite of the Range, if that was on a Heavy Load, then an Empty Boat could result in Over Revving, meaning not enough Prop. You only have a 400 rpm Range so keep an eye on it if you have a few hundred Pounds difference in load, either up or down. No not form an Opinion that if you are Underpropped and the Engine is Over Revving, to simply pull back on the Throttle, is the Answer, as that changes the Fuel Delivery Curve-Calibration in regards to Ignition Advance, which might lead to Detonation.
Marine Engines run under a Very Different set of Operation Conditions than a Similar Engine operated in a Car or Truck on the Road. A Boat running 35mph on a Lake is similar to driving a Car/Truck up a Hill at 80-90 mph, whilst towing a 40 ft Holiday Trailer

Heavy load. No towing but there was the max 6 people on board. Here they are returning from the run after (same people, but with the tube):



Two average adult men, one large adult man, one skinny woman, and two skinny 16-year-olds. I'm a very large adult man (oink!) FWIW.

Might explain a lot. I'll have to try it on the river next week(end) (weather and trailer repair permitting) with just my wife and I and see.

 

[jimmbo]

I liked that Hull, Starcraft also made a 15ft Outboard Version of the Hull, which was Flipped by a Canadian Boat Maker back in the Late 70s

 

[ratdude747]

Took it out yesterday with my wife, the best friend, and his buddy (the same crew we had on the first outing this year).
I didn't get it to WOT (water too rough on Monroe Lake) but the performance was about the same (if not better) than the prior prop. Did seem to be a consistant 10-to-1 ratio of RPM to GPS indicated speed. And didn't seem to guzzle fuel.

Also will note the last outing only used 12.7 gallons of fuel (against a 20 gallon tank). I sized the sending unit per the MFG suggestions (1" less than full depth, so I bought a 5" unit for my 6" tank)...

All considered, I'm calling this case closed. Seems the current blades are doing what I want.

 

[ratdude747]

Finally had a good chance to get it to WOT.

4050RPM, 41-42mph. Not as fast as new but for what I'm doing it's fine. Decent hole shot.