Prop size questions; lots of details

[dustinsapple]

I recently bought my first boat, its a 1988 Renken Cuddy Classic 2000 19FT.

OMC 4.3 Cobra Inboard/Outboard

I bought the boat with a cracked block so I removed the motor and rebuilt it with a newer block but reused all the marine parts and heads, flywheel, balancer etc.

Aftering tinkering with the boat a bunch afterwards to get it to run right, I have the timing set at 6* at idle, and it sits around 18* at 3000rpm. I don't have a digital timing light just a basic and had to use a piece of tape to mark out the degrees to transfer to the balancer with whiteout since it only has 1 TDC mark. So my timing is aprox due to the fact that I just took the circumference of the balancer divided by 360 and layed out marks onto the balancer with whiteout. I have good vacuum at idle (20HG) so I believe the carb is setup fairly well, and I do not hear any tapping or funny noises from the rocker arms.

I also had to use an electric fuel pump with my new block, package says 4-7psi and seems to be getting enough fuel.

Anyways, so WOT I am maxxing out at 4000RPM with 15x17 prop and reaching 37-38mph on the water. I read online that I should be able to achieve 4200-4600rpm with this 4.3L OMC.

My question is, is my prop the wrong size for this boat, it obviously came on the boat however I do not know if it is original to the drive or not. I do not have trim tabs so I can only trim up/down with the out drive. When I am WOT it does however plane out fairly well. I am looking to get a little bit more top end, and be able to run the motor WOT and reach the 42-4600RPM range. Do you think my 4000RPM max is related to the prop size?

I do not know the weight of my boat, but its sitting on a single axle no brake trailer so I highly doubt its more than 3500lbs

Thanks!

 

[wiigelec]

Is your distributer electronic? How did you set the timing? Why did you select that number of degrees? What octane fuel are you using?

I get over 5000 rpm WOT with a 15x17 on a similar though probably slightly lighter boat...

 

[wiigelec]

If you have an EST distributor you should find the following helpful:

https://f01.justanswer.com/oldnewenglandmarine/6e934c0e-8bba-4f60-ab66-d19879033c03_Delco_EST.pdf

 

[dustinsapple]

My distributor does not have any electrical plugs, just receives the spark from a single coil which only has 2 wires (pos/neg) so it is not electronic. I came up with the 6* at idle BTDC and 18* BTDC from other posts on this forum. I am putting 91 premium in the boat.

I set the timing with a basic timing light and marks on my harmonic balancer, by adjusting the distributor.

I did adjust the throttle cable this morning, to the point where it idles on muffs at 1k rpm and the idle screw now would only make the RPMS go up, no room to make the idle speed go down now as the cable is tightened to that spot. I did this after some friends suggested my throttle cable needs adjustment and I was not getting a true WOT.

 

[dustinsapple]

Also in Neutral while on the way with throttle cable disconnected I can easily rev the engine to 5k burping the throttle, which makes me believe its something to do with the prop size.

 

[GA_Boater]

Also in Neutral while on the way with throttle cable disconnected I can easily rev the engine to 5k burping the throttle, which makes me believe its something to do with the prop size.

This means nothing because there is no load on the motor. Is the new motor a car, marine or truck motor?

Have you done the basics? Compression and spark on cylinders and rebuilt the carb.

Also the boat may have added weight over the years.

 

[dustinsapple]

2002 Vortec motor from a truck, I only used the bottom end. Reused everything else from the original OMC motor. The only questionable thing I guess would be the carb. I was able to trim up a little today, it reaches 4100rpm when trimming up a bit at WOT.

 

[dustinsapple]

Another thing to add, which I found today is that the prop nut was loose, only being held on by the cotter pin and castle nut. Without this being torqued to OMC spec (75ft/lbs) would that cause much more slippage in the prop than is acceptable?

 

[wiigelec]

only one way to find out torque it to spec and go boating!

 

[ahicks]

Another thing to add, which I found today is that the prop nut was loose, only being held on by the cotter pin and castle nut. Without this being torqued to OMC spec (75ft/lbs) would that cause much more slippage in the prop than is acceptable?

The prop nut is there to keep the prop from sliding off the end of the shaft, not to prevent slippage. Loose like you describe could cause some banging as the prop slams forward and backward.

 

[dustinsapple]

I picked up a 14.25x21 prop to see what kind of difference I get in RPM at WOT and speed. Will report back.

 

[highpowerdad]

If it couldn’t swing a 17pitch to a high enough rpm, I doubt it will do well with a 21 pitch , even if the diameter is a little less, you needed to go down in pitch...not up...

 

[dustinsapple]

If it couldn’t swing a 17pitch to a high enough rpm, I doubt it will do well with a 21 pitch , even if the diameter is a little less, you needed to go down in pitch...not up...

Yes this is true.

I got a good deal on this prop, its an original from an OMC like my 15x17 so it wont hurt to have an extra prop anyways.
From what I've read online, the 3/4" dia drop will result in an increase of maybe 400rpm, but then decrease 100rpm for ever inch of increased pitch (all approximately) so realistically there shouldn't be too much difference maybe a couple hundred RPM or so. What I am interested to see is how much this affects my top speed if any at all, and will my RPMs be capped at the same spot 4 to 4100 RPM or not.

I also removed the water separator for the time being, as I was wondering if it being plugged or full of crap was affecting how much fuel my electric pump was drawing. I was getting the occasional bog down when going WOT which seems to have be cured by taking this out and running the tank line straight to the pump.

I ordered brand new locking nuts from GM for the rocker arms as one of my friends suggested running the motor with the valve covers off and using new nuts and adjusting all the push rods once more. I only adjusted them by turning the motor on the engine stand. But I did end up thinking, what if there is a chance that I adjusted all the push rods on the exhaust stroke, I wonder if that would be limiting my top end, but at the same time how could the motor even run as good as it does upto 4k RPM is they were that far off.

Either way, it has something to do with when the motor is under-load obviously as in neutral I can rev 5k easily as stated before.

One other note is that the hot air choke, it had a little copper line that did a loop inside the intake manifold has been plugged off. I don't plan on using the boat in the cold so its not needed. Should I remove the arm on the side of it that controls the top butterfly? Pretty sure the vacuum controlled diaphragm on the back of the carb opens everything up anyways under throttle.

Reason the hot air choke has been removed, is that the little copper line that looped inside the intake manifold was in the way of the balance shaft, well it looped inside the little chamber that allows the steam ports (whatever their called) to breath without allowing the oil to splash around in there. Not sure if I'm explaining it right for everyone lol but theres a bubble chamber with 2 holes that was in the way of the balance shaft. I was told anyways that you can weld the steam ports shut on the intake manifold, (which i didn't do, funny because I'm a welder by trade lol) so I just "when it doubt hammered it out" hammered it flat and added a couple extra holes for the steam ports to still breathe without oil sloshing around and getting down in there. Anyways from everything I read, the steam ports and hot air choke are essentially for cold starts in colder weather climates.

I'll post back again and keep a good log of everything I'm doing, hopefully it will help someone out in their quest in the future.

OH - you know whats annoying, you can't access the air/fuel screws on the carb while the motors running because the thermostat housing is right in the way, anyone got any tricks for this? Would love to be able to use my vacuum gauge and start fresh with the carb, considering its air fuel mix setup is still set from previous engine, not sure if there is that much difference from engine to engine or not.

 

[dustinsapple]

On choppy water today, got it to 3900 rpm, feel like I i I could get more anyways. Also probably 6 mph faster anyways