45hp Chrysler Prop / Top speed mystery

[8ender]

Chrysler Prop / Engine mystery

I have a 45hp 1974 Chrysler Outboard I've gone over with a good carb clean, new plugs, etc. The motor runs wonderfully at all throttle openings. It's on a very small and light Chrysler Cadet runabout, which is 14 foot long and, according to Chrysler, 330lbs dry when it was made in 1969.

It came with a 10 3/8 x 11.5 prop that looks newer. I also have a 12.5 and 13 pitch prop of the same size.

So here's the mystery: I figured the 11.5 prop to be a low pitch for such a small boat. I fitted a tiny tach and decided to do some prop science and got some weird results.

11.5 Prop: Around 4650 rpm max, 25.4 mph top speed
12.5 Prop: Around 4440 rpm max, 26.7 mph top speed
13 Prop: Around 4300 rpm max, 24.8 mph top speed

Any ideas on this? The boat is tiny and we confirmed the weight today by lifting one side with just two people on the trailer. The manual calls for 4500-5500 rpm range for the 45hp and I'm only just barely in that range with the 11.5. I'd have figured I'd be high on the range or even over revving. The engine seems very healthy and the boat gets on plane real quick.

 

[jimmbo]

Your tach might not be accurate. I don't know where Chrysler engines get their signal for the tach, but you could verify the readings with one of the Mini Digital Tachs that get a signal from a plug wire.

 

[8ender]

Your tach might not be accurate. I don't know where Chrysler engines get their signal for the tach, but you could verify the readings with one of the Mini Digital Tachs that get a signal from a plug wire.

That's what I'm using right now. It's not a Tiny Tach exactly but the same concept. Wraps around the spark plug wire.

 

[jimmbo]

Looking in the 1974 Chrysler Brochure.
Your engine has a gear ratio of 13:21 or 0.619.
45hp@4750 rpm
A WOT range of 4400 - 5100 rpm
3 different ignition systems: Magnapower; Battery; Magneto

4650 rpm with the 11.5" would be 28mph with 10% slip and 26.8 mph with 15%
4440 rpm with the 12.5" would be 29.5mph with 10% slip and 27.6 with 15%
4300 rpm with the 13" prop would be about 29 mph with 10% slip or 27 mph with 15%

 

[8ender]

Thanks for finding the gear ratio specs, I was looking around everywhere for them. My knowledge of props and outboards isn't good enough for me to interpret your results though. Do your results mean my motor is operating correctly? I'd have figured it would be able to push this small boat much faster.

 

[WesNewell]

engine mounted too low causing excessive drag?

 

[8ender]

engine mounted too low causing excessive drag?

It's the stock engine for the boat. The cav plate seems to be right inline with the bottom of the transom.

 

[Tassie 1]

I'd be a little wary of subjecting a 45 yr old outboard to those sorts of revs constantly,
specially for just trying to get the " best " prop for an elderly motor for really little gain,
you may shorten it's life considerably
just use the standard prop,

How are you doing these tests?
hopefully not with muffs

 

[8ender]

Cross posting this from the prop forum as you folks might have an idea of what's up:

I have a 45hp 1974 Chrysler Outboard I've gone over with a good carb clean, new plugs, etc. The motor sounds wonderful at all throttle openings. It's on a very small and light Chrysler Cadet runabout, which is 14 foot long and, according to Chrysler, 330lbs dry when it was made in 1969.

It came with a 10 3/8 x 11.5 prop that looks newer. I also have a 12.5 and 13 pitch prop of the same size.

So here's the mystery: I figured the 11.5 prop to be a low pitch for such a small boat. I fitted a tiny tach and decided to do some prop science and got some weird results.

11.5 Prop: Around 4650 rpm max, 25.4 mph top speed
12.5 Prop: Around 4440 rpm max, 26.7 mph top speed
13 Prop: Around 4300 rpm max, 24.8 mph top speed

Any ideas on this? The boat is tiny and we confirmed the weight today by lifting one side with just two people on the trailer. The manual calls for 4500-5500 rpm range for the 45hp and I'm only just barely in that range with the 11.5. I'd have figured I'd be high on the range or even over revving. The engine seems very healthy and the boat gets on plane real quick.

 

[racerone]

The engine " seems healthy "-----What does that mean ?-----Post the results of a compression test first.

 

[GA_Boater]

Cross posting this from the prop forum as you folks might have an idea of what's up:

If you want a thread moved, flag it and we will move it. We ask that one problem has only one thread because we are easily confused and you will get duplicated replies.

Both threads have been combined for clarity.

 

[8ender]

The engine " seems healthy "-----What does that mean ?-----Post the results of a compression test first.

I got around 150 each cylinder when I tested a few months ago.

For context the engine has new spark plugs, I've gone over the fuel system with new hoses and checked the fuel pump diaphragm for cracks. I followed the service manual and set the linkages so that that butterflies are horizontal at WOT and adjusted the idle screw to about 7/8 of a turn out which it seemed to like the best.

How are you doing these tests?
hopefully not with muffs

Oh no, I'd need to do them in the water to measure the top speed, and muffs don't really work on the Chrysler outboard so I use a bucket when testing idle. I'm not really pounding on the motor here, it's operating range is 4400-5500 rpm, it's meant to run that high.

 

[jerryjerry05]

Outboards are designed to run wide open.
They can be set up to run 6000 but best results are around the 44-5500 like the factory suggests.
Is the metal tag still on the lower cowl? Riveted in place in front of the cowl?

You results are a bit low? maybe a smaller prop? maybe it's over loaded?

Is the throttle linkage working right? Opening the linkage all the way?

Do another compression and spark test.
You'd be surprised how well these run on just one cyl.

 

[8ender]

Thanks for the tips JerryJerry05.

I put it in the bucket again today and ran some tests but didn't get much further on my mystery. I'm definitely getting spark on both cylinders. I popped the flywheel and checked the timing and it looks like exactly 32 degrees off TDC. Point gap on the top cylinder was a little tight so I correct that and gave them a good cleaning. Condensers look almost new.

I checked the fuel pump diaphragm again and it's solid and rubbery. Checked the linkage again and I'm getting a nice flat horizontal butterfly at wide open throttle. It idles like a swiss watch and has no issues revving.

I did notice a little bit of gas/oil sitting in the carb throat, float needle issue?

 

[DunbarLtd]

Make sure motor is not tilted up before this test.

Squeeze the fuel primer bulb until firm (dont force it any further than you need to) and watch for leaking fuel from throats. If everything is good no fuel will come out. If fuel comes out then Id suggest rebuilding the carbs and adjusting floats. I had the same issue and a cleaning and a rebuild fixed my problem.

Seeing fuel in the carbs after running is normal. Not all of it gets burnt. Most have a shroud to catch this unburnt mix and reroute it back to the intake. Also when tilting the motor up the needles tend to release and spill fuel.

 

[8ender]

Make sure motor is not tilted up before this test.

Squeeze the fuel primer bulb until firm (dont force it any further than you need to) and watch for leaking fuel from throats. If everything is good no fuel will come out. If fuel comes out then Id suggest rebuilding the carbs and adjusting floats. I had the same issue and a cleaning and a rebuild fixed my problem.

Seeing fuel in the carbs after running is normal. Not all of it gets burnt. Most have a shroud to catch this unburnt mix and reroute it back to the intake. Also when tilting the motor up the needles tend to release and spill fuel.

This is probably the case then. It wasn't a flood of fuel just a tiny puddle in the carb throat. I've been tilting and running the engine all day.

 

[jerryjerry05]

That little bit of fuel is nothing.
Running in a bucket is ok for minor things.
You'd need a test tank that holds a few hundred gallons of water and have a testing wheel.
It sounds like the motors running as good as the load and prop will let it.
You want to bring up the rpm's?? get a smaller pitch prop. 10.5p probably bring up the rpm's by 500

 

[8ender]

Checked compression again today. Around 140 pounds on both cylinders. I’m still mystified why this perfectly healthy engine can’t get my tiny boat to 30mph.

One last request: Does anyone know if the lower unit gear ratios changed with the Force engines? I have a tail shaft from a 1988 Force 50 on my engine due to it coming with a cracked original. Would that make a difference?

 

[Nordin]

Same gear ratio at the 50Hp Force 13:21 until they started to put Mercury LU at the Force and that was in 1995.
Actually the same gear case from 1974 and up, the earlier LU with nose cone also had ratio 13:21 but another design.
They also fit but Another degign of the shift rod intermediate.

I do not know the design of your hull but the last 2-3 MPH can depend on the design of hull, the mounting of the engine (height) etc.

As jerryjerry says, I think you should be happy with how it perform as it is 45 years old.
Maybe go down in prop pitch about 1-2 in if you are able to find a good prop with 10-10,5 pitch.

 

[8ender]

If it's doing 4650rpm in with the 11.5 then I'll take it because its in the operating range. I'm just a little disappointed in the performance. I understand it's a 45 year old engine but that doesn't stop my Kawasaki KZ650 from kicking ass. If it's taken care of even the oldies can run up to original spec.

I get a week on the lake next week so I'm going to try some experiments. I think my trim is off a peg and I haven't been able to test it since I fixed the points and refreshed the plugs. Even if I don't break 30mph boat science sure is fun.