Evinrude prop way to small??

Unruh · Oct 6, 2012

i have a '88 Bayliner that the previous owner put an '86 Evinrude 70 hp 3 cyl outboard on. Motor runs great on muffs but on the lake only 2400 rpm- about 7-8 mph. The prop is a 13 3/8 x 17. I've read some other threads about motor height so built a bracket to raise the motor several inches but didn't help. How much do I need to downsize my prop or is there another problem? I believe the boat weighs 1300-1400 lbs.

steelespike · Oct 6, 2012

Re: Evinrude prop way to small??

While you may have to make a prop change at some point your prop isn't the problem.
When a motor runs on muffs it only means it runs.There is no load and no back pressure.
Could be any number of probems. Check that the throttle opens all the way.
Check that it is running on all cylinders.Do a compression check.The carbs may need a rebuild.

Unruh · Oct 6, 2012

Re: Evinrude prop way to small??

what should the compression be?

glenn property of pam · Oct 7, 2012

Re: Evinrude prop way to small??

compression testers are a little hit and miss so as the engine runs we know that it has compression on a few cylinders at least so you are realy testing that all 3 cylinders are the same number. about 120 would be a good number but as long as they are all the same it should be ok.
generally people start with compression as thats a major because bad compression may not be worth fixing. next is ignition and testing the spark to each cylinder then fuel/carb. if the engine can only rev to 2400rpm with a 17 pitch it does sound like you have one cylinder thats not working. the engine should rev to 5000 with the 17 pitch

steelespike · Oct 7, 2012

Re: Evinrude prop way to small??

Its important the cylinders are within 10% of each other.100lbs minimum.
What size and model Bayliner? Any test runs should be done lightly loaded. We will need the wot rpm and speed.

Unruh · Oct 7, 2012

Re: Evinrude prop way to small??

its a 1988 17' Bayliner Trophy all I can get is 7.5 mph @ 2400rpm. going to try to do a compression test tomorrow

Unruh · Oct 8, 2012

Re: Evinrude prop way to small??

Compression tested from top to bottom 95 90 100. have spark on all 3 cylinders

glenn property of pam · Oct 8, 2012

Re: Evinrude prop way to small??

was that a warm compression test or a cold test?

Unruh · Oct 8, 2012

Re: Evinrude prop way to small??

cold

eavega · Oct 9, 2012

Re: Evinrude prop way to small??

Not to distract you from the problems your motor is having, but what is the max recommended HP for that boat? Seems its really heavy for a 70 HP outboard. a 1300-1500 Lb boat should need somewhere in the neighborhood of a 90+ HP motor. I had a small boat with a weight of under 1300 Lbs max capacity, and it was rated for 70 HP max.
Back to your motor, though, you need to check spark and fuel to each of your cylinders. Do a spark test and verify that spark is good to all three cylinders. You should also do a cylinder drop test to see which one of your cylinders are not firing (on first blush it looks like you are not running on all three cylinders).

Rgds

steelespike · Oct 9, 2012

Re: Evinrude prop way to small??

Your really on the bottom edge and while within 10% 2 cylinders are below spec.
I feel that the boat should plane,may not be real fast but at 7 mph your only slightly over displacement speed.
You could do that with a 5 hp.
Spark should jump 3/8-7/16" on a tester.
Be sure the throttle opens all the way and the link and sync are correct.
You may have to rebuild the carbs.

Unruh · Oct 10, 2012

Re: Evinrude prop way to small??

spark jumps 1/2 in. I believe the throttle is opening all the way but not positive outboards are fairly new territory for me. max rated hp is 90.

Unruh · Oct 10, 2012

Re: Evinrude prop way to small??

also idled motor and disconnected plug wires- motor died or nearly died on all 3

steelespike · Oct 10, 2012

Re: Evinrude prop way to small??

If the motor is healthy and at the correct idle speed it shouldn't die or almost die with one cylinder dead.
Normal idle is about 700-750
A healthy 2 cylinder motor might not give any clue to being on one cylinder except for the lack of power.
Your 3 cylinder shouldn't be bothered except for the same lack of power.
It would seem to me that the motor should at least be close to planing and at 7 mph it isn't.
It is possible going to a 15" prop might get it on plane and we can evaluate the motor a little better.
You may want to evaluate the things I suggested before buying a prop.

Unruh · Oct 11, 2012

Re: Evinrude prop way to small??

Would pulling plug wires while running on the lake tell me anything? The idle is a bit faster then that- 850-900. Or would it be a good idea to just rebuild the carb? Got an hour and half drive to the lake out here in west texas.....

jestor68 · Oct 11, 2012

Re: Evinrude prop way to small??

Sorry to be the bearer of bad news; but that is considered low compression. Your motor needs to be rebuilt.

Now you know why the previous owner wanted to sell it.

steelespike · Oct 11, 2012

Re: Evinrude prop way to small??

I think unhooking plug wires may damage the ignition at speed. The carbs should be timed to open as the timing advances.
They need to open exactly the same.If your going to be diving into this motor you need a factory service manual
You may find a reprint factory manual here.Pricey but worth every penny.
You could do a good test run and may be able to carefully feel the spark plugs if not firing they may be cooler.
Also pull the plugs and compare condition.
Not sure how the choke is applied but you might see if a little choke helps. Also see if the primer bulb helps.

steelespike · Oct 11, 2012

Re: Evinrude prop way to small??

jestor68 said:
Sorry to be the bearer of bad news; but that is considered low compression. Your motor needs to be rebuilt.

Now you know why the previous owner wanted to sell it.

I fully agree it's time for a rebuild but it does seem it ought to plane if everything is right.
It only makes 7 mph I think you could do that with a 5 hp.
Is it worth all the effort to end up with a weak motor?
I think I would be moving on to a known good motor.

Mi duckdown · Oct 11, 2012

Re: Evinrude prop way to small??

And wondering if that boat is water logged? .02 7mph. something is seriously wrong.

Unruh · Oct 11, 2012

Re: Evinrude prop way to small??

Ok just so im clear on this- I need to rebuild the motor and possibly still have another problem? Any estimates on cost if I do the rebuild myself? No experience with this just a buddy with lots of 2 stroke (no outboards) experience but would like to learn...

Evinrude prop way to small??

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