35hp evinrude on 15' am I doomed?

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

In response to an earlier reply... The boat will not rev under load. It doesn't seem to be the prop hub. It will rev and idle just fine with the muffs. (don't worry it was a very short time we didn't want to damage anything, we just wanted to see if it would do differently than under load.

 

[fireman57]

Re: 35hp evinrude on 15' am I doomed?

I agree with CAT. Sounds like you are dropping a cylinder. Check spark with a timing light when under a load. Or you can take a friend that you don't need anymore and give him a pair of insulated pliers to pull and replace the plug wires one at a time when under a load and it does this. You may show spark on your spark tester but if it is yellow or orange it is too weak.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

so if I do have poor spark, what do I do? I believe this engine has a magneto not points, but i'm not sure. could old s/plugs attribute to this problem?

 

[BF]

Re: 35hp evinrude on 15' am I doomed?

As others have said, it sounds like its running one 1 cylinder... they can run amazingly smooth well on 1 cyl, just no power. To test, start engine and pull off one plug wire, see if idle changes... replace it, do the same with the other. If pulling one makes no change, it's not firing on that cyl. Even if you have spark when you pull the plug out, weak spark can fail under load.

If that is OK, then I'd think about the fuel system. Does the primer bulb go flat when you give it more throttle? That would point to a fuel restriction at or before the primer bulb... If you have another fuel line and tank, try it to rule out a bad primer bulb or clogged fuel uptake. Or it could be crud has clogged up the high speed jet.

Edit <oops a bit redundant with the other posts.... sorry>

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

Thanks bf. I'm going to do that check after I get done with work.

 

[CATransplant]

Re: 35hp evinrude on 15' am I doomed?

I noticed a 13 on the prop last night. I'm thinking that the prop has a 13 pitch. Too much?

Any other prop recommendations?

The 13" pitch on that prop is probably just right for a 35 hp and your boat. You should get about 25 mph or a little more, wide open. Once the engine is running right, you'll move right along.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

k thanks. I'll update after a few more tests.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

Replaced spark plugs and pulled wires while in forward gear in a big garbage can. Neither showed a lack of change when pulled. They both had a nice popping spark. I gapped the plugs to about .030. Is that about right? The engine seemed tp run smoother with the new plugs.

Also... update on the prop

Its actually a 10x15 if that helps any.

Hopefully I'll be able to put it in the water soon to see if I get any change.

Thanks

 

[CATransplant]

Re: 35hp evinrude on 15' am I doomed?

Replaced spark plugs and pulled wires while in forward gear in a big garbage can. Neither showed a lack of change when pulled. They both had a nice popping spark. I gapped the plugs to about .030. Is that about right? The engine seemed tp run smoother with the new plugs.

Also... update on the prop

Its actually a 10x15 if that helps any.

Hopefully I'll be able to put it in the water soon to see if I get any change.

Thanks

Helps a lot. 10x15 is way too high a pitch for a 35 of that vintage. 12 or 13 is more like it. Your Big Twin just can't turn a prop with that high pitch at proper RPMs. Get a prop with a pitch from 11 to 13 and you'll see a remarkable change in performance.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

So what would be the difference between an 11,12 or 13 pitch. Thanks for the info.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

What's the difference between a 10x11 and a 10.5x11?

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

Is there any special way to put on a new prop. I see the cotter pin and nut. Is there anything else I need to do when I swap the prop?

 

[Harker]

Re: 35hp evinrude on 15' am I doomed?

Put some grease on the shaft is all..helps later on if you want to remove it.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

Thanks. How much should you torque the nut?

 

[CATransplant]

Re: 35hp evinrude on 15' am I doomed?

So what would be the difference between an 11,12 or 13 pitch. Thanks for the info.

As the pitch increases, it takes more power to turn the prop at a given RPM. You want your engine to max out at about 5400-5500 RPM. The proper pitch will be the one that lets your engine reach that speed, but no higher.

Your next message is similar. A 10.5" dia. prop will take slightly more power to turn at your target RPM than a 10" prop with the same pitch.

I wouldn't spend too much time worrying about that until you get the engine running right. A 12 or 13" pitch prop of either diameter will do you just fine with that combination of boat and motor. The difference between them will be minimal, frankly, and will only mean one or two MPH for you.

More important will be the trim you set on your outboard, where the anti-ventilation plate sits in relation to the hull, and other factors.

I'd forgotten the year of your motor. I'd definitely skip the 11" pitch on that one. I'm guessing the 13" pitch will be the best choice, once your engine is running right.

 

[BF]

Re: 35hp evinrude on 15' am I doomed?

seems like you might not know what the prop numbers refer to... first number is diameter, second is how far the prop would move forward in one rotation (if there was no slip). Therefore a 10 x 13 is 10" in diameter, and would go 13" in one rotation. A lower pitch, e.g. a 10 x 11 would have a shallower pitch on the blades, so the engine would need to do less work to spin it one rotation. Sometimes on larger motors people use a low pitch as a "ski prop" (good for pulling) and a higher pitch prop as a "speed prop" (boat will go faster as long as engine has enough power to spin it up to proper full throttle RPMs).

If your boat has a prop with too much pitch (15" may be too much?), it is "over propped" and the engine will not have enough power to spin it to recommended rpm's at full throttle... if it is way off, even a healthy motor may not have enough power to get the boat on plane (which could be your problem ?).

HTH

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

I understand the difference between the 2 numbers. I just didn't know how much rpm difference that would make if any.

All the checks that I have done on the motor have checked out. (spark, compression, wot when handle all the way up, no spark loss under load) It seems like i'm borderline planing. It sounds like i'm 500-1000 rpm off the 5k mark. (no tach) It just seems almost there to plane. How much rpm should I expect to pick up for each 1 pitch number assuming I stay with a 10" prop?

 

[CATransplant]

Re: 35hp evinrude on 15' am I doomed?

If your prop is a 15" pitch, I'd drop back to 13". That should bring the revs up 1000 RPM at least. The thing is, with the 15" prop, you're not near the power band of that engine, so it just doesn't have the poop to spin it up any faster. The 13" prop will get you into the power band right away and will then be on plane and finish winding up.

Again, though...it could just be that a 12" pitch prop is the right one. If it were me, I'd try to find both, eventually and use the one that works the best, and keep the other as a spare.

Or, use the 12" pitch when the boat is heavily loaded and the 13" when it's not. Changing props is a snap. I'm sure the 11" pitch prop isn't what you want, though. It might be ideal for pulling a tube around or dragging two skiiers out of the water, but for general use, you'd be underpropped and wouldn't get the most out of it.

All of this, of course, is assuming that your trihull isn't waterlogged in the foam. If that's the case, then it's difficult to predict.

My 14' 6" Glastron was perfect with the 13" pitch. It jumped on plane super fast, and ran up to around 25-26 mph with 2 guys on board. And that was with a 1958 Johnson 35.

Your boat is a little heavier, but the engine is actually a little more powerful, at least at WOT RPMs. The old one probably had more torque at lower RPMs than yours.

12" or 13" Either one will do the job.

 

[cmuman]

Re: 35hp evinrude on 15' am I doomed?

Thanks for the input. How would I determine if my foam is waterlogged? any tricks? I know that I can shift the boat on the trailer pretty easily if it loads off center by myself.
You guys rock. You've been very helpful an patient with my novice boating skills. I really appreciate it.

 

[BF]

Re: 35hp evinrude on 15' am I doomed?

Hi again...

what you've just said doesn't seem right to me. Earlier you said:

In response to an earlier reply... The boat will not rev under load. It doesn't seem to be the prop hub. It will rev and idle just fine with the muffs. (don't worry it was a very short time we didn't want to damage anything, we just wanted to see if it would do differently than under load.

Now you say that you think it will reach 4000-4500 RPM's while pushing a plowing boat (not on plane)? Either your estimate of RPM's is way off (they're really much lower), or your prop hub is spun. If you really were running those rpm's with a 15" prop attached (and not slipping), you'd be planing the boat.

Since you're on the verge of getting it planing, have you tried moving some weight forward (e.g. a passenger) and see if that allows it to plane. Once it gets on plane, the workload required of the motor drops dramatically and the rpm's (and speed) should pick up dramatically, even if it is over-propped. If overpropped it won't reach the recommended RPM's, but it is really tough to guess WOT RPMs by ear well enough to tell if the prop is the right one or not.

As for you Q about how much change in WOT rpm's a change in pitch would make, a guestimate would be that a 2" decrease in pitch would cause about a 300 increase in WOT rpm's... BUT, this varies depending on the slipage (how heavy the boat is) and more importantly, is for WOT RPMs of the boat on plane. As I said above, there will be a jump in rpm's once you get the boat planing on top of the water instead of pushing it. Maybe you're so far enough off in pitch that it just doesn't have the power to spin the prop enough to get out of the hole (?). In which case a 2" drop in pitch might allow the boat to plane and cause an large increase in WOT rpm's compared to what you have now.

Hope this helps.