Need some help- Motor bogging at full throttle

[thill]

I'm putting together a deep-V aluminum boat, a 1957 Crestliner Viking 710.

This is a nice, heavy, well-designed hull, factory equipped with either a 40 HP or a 60 HP Evinrude. Prev. owner gutted the boat and used it as a jon, with a 9.9 tiller. I've added a front deck, trolling motor, new transom, dual batteries, etc. Probably similar in weight as the original design.

To this, I have mounted a 1994 Evinrude 25 HP tiller motor on it, with whatever prop it came with.

I cleaned the carbs, and the motor starts and idles great and jumps up on plane fine, and loaded normally, it reaches 23 MPH GPS, which I'm happy with.

My PROBLEM is that the boat reaches max speed/RPM's at around 3/4 throttle. Any more throttle, and it gains no more RPM, and after a certain point, (when I give it full throttle) it actually bogs down and loses some RPM. No tach, so I don't know what the actual RPM is, but it sounds a little low.

My guess is that my prop pitch is too high for this load. Does this sound correct?

I just want a few other opinions before investing in a lower pitch prop.

Thanks guys.

-TH

 

[keelme]

could be air leak around fuel line fittings or fuel pump maybe not pumping enough fuel at top end

 

[Pappy]

You could be correct on the prop pitch. However, it really should at least hold RPM and not drop or bog.
You can effectively eliminate the fuel pump by simply squeezing the primer bulb at the RPM where the problem occurs. Doubt this is the issue.
Although most carburetors have a roll pin driven through the throttle shaft and a cast-in stop on the carburetor main body do a check (engine shut off and in gear) to make sure this pin is near vertical when you advance the throttle to WOT. If for some reason it is able to go past vertical this may produce the symptoms mentioned. Doubt you will find this.
What I can tell you is that, in the past few years, I am finding more and more engines with high speed jets that are eroded internally. These jets will typically be from .003 to .006 larger than the jet size stamped into them. When this happens the symptoms are exactly as you describe. The engine becomes over rich and will slow down at WOT throttle settings.
Since you already know your way around the carburetor my advice to you would be to purchase a new high speed jet and install. BTW - if you did not do it initially purchase an OEM carb kit as well. It will have the float with it as well as a new packing for the low speed needle and alcohol resistant gaskets.
You did put the high speed nozzle gasket back in place prior to re installing the fuel bowl I hope? Usually if not the symptoms are much worse than you describe.

 

[thill]

Ahhh... Some interesting and thought-provoking replies!

I replaced all the fuel lines when I did the carbs, and did all the connections myself. Very confident that they are tight. But I did not replace the carb to block gaskets, so it's possible some air may be getting in... Will have to look at the plug condition to see if it looks like they are running lean. Good call!


ALSO, I just replaced the spark plugs yesterday, properly gapped. Champions, as per the manual.


When I cleaned the carbs, there was obvious Ethanol phase separation in the bottom of the bowls. You could see the line where water sat. This absolutely WILL cause corrosion/erosion of the main jet down there! Jets are $8 each. I'll order a pair ASAP. Good call!

As far as the bogging, let me clarify a bit. She hits max RPM at about 3/4 throttle, and it stays there the rest of the way, until you pin it open, and there she actually slows down a little. Not a ton, but enough to lose maybe 2 MPH if you hold it there. So your description of the eroded jets fits!

So what I've been doing is run her until I reach 23 MPH, and then hold it there or a little less. My reasoning is that if pouring extra gas in the cylinder isn't increasing output, it's just leaving unburned fuel and creating heat. I don't want to burn up my motor!

Getting ready to go out and check my prop pitch. Very curious as to what it is...

Thanks.

-TH

 

[thill]

OK, went out and checked the prop. It's a 10" x 13 pitch aluminum. Not in bad shape at all, but it is missing the condensing ring around the back end of the hub. Would this cause any problems???

From what I remember, I think other guys are running an 11 pitch with their 25 HP's on heavier boats.

Any thoughts?

-TH

 

[Boat2fast]

You can't be that far off on the prop pitch. It still needs to be set-up with a tach. I can't even sleep at night if I put on a new prop and don't check it with a tach.

Pappys comment made me think of something. I see this occasionally. Sometimes an inexperienced mechanic will get in there and make adjustments to the throttle linkage. On your engine it's real easy to mis-adjust the linkage that operates the carb. It can push the carb right past 'wide open' until it's actually closing it down again. This adjustment needs to be checked whenever the throttle linkage is disturbed at all. You can tell by the little roll-pin in the throttle shaft, see if it's vertical at WOT

This adjustment is called "linkage adjustment and synchronization" or 'link 'n sync'. The procedure needs to be followed step by step as laid out in the factory service manual. I have the manual for your engine. If I can figure out how to scan the procedure I will forward it to you.

 

[Pappy]

thill » Yesterday, 10:57[/url]"]
I replaced all the fuel lines when I did the carbs, and did all the connections myself. Very confident that they are tight. But I did not replace the carb to block gaskets, so it's possible some air may be getting in... Will have to look at the plug condition to see if it looks like they are running lean. Good call!
-TH

You can pretty much forget that as a cause. The symptoms would be very evident at an idle if that gasket was leaking air.
Also, there should only be a single high speed jet to purchase.

 

[thill]

I read your post and went out to check the carb movement, and it's right on. Completely closed to full open, right on the button, everything aligned properly.

But then I thought about it... The timing. What I'm probably hearing is the timing not advancing enough or advancing too much. That would certainly explain the lagging engine speed when I go past a certain point.

It's only 12 degrees outside right now, but when it warms this afternoon, I'm putting a timing light on that baby! Maybe it's that simple!

You guys are great! Will report back later.

Thanks.

-TH

 

[Pappy]

Wont be timing either.
The timing is fixed as part of the mag plate. Wherever the mag plate stops so does the timing. Check it if you want....will only make you more familiar with the engine so never a bad thing.
Also....on a 2-stroke.....advance the timing a few degrees too much and you can take the piston out in pieces usually!

 

[Gotem]

Could it be a powerpack?

 

[thill]

OK, next update...

Boat2Fast sent some helpful information, (thanks!) and I went out and double checked the sync and link. Again, it was dead on the money.

But while under there, I was reminded that everything under the hood is really gummy and sticky. Someone sprayed storage grease on everything, and it's dried out and gotten stiff. Even my recoil start cord retracts VERY slowly, taking maybe 5 seconds to recoil between pulls.

So I took a pump bottle of WD40, and sprayed everything down, and exercised it back and forth. Within a few seconds, everything loosened up and is moving smoothly, as it should. I'm wondering if that wasn't half of my problem? Maybe the sticky stuff wasn't allowing my timing plate to advance fully?

Something then occurred to me... I THINK when I ran the boat in warm weather (only once) it reached full RPM's, and this problem started when it got cold. No adjustments at all since then. So the sticky stuff under there MAY have something to do with this?

But either way, as you say, I need to learn how to check/adjust timing. A few questions:

1. How/where do I connect a tachometer to test with?
2. Can I use a regular tach with alligator clips, or does it have to be a diagnostic unit?
3. Does it need to have a stator for the tach to work properly? I have a stator, terminal strip and rectifier that I plan to put on the motor eventually. Should I put that on first?
4. Would it be OK to test WOT while tied off on a dock? I don't have a test wheel to use in my tank.

Thanks, guys!

-Tony

 

[thill]

Gotem » Today, 13:29[/url]"]Could it be a powerpack?

Sorry, didn't see your post!

I suppose it could be the powerpack getting "weak" but it runs great all day, with the exception of when the throttle is cranked all the way, so I'm not sure about that. You would think it would get weaker as time went on, from it getting hot.

I'm also thinking that I should pull the bowl and make sure there is nothing gumming up the main jet or tube. I cleaned the carbs carefully, but maybe a piece of hose got in there before I changed them out? Those hoses were pretty bad, and were shedding chunks on the inside.

This is an interesting search! Hopefully, I will learn something before it's over. I'll be sure to post all progress, so as to possibly help someone else while I'm learning.

Thanks!

-TH

 

[Pappy]

You seem to be forgetting about replacing the main jet.
Your mag plate is a straight mechanical connection. Gummy or not it will advance to a pre set limit. Also the lubrication for that plate is in a place where a sprayed on corrosion preventative cannot reach.

 

[thill]

Pappy,
No, I have absolutely NOT forgotten the jet! That is a simple, easy, inexpensive fix, and it fits the symptoms. I just want to eliminate anything else that might be contributing in the mean time. I would have ordered it already, but the jet is $8, and shipping is $12! Should cost maybe $1-$2 for a padded envelope. Plan to call the local OMC shop to see if they have that jet in stock.

But that spray grease stuff had EVERYTHING sticky, including the timing plate, linkage, and even the pull start recoil, which is way up under the cover, and really hard to even see. Who ever applied that spray, really got it up in there!

In the meantime, I need to learn how to time a motor properly, and as they say, no time like the present.

Hopefully on Monday, the marina will have a main jet for me, and I can swap them out.

Thanks again!

-TH

 

[Charger25]

I'm also thinking that I should pull the bowl and make sure there is nothing gumming up the main jet or tube. I cleaned the carbs carefully, but maybe a piece of hose got in there before I changed them out? Those hoses were pretty bad, and were shedding chunks on the inside.

I'm a big fan of keeping it simple. If all the mechanical parts are moving freely ,link n sync are good , electrical parts are good (no frayed wires) I would look at some kind of fuel restriction somewhere, from the tank through primer bulb through hoses and out of the back of the carb. Even with too high a pitch prop you would not lose RPM's you just won't get to top end. I would rule out the prop though simply because it was stated that it jumps up on plane.

 

[Pappy]

Chances are they will not have it in stock but the nice part is that they can include it with their normal parts order and no shipping costs to you!
Good way to check and see if the engine is running lean, since it was suggested, is to engage the primer at the RPM where the issue occurs or if not primer equipped a spray bottle with a mixture of 50:1 can be used and sprayed down the carb throat. If the engine picks up and runs you have a restriction.
Which brings me to another question. Does your engine have a choke or primer? If it has a primer make sure the primer is locked in the fully closed position or it will flow fuel through it and richen the mixture. Push it in hard and see if it locks closed. A second way to tell if the primer is leaking internally is to simply pull the hose off the barb above the carb or pinch off the supply line to the primer with a pair of pliers, again at the RPM where the issue is occurring. If it picks up now then the primer was leaking fuel through it.

 

[thill]

Pappy,
Yes, this engine HAS had primer issues.

My engine has the choke/pump primer. The kind where you pull it out and it sprays gas into the carb like a pump bottle. Recently, that got clogged and locked up from the rubber bits from the old hose.

When I forced it, it blew the primer hose off, spraying fuel everywhere. When I pulled the hose, I found that little orifice clogged with rubber bits. I cleaned it out several times, until everything worked well.

But YESTERDAY I noticed, that the primer hose was dripping/spraying fuel again. Your possible theory would explain a lot!

On my next trip out, I will bring a pair of hemostats and test it. Thank you again!

-TH

 

[Boat2fast]

If there are rubber hose chunks all throughout the fuel system, then that needs to be addressed all at once. Everything from the connector all the way to the main jet. Any piece of material left in the system will move downstream to the largest hole it can't fit through. A little piece of that in your float needle will give you fits.

 

[thill]

Yes, that stuff has been a pain! Like black flour, gumming stuff up. I've cleared it out 2-3 times already, and it finally seemed to be gone, until I found that bit the other day.

It is some frustrating stuff! Not sure where it was hiding, but hopefully, another cleaning will be the last of it.

We are up to about 8" of snow now... I had planned to pull the carb today, but I don't want to uncover the boat in all this stuff. I guess it's time to work on the wife's honey-do list... <sigh>

-TH

 

[Pappy]

thill » 02 Mar 2014, 21:37[/url]"]Pappy,
When I forced it, it blew the primer hose off, spraying fuel everywhere. When I pulled the hose, I found that little orifice clogged with rubber bits. I cleaned it out several times, until everything worked well.
-TH

Sounds like you are getting a good handle on this stuff!