Need some help- Motor bogging at full throttle

[thill]

I don't know if I would call it a good handle, yet... More like barely hanging on!

I'm still learning the basics. And that's why you fellows, who are kind enough to share their experience, are SO appreciated! Sorry if I keep saying thank you too much, but THANK YOU!


Side question... What kind of boat is that under your scree name?

-TH

 

[muddywaders]

Hey guys the motor symptoms described have me intrigued.I agree with pappy "bogging" is usually a sign of a too rich condition however at 3/4 thottle the motor should be running on the main jet so perhaps something other than a bad main jet is causing the problem.Not too familliar with O.M.C. stuff but I do recall some motors being equipped with a cold enrichment circuit instead of a choke.Could it be possible that at higher rpm too much fuel is drawn through this cold circuit which I believe has a valve of some kind on the primer?let us know what the problem was after you fix it.Cheers M.W.

 

[thill]

...and that makes two, pointing toward the primer system.

Thanks for the insight, Muddywaders. At the top of page 2, Pappy made the same call. That really verifies the thought.

And at this point, I have to make a confession... This is the first motor I've ever had with this kind of primer system. With all the pulling to get her started, due to the rubber stuff in the system, I am NOT absolutely sure if I pushed the primer in all the way, after getting her started. I THINK I did, but I'm not 100% sure.

It pains me to say, but this MIGHT be user error. Half of me hopes so, but my pride hopes not! This makes me feel like a real idiot!

As soon as the snow melts and we get dug out, I'm running to the lake to test out, and will report back.

However it it goes, I owe you all a big thanks!

-TH

 

[Pappy]

Well, I had a little time today.
Repaired a 1988 Johnson 30hp. This engine is close enough to your 25hp to do a comparison with.
Once running on the hose I shut it off and backed it in a little way into the water and while on the trailer I throttled it up. Ran nice and steady which it should.
Okay....satisfied I can now give it back to the customer I went ahead and pulled the primer out and left It out. Ran the engine to WOT again and, once steady, I locked the primer back into the closed position. No difference in RPM.
Primer was nice and clean as were the two nipples (had the intake manifold off and checked carb and intake while there) and functioning normally.
Just an FYi. for you......

 

[thill]

Pappy,
Thanks for the actual field testing! That's going above and beyond, for a guy you've never met. I really appreciate that!

I'll give mine a test run anyway, possibly tomorrow, if I can get time, but I'm guessing that I'll be back to checking for debris in the bottom of the bowl causing a fuel obstruction, and then replacing the main jet. Thankfully, both are easy to do.

Hopefully, I can find it immediately, now that I know what I'm looking for.

Thanks again. Will report back as soon as I do my testing.

-TH

 

[Boat2fast]

You guys are having all the fun! Nothing around here but ICE.

 

[thill]

I'm sure it's much colder up there, but snow and freezing rain is falling right now. At least the big lake here has open water to test boats on.

But you probably won't be complaining this summer, when it's 102* down here with 95% humidity. :shock:

 

[thill]

Update... Weather has finally moderated. Snow mostly melted.
I finally got some free time this afternoon, so I snuck out back to play with the boat.

Opened her up and started by examining the primer system. Pulled and pushed on the primer, and noticed some fuel dripping/shooting out of the lower return hose underneath the carb. HEY!!! Sure enough, one of the tiny lower hoses that I did not replace last time had a CRACK in it. Could that be my problem? Don't know, but I replaced it.

Then I pulled the carb. Everything looked mostly clean, but there was a little powdery stuff here and there. Cleaned everything very carefully and reassembled. Replaced one other hose, just in case. Put everything back together, and still had an hour before dark. Just enough time to hit the lake. WAIT.... Put fresh gas/oil in the tank, with a shot of Seafoam, just in case...

In a rush, I forgot to bring my GPS. But the engine started up first pull and idled well, but a bit smoky. Maybe set a little rich? Or was it the Seafoam in the fuel, burning off carbon? Probably the latter. Nothing to worry about right now.

This time, I made sure I pushed in the primer back in all the way! She took off strong an got on plane. She ran all the way up to WOT, NO BOGGING at the high end! YESSSS!!!!! =D> =D> =D>

By my ear, my RPM's still seem a little low, but NO bogging. THAT is what I would expect, if I needed to go down a little in prop pitch, but will have to check for that later. Ran around the lake until dark, then back on the trailer and back home.

So what was my problem? Moisture in the fuel? The crud in the carb? That cracked return hose? The choke handle being left out? Maybe all of them, but I really can't be sure. But I AM sure that you guys helped me out! Thank you SO much for the help and suggestions! I really appreciate it!

If I get time tomorrow, I'm going to bring the GPS and my new tiny tachometer, and take it back for more testing. Very curious to see what my revs are at WOT, and what the speed currently is at WOT. Will report back.

-TH

 

[MDCrappie]

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By my ear, my RPM's still seem a little low, but NO bogging. THAT is what I would expect, if I needed to go down a little in prop pitch, but will have to check for that later.
-TH

Tony - if you do wind up needing a prop I do have a like new 12". The 12" was a bit too small for my 16' and I went up to a 13".

BTW-so you don't knock your brain too much- this is Rob from Baltimore

 

[thill]

Rob,
Hey there!!! Good hearing from you! How are you? Still working in the same place? Every once in a while, I look at that prop and hope you are staying safe.

I still owe you a fishing trip. I haven't forgotten, but I have not fished much these last few years.

The CCNP power plant has fish right now, but the harbor is probably is loaded up, too. I live on Lake Anna, and run down to Virginia Beach often, especially in May, when the big breeders are leaving the Bay. Let me know when and where you want to go, and I would be happy to host a nice trip.

This is the second time you have offered to save me prop-wise. That Turbo was perfect for my boat, once I lifted the engine a hole. But then it spun a hub, and not long afterward, my ECM went bad on that motor, and it's been sitting under the cover ever since. I plan to get her going this spring.

[email protected] is my email, (540) 219 - 9818 is my cell

Talk to you soon.

-TH

 

[MDCrappie]

Doin OK - going on 27. For the past 2 I've been inside, downtown, preparing for retirement. Will be sometime between this June and Jan '15 - we'll hook up when it happens.
Haven't been fishing much this year yet due to the ice, but getting ready to start this week.

 

[thill]

Yes, it's been a cold year.

Hooked up my new mini-tach and hit the lake this afernoon, but I did a BONE-headed thing...

After hooking up the tach and mounting the display, I was so excited to go, that I raced off to the boat ramp (was getting dark) without putting the cowl back on.

Got to the lake and realized my mistake, but figured that it couldn't hurt too much... right?

She idled at 750-800 RPM in neutral, 650-700 RPM in gear. I think that was about right. Hit the gas, and she accelerated strongly. Stronger than the day before, it seemed. Almost screaming, actually. I looked over, and she was running 5710 RPM! I backed off to about 5200 RPM, and she sounded much better.

I rode around like this for awhile, enjoying the scenery and the warm breeze. But when I slowed down a bit she did a lean-pop, which had never happened before. I sped back up, and all was well, but when I slowed, she lean-popped again, and cut off. Uh-ooh... What was going on? Why was she running lean? I tried to restart, but nothing. Pulled the primer once, and she started right up, ran well for a minute, then sneezed again.

And that's when it hit me... without the cowling air was being forced down her throat, and causing her to run lean. NOT GOOD!!! What am I doing?!? I know better that this! I started up and she ran fine at idle, so I eased back to the ramp. Hopefully, I didn't do any damage.

Pulled the primer several times, and ran her a little rich, back at home, just to make sure there is a good coating of oil inside the motor. Will have to test her out soon. Will check compression first.

Boy, I'm really missing on this one!

-TH

 

[MDCrappie]

thill » Mon Mar 10, 2014 9:00 pm[/url]"]

And that's when it hit me... without the cowling air was being forced down her throat, and causing her to run lean.
-TH

Tony - I don't think this can happen, unless your fuel is being restricted. The more air going into the carb, the more fuel that will be drawn thru the Venturi. It will run better due to this increase, but I'm pretty certain the fuel mixture will stay the same. Otherwise wouldn't "ram induction" motors be blowing up all the time?

 

[thill]

Rob, I really think it makes a difference. I noticed that when I got the lean-pop, I was heading into the wind. The carb is right there, catching the wind, no cover. Look:



But thinking about it, this would be a GREAT way to add HP to a small motor, if you could figure out which bigger jets to use. Force air in (like a supercharger) then add more fuel = VROOM.

But my current jets are sized for the restricted and directed airflow presented by the cowl. Without it, I don't know... Maybe my mind has been poisoned. Over on BBC and on the hull truth, I've read how guys have scored cylinders and blown up motors by running them hard with the cowl and/or baffles off. And if I recall, they commented how much faster they were running... just before they blew.

I doubt if it's a fuel restriction, especially since she ran so hard at WOT. (Maybe a weak fuel pump?) And I really should have known better! The motor was screaming, and I was flying, and a little voice in the back of my head was telling me that something wasn't right. Then I looked at the tach, and it HIT me. DUH!!!!

I just went outside, and she doesn't want to idle for more than a few seconds, before a cough and she dies. I've got a bad feeling about this.

My current plan is to add double oil to the gas, and try to keep her idling for awhile. Sometimes, the extra lube can smooth out very minor stuff in a cylinder, but if it's damaged, it might not come back. If I can get her running well, I'll take her back out on the lake with the double oil and see what happens. THEN I'll check compression. I'll report my findings.

-TH

 

[MDCrappie]

thill » Tue Mar 11, 2014 7:59 am[/url]"]It would be great, with bigger jets. THAT might be a way to add more HP to a small motor. Force air in (like a supercharger) then add more fuel = VROOM!

But the current jets are sized for the restricted and very directed airflow presented by the cowl. I know that over on BBC and on the hull truth, guys have scored up cylinders and blown up motors by running them with the cowl and/or baffles off. And if I recall, they commented how much faster they were running just before they blew!

-TH

But the air is not being "forced in" under any "high" pressure. Any of those guys get "confirmation" on the air causing the explosion? My guess is that they were having problems prior, hence the reason for running w/o cowling.

I'd be fairly certain that the stock jet supplied with the motor is sufficient to supply the amount of fuel necessary for any "non-compressed" air flow amount coming thru that carb throat.

I mean how much "pressure" is on that air when you're moving 25mph? I mean is it any (noticeably) harder for you to exhale while driving?

 

[thill]

You must be online, as you were able to respond while I was still editing.

But you make a good point. I was doing about 25, and the wind was blowing maybe 10 MPH. So the air pressure was the equiv. of putting your hand out the window when driving 35 MPH. That is considerably more than with the cover on, but I hope you are right about the jets being enough to compensate

Going to run out to the lake right now. Will report back shortly. Thanks!

-TH

 

[MDCrappie]

If you were getting the lean pop with the cover off, then put the cover on at home and now it's won't stay running you may have an exhaust leak. I had this happen to an old 40 I had.

 

[thill]

OK, I'm back.

Went to the lake, and started her up. She popped, sneezed and shut off. Triple primed her, and she ran longer, then died. Hmmm.... Pulled the primer about six times and started her again. This time, she ran for awhile, then died. Good enough for now.

Long story short, I kept having to give her gas, but the longer she ran, the better she ran. After running her around for maybe an hour, she FINALLY stopped that "rattly" sound, and would idle properly again without sneezing. It still seems just a little "off", but it's much better than when I first started.

With the hood on, she ran up to 5,520 RPM, which is pretty perfect. Yesterday, with the hood of, she was passing 5700 by with no problem.


AND GET THIS... When loading up, I ran into an older gentleman ("Gary") with a home-made plywood racing boat behind his truck. It was quite the beauty! It had a 30 HP Mercury on the back, without a hood to keep from restricting the air flow. That caught my attention, so I asked him about it.

He says that 30 HP will run 8,000 RPM and push the boat up into the mid 60's. His 40 HP motor will run well up into the 80's, but he says he can't handle that anymore, at his age. He's about to turn 80. WOW. I got some pictures, but my phone is a pain to download from, so I'll do it later.

He was nice enough to take a look at my rig and give me some suggestions. In my case, do NOT run it without the hood anymore, unless I re-jet the carbs, and also put a 1/2" shim under my motor, to stop that spray from the front of the cavitation plate.

Gary is a very interesting guy. Very glad I met him. Also glad my motor is running better.

After talking to Gary, my theory is that my motor was running lean, and it was getting dry inside the crankcase, which explains the rattly sound and the sneezing. Once the oil film got built back up, things are mostly back to normal. She's still not as quiet as she was two days ago, but I'll take what I can get.

-TH

 

[Pappy]

Thill.....do yourself a favor at this point and check compression on both cylinders please?
OMC jetted engines to be able to run either with or without cowlings on all engines at any time....with the possible exception of some Military Spec engines that I am not familiar with.
If this were not the case it would have been clearly outlined both in Owner Manuals and Service Manuals.......it is in none of the above and never has been.
A lot of my troubleshooting time through the years has been spent running engines wide open with no cowl and no airbox on the front. Open carbs out in the breeze and in some wicked fast boats.
Your RPM difference may have been due to the air temp difference as well as the open cowl since the large main air entrance is in the top rear and goes around a hot engine to get to the carbs. Or something else was going on. Between that entrance and the one below the carb there is no restriction whatsoever.
When cowlings are developed part of the development sequence is to install a manometer in the cowl to check for restriction.
I never experienced as much of a change as you have outlined. But the fact that it quit idling and slowly gained back idle quality the longer you ran it is not good. ........................So check compression.

 

[thill]

It WAS strange... The motor was running great to start with. But the longer I ran it without the cowling, the worse it ran, to the point where it was sneezing and lean-popping and stalling.

Then, the longer I ran it with the cowl back on, the better it got. That's why I was theorizing about the crankcase getting dry inside. Is there any other seal failure that could cause that?

When I get back this afternoon, I will check compression.

-TH