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  1. #1

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    K&B 3.5 Outboard



    Hello everyone

    I have a K&B outboard I haven't run in two years, the reason is I never ran a full tank through it. It would run about 2 minutes then shut off like its out of gas. I changed tanks and tubing several times any suggestions.

  2. #2
    Moderator w8ye's Avatar
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    RE: K&B 3.5 Outboard

    Try running the engine a little richer. If result don't improve, try some different fuel or a different glow plug.
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
    Recipient, Mangledhand award August 2008
    Club Saito Member #7
    Original AMA #31261

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    RE: K&B 3.5 Outboard

    I ran the boat about four turns out for break in and always used the suggested long K&B plugs. The fuel was the recommended 30 % nitro

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    RE: K&B 3.5 Outboard

    Hi!
    Well have you checked the ballbearings...Two years is a long time and chances are that they need replacement.

    Regards!
    Jan K
    Sweden
    Jan Karlsson - Supplier MVVS Products

  5. #5
    Sport_Pilot's Avatar
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    RE: K&B 3.5 Outboard

    Wow! 30% nitro for break in? Dontcha think that the extra heat of friction during friction could cause the nitro to detonate? Try 15%, then switch back after break in.

    Four turns out with 30% nitro is running way too lean on some of my airplane engines. W8ye said to run it richer, he really didn't care how many turns out, if you had it eight turns out, it sounds like it is running lean.
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    piper_chuck's Avatar
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    RE: K&B 3.5 Outboard

    ORIGINAL: gjpratt



    Hello everyone

    I have a K&B outboard I haven't run in two years, the reason is I never ran a full tank through it. It would run about 2 minutes then shut off like its out of gas. I changed tanks and tubing several times any suggestions.
    1. As has been suggested, you might need new bearings. If you ran the fuel out of it and then lubed it up well, they should be ok.
    2. If you did not remove, dry, and regrease the prop shaft, you will most likely need a new one.
    3. If it has a good idle and transition to high speed while on shore, it's too lean. If it sputters and spits out fuel, and you need to keep revving it up to prevent stalling then it's ok. Note, DO NOT keep the engine running at high RPMs out of water, it will throw the rod. However, it's ok, and often necessary, to keep blipping the throttle so it doesn't load up while idling.
    4. There's nothing wrong with running 30% or higher nitro. I used 25% on the one I broke in this summer, but ended up switching to 50% so I didn't have to carry 2 different fuel bottles.
    5. Are you using a single tank, or a main and header tank? Just to confirm, you are using the exhaust pressure, right?
    6. What prop are you running? It is sharpened and balanced, right?
    7. You might also try a different glow plug. Nothing wrong with the K&B ones, but I've also had good luck with McCoy.
    AMA 87959

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    RE: K&B 3.5 Outboard

    What size bearings? From what I understand these engines are not like plane engines and the must use the higher nitro. I had a nitro car that ran on 15.

    Chuck
    I am using one tank a sullivan, and the pressure fitting from the lower unit leads back into the tank. I have heard they dont produce much pressure though. is there a better way to hook up the tank?

  8. #8
    Sport_Pilot's Avatar
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    RE: K&B 3.5 Outboard

    There's nothing wrong with running 30% or higher nitro. I used 25% on the one I broke in this summer, but ended up switching to 50% so I didn't have to carry 2 different fuel bottles.
    I am aware that many marine engines use high nitro, but since most engines develop more heat during break in, I would recommend a lower nitro fuel for the initial break in. Especially if it is a ringed engine. That would eliminate detonation as a posiblility. You can then switch to the recomended nitro.
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    RE: K&B 3.5 Outboard

    I have the aircraft version which is the same engine with fins in a Hustler delta. Actually it's a DF engine. Can't get it past half throttle without 25% nitro because of the advanced port timing.Piston and cyl are the same as I used marine version in mine when I smoked one.
    Gord
    Dreamed I was a muffler. Woke up exhausted.

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    Sport_Pilot's Avatar
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    RE: K&B 3.5 Outboard

    You should be able to get any engine to run on 0% nitro. Idleing and transition may be a problem, but you should be able to start it and run it wide open. You will need a hotter plug with low nitro. However I think his main problem is having the needle only open 4 turns. I recall about 5 turns out with high nitro fuels on my airplanes.
    Glow Head Brotherhood #15

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    RE: K&B 3.5 Outboard

    ORIGINAL: Sport_Pilot

    There's nothing wrong with running 30% or higher nitro. I used 25% on the one I broke in this summer, but ended up switching to 50% so I didn't have to carry 2 different fuel bottles.
    I am aware that many marine engines use high nitro, but since most engines develop more heat during break in, I would recommend a lower nitro fuel for the initial break in. Especially if it is a ringed engine. That would eliminate detonation as a posiblility. You can then switch to the recomended nitro.
    It's not ringed, they've been ABC for a very long time. Here's the fuel recommendation, straight from K&B (different quotes for different models): " We recommend using K&B 525 (25% nitro) or K&B Speed Fuel 550 (50% nitro)" or "We recommend using K&B Fuel with 15 to 25% nitro. For competition nitro content may be increased to 50%." Given this, I see absolutely nothing wrong with 30%. But what do I know, I've only got 4 K&B 3.5 engines at home.
    AMA 87959

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    RE: K&B 3.5 Outboard

    ORIGINAL: Flypaper 2

    I have the aircraft version which is the same engine with fins in a Hustler delta. Actually it's a DF engine. Can't get it past half throttle without 25% nitro because of the advanced port timing.Piston and cyl are the same as I used marine version in mine when I smoked one.
    One of my aeros (true aero, not DF) is in a Shrike .10. It screams.
    AMA 87959

  13. #13
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    RE: K&B 3.5 Outboard

    ORIGINAL: gjpratt

    What size bearings? From what I understand these engines are not like plane engines and the must use the higher nitro. I had a nitro car that ran on 15.

    Chuck
    I am using one tank a sullivan, and the pressure fitting from the lower unit leads back into the tank. I have heard they dont produce much pressure though. is there a better way to hook up the tank?
    Two sources for bearings, http://www.racecraftbearings.com/ and Boca Bearings. The first is someone that's active in R/C boat racing, he knows his stuff. Boca is ok too. You could also go directly to the source, K&B http://www.mecoa.com/kb/index.htm K&B made a bunch of different versions of outboards, it's helpful to know your exact model number before ordering bearings.

    If your tank is significantly lower than the carb, it can have trouble drawing fuel. If the engine seemed to be running well and then started dragging a bit and stopped then it's probably lean. The first "fix" is to try richening it. Another thing to consider is raising the tank as high as possible or adding a hopper tank. For an outboard, the hopper would be a small (1 or 2 oz) tank that you'd mount on top of the main tank. Then you'd plumb them so the engine draws fuel from the hopper rather than the main tank.

    What part of GA are you in? There are several active boat clubs down there. You might check IMPBA district 13 to see if one is near.
    AMA 87959

  14. #14
    Sport_Pilot's Avatar
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    RE: K&B 3.5 Outboard

    Piper,
    Do you read? I have not said you cannot run the engine with 30% nitro! I have not disputed that at all! I suggest you go back a read some of the past posts! I am only suggesting using lower nitro for break in, as I do for low nitro engines also. If the engine normally runs on 20% nitro, then use 10% nitro. Yeah, 90% of the time you won't have any problem using the higher nitro. But get one that is too tight, it runs hot, then the nitro detonates from the heat, then fried engine. Have you broken in a hundred of these engines?
    Glow Head Brotherhood #15

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    piper_chuck's Avatar
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    RE: K&B 3.5 Outboard

    Sport, yes, I read, you suggested that 30% would cause problems while breaking in this engine. Since you didn't know whether it was ABC or ringed, I wonder how many of these you've broken in? How many K&B 3.5 outboards have you actually run? For that matter, how many have you even seen run? Obviously not hundreds. When was the first time you ran one? My first was the early 80s. How many ABC boat engines have you broken in? Fuels in the 30% range are not at all high for boat engine standards. In fact, people were surprised that I was using something as low as 25%. The engine is designed for higher nitro fuels than the typical sport airplane engine, and detonation should not be a problem. Also, people who run boat engines don't usually talk about idling and transition, those terms have little meaning for this kind of boat.
    AMA 87959

  16. #16
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    RE: K&B 3.5 Outboard

    I do not recommend running a lower nitro content for break-in. Use what fuel you plan on running. If you want, add a little extra oil to the fuel to help lube during break-in to bring it up to a max of 20% oil content, no more. If you are 4 turns out, you should be slobbering rich. The easiest way to tell is from the sound of the engine.

    If your bearings are bad, you should be able to feel roughness when you turn the engine over by hand. Don't change them needlessly. Find a friend that has experience in tuning marine engines that can help (no offense, not insinuating you aren't capable, but two minds can be better than one, especially if experienced).

    I'm sure you already know this, but make sure you don't run it out of the water for more than a minute or two, as the shaft and lower busing will burn up and wear out quickly without the water to cool it, plus the engine will heat up without the water cooling.

    Check for good fuel tank and lines (no gooped up mess to restrict flow), check for any foreign particles in needle valve assy, use a filter in your line. Try to get your tank centerline as close to centerline of carb so you are not trying to pull fuel uphill too much. Don't use too big a tank for the small engine (6 or 8 oz should be good). 25% nitro is good sport fuel for marine use, don't use 50% unless you are racing and don't care how fast you wear out the engine.

    I'll start a holy war here, but... If you break in an ABC engine with a low nitro fuel, the cylinder does not thermally expand as much as it would with a higher nitro blend. So if you break in with 10%, it wears in with less expansion due to heat than it would with 50%. Then if you switch to a higher nitro, it expands more and gives you a sloppy piston fit. A good rule of thumb is to start out with the highest nitro content you plan on regularly plan on using. As the engine wears (sloppy piston/cylinder fit, low compression), you can switch to a lower content nitro and theoretically get less cylinder expansion due to cooler running, and gain better compression. I never recommend running a low nitro for break-in then switching to a higher nitro. I do not think moderate switching makes a big difference (ie: I'm out of 20%, so I'll run 15% today).
    I know just enough to be dangerous

  17. #17
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    RE: K&B 3.5 Outboard


    ORIGINAL: FlyingPilgrim

    I do not recommend running a lower nitro content for break-in. Use what fuel you plan on running. If you want, add a little extra oil to the fuel to help lube during break-in to bring it up to a max of 20% oil content, no more. If you are 4 turns out, you should be slobbering rich. The easiest way to tell is from the sound of the engine.
    There is nothing wrong with breaking in an engine on low nitro, especially if you are unfamillar with the engine. Extra oil will do little for a hot lean detonating engine

    If your bearings are bad, you should be able to feel roughness when you turn the engine over by hand. Don't change them needlessly. Find a friend that has experience in tuning marine engines that can help (no offense, not insinuating you aren't capable, but two minds can be better than one, especially if experienced).
    I have never been able to actually feel roughness on a bad bearing, usually they are detected by their whining noise. The only time I have felt roughness is from congealed oil, or a close fitting crank and dust between the crank and housing between bearings.
    I'm sure you already know this, but make sure you don't run it out of the water for more than a minute or two, as the shaft and lower busing will burn up and wear out quickly without the water to cool it, plus the engine will heat up without the water cooling.

    Check for good fuel tank and lines (no gooped up mess to restrict flow), check for any foreign particles in needle valve assy, use a filter in your line. Try to get your tank centerline as close to centerline of carb so you are not trying to pull fuel uphill too much. Don't use too big a tank for the small engine (6 or 8 oz should be good). 25% nitro is good sport fuel for marine use, don't use 50% unless you are racing and don't care how fast you wear out the engine.

    I'll start a holy war here, but... If you break in an ABC engine with a low nitro fuel, the cylinder does not thermally expand as much as it would with a higher nitro blend.
    Unless you are running it too lean it will be hotter on low nitro fuel. Because nitro likes to run rich it will be slightly cooler when you are rich of lean. However if too lean nitro will run much hotter, so it will start to detonate, then get even hotter and fry the engine. I have ruined an engine this way!
    So if you break in with 10%, it wears in with less expansion due to heat than it would with 50%. Then if you switch to a higher nitro, it expands more and gives you a sloppy piston fit.
    Actually there is very little differance in wear or expansion, the C/L people run ABC engines sloppy four cycle rich which breaks into a leaner two cycle run with no problems. Also there is a thread here where a test showed no problems breaking and ABC engine super rich. In fact if you break in an OS LA aircraft engine per the book it will be sloppy rich.
    A good rule of thumb is to start out with the highest nitro content you plan on regularly plan on using. As the engine wears (sloppy piston/cylinder fit, low compression), you can switch to a lower content nitro and theoretically get less cylinder expansion due to cooler running, and gain better compression. I never recommend running a low nitro for break-in then switching to a higher nitro. I do not think moderate switching makes a big difference (ie: I'm out of 20%, so I'll run 15% today).
    Bad idea, and expensive, especially if you are new to the hobby. A lean run on 50% nitro will ruin an engine in a hurry! The rule is to swith from low to high nitro as it wears, but this is bacilly just restoring lost power from poor compression.
    Glow Head Brotherhood #15

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    RE: K&B 3.5 Outboard

    Sport, yes, I read, you suggested that 30% would cause problems while breaking in this engine.
    Wrong! I suggested that high nitro MAY cause problems IF you run it too lean, or for some reason ran hot, such as being too tight! This is only more likely if ringed, it can happen to ABC engines.

    Since you didn't know whether it was ABC or ringed, I wonder how many of these you've broken in? How many K&B 3.5 outboards have you actually run?
    I have not broken in any marine engine, nor desire to. BUT, I have probably broken in the aircraft version of that engine. And probably more ABC engines as well. As someone said the marine engine is the same as the ducted fan aircraft engine. I assume they were correct.
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    RE: K&B 3.5 Outboard

    ORIGINAL: Sport_Pilot

    Sport, yes, I read, you suggested that 30% would cause problems while breaking in this engine.
    Wrong! I suggested that high nitro MAY cause problems IF you run it too lean, or for some reason ran hot, such as being too tight! This is only more likely if ringed, it can happen to ABC engines.
    [/quote]
    You said "Wow! 30% nitro for break in? Dontcha think that the extra heat of friction during friction could cause the nitro to detonate? Try 15%, then switch back after break in. " Nothing about too lean, nothing about too tight. Without any experience operating these engines, you just asserted that 30% could cause detonation and that 15% was a better choice.
    Since you didn't know whether it was ABC or ringed, I wonder how many of these you've broken in? How many K&B 3.5 outboards have you actually run?
    I have not broken in any marine engine, nor desire to. BUT, I have probably broken in the aircraft version of that engine. And probably more ABC engines as well. As someone said the marine engine is the same as the ducted fan aircraft engine. I assume they were correct.
    "Probaby", "assume"? I guess I should yield to your vastly superior uncertainty.

    While some of the parts on the outboard (water or air cooled), inboard, aero, and ducted fan versions of the K&B 3.5 may be interchangable, that does not mean they are the same. It's well known that over the years K&B has made a variety of little tweaks. To someone who doesn't know any better, the parts may look to be the same, but that doesn't make them identical. People running boats tend to run different fuel (higher nitro) than aero people, so the engines are slightly different. The marine engines are built for higher nitro.
    AMA 87959

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    RE: K&B 3.5 Outboard

    Agreed that one way of noticing bad bearings is the screeching sound (the original post never mentioned hearing noise).

    Marine engines are generally not broken in on a test stand like a lot of us do with our airplane engines, so the arguement about break-in with the fuel you plan to use being expensive is not a valid one. Break-in for most marine enthousiasts is simply to run it on the pond slightly rich, so it's not like you're going through a gallon on a test stand.

    None of my suggestions addressed lean running, and nothing you can do will eliminate damage from lean runs. The only thing you can do to prevent lean run damage is not to run too lean (duh!). When in doubt, get assistence from a more experienced modeler that can help you tune it.

    There are many opinions of how to properly break in a model engine and switching nitro content in fuel, that is why I prefaced my statement with the comment about starting a holy war. For every person you find with one "sure way" there will be another who swears you're all wrong. My suggestion to any inexperienced modeler is to read as much as you can in the magazines and books, and take all you read on these forums with a "grain of salt" so to speak. Although many of us hear are experienced modelers and use what works best for us, our opinions are for the most part just that- opinions. Look for advise from pros like Clarence Lee on engines, Jerry Dunlop is one of the most knowledgable on outboard boats.

    I don't want to start a urinating contest, so I'll get off my soap box now. Good luck on getting your outboard to run right, they are a ball once you get them dialed in.
    I know just enough to be dangerous

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    RE: K&B 3.5 Outboard

    ORIGINAL: Sport_Pilot


    ORIGINAL: FlyingPilgrim

    I do not recommend running a lower nitro content for break-in. Use what fuel you plan on running. If you want, add a little extra oil to the fuel to help lube during break-in to bring it up to a max of 20% oil content, no more. If you are 4 turns out, you should be slobbering rich. The easiest way to tell is from the sound of the engine.
    There is nothing wrong with breaking in an engine on low nitro, especially if you are unfamillar with the engine. Extra oil will do little for a hot lean detonating engine
    You're assuming that the engine was detonating. Since it was designed for the fuel the OP was using, this is unlikely.

    If your bearings are bad, you should be able to feel roughness when you turn the engine over by hand. Don't change them needlessly. Find a friend that has experience in tuning marine engines that can help (no offense, not insinuating you aren't capable, but two minds can be better than one, especially if experienced).
    I have never been able to actually feel roughness on a bad bearing, usually they are detected by their whining noise. The only time I have felt roughness is from congealed oil, or a close fitting crank and dust between the crank and housing between bearings.
    If they're rusted from being stored with fuel in them it's usually pretty easy to feel the roughness.
    I'm sure you already know this, but make sure you don't run it out of the water for more than a minute or two, as the shaft and lower busing will burn up and wear out quickly without the water to cool it, plus the engine will heat up without the water cooling.

    Check for good fuel tank and lines (no gooped up mess to restrict flow), check for any foreign particles in needle valve assy, use a filter in your line. Try to get your tank centerline as close to centerline of carb so you are not trying to pull fuel uphill too much. Don't use too big a tank for the small engine (6 or 8 oz should be good). 25% nitro is good sport fuel for marine use, don't use 50% unless you are racing and don't care how fast you wear out the engine.

    I'll start a holy war here, but... If you break in an ABC engine with a low nitro fuel, the cylinder does not thermally expand as much as it would with a higher nitro blend.
    Unless you are running it too lean it will be hotter on low nitro fuel. Because nitro likes to run rich it will be slightly cooler when you are rich of lean. However if too lean nitro will run much hotter, so it will start to detonate, then get even hotter and fry the engine. I have ruined an engine this way!
    So if you break in with 10%, it wears in with less expansion due to heat than it would with 50%. Then if you switch to a higher nitro, it expands more and gives you a sloppy piston fit.
    Actually there is very little differance in wear or expansion, the C/L people run ABC engines sloppy four cycle rich which breaks into a leaner two cycle run with no problems. Also there is a thread here where a test showed no problems breaking and ABC engine super rich. In fact if you break in an OS LA aircraft engine per the book it will be sloppy rich.
    The thread you were talking about is one person's experiment with a cheap engine that he didn't mind ruining. Suggesting that the results of that person's very limited testing would apply to other, and significantly more expensive, engines puts peoples engines at risk. It's kind of scary that you so casually give such advice to beginners. It's way better to follow the directions of the manufacturer and people who have experience with these engines.
    A good rule of thumb is to start out with the highest nitro content you plan on regularly plan on using. As the engine wears (sloppy piston/cylinder fit, low compression), you can switch to a lower content nitro and theoretically get less cylinder expansion due to cooler running, and gain better compression. I never recommend running a low nitro for break-in then switching to a higher nitro. I do not think moderate switching makes a big difference (ie: I'm out of 20%, so I'll run 15% today).
    Bad idea, and expensive, especially if you are new to the hobby. A lean run on 50% nitro will ruin an engine in a hurry! The rule is to swith from low to high nitro as it wears, but this is bacilly just restoring lost power from poor compression.
    It's not at all a bad idea to use the fuel the engine was designed for. If it was designed for 5-15%, use that. If it was designed for 25-50%, use something in that range.
    AMA 87959

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    RE: K&B 3.5 Outboard

    Without any experience operating these engines, you just asserted that 30% could cause detonation and that 15% was a better choice.
    I have more experiance than you could shake a stick at, but admitedly no marine experiance. You took that out of context of this thread. I explained why I made that recommendation. That post was a near lie.
    Glow Head Brotherhood #15

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    RE: K&B 3.5 Outboard

    While some of the parts on the outboard (water or air cooled), inboard, aero, and ducted fan versions of the K&B 3.5 may be interchangable, that does not mean they are the same.
    Ok, what is differant about this engine that makes it so differant from model aircraft engines?
    Glow Head Brotherhood #15

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    RE: K&B 3.5 Outboard

    Break-in for most marine enthousiasts is simply to run it on the pond slightly rich, so it's not like you're going through a gallon on a test stand.
    Good point. But it will also be impossible to richen up the needle till you bring the boat back.

    None of my suggestions addressed lean running, and nothing you can do will eliminate damage from lean runs.
    I have never melted a piston from a lean run on low nitro fuel. But it has happened with high nitro fuels.

    For every person you find with one "sure way" there will be another who swears you're all wrong.
    And you will find people having success with both methods. No reason to get upset about it. I would be talking in a calm voice if this was in person.
    Glow Head Brotherhood #15

  25. #25
    FlyingPilgrim's Avatar
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    RE: K&B 3.5 Outboard

    Come on guys, lets keep this on track. No one cares who has more experience, if you must argue, use PM's.

    Back to the original problem, my guess is one of two things:
    1. fuel delivery problem
    2. getting hot from either running out of water too long or blockage in coolant line (or maybe even the silicone tube is not on!). Is the brass pickup tube in good shape and not crimped/bent?
    I know just enough to be dangerous


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