k&b won't stay running

[James_Simpson]

k&b won't stay running

Help I have a 3.5 k&b OB mounted on a prather spitfire. It starts and runs great but always dies after 50 yards or so after getting on plane. I have adjusted the needle in and out from the recomended 3 1/2 turns. and tried different props from an x437/3 to 1740/3.

My fuel is 15% nitro and a year old. What gives, I am clueless.

 

[Propjockey]

Re:k&b won't stay running

Does it run hard...then slow down and stop? Or just die? Watercooled?

 

[MarkScott]

Re:k&b won't stay running

Is the prop too deep / angled correctly. Believe it should run on 15%, but you might want to try a bit higher if you can, 45% - 55% and it'll wake up... 3.5 love nitro Once fully broken in, go even higher if you want. Did this condition just start? Been running good, but now does this? Fuel lines old? Fuel should be good. What brand? Could be a million things.

Mark...

 

[James_Simpson]

Re:k&b won't stay running

Thanks for your quick replies.

The motor is watercooled and the outlet fitting is missing, do you think it could be over cooling?

I did try diferent prop depths and angles. It seems like once the prop loads up when the boat gets on plane it pulls the revs to low and it dies. I bought the boat second hand an it has hardly ever run more than 20 seconds on the water without stalling. On land it seems to run good. The motor is very tight and the piston & liner look good. Also the bearings feel good. Tank and fuel lines are new.

 

[topfuel443]

Re:k&b won't stay running

pull the needle out of the carb and make sure there is no built up gunk on the needle, also make sure you can see a nice slean hole where the needle gpes into the carb.

Did you say that the 15% fuel is a year old? Nitro looses its kick after its been exposed to air for to long, not to mention you only 15% nitro to start off with. Id recommend getting some new 30 or 40 % nitro fuel, put the carb back together and richen it up until will almost not run from being to rich. then see if it will run a tank of fuel out. If it runs the tank out, then you can start to lean on the engine 1/8 turn at a time until it starts to die again, then find a happy medium.

With all of that said, we (Intlwaters tech. group ;D) can only assume that mechanically the eninge is sound, i.e. timing, compresion (P/L seal) , head clearance, type of plug, bearings all need to be correct before the engine will run correctly. Can you notice any fuel coming out of any seal or especially the "top" bearing (the one under the starter cone)? If so, its probably got an airleak wich will cause a lean condition every time no matter how rich you run it. You may need to check the crank/front housing clearance. if a bearing was bad at one time, it could easily make the two rub together and will in turn cause it to loose the close tolerance it needs to prevent an air leak.

Could be tons of different things honestly, but thats a few you can try.

DONT GIVE UP ON IT!

~ James

 

[[email protected]]

Re:k&b won't stay running

Close the needle all the way in and set it at three turns. Check the idle adjustment make sure it's not screaming. Replace all the fuel line, yes replace it, second hand boats always have old brittle or discolored line. Check to make sure the fuel tank cap is tight and does not leak. Blow into one end of the tubing and pinch the other. Make sure the tank swell up hard. If it doesn't you have a leak. That's some advice. Also go up to about 40% to 50%. 3.5 K&b really needs the high nitro.

 

[[email protected]]

Re:k&b won't stay running

Are you the guy with the Prather Spitfire 3.5 for auction on ebay??

 

[James_Simpson]

Re:k&b won't stay running

no I do'nt have it on e-bay.

 

[GTR]

Re:k&b won't stay running

Hi James

Your Nitro is probaly a bit on the low side as others have suggested and I doubt it could pull a 1740 on that %. When you start it and take the power of the glowplug do the revs drop? If they do you need a new plug/ a hotter plug - or both.

Get a water outlet fitting for it and solder up the end and drill a hole about half the diameter of the standard one. The WC motors nearly always run to cold unless you run to lean. Oh yeah and the sysmtems you describe are mostly from being to lean for some reason or a dud plug.

Keep going though - OB OB OB more power too ya!

GT ;D

 

[[email protected]]

Re:k&b won't stay running

oooowww, cold, water-cooled, runs, lean. So lean is my secret. I guess that's why I love the water cooled engines. Very Lean friendly! Lean is Mean! 8)

 

[nitrotoys]

Re:k&b won't stay running

Get some good 40% fuel, get rid of that old stuff.

I had that problem, Check the NEEDLE BASE SCREW TO BE SURE ITS TIGHT.

snug it up against the carb and fresh fuel.

See how all that does, and i know the lines are new, but replace anyways, a pin hole could spoil your fun at the pond.

NT

 

[Kris Flynn]

Re:k&b won't stay running

hmmm i run on 10% fine! i find that it screams!!!!

i suppose low nitro could be contributing to my reverse rotation problem also?

Kris

 

[GTR]

Re:k&b won't stay running

Hi Kris

I don't think your backward running is related to the nitro %. Hey you realise water runs down the drain the other way here in Oz maybe the engine thinks its in the US and is confused and trying to spin the wrong way. Could be Kaos theory physics at work. : I think I'm loosing it.

I told you if you can't get it licked send it up here and I'll whoop it's butt into submission. It will come back like a trained puppy.

GT ;D

 

[James_Simpson]

Re:k&b won't stay running

ok I have a spare plug but never even thought to try it. Both plugs are new. How do you tell if you have an air leak? I have had the motor apart and changed the rear bearing. How do you check the crank to front housing clearance? Also I don't understand how the PTO is sealed. Could the PTO be leaking? I will try another glow plug and maybe new fuel tomorrow at the pond.

 

[duc]

Re:k&b won't stay running

I know you guy's on the other site of the pond like nitro but I say this once more: K&B's wil run perfectly on 10%. I did use 10% for many years. However it wil have a bit more oooomph when using higher nitro and maybe starts better. So I use 25% now.

 

[dale]

Re:k&b won't stay running

Yes i had one of these boats once, same problem i found that if the prop is not at the perfect spot up or down it just would not go, get that prop up.here's the best thing to do but a small 3.5 tunnel youll have much more fun and go alot faster.we guned my spitfire at34mph best it ever did. but its long gone now it was a pain :-X :-X Good Luck.

 

[dale]

Re:k&b won't stay running

buy a tunnel ;D not but one sorry :

 

[dale]

Re:k&b won't stay running

Try a x440 two blade, 3 blade i think will put to much drag on the motor, i never ran a three blade on mine.

 

[lohring-miller]

Re:k&b won't stay running

A very common reason this happens is the needle is too lean. The engine should run even very rich. If you slowly lean the engine and it stops before leaning down all the way, you probably have an air leak. Either the front or PTO seal could be worn, especially if you have had a bearing failure. Dissassemble the engine and try blowing through the seals. the leakage should be very slight, even with no oil in the seal area.

Lohring Miller

 

[James_Simpson]

Re:k&b won't stay running

Ok thanks for all the suggestions. I replaced the missing cooling outlet and it ran three laps and then I brought it in adjusted the needle valve an away it went again. I got 3 or 4 runs with out it stalling! Then we tried 40%. I could not see any difference in speed with the 40% fuel. I was running an x437/3 prop. Then I tried a x438 and it would not get on plane no matter how hard I launched it. Next time I will try a 1740/3 prop. I don't have anything in between.