K&B 7.5cc 2 second did out

[CamcorderRacer]

Hi, I have been struggling with this motor for some time now. It won't run for more than a few seconds than dies. I have put in a new piston/sleeve, bought new 30% nitro fuel, new fuel tubing, made a hopper tank, and new K&B glow plugs. I have gone from three and a half turns to seven turns on the high speed needls. The low speed jet is flush. Bearings are good fuel gushes out when I spin the motor over whle the glow plug is out. The motor is old but bearings are fresh, etc. I'm stumped. Any suggestion would be greatly appreciated.

 

[MarkScott]

Air could be leaking somewhere. Are the gaskets good shape? Any cracks in the case? Carb might have junk inside it. Might need to take the spray bar out and give it a good cleaning.

 

[Mike Larson]

This is going to be off the wall! ( but, I saw it once, and fell over) try running the motor the other way just once, and not for very long..... it's possible you may have an inboard crank in that motor..... ( parts build on a 3,5, drove a freind nuts once, till I had an ideer) careful............ mike

 

[Jerry Wyss]

This is going to be off the wall! ( but, I saw it once, and fell over) try running the motor the other way just once, and not for very long..... it's possible you may have an inboard crank in that motor..... ( parts build on a 3,5, drove a freind nuts once, till I had an ideer) careful............ mike

Oh, that'd be about as funny as a wussy on a man wood'nit :lol:

 

[Tim_Duggan]

Oh, that'd be about as funny as a wussy on a man wood'nit :lol:

:lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:

 

[CamcorderRacer]

The topic should be 2 second die off instead of 2 second did out.

I ran the boat last season so I know it will work. I replaced the piston/sleeve due to the 2 second run time I've experienced. I even sent it to K&B for the repair. I've blown air into all the passage ways in the carb, etc. Fuel squirts out the glow plug hole and I can see fuel movement in the tank as I spin the motor. The motor is an outboard and I don't want to try it backward and ruin the flex shaft. I have been using the same case gasket since it still looks good on reassembly.

 

[Eric Bourlet]

Seen this one many times on K&B Pro motors. Check the carb barrel I bet it is 180 degrees out. Make sure the hole in the spray bar and the slot in the sleeve that covers the spray bar are lined up.

 

[topfuel443]

If you cant get it figured out, send it to us. I would be happy to look at it for you. Im sure its something simple. It could be many things, I know you said you got a new hopper tank, but what about the main tank? if its cracked or leaking it will lean out everytime. You could try to run it just on the hopper, and see if it helps. I know the hopper is small, but you should be able to determine if the motor will run longer that a few seconds. Re-route your fuels line so that just the hopper is the main tank.

which carb does this have? which motor? Can you post a picture maybe?

~James

 

[waltr]

I like James approach.

I've seen the brass pick up tube in the main tank end up with a small crack in it, usually caused by a kinked bend inside of the tank. That will suck air also.

Defiantly worth the try, but surely don't rule out the other options.

 

[The Clock Guy]

A number good ideas so far.

Carb barrel out 180 deg.

Spray bar out 180 degs or just out of adjustment.

Cracked case and or leaking gaskets.

one more for you

Check that the piston and or sleeve are in correctly. For the sleeve exhaust port should line up

and piston skirt must keep the exhaust port closed at TDC. Someone did this one to me a 3.5

good luck.

 

[Dan Hydro]

The topic should be 2 second die off instead of 2 second did out.
I ran the boat last season so I know it will work. I replaced the piston/sleeve due to the 2 second run time I've experienced. I even sent it to K&B for the repair. I've blown air into all the passage ways in the carb, etc. Fuel squirts out the glow plug hole and I can see fuel movement in the tank as I spin the motor. The motor is an outboard and I don't want to try it backward and ruin the flex shaft. I have been using the same case gasket since it still looks good on reassembly.

Cam,

Never assume, just because K&B did the work they may have made a mistake. Check everything over.

Just my 2 cents worth.

Dan.

 

[Jerry Wyss]

I probably should add something constructive here h34r: Most of the time I've had a K&B run like that

it was a front bearing problem or in that area. Friend had one that the bore where the crank rotor resides

was scored so bad that the engine lost its pumping seal and would require a "falling out" needle to start

at all.

Many had the front bearing go bad, never assume its good and leave the possibility out totally.

Another had the piston pin clip between the piston and liner.

A sign that there's an air leak is the fuel dropping back to the tank from the carb after you stop using

the starter. Check and seal all bolt on parts to the case.

Possible pto area leak thru the shaft area.

JW

 

[Richie R]

I know they said check for cracks but check the seam of the crankcase. I had one that split there and took a little while to find.

Motor would run great then just die.

Hope this helps.

Rich R

 

[Josh Stollfuss]

Runs and dies when to toss in it right away in my experience is usually bad bearings. But I would check the carb and spray bar and make sure they are aligned correctly. I had the happen to me on my 1st K&B outboard when I was just starting out in R/C boating.

Josh-

 

[CamcorderRacer]

Runs and dies when to toss in it right away in my experience is usually bad bearings. But I would check the carb and spray bar and make sure they are aligned correctly. I had the happen to me on my 1st K&B outboard when I was just starting out in R/C boating.
Josh-

Thanks for all the help. I'm going to try each and every suggestion and will look at my bearings again. They spin just fine and if I remember right two years ago I replaced one of them. Tomorrow is Saturday which should be time to do everything listed. I'll report back after a through run thru.

I'm still a newbie at this,

FM

 

[CamcorderRacer]

If you cant get it figured out, send it to us. I would be happy to look at it for you. Im sure its something simple. It could be many things, I know you said you got a new hopper tank, but what about the main tank? if its cracked or leaking it will lean out everytime. You could try to run it just on the hopper, and see if it helps. I know the hopper is small, but you should be able to determine if the motor will run longer that a few seconds. Re-route your fuels line so that just the hopper is the main tank.
which carb does this have? which motor? Can you post a picture maybe?

~James

I haven't a digital camara to take a picture but it is a StockModel K&B # 8701 7.5cc outboard marine engine. I tried running it off the hopper tank and noticed bubbles in the fuel tank. I've looked at all the responses and have gone through all of them. It seems to historically to just run the fuel from putting my finger over the carbureter inlet while spinning the motor over.

 

[Eric Bourlet]

Pressure test the fuel system. Take the line off the carb and plug the pressure line that goes to the muffler or pipe. Blow into the line that usually connects to the carb . Pinch off the line you are blowing in, after a short time stop pinching the line it should hold the pressure and release it when you unpinch the line.

 

[Duck899]

Are you saying that the engine will not run longer than 2 seconds on the bench? This is a lean condition or an overly rich condition. What does the engine sound like when it starts? Does it die at a screaming high rpm? Or, does it die slobbering rich? Where is your fuel pick-up in the hopper tank and the main? It should be in the rear left corner of both tanks. You could also check the fuel system as Eric mentioned and also remove the fuel line to the carb inlet and use another piece of fuel line about 12" long and attach it to the fuel inlet nipple on the carb. Turn the radio on and while rotating the barrel of the carb from idle to full throttle, blow air in the carb nipple and simulate the fuel. It should almost be impossible to blow through at idle and get easier as you go from idle to full open. If your needle (high speed) is at 5 turns open from fully closed, you should be able to blow through easily. I have seen little pieces of crap in the spraybar assembly cause the symptoms you describe. Also by blowing into the carb, you can rotate the crank and close off the air to the crankcase and put your finger over the carb air inlet and check the carb body for leaks......Does this engine have the old grey cast aluminum carb with K&B engraved into it?

 

[CamcorderRacer]

Pressure test the fuel system. Take the line off the carb and plug the pressure line that goes to the muffler or pipe. Blow into the line that usually connects to the carb . Pinch off the line you are blowing in, after a short time stop pinching the line it should hold the pressure and release it when you unpinch the line.

All I'm doing is blowing bubbles. I've read other posts with similiar conditions and now I'm confused. I keep thinking "Buy another engine and everything will be rosy" but it is not.. I'm going to keep on trying.

 

[moparbarn]

if you disconnect your fuel lines, and block off the ex./pressure hose, blow in the carb/feed hose and get bubbles, you have a split line or tank. tanks are cheap, put a new fuel system in that boat. sounds like that's what will fix it. my theory is, if one part of the fuel system has failed from age, replace all of it, it's all the same age, what will go next? fix it right, fix it once. you still won't spend $20 doing it.