Robin Brush Cutter TK diaphragm carb will not keep going

Hello please help;
I have a Robin brush cutter model NB02, it has a TK carb, it stopped running this summer. The compression is excellent. Spark is ok and on time. Carburetor was dirty, I stripped it, cleaned it w/ solvents, carb cleaner and WD40. All passages were cleaned with a copper wire or a pipe cleaner and air pressure. I even used paint stripper to make shure no deposit left behined.
The check valve in the fuel tank is clean and working, as well as the one at the enterance of the fuel pump. I can hear the ball ratteling.
Used new diaphragm/gasket kit, assembled it according to the exploded view in the manual and the notes I took before I took it apart.
I set the mixture screw to factory specs; 1.5 truns off closed, and the idle at 1 turn off closed.
Using fresh gas, mixed at 4%; I prime it. it runs after a couple of pulls, soon it starts to stutter and stops, no matter where I position the chock, or the needle in the slider, or the mixture screw.
I went over the carb several time, all passages are clean. Diaphragm & gaskets are assemble correctly.
I operated the diaphragm pump by push-pull little air using a tube between my mouth and the air port. The pump air port (cylinder side) is clean and sends air pressure variation when I crank the engine (attached an old fasion vac gauge to it.)
Even if I contine to pump the primer, it will not keep going.
I tried gravity feed, gave it a head of 3 feet, it will not keep running beyond the 10 to 15 seconds
Please help with any suggestions, or ask if you need more information.
Thank you all.
Logan

 

Does choking it affect the amount of run time?

 

Not much difference, in run time, with or without the chock
Logan

 

Sorry, I see I missed that in your original post. Are the fuel lines new? All I can think of is that it is pulling air instead of fuel. If there is a tiny pinhole or crack in the fuel line, it could cause the problem. Also if the fuel line is pinched or kinked it could do this.

 

Hello;
Yes the line between the primer & carb is new. The filter inside the tank is new, check valve in the suction tube is working. Tested the fuel delivery collectively by pushing air through the little hole in gas tank cover (the hole is clear) fuel showed up after the pump therough fuel delivery port.
I just do not know what I could have missed.

Thanks

 

Hello Cheese;
A new development since I wrote the last reply. I can keep the machine running if I push the "tickler" partially. This is the button at the bottom of the carb and it pushes on the main diaphragm allowing the needle valve to let gas go through.
One thing I have noticed, and did not mentioned in the past, is that the main diaphragm pushes on a lever that opens the needle valve. This lever is opposed by a spring loaded plunger that does seem to have any adjustment. then there is the little spring that keeps the lever up. During cleaning, when I pushed on this plunger, it seems to have much resistance. When I cleaned the carb, the "drain" hole that connect to the little short tubing was blocked. When it was opened eventually, I would apply some WD40 and use little air pressure (with tube, by mouth) WD40 would show up at the plunger then little bubbles. I will try to get the carb exploded view and send it to you, if this system allows attachement.
Can I bend the lever slightly to make it easier for the diaphragm to open the needle valve???
Many thanks.

 

Yes, the lever can be bent, but it shouldn't have to be. It ran in the position it is in now, and should again. Is the diaphragm identical to the one that was removed? Including the button that presses the lever? Is the needle free to move in the carb, save for the tension of the spring? I can't imagine the plunger would keep it from getting gas. Might cause it to get too much, but shouldn't stop it from getting any. Is the diaphragm possibly installed upside down?

 

Hello Cheese;
The diaphragm is identical to the original. It is not possible to install it up-side-down, it has four holes for the screws, in the corners, and two locating holes (opposite corner from each other) that mate with pins on carb body. I took notes of all parts when I disassembled the carb and consulted with the exploded carb view, the diaphragm and the gasket are mounted as the old ones and the as exploded view. The nickel-size washer on the diaphragm towards the carb body.
The needle moves freely and it does seats properly so there is no leak when closed.
The spring loaded plunger, I referred to, is mounted in the carb body right at the center, it opposes the lever that operates the needle valve to keep it closed. When the diaphragm operates, it pushes on the lever that pushes on the plunger and sea-saw around the fulcrum to open the needle valve.
As I said, I suspected it because I was able to keep the engine running while pressing gently on the "tickler"
By the way, do you know what causes the diaphragm to operate, I did not see a hole which would channel vacuum in that area, let alone that this side of the diaphragm is filled with gas, the other side is exposed to atmospheric pressure through the hole in the tickler button.
Thanks again.

 

Pressure/vacuum pulses from the piston moving up and down in the cylinder should be sent through a port going from the cylinder, through the intake piece, through the carb body, through a hole in the gasket and diaphragm, to the back side of the diaphragm to operate it. I don't ever recall seeing one of these vented directly to atmosphere. I haven't had any luck finding a diagram of your carb to see it. I recall seeing them and working on them, but it's been a long time and i don't remember details. I'll look out at the shop, I think I have one of these carbs on an old trimmer out there.

 

Hello Cheese;
Your description fits the way the pump operates; it is the smaller set of gasket & diaphragm in the picture at this link

Bailey's - Oregon Diaphragm and Gasket Kit for Teikei Carburetors on Fuji Engines

The larger gasket & diaphragm are the main diaphragm that allow so much fuel into the little cavity above the diaphragm which supplies the jet with fuel. I figured it out this morning when I took it apart, yet one more time, that as the fuel is sucked into the carb through the jet, the little vacuum generated by the absence of the fuel causes the large diaphragm to operate the needle valve and let little more fuel in.
You can see in the picture of the large diaphragm the little extra locating holes I described which dictate how the diaphragm will fit.
How can I send you a picture of the carb exploded view?

Thanks again

 

Question on the location of the diaphram and the diaphram gasket. Some of these units had the diaphram next to the the carb body where others have the gasket and then the diaphram. If you can lightly press the button and it will run then you are allowing the fuel to enter the area by the diaphram. To me it sounds like the diaphram is not touching the needle valve lever to let in fuel. Try it with the diaphram next to the carb body and then the gasket and cover. This carb was used on the old suzuki trimer engines for toro.

 

Hello;
I have tried placing diaphragm next to body, the engine was harder to start and it ran with up-n-down speed then it stalls. It smelled like too rich a mixture, tried various setting on mixture screw without luck.
I have noticed that the new main gasket is only ½ mm. The older one is 1 mm. When I assembled the carb with gasket next to body as in the exploded view and according to my notes, there is no play between the diaphragm and lever. As you know there is a nickel-size washer and a rivet holding it to diaphragm that takes up the space. When I touch the diaphragm gently I do not feel a gap.
With older 1 mm gasket next to body, then diaphragm, the engine ran ok after it warmed up at idle, half-speed (throttle button pushed in) and at max speed, only every few seconds it will struggle and I had to touch the “tickler” just a little.
I do not know what else I could have missed, please suggest anything that comes to mind.

Thank you for your help.

 

Hello everyone;
You may find these sites invaluable for taking care of Walbro (or nay other diaphragm carburetor). I ran into these site while I was searching for a solution for my Teikei carb on a Robin brush cutter. I know these guys are tuning carbs for airplane engines, but it does not hurt to learn the technique of overhauling a carb;

On tuning a diaphragm carb;
Tuning a Diaphragm Carburetor
Walbro carb tune up;
Walbro Carb Tune up!

I am still looking for help with my machine mentioned above; I can start it and it will run for 5 to 7 min then stalls. After that I can't start it no matter what till next day.

 

Logan,
Make sure the needle valve is in the slot of the metering lever, not just sitting on top of it. Also check the insulator block is properly screwed onto the head, the 2 screws there often work loose due to vibration and heat cycling. This can show up as hard start when hot as the screws lengthen due to heat expansion and you get air leaks for the fuel pump.

TJ

 

As mentioned in my previous posts, there is a center brass “plunger”, a spring-loaded brass knob (about 3/32” dia) at the center of the metering chamber. It is immediately under the tip of the lever that operates the gasoline needle. There is also the usual spring that pushes up on the lever. The “plunger” had clearance around it; this led any access gas to a little tubing that discharged to the air. A bad idea for the environments, and my pocket.

Since the engine had a good compression and a good spark, even after I heated the ignition coil with a hair dryer to make sure it is not failing when hot. So it had to be carburetion. As mentioned before, I cleaned the carb with solvents, WD40 and air, all passages are clean and inspected with a copper wire –used a new gaskets and diaphragms for metering chamber and pump, but it will always run and stop in few, not even allowing for any adjustment of the idle air (the idle adjustment on this TK carb is just an air bleed to lean or enrich the idle mix by allowing more or less air to what the slide-throttle and fuel jet were sending to the engine). The mixture adjustment screw on this carb allows more or less fuel to the jet where the tapered needle dips into. BUT, I could not find any technical information about this carb (Teikei 1E10B) or TK for short; I found nothing about the “plunger” or its roll. So I studied other diaphragm carbs and none of them had such a “plunger”. I was impressed with the generous information from Walbro carburetors. See the links posted above.

In the tech report from Walbro, it stated that the metering lever, the link between the diaphragm and the needle, has to be at the correct height so the diaphragm pin touches it, and the spring pushing it up must be of the correct tension. They even have gages for their carbs and various springs to adjust or repair used carbs. So I decided the “plunger” had to go. I was not sure enough to decide to remove it. So I tried to depress it and keep down, to get it out of the way, so it would not resist the lever when it is being pushed by the diaphragm by jamming a stiff thin steel wire, but it kept slipping up. When I tried to jam it down with a tooth-pick it stayed down but when I removed the pick, it broke and the “plunger” stayed down. This increased the clearance around it and more gas was going through the tube. So I drilled a hole in the upper side or the tank (a diameter just under the diameter of the tubing), tapered the tubing and pushed it in. Thus, I had a gas return path, less pollution and more gas savings.

I chose a medium tension little spring to fit under the lever to replace the original weak spring (in the normal place half way between lever tip that the diaphragm pin that pushes on the lever and the fulcrum point). Assemble the carb with gasket against the casting 1st as suggested in Walbro manuals and as it was when I took it off, then the diaphragm. Primed the carb, the engine started on the 2nd pull and kept running, although a bit rough. When it warmed up, I adjusted the idle then ran ½ w.o.t. and adjusted mixture screw, used it for some 15 minutes and opened it w.o.t. and re-adjusted the mixture screw. (on Walbro carbs there is an L screw to adjust the amount of fuel going through the idle jets, then there is an H screw for high speed fuel adjustment. See Walbro manual). I have used it many time since, it starts in a couple of pulls. I do not even have to prime it. Enough fuel is staying in the pump and metering chamber to start it.

Sorry about the lengthy letter, I am hoping it would help someone tracing a specific fault in an orphan carburetor.

Many thanks for all who helped me in the past.