30 replies to this topic

[Mechengine]

Hello All,

 

I'm having some similar issues. I have a 92 TDM850 It had been stored for about 10yrs. I took it out started going through the bike fluids etc. I started the beast using a bottle with the tank off and low and behold she came to life. Then she started to bog and misfire I noticed fuel coming from both overflow tubes and turned off the fuel valve. She cleaned up and continued to run perfect. So I figured it was time for a carburetor rebuild. I broke them down and did a complete rebuild all new seals, o-rings, jets, needles, diaphragms and needle valves. The kit did not include the seats. Reinstalled the carbs she fired up and started leaking fuel again from the overflow tubes. I have checked, set, reset retested still leaks. I have replaced the o-ring on the seat checked for a cracked bowls and all is good. In over 40 years of working motorcycles I have never been stumped like this. I have removed the float assembly connected it to a syringe via a hose and under pressure tested the needle and seat and it will flow and seal even against some pretty good pressure. It doesn't appear to be the needle and seat and I have float level about 2mm under the recommended levels and it still leaks. I can't find any cracks in the bodies or the bowls but both carbs still pour fuel even when I run them on gravity and don't use the pump which is still working. I know I dumped a lot or info but does anyone see anything I'm missing? It runs perfect when I regulate fuel valve.  Any thoughts would be appreciated. 

[Studley Ramrod]

Hi. A few things to look at. Firstly, make sure the floats are positioned correctly as they will go in at a slight angle and if that happens the float can snag on the carb body. Secondly, make sure the main jet housing is secured correctly, it is possible to secure them in a way that allows fuel past the gasket. Lastly, make sure the main jets are seated fully and re-check the o-rings on them.

 

You could try lowering the fuel levels.  Do you measure fuel level or float height ?  Checking the fuel levels is always a good first indicator of any potential problems.

[Mechengine]

Thanks for the reply. I measure the floats with a float gauge, I know that's old school, not the the way the Yamaha service manual says and awkward because they sit at an angle. Because I have a leak I cannot measure it with a tube against the float bowl cover. The main jet housings are secured with new gaskets. I will re-check the main jets and make sure they are properly seated and there are no defects in the new o-rings on them. Anything else I should re-visit? It seems strange that both carbs are leaking and have the same symptoms. I would think that the needle seat would stop the fuel flow once the float rises and the needle valve closes. It appears to keep flowing is there another possible path from the inlet tube to the float bowl? 

[Mechengine]

I have an update, as I initially stated when I tested float assembly connected it to a syringe via a hose and under pressure tested the needle and seat and it will flow and seal even against some pretty good pressure. So I tested the float assembly by removing the bowl and applied fuel from the bottle and when a moved the float it appeared to stop the fuel coming from the float assembly but fuel appears to be coming from a port in the body. I have included a picture, does anyone know what this port is and why it would be a unregulated source for fuel?   

 

 

Attached Files

•  Needle and Seat Port_.jpg   169.24KB   1 downloads

[Mechengine]

 Needle and Seat Port_.jpg   169.24KB   0 downloadsDoes anyone know why fuel will continuously from this hole. The needle and seat flows fuel but will close and stop flowing from the needle and seat at the appropriate level. However fuel flows this hole continuously and floods the carb. Has anyone experienced this and if so what is the fix?

[Snowbird]

That looks like a float bowl breather to me, is there anything else that has breather pipes linked that could leak and get back to the float bowl that way (fuel pump?)? Sorry I'm not familiar with the 3vd carbs, for that matter I'm not familiar with my own 4tx ones yet.

[madmopedracer]

just an off chance but do any of the floats have a leak making them stay down sometimes was a problem with the old brass ones bit clutching at straws 

[steveearwig]

 

The kit did not include the seats...I have replaced the o-ring on the seat

Do you mean the o-ring where the seat part of the assembly pushes into the carbs? Or did you find a cheap way of replacing the seats? The seats leaked on mine, in bits it was fine but put it back together and attempt to go for a ride and fuel everywhere. After taking the bike to pieces about 400 times and finding nothing, rebuilding the carbs (no seats of course) I took it to someone that knew what they were doing and by this time it needed new rings too.

 

Symptoms: bike horrific to ride in traffic due to coughing and farting, cylinders coming and going, but open the throttle and everything's fine.

[Mechengine]

The floats are not leaking I tested them thanks. Yes, I have replaced the o-ring where the seat part of the assembly pushes into the carbs, you are correct. I thought this is where it is leaking from because when I wet tested the float assembly it works fine allowing fuel when open and sealing when closed. The port in the picture appears to me a fuel inlet port. The port ties into boss on the casting of the Carb that feeds the float assembly and goes to the fuel inlet Tee, it doesn't make sense that it is before the float assembly that makes it unregulated. It is also in the pictures in the factory Yamaha manual so it is designed to be there. I can remove the float assembly and plug the spot where it goes but when I apply fuel it just runs out the hole marked by the arrow. I've been looking for but can't a Mikuni BDST 38 fuel circuit map but have not ran across one the net yet does anyone know of one? 

[dapleb]

Hi mech,

I am sure you are already passed this stage but the two floats are diff heights. Factory Pro suggest measuring 15mm from the rectangular float (NOT diamond) as there are a few mm height difference. They suggest 15mm, I think I went for 13mm...been a long time since I have done any adjustments.

Here was my setup http://www.carpe-tdm...-1144949778.jpg


Factory pro are relatively close to you (compared to the majority here) and have been very helpful to carpe in the past, so it might be worth a call to try and get some tech info on fuel flow map etc. Linky dink http://www.factorypr..._procedure.html

I will check the workshop manual when I get a chance but sounds like you have already done that.

[Studley Ramrod]

I can't see it being a fuel inlet port, just doesn't make any sense really. The problem with testing the floats the way you did is, the float bowl acts to secure the floats in place,preventing it from moving and ensuring the seal remains in place.  Once the bowl is removed the float seal can and will leak fuel.

 

I would empty the carbs, rig up a fuel supply and use some clear tubing to check the fuel levels, you might then get an idea if you set the float heights correctly and also which carb is overflowing.

[Mechengine]

Dapleb thanks for the picture.

 

I have been testing the floats like you picture depicts only on the bike to maintain the angle and be able to apply fuel. I will use your picture as a reference for setting the floats again. I currently have them set well under 13mm. The needle valve only moves about 0.030" before it is closed. I  wanted to know if it possible that testing and running the bike from a test bottle is causing the problem? Has anyone else run or tested their carbs while using a test bottle? 

[Mechengine]

Ramrod,

 

 I turn the bowl over and reattach it to the carb to hold the floats in place like Dapleb's picture shows, this holds the float assembly in place. I will reconnect the clear tubing and measure the height of the fuel in the tube relative to the centerline of the bowl like the manual says. Maybe I missed something the first dozen times I did it. When frustration sets in mistakes increase.  I might even set the tab on the float so the needle is closed at all times and does not open. This will either confirm that needle and seat are not sealing or fuel is entering the bowl from another source. Can you think of anything else I might have botched, missed or flawed techniques?

[Snowbird]

Fuel from the hole in the pic is screaming a pipework issue to me, try disconnecting any breather hoses and see what happens.

Edited by Snowbird, 29 June 2022 - 03:57 pm.

[dablik]

Dapleb thanks for the picture.

 

I have been testing the floats like you picture depicts only on the bike to maintain the angle and be able to apply fuel. I will use your picture as a reference for setting the floats again. I currently have them set well under 13mm. The needle valve only moves about 0.030" before it is closed. I  wanted to know if it possible that testing and running the bike from a test bottle is causing the problem? Has anyone else run or tested their carbs while using a test bottle? 

Yes Mech i've used a test bottle on many, too many occasions, any overflow problems were always related to dirt (very small) in the float needle recess, i would re-check float heights using the fuel height method to rule out that potential problem.. i'm not a whiz on carbs although always resolved my issues with the chaps knowledge on here..frustrating, hope it gets sorted.

[Mechengine]

Ramrod,

 

I have tried to set the floats using the clear tube no luck. Even with the float set so that needle valve is fully closed all the time the fuel in the tube rises to the same height as my fuel source. So the float assembly is doing nothing to stop the fuel. I even tested the needle and seat and it was air tight prior to installing it. So fuel is running into the carb unregulated. When the engine is running the fuel pump, pumps the fuel in and when on an external fuel supply gravity runs the fuel in. When I lower the supply it lowers the level in the clear tube even with the float closed. When I installed the bowl like the picture Dapleb posted and apply fuel from the bottle while manually operating the float, the needle and seat stops at the needle and seat. However fuel continues to flow from the port in question above. When a hang the bottle on the side of the bike and go for a ride it runs perfect as long as I manage the fuel valve with one hand while I operate the bike with the other. It will even spin the tire with my 250lb bottom on it holding on with one hand. If I didn't like the bike so much I would set a torch to it. ???

 

Attached Files

•  Air tight float.jpg   140.18KB   0 downloads

[Mechengine]

I saw in a post from tdm850sf back in January 2015 he had a similar issue but it mysteriously resolved it's self. That isn't happening for and the Carb fairy isn't sprinkling her magic carb dust over it.

[Snowbird]

Going back to your pic that looks like a breather pipe connects directly above the fuel connection, double check you are connected to the right one.

It's one of those daft mistakes that are easy to go blind to being so close together and only get picked up with a different pair of eyebollocks.

Another trick is to use a small pipe and blow up the offending port and see where the air blows out from, looking at Dap's float height pic I recon it's a breather that links to the valve on the side of the carb body (which I think is an air cut off activated by the pressure in the CV top but can't quite remember what for).

Failing all that there must be a split in the body between those ports somewhere.

[Mechengine]

Thanks Snowbird,

 

The breather pipe is the first pipe in the picture it has a cap on it from the factory not sure why. The next pipe up is the fuel pipe connection Tee. After investigating this further I could have the Breather and the fuel supply reversed. The pictures in the manual are not very clear. Can anyone confirm the order of the Breather Tee and the Fuel Tee? Is the Breather/Ventilation Tee on top and the Fuel/Delivery Tee on bottom? Also what is supposed to connected to the Breather/Ventilation Tee, mine has a vacuum cap covering it and I don't have any pipes that aren't connected, all connections are accounted for?

[Snowbird]

The cap on the breather should be a filter not sealed, the fuel in the bowl needs to be open to atmospheric pressure for a carb to suck fuel or their would be no pressure difference in the venturi to draw the fuel through the jets.
Swap them pipes and try it just to shut me up, feel free to stick your tongue out at me if I'm piddling up the wrong tree.