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Discussion Starter · #1 ·
Hey everyone,

In the diagram below part #40 is broken on my 400ex. #43 and #45 are no where to be found. Where does that hose run/connect to? #11 is also busted off. What adjustment is that?



Here's what's left:



Also I found that one throttle cover screw and all the float bowl screw are rounded out completely.

 

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Discussion Starter · #3 ·
The broken tube (missing hose) is the float bowl vent - it went up over the top of the carb and the end of the hose pushed into a hole in the splash guard behind the fuel tank

The other piece (#11) is the idle speed adjustment screw
Excellent response thank you. Do you think that the float bowl vent being broken could cause an overflow condition to happen? The whole reason I'm ripping this apart is because the lower vent tube that exits near the rear shock was pouring gas out. It ran great with the exception of backfires on decel, but it idled and ran fine otherwise.

Could that prick be my whole issue here? Because getting the float bowl off this carb is going to be a nightmare thanks to the messed up screw heads.
 

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No - those tubes let air in and out of the float bowl so the fuel level can rise and fall. If it's overflowing, it is a problem that is going to require the float bowl be removed.

If you want an easy way to get the screws out, here's how I do it - I have a collection of left hand twist drill bits - I use a bit slightly larger than the shank of the screw - for a 4mm screw I use a 3/16 drill bit - as the head of screw is drilled off the shank, sometimes th bit will "catch" and back the screw out of the hole - if that doesn't happen, once the head of the screw is off the shank, simply remove the float bowl then remove the shank of the screw from the carb body - usually you can spin it out with your fingers - put a new "proper" screw back in when you are done

Backfiring on deceleration is usually caused by a decrease in back pressure when a "performance" exhaust is installed or the OEM muffler has been modified for less back pressure. Backfiring out the exhaust won't hurt anything - it's just annoying - you can get rid of it by enriching the idle mixture - turn the fuel screw out about 1/2 to 1 full turn - if the air filter has been replaced with a K&N and/or the air box lid is off, you may have to have the carb completely rejetted
 

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I don't know - I take them apart, all removable parts (choke plate and throttle plate are considered non-removable) and soak them in commercial carb cleaner - I'm not rich enough to have a sonic cleaner - but before you do anything else, inspect the overflow pipe in the float bowl - they can split - I check them by filling the bowl with liquid and put compressed air to the nipple on the bottom of the carb - if the tube has split, there will be bubbles in the liquid along the side of the pipe
 

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Discussion Starter · #7 ·
Thanks for the tip on the overflow pipe. Checked out good. I'll get the replacement screws and some more carb cleaner tomorrow. Checked the fuel pump too it was in good shape but was covered in grains of sand.

You don't need to be rich to own a sonic cleaner. It's a fairly inexpensive tool that's pretty useful. I clean all sorts of things in mine.

This is the one I have: Ultrasonic Cleaner - Save on Ultrasonic Cleaners at HFT
 

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Discussion Starter · #8 · (Edited)
Well, got it back together and now it doesn't idle! Sounds like it's really struggling. Backfiring and dying. It will idle for a bit with the choke on then starts clicking, backfiring and dies. I adjusted the idle up a bit and that still resulted in the noises and it still dies

Seems to not be leaking anymore however

Later update...

I took it back apart and found a little sand in the pilot jet. Cleaned it well.

Took it for a ride. It runs ok with the choke on until it warms up, then once you have to turn the choke off it doesn't run well at all, even with more throttle - it backfires and bogs. Still seems like getting fuel is the problem. I replaced that broken bowl vent connector with a vacuum line t which is smaller in diameter before starting it either time last night. Could it be that the airflow in that t is too restrictive and because of that it isn't getting enough fuel quick enough?

Dyno jets 155 main 38 pilot
 

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No - not too restrictive - the purpose of the hose, wye and all that is to keep water and dirt out of the carb - if you think the plumbing is the cause of the problem, just pull the hoses off the carb and try it. My guess is you got dirt in the pilot jet.

There's basically 3 circuits in the carb - idle, mid-range and full throttle - of course idle is the most critical and sensitive. Another reason I completely disassemble a carb to clean it

Whenever you have to run with the choke on it indicates a lack of fuel (you are shutting off air to get a richer mixture) - take the carb off and concentrate on the pilot jet as the most likely culprit
 

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Discussion Starter · #10 ·
No - not too restrictive - the purpose of the hose, wye and all that is to keep water and dirt out of the carb - if you think the plumbing is the cause of the problem, just pull the hoses off the carb and try it. My guess is you got dirt in the pilot jet.

There's basically 3 circuits in the carb - idle, mid-range and full throttle - of course idle is the most critical and sensitive. Another reason I completely disassemble a carb to clean it

Whenever you have to run with the choke on it indicates a lack of fuel (you are shutting off air to get a richer mixture) - take the carb off and concentrate on the pilot jet as the most likely culprit
Thanks again it will come back off tonight. The only part that I did not remove beside the plates is the throttle plunger/needle. I did pull the main out and sprayed a bunch of cleaner up into and around the needle however and the jets looked clean when I reassembled.
 

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I did pull the main out and sprayed a bunch of cleaner up into and around the needle however and the jets looked clean when I reassembled.
Looking clean is not good enough - you have to be able to see through them - very small hole in the idle jet, measured in thousandths of a millimeter - spray carb cleaner certainly will not damage this carb (it will damage CV style carbs) except for the accelerator pump diaphragm and the air cut valve (the air cut valve's purpose is to cut the air to the idle circuit during deceleration to prevent backfiring in the exhaust) - if you sprayed carb cleaner into the valve when you were cleaning the carb, you may have damaged it - that would affect idle and increase backfiring - you can check the valve by sucking on the hose going to it, it should hold vacuum - if air passes through it, you need to replace it - part number 16510-MZ2-650, about $69.50 from Honda - I hope you didn't do that - the accelerator pump diaphragm can be damaged by carb cleaner also - if the engine stumbles on acceleration, check to see that the pump is shooting a shot of fuel into the throat of the carb when the throttle is pumped, if not, take the carb off, take the three screws out of the pump (don't lose the spring or small D-ring) and inspect the diaphragm for holes or hardness (it needs to seal and be flexible) - if needed, a replacement diaphragm part number 16021-HM3-L00 can be gotten on Ebay or Amazon (about $53) or from a local dealer for about $65

Good luck
 

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Discussion Starter · #12 ·
I sprayed that stuff everywhere, in every spot I could find. I actually took the pump apart, and didn't expect it to fly apart when I did. I didn't see anything other than the cap, spring, and diaphragm. The rubber on the diaphragm was flexible and it pumped carb cleaner when pressed. I will recheck. I don't see the D ring you mention on the exploded part diagram in my first post here, where does that go/fit into this? Or is that attached to the diaphragm? If so it may not have fallen off.

I will check for vacuum tonight on that air cut valve. That sounds like a solid possibility and just my luck! Thanks again for the excellent advice!
 

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On some models the D-ring was (is) part of the diaphragm - from the picture in the service manual, the D-rings (2 of them) are part of the diaphgram

With the carb upside down, put the diaphragm into the carb body, set the spring on top, then put the cover on - then test with gas, lacquer thinner, kerosene or diesel fuel - anything other than water or carb cleaner
 

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Discussion Starter · #14 ·
Took it apart again, replaced the 38 slow jet with one from my rebuild kit. Sprayed cleaner into the slow jet side. Took out the needle and cleaned the seat, and cleaned the inside of the main jet again. Checked that pump - it pumped gas but had to be pressed many times before it primed and started pumping. The diaphragm is still in decent shape and looks like it would seal. I did your test on the air cutout valve as well and it held vacuum. I couldn't pull air through it.

Put it back together and fired it up. Same issues. Won't idle without the choke on. It idles beyond well with the choke on and revs and sounds perfect. Flip the choke off and it will run ok for about 5 seconds and then start struggling. Within 10 seconds it'll die for sure. Just seems like maybe the bowl doesn't fill with fuel quick enough or something to keep up.

Is there anything else I should be checking?
 
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