what happened? carb issues for yz400f.

8 replies to this topic
  • CRF100biker

Posted February 03, 2012 - 02:59 PM


so i took apart my carb to clean it because at the end of the season the bike was running a bit rough and wasn't as responsive as it used to be. i took it apart according to the factory manual, cleaned it, and made sure i put it back together correctly. set everything according to the manual and i'm getting nothing. i can get it to sputter over a few times with the hot start knob pulled out. what am i doing wrong here? about all i can think of is a bad spark plug or bad gas. the valves are spot on and the bike ran without a flaw before this. always started on the 4th kick with the choke turned on when it was stone cold and would always start on the first kick without even needing the hot start once warmed up no matter what unless if it was dumped and was sitting on its side for a few minutes, then it took 2.

Edited by CRF100biker, February 03, 2012 - 03:05 PM.

  • grayracer513

Posted February 03, 2012 - 03:11 PM


So is your bike a 1956 Velocette Thruxton, or a Moto Guzzi 500 twin?

  • briangriff321

Posted February 03, 2012 - 04:44 PM


I think its a 1930 excelsior 750

  • RasmusDK

Posted February 03, 2012 - 06:04 PM


what happened? carb issues for yz400f

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  • grayracer513

Posted February 03, 2012 - 07:15 PM


When you reassembled it, how did you set your idle speed and pilot screw?

  • CRF100biker

Posted March 11, 2012 - 06:37 AM


When you reassembled it, how did you set your idle speed and pilot screw?

pilot screw is set to factory spec. i'm not sure how to set the idle speed and the manual doesn't tell me how either, except for when it's running, which doesn't help me since my bike doesn't run. the bike will cough as if it wants to fire, but never will fire. earlier i had a leak from the float bowl, so i decided to replace all the gaskets and o-rings in the carb. it still leaks out of the float bowl and it looks like some is coming out of the accel pump cover too. it's definitely the gaskets causing the leak as EVERYTHING else in the carb checks out. there's no reason (that i know of anyways) for this thing to be malfunctioning. i have no idea what i am doing wrong, i am simply doing what the factory manual tells me to do. is there a glue or bonding agent i am supposed to use for the gaskets/o-rings?

i'd check my spark, only i don't have the tool to do it with this bike. anyone know what plug tool will fit in this bike?

Edited by CRF100biker, March 11, 2012 - 06:41 AM.

  • grayracer513

Posted March 11, 2012 - 07:58 AM


Never use any sealer of any kind on any carb seal. Fuel leaking from the pump cover or around the actuating rod indicates a perforated pump diaphragm.

The trick to setting the idle back to it's original setting, or close to it, when you pull the carb apart is to find the largest drill bit that will fit under the slide without lifting it before you take it apart. Then on reassembly you put that bit under the slide and turn the idle speed screw tighter until the bit drops free.

If it will start when you open the throttle, can you keep it running by holding the throttle off idle? If so just turn the screw in until it will support itself.

The two other things that occur to me are that you may have no gotten all the dried fuel out of the pilot jet or that you might have the vacuum release plate on the engine side of the slide installed upside down.

In This Illustration, did you replace the 3 seals at the fuel inlet, shown as #63? (PN 3TJ-14564-40-00)

  • CRF100biker

Posted March 11, 2012 - 02:20 PM


i'll try replacing the pump diaphraghm.

i'm not sure about holding the throttle to get it to run. it doesn't really run, just kinda coughs like it wants to for a second and then it stops. the bike never needed to have the throttle touched for it to start anyways.

all the jets are completely clear. if i remember correctly the plate was only able to go one way when i installed it.

i did not replace those seals, but then again i didn't take that apart. nor is it leaking any fuel from there.

Edited by CRF100biker, March 11, 2012 - 02:21 PM.

  • grayracer513

Posted March 11, 2012 - 05:05 PM


There are two ways for the VR plate to fit on the slide, and only one is right. The "square" edge with the hole in it goes down.

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