Just cleaned carb, can't get it to start


14 replies to this topic
  • BASSic

Posted May 21, 2008 - 11:02 AM

#1

I recently took the carb off my 01 YZ426 to clean it. I reinstalled it and now I can't get it to start. Normally when the bike is cold, I'll pull the choke, give it a twist of the throttle, and it fires up in one kick.

While cleaning the carb, I replaced the fuel screw with an adjustable one. I'm almost certain that's the source of my problems; the only issue I'm having is determining if I need to richen or lean the idle mixture.

I've tried with the choke, with the hot start knob, and I started with the fuel screw at 2.5 turns out and have moved it in at 1/4 turn increments and still no luck.

I'm about to try some ether just to get it running and warmed up so I can set the fuel screw properly.

  • DrThumper

Posted May 21, 2008 - 11:56 AM

#2

A common mistake or overlook when people clean there carbs is re-setting slide or idle adjustment. What happens is that when they are finished the slide is all-the-way down due to removing the idle adjust screw, but for the bike to start it must be slightly opened, hence the idle adjustment.

Also, you mentioned that you have to give your bike a slight twist of the throttle for it to start. I believe your carb wasn't 'right' to start with. You shouldn't even touch the throttle during the starting ritual on a thumper, that's an old 2-stroke protocol.

  • angrybob

Posted May 21, 2008 - 12:03 PM

#3

Also, you mentioned that you have to give your bike a slight twist of the throttle for it to start. I believe your carb wasn't 'right' to start with. You shouldn't even touch the throttle during the starting ritual on a thumper, that's an old 2-stroke protocol.


Exactly what I was thinking.

After its sat for a while, try it with no throttle input. Put your right hand on the m/c reservoir (ask me how I know this :thumbsup: ) to ensure you did not touch the throttle.

If its not starting with no fuel, open up the fuel screw more. If you're too lean, you may even have to hold the throttle 1/4 open while kicking. If so, you may even need to up the idle jet.

  • BASSic

Posted May 21, 2008 - 12:04 PM

#4

Also, you mentioned that you have to give your bike a slight twist of the throttle for it to start. I believe your carb wasn't 'right' to start with. You shouldn't even touch the throttle during the starting ritual on a thumper, that's an old 2-stroke protocol.


As you probably know, the Keihin FCR has an accelerator pump. Everyone I know who has this carb has to twist the throttle prior to kicking to shoot a nice stream of fuel inside the engine when starting it cold.

I pulled the filter and sprayed some ether into the boot, and I still couldn't get it to start. After some trial and error I finally got it to start: my idle screw was set too low and I think the residue from the carb cleaner had to burn off. After turning the idle screw in, the bike started but was running a bit rough until all the junk burned off.

It's running just like before, actually better than before now that I have the fuel screw set properly and the accelerator pump pushrod cleaned up (it was sticking before, moving too slow and causing the bike to bog when the throttle was abruptly opened, even with the O-ring mod)

  • matt4x4

Posted May 22, 2008 - 07:12 AM

#5

I find if my bike sat for a week or more, a small throttle twist will shoot a little fuel from the AP into the intake and it fires first kick, if it sat a day or two, it does not seem to need the throttle twist.
What likely happens is that the pilot circuit drains/dries down requiring an extra kick or two to provide adequate fuel for starting if it sits more than a few days, the AP squirt makes up for that.
Glad you found your problem.

  • grayracer513

Posted May 22, 2008 - 08:33 AM

#6

Also, you mentioned that you have to give your bike a slight twist of the throttle for it to start. I believe your carb wasn't 'right' to start with. You shouldn't even touch the throttle during the starting ritual on a thumper, that's an old 2-stroke protocol.

Twisting the throttle to prime the engine for a cold start is the prescribed procedure, and always has been.

http://www.yamaha-mo...troke_vid_a.mpg

  • Bigcr

Posted May 22, 2008 - 08:58 AM

#7

When I cleaned the carb on my '03CRF450 it wouldn't start afterwards either. I took the carb back apart and found that I had installed the plate that attaches to the slide upside down. You may want to look in that direction.

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  • grayracer513

Posted May 22, 2008 - 09:45 AM

#8

When I cleaned the carb on my '03CRF450 it wouldn't start afterwards either. I took the carb back apart and found that I had installed the plate that attaches to the slide upside down. You may want to look in that direction.

That's a very common mistake, made more common by the fact that it looks more like it should be upside down than right side up.

  • BASSic

Posted May 22, 2008 - 11:33 AM

#9

I posted a follow-up reply yesterday and indicated I got it running fine.

My idle screw was set too low. After turning it in, the bike started but was running a bit rough for about 30 seconds until all the carb cleaner residue burned off.

  • FreshDonutsCAN

Posted May 22, 2008 - 01:31 PM

#10

Twisting the throttle to prime the engine for a cold start is the prescribed procedure, and always has been.


True True :thumbsup:

  • BASSic

Posted May 23, 2008 - 10:28 AM

#11

I can't get it to start again, and my right leg feels like it's going to fall off.

Is that twist to get a squirt of fuel from the AP necessary in warm (70+) weather? I have a feeling that's where I went wrong - too much fuel and my plug may be wet.

  • matt4x4

Posted May 23, 2008 - 11:18 AM

#12

Like I said in my earlier post, I don't twist unless it sat for a week or more, I find it unnecessary if I recently ran the bike.
Did you follow the proper procedure for setting your fuel screw after you got your air screw set up? Try turning your air screw in another half turn see if it fires or try using your choke (normally not needed at 70), if that's the case, your pilot mixture may still be a little off. There's some good threads that explain how to do it right on TT.

  • BASSic

Posted May 23, 2008 - 01:54 PM

#13

Where's the air screw?


I'm only aware of a fuel mixture screw (on the bottom of the carb) and an idle speed (throttle stop) adjustment screw.

  • grayracer513

Posted May 23, 2008 - 02:06 PM

#14

There is no air screw in the same sense as in a two-stroke carb. What he is probably referring to is the idle speed screw, which holds the slide partly open, and in that sense allows or restricts incoming air.

You can fiddle with raising or lowering the idle speed setting in fine tuning you carb to start better, but you can accomplish the same thing with the throttle IF you can hold it at one steady setting as you crank the bike, and not twist as you kick.

Except for the initial squirt of the fuel pump, opening the throttle at starting speeds reduces the amount of fuel delivered by reducing the vacuum at the pilot circuit discharge port. If you hold the throttle open about 1/4 to 1/3 on an engine that has refused to start up to that point, and the engine responds with one or two weak "blups" out the exhaust, it may have been flooded, and it may now start with the throttle back closer to closed.

Also, spark plugs are a premium item in a 426, and you may want to try a brand new one, if you haven't already.

  • BASSic

Posted May 24, 2008 - 03:14 PM

#15

Prior to cleaning the carb, my 426 started in one kick. I think my ritual of one twist to get fuel from the AP worked fine before because the AP pushrod was not moving freely. After cleaning my carb, the AP pushrod now moves freely, resulting in a nice jet of fuel being squirted (much more than before). This massive amount of fuel flooded my bike.

I pulled the plug (which only has a few hours of runtime on it) and lo and behold it was saturated with fuel and also black.

I had a little issue with the plug which may have been defective (I'll post another thread on that subject once I can get some pictures up), but after putting in the old plug which was clean and dry it started in two kicks, choke out and NO throttle twisting.





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