1998 yz400f hotcams and yz450f carb hard to start now.


7 replies to this topic
  • namaher1

Posted March 08, 2015 - 01:27 PM

#1

1998 yz400f. I just got through putting both intake and exhaust hot cams in it, I rebuilt and installed a 2007 yz450f carb on it also. It used to start first or second kick. Now it takes 20 or more but it runs better then ever. What's your guys starting technique with these auto decompression cams?

  • G-BRO

Posted March 08, 2015 - 05:47 PM

#2

1998 yz400f. I just got through putting both intake and exhaust hot cams in it, I rebuilt and installed a 2007 yz450f carb on it also. It used to start first or second kick. Now it takes 20 or more but it runs better then ever. What's your guys starting technique with these auto decompression cams?


It should kick over normally with the auto cam. Just get a full stroke and it should fire up. Does it start 1-2 kick if you do the old "drill "?

  • namaher1

Posted March 08, 2015 - 06:14 PM

#3

Thanks for the reply. Well with trying to start it by the old "drill" it seems to want to start better ( I get more pops of it wanting to fire) but still takes a lot of kicks. It seems the best way I have found is to blip the throttle 2 times, find the compression stroke, then I have to hold the throttle off idle and kick to get it running. Even with doing this it still takes a lot of kicks. I've also found that I had to slow my kick down a little bit. (I'm still use to giving her all I got!) And when hot it definitely needs the hot start pulled and throttle off idle.I guess It still feels like you need to find "that sweet spot" and do everything right for it to run which should not be correct with the AD cam in it. I put the carb jetting all back to stock. Pilot screw 2 turns. Runs great with normal light decel pop. Are these 07 carbs touchy at start up? It almost seems to me like it isn't getting enough fuel at start up but I'm not too sure yet.

  • grayracer513

Posted March 08, 2015 - 06:45 PM

#4

First, if the carb sat for some time before the rebuild, you might have a blocked pilot jet.  Read:

 

http://www.thumperta...o/#entry6879695

 

Regarding using the "old drill" with an Auto Decompression cam won't work very well, really.  With the old cam and the old drill, the piston came up against compression (the "hard spot") at about 120 degrees before TDC.  Then you used the compression release to move the kick starter about an inch.  When you did that right, it moved the piston up to more like 20-40 degrees before TDC.   From there, you can pop it through the rest of the compression stroke and have it start.  What the AD cam does is hold one exhaust valve off its seat until the engine gets to around 20 degrees BTDC already, from which point you should just reset the lever and kick.  If you move another 80 degrees past that by employing "the old drill", you'll be more than a full turn and a half of the crank away from the next compression stroke.



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  • namaher1

Posted March 08, 2015 - 07:02 PM

#5

Nice to hear your input on this Gray. I have to say that this forum has helped me tremendously with this old, but trusty, 400. And I always learn a lot from your posts. About 3 years ago I did a complete rebuild and now a little more improvements. The carb is all cleaned up with all new stock size jets, needle, seals and auxiliary pump diaphragm. I don't believe the pilot circuit is clogged because I was getting good flow with carb cleaner through it.

Regarding the old school drill. You are absolutely correct on this Gray. I tried starting it by doing the "drill" exactly(compression, 20 degrees before top dead) and it didn't seem to like it. So then I found if I find the bottom of compression stroke (hard spot) and kick it from there it seems to want to start easier but still requires a lot of kicks. And also of course just kicking regularly she doesn't like either.

Edited by namaher1, March 08, 2015 - 07:26 PM.


  • import silvia

Posted March 09, 2015 - 07:35 AM

#6

No input on the cam swap, but I put an '07 carb on my '99 and it starts first kick every time.


Unless the fuel is shut off as I frantically try to figure out what the hell is going on cause it was running perfect just a minute ago, then it takes more kicks.



  • namaher1

Posted March 11, 2015 - 04:19 PM

#7

So with a little more experimenting, I found that it starts 1st/2nd kick if I turn the idle up about 3/4 turn and kick a full kick from the hard spot in the stroke, which I believe is about 20 or 40 degrees BTDC. I'm wondering now if something is in fact going on with the pilot circuit?
It just crossed my mind that if I have to turn the idle up which allows more air flow, maybe my air filter is restricted.:)oops::(. Haha it's only had a couple rides on the last cleaning but maybe I will get lucky and try cleaning it. I will post back the results.

Edited by namaher1, March 11, 2015 - 04:32 PM.


  • namaher1

Posted April 19, 2015 - 10:54 AM

#8

Pulled the carb and verified pilot circuit was not clogged, everything looked good. Put a 48 pilot in it. Fuel screw 1 1/4. Starts cold second kick with a good prime of about 5 throttle twists. Starts first kick hot with no throttle and hot start pulled. Runs better then ever. Now some suspension work!





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