yz400f idle speed problem

10 replies to this topic
  • poisoner

Posted September 08, 2009 - 01:03 PM


I already read some topics about that but I still cannot fix my problem.

The problem is:
- no idle speed when engine is hot
- engine stops sometimes when opening the throttle 0 - 1/4 and more slow or fast

What I did:
- cleaned the carburetor and all the jets
- new iridium spark

I moved the position of jet needle from 6 to 4 (standard).
And tried to adjustment pilot screw, I turn in pilot screw until it is lightly seated, and then trying to turn out 1/2 turn and start the engine. Until 3 turns the engine has no idle. I read it should be max 2 turns out, if it is more I need to replace the pilot jet with one size bigger.

When with 3+ turns out of pilot screw the engine runs on high rpms then I need rich fuel, right?

Is it possible without pilot jet replacing, but with jet needle position change to make rich fuel? Maybe that because it was on position 6 from previous owner.

  • grayracer513

Posted September 08, 2009 - 01:19 PM


The jet needle does not affect idle jetting. By the way you describe the bike's behavior, you need a richer pilot jet (larger, probably by one size).

The stumble when opening may be cured by that, but if not, then look at the accelerator pump.

  • poisoner

Posted September 08, 2009 - 11:38 PM


I forgot, also cleaned the accelerator pump before all other, because it was not working, was blocked from dirt in the up side. Now the AP also work, but the bike not working better.

I checked my valve clearance
intake 0.10 - a little tight maybe
exhaust 0.20

Is the problem can be from the valve clearance or from TPS?

  • grayracer513

Posted September 09, 2009 - 07:12 AM


The valve clearance is fine.

  • poisoner

Posted September 09, 2009 - 11:28 AM


With 4 turns of the pilot screw the bike runs fine.

I will go for larger pilot jet, when I see what size is already installed.

How can I check is everything with TPS is OK?

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  • poisoner

Posted September 20, 2009 - 08:57 AM


I got hanging idle, when open the throttle and then close it the rpms getting high, i think that is another sign I need larger pilot jet.

Thats are my current jets
Main Jet 190
Pilot jet 45
Starter Jet 65
Pilot air jet 200
main air jet 100


I am watching this but is there something wrong 20 is like starter jet but standard is jet(#45) STD, mine is 65.
21 is like pilot jet, but standard is JET(#65) STD, and mine is 45.
Something is wrong, I wrong or the scheme?

  • grayracer513

Posted September 20, 2009 - 09:18 AM


That illustration has the reference numbers for the starter and pilot jets reversed. The jets listed in the parts list as item 20 (3TJ-14948-**-**) are the pilot jets.

  • poisoner

Posted September 20, 2009 - 12:12 PM


yes grayracer513 they are reversed.

hmm I need to find some other store because I need to pay $38 min only for shipping of one little pilot jet.
Somebody to know some UK, or Europe online store?

  • poisoner

Posted October 10, 2009 - 10:08 AM


grayracer513 thanks for your help in the thread.

I installed larger pilot jet 48 (the old is 45), and there is the same problem, when the engine is warm it doesn't idle and engine stops. Pilot screw 1.5, 2, 2.5, 3

Then I removed the TPS and it looks like the engine idles better, and not stops when rpms are not so low. Pilot screw at 1.5

I think my problem is TPS. How I can measure the TPS, I mean on started engine or not?
Can I ride without TPS? (i know it will have low power, I didn't try to ride it without TPS)

  • grayracer513

Posted October 10, 2009 - 10:44 AM


The TPS does not affect the full throttle power output. It's intended to advance the timing at part throttle for improved fuel efficiency while cruising and to improve part throttle "crispness". If you think it's at fault, disconnect it.

Testing procedures are in the manual.

  • poisoner

Posted November 01, 2009 - 03:00 AM


I made a test ride in the mountain,
started from Min Elevation: 238 m (780 ft) up to Max Elevation: 1639 m (5377 ft). In the beginning the bike worked very good, no problems with idle speed, and riding without any problems. But at the high elevation it started to make problems and on idle engine stops, I kick to start it, and it stops after a second.
Why it works good good down and not works up at the mountain?
Maybe I need to set pilot screw in the mountain and not in my garage.

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