2005 YZ450F Idling Problems

Hey guys, I have been searching through the forums for my problem and have found a few individuals with similar cases. I tried to remedy the problem with some of the recommended fixes but have come up with nothing. The issue is as follows:

Bike will start up with choke pulled out and idles quite nicely. If you push the choke in at any time it immediately dies. If you try to apply throttle while pushing in the choke it bogs heavily, sputters, and eventually quits with an occasional backfire as well. I bought the bike from an individual who obviously knew of the problem and just wanted to get rid of it. Originally I believed it to be carb related as well as most would, I believe. I have taken the carburetor off and thoroughly cleaned all passageways with special attention to the pilot jet circuit (from what information I received from other members posts). I have replaced all jets back to new O.E. jets. I replaced my pilot screw with an aftermarket toolless version. Tonight I checked the TPS and CDI Magneto resistances and voltages while running and those checked out within limits. I have inspected the valve clearances and those were all within limits. The right exhaust was right at the lower limit (very tight). While the engine is running at idle with choke on it doesn't make any abnormal sounds (that I am aware of). Does the accelerator pump work while choke is on and off or should I be worried about the diaphragm? I just can't believe it is anything other than the carb because the problem does not exist when choke is on. I am about to drain the brand new oil I put in and start taking this engine apart because I am completely baffled. I hate not knowing what kind of terrible maintenance has been done on this bike and would appreciate any help I can get with this! Thanks much guys!

It sounds like you know what you're doing but one thing to check is on the slide there is the piece that is removable and the way it looks like it should go is actually upside down and the way it looks like it should not go it does. I took mine all apart and went off memory and made this mistake. I think it would idle with choke in this condition but would die fast and back fire once in a while without choke. Also it was running really lean and getting hot fast.

FYI my buddy is having the same problem with a wr 400 and the carb and engine is perfect. We both checked his carb and like you he gave up on it being the carb or electrical(he should have investigated further before next step) and he pulled the motor down and all looks good there to. I checked his stator and I got bad values for it and now were waiting for him to get a new stator and try again. Again though you said you checked electrical components so that leaves me stumped but if I were you I would get another set of eyes on it before pulling it down.

It sounds like the carb, all right. Something in the pilot circuit, or an air leak/lean condition caused by something outside the carb, like possibly tight intake valves.

The accelerator pump operates regardless of the choke status.

check the hotstart button .

Edited by etuke

Cable, not button, but yes, that could be the problem as well. If the hot start isn't closing, or the seal on the plunger is bad, that could do it.

O also on my buddies bike we found the intake boot (carb to head) was cracked and had a small air leak. did not fix his problem

Thanks for the quick responses guys. I plan on checking these out tomorrow in the easiest order to the hardest. I remember the seal on the hot start cable seemed a little strange. As in if it is suppose to be a regular o-ring it seemed to be old and definitely not rounded like an o-ring. Is this seal suppose to be a special kind that looks different than a typical o-ring?

The hot start seal is a lip seal, not an O-ring. Lip faces up in the carb.

time to take apart the carbs and clean the jets and everything else inside. also fresh plug, fresh gas, and new air filter.

My 2006 YZ450F sounds like it is having the same problem, I Tore down the carb clean all passages, put it all back together, the bike back fired once on the first kick, I thought we had "GREEN LIGHTS!" pulled the choke, bike fired up idled great, after giving it few seconds with choke on it died, tired it again, and the same thing happened..... We brought it back in to work on the idle screw, nothing changed! With some throttle, the bike did idle, but back fired(popped) more then usual after the bike was "warmed up." Now we're looking at the pilot jet and/or taring the carb back down for a thorough inspection!!

-pilot jet misadjusted

-Hot Start Seal

-Change spark plug, clean/fresh air filter,..

any thing else?? Thanks

Edited by jeeperHolly

My 2006 YZ450F sounds like it is having the same problem, I Tore down the carb clean all passages, put it all back together, the bike back fired once on the first kick, I thought we had "GREEN LIGHTS!" pulled the choke, bike fired up idled great, after giving it few seconds with choke on it died, tired it again, and the same thing happened..... We brought it back in to work on the idle screw, nothing changed! With some throttle, the bike did idle, but back fired(popped) more then usual after the bike was "warmed up." Now we're looking at the pilot jet and/or taring the carb back down for a thorough inspection!!

-pilot jet misadjusted

-Hot Start Seal

-Change spark plug, clean/fresh air filter,..

any thing else?? Thanks

If you cleaned the carb passages but did not clean the pilot jet, you need to take it apart again. Cleaning the pilot will most likely cure the problem.

Well, I broke down and decided to rip off the carburetor again for another inspection and cleaning. This time I sat down with all the parts and had the manual open and followed each step as if my life depended on it. I 'believe' it is in perfect mechanical functioning order. All jet passageways have been cleaned again with carb cleaner and compressed air. All jets were perfectly clean as well. The hot start plunger seal didn't seem like it protruded out which would seem like it could possibly cause a leak. Though I have never seen one brand new so I was hoping somebody here might have a picture of a new hot start cable? It was also starting to crack a little bit on the plastic threads for the piece that mates to the carburetor. So my next question is: Is there a way to get the o-ring/seal for the plunger without buying an entirely new cable?

If I have to I will purchase the cable but I would rather only replace parts that are truly needing it. I am going to have to check for air leaks tonight around the carburetor so that I can slowly weed out potential problems. I am almost to the point of dropping it off at the shop and paying to have this diagnosed. I definitely wanted to do a leak down test so I think I am going to get that done at the very least. Thanks for all input thus far guys and I appreciate it greatly.

Well, I broke down and decided to rip off the carburetor again for another inspection and cleaning. This time I sat down with all the parts and had the manual open and followed each step as if my life depended on it. I 'believe' it is in perfect mechanical functioning order. All jet passageways have been cleaned again with carb cleaner and compressed air. All jets were perfectly clean as well. The hot start plunger seal didn't seem like it protruded out which would seem like it could possibly cause a leak. Though I have never seen one brand new so I was hoping somebody here might have a picture of a new hot start cable? It was also starting to crack a little bit on the plastic threads for the piece that mates to the carburetor. So my next question is: Is there a way to get the o-ring/seal for the plunger without buying an entirely new cable?

If I have to I will purchase the cable but I would rather only replace parts that are truly needing it. I am going to have to check for air leaks tonight around the carburetor so that I can slowly weed out potential problems. I am almost to the point of dropping it off at the shop and paying to have this diagnosed. I definitely wanted to do a leak down test so I think I am going to get that done at the very least. Thanks for all input thus far guys and I appreciate it greatly.

Yes you can just buy the seal I just did about 2 weeks ago. If the threads are cracked I would change out the cable as well, not sure if it will really cause a problem but it would suck to break out on the trail.

Well, I broke down and decided to rip off the carburetor again for another inspection and cleaning. This time I sat down with all the parts and had the manual open and followed each step as if my life depended on it. I 'believe' it is in perfect mechanical functioning order. All jet passageways have been cleaned again with carb cleaner and compressed air. All jets were perfectly clean as well. The hot start plunger seal didn't seem like it protruded out which would seem like it could possibly cause a leak. Though I have never seen one brand new so I was hoping somebody here might have a picture of a new hot start cable? It was also starting to crack a little bit on the plastic threads for the piece that mates to the carburetor. So my next question is: Is there a way to get the o-ring/seal for the plunger without buying an entirely new cable?

If I have to I will purchase the cable but I would rather only replace parts that are truly needing it. I am going to have to check for air leaks tonight around the carburetor so that I can slowly weed out potential problems. I am almost to the point of dropping it off at the shop and paying to have this diagnosed. I definitely wanted to do a leak down test so I think I am going to get that done at the very least. Thanks for all input thus far guys and I appreciate it greatly.

my hot start broke too....the plastic threads just stripped right off. So I bought an aluminum one. It works way better and seals nice and tight.

Just checking but how old is that gas ? It doesn't take too long before it can cause problems. Also, did you check the plug ?

Hey guys, I ordered the hot starter cable from Motion Pro only to realize upon arrival that it does not come with the Hot Starter Plunger. Does anybody know where I can order the plunger itself? I appreciate it if anyone knows!

A Yamaha dealer.

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