450 has me at the end of my witts


72 replies to this topic
  • Diode663

Posted November 12, 2005 - 03:56 PM

#1

Situation:
I bought a 450 used and it ran fine for a 2 days of riding. Then I take the bike to upstate NY and play around the roads for about two hours. Everything is fine. Then the next day I play around for another hour and then its time to go on some serious riding. Top the bike off with 94 (the previous owner used half U4 half 93). I take off down the road and the bikes running good and then starts to lose power suddenly, then the back wheel locks up. I kicked it for about an hour total that day with only backfirering.
Upstate was about 25 degrees difference colder and within 500 feet of my home altitude.

Solutions tried in order:
Changed spark plug -no effect
Checked spark -plenty
Tried different gas -no effect
Pulled flywheel - keyway intact
Checked Timing - Correct
Checked all electrical with multimeter -Everything fine but coil (100 ohms out)
Replaced CDI with revbox (manuals last option) - We got it to run but only for 7 seconds and then not again
Cleaned Carb thouroughly - No effect but 178 Main (seems large? led me to belive I should try straight U4.)
Got raped on U4 -got the bike to run twice once bike idled low with choke on fuel screw 2 out then stalled on its own. Then got bike to run with gas off fuel screw seated no choke and holding the hotstart. Got a nice solid idle, then touched the gas and the bikes rpms shot up and stayed there so fearing leanness I prematurely ejaculated and hit the killswitch instead of working it out.

Bike has not started since under any of the same conditions and just lets out some nice deafening backfires complete with fireball. The motor still has serious compression ( Between me and my friend who is well versed in motors we kicked the thing over 3000 times and it never got any easier).
The coil measured 100ohms over the specified 100 ohms does anyone belive that could cause this kind of problem? I personaly do not. I forgot to note the other jets in the carb but 178 seems kind of rich even for oxygenated fuel. Im at around 1200 feet in my home town where the bike ran perfectly.
All I can think is and Im exausted and have run out of curse words and ideas. My friend who helps me keeps telling me I should have gotten a Honda and I really dont want him to be right. Im about to light the thing on fire and ghost ride it off a cliff.

Thanks for the help. :applause: :ride:

  • ethan

Posted November 12, 2005 - 04:28 PM

#2

Sounds like you checked everything but the valves. I would go there next for sure. If those check out ok, your cdi could be toast. What is a rev box? Does that replace the cdi? Even though the ohms check out and the spark is good, the box could be on the way out. If you have a friend or know someone with the same year you could try their cdi and see if that is the problem. I hope you figure it out, it sounds very frustrating, but you'll get it.

  • 642MX

Posted November 12, 2005 - 04:30 PM

#3

You need to find out what pilot jet is in there. It should be a 42. I would put a 160 main jet in it and turn the fuel screw 1 turn out. Check the needle, make sure it is at the 3rd or 4th clip position. If you have plenty of spark, I think the coil should be okay. Forget the U4 just run pump gas for now. Sounds like its too rich, four strokes can run very lean without damaging anything, don't be afraid to play with the fuel screw once it lights up.

  • Chazlom

Posted November 12, 2005 - 04:34 PM

#4

Check for exhaust leak, I went threw the same thing a few weeks back swearing it was some valve or timing issue, come to find out it was the muffler to header clamp was loose, once tightend it started and ran great.

Hope that's all it is.

Chaz

  • kizzle426

Posted November 12, 2005 - 05:24 PM

#5

Could it just simply be the piston losing compression? Or is that an extremely rare occurance on these bikes?

  • Diode663

Posted November 12, 2005 - 05:35 PM

#6

Ethan- I was thinking bent valve but there is still load of compression even with a slightly bent valve the bike should still run. Yes I thougth the cdi was toast so I got an aftermarket CDI "revbox".

Chazlom- header bolts tight. But I have a Yoshimirua and it just slips onto the header and is held with springs.. thats pretty solid.

642mx- I will take the carb apart tomarrow, I really didnt wanna run U4 It just seemed like it would be the solution judging on other peoples jet sizes and the fact that thats what it ran fine on before. Should I pick up a Zip-ty or JD jet kit and give that a whirl? And we got the thing running the second time with the hotstart held in and then turned the fuel screw in from two turns out.. then I could let go of the hot start and it ran rock solid til I killed it. I just cant fathom what could have changed that makes the old jetting that worked amazing not work at all in the same weather conditions with the same fuel. Surely it has to be something God himself did because it doesnt add up.

Thanks a bunch for your replies I appretiate it... if there is any more input or speculation maybee one of you will come up with something we havent thought of.

  • Diode663

Posted November 12, 2005 - 05:40 PM

#7

Kizzle im not sure what you mean? The bike has good compresion, when it shut off it locked the rear wheel while I was going 70. Im gonna have to get some numbers for you guys tomarrow. And I really dont know too much about these bikes other then what Ive learned in my crash course of 4 strokes fixing this thing. Ive always rode a 2 stroke. Hell I totaly forgot about the accellerator pump and shot myself in the face with it. So any information will probably be news to me.

  • DaveJ

Posted November 12, 2005 - 06:08 PM

#8

If this bike was in front of me here's what I would do.

1. Replace the spark plug with a new one. Don't use anything other than a NGK or ND. BTW, a spark on the outside does not mean a spark on the inside.

2. Pull the carb and check for water and any contamination.

3. Check the float level.

4. Remove the petcock and clean the filter screen. If the screen is dirty, flush the tank.

5. Connect carb to line, fill and verify accelerator pump delivery and a full carb bowl.

6. Remove the valve cover and verify valve timing.

7. Remove the left case and verify flywheel position against marks and to piston position.

8. Verify CDI and coil including connections per manual.


With that said, I can't count the number of times someone has had similar mystery problems, only to find that when the bike was brought to me that although they checked something or everything, they didn't check it right or made some assumption along the way. You have to be precise and know without a doubt that each of these steps has been verified.

Now, every once in a while I come across a bike that seems to check out but is still possessed with some sort of a weird problem. In these cases, I usually put on a new or known carb and test. Then if necessary, do the same with the CDI module and coil.

Sometimes things check out but can still fail under load or when put into operation. It can happen on Yamahas, Hondas and just about every bike ever made.

It just sucks, but you'll get through it.

  • kizzle426

Posted November 12, 2005 - 06:44 PM

#9

Kizzle im not sure what you mean? The bike has good compresion, when it shut off it locked the rear wheel while I was going 70. Im gonna have to get some numbers for you guys tomarrow. And I really dont know too much about these bikes other then what Ive learned in my crash course of 4 strokes fixing this thing. Ive always rode a 2 stroke. Hell I totaly forgot about the accellerator pump and shot myself in the face with it. So any information will probably be news to me.

My bad.....I forgot about where you said it still had good cmopression. I was just wondering.

  • WR450F_RDR

Posted November 12, 2005 - 07:07 PM

#10

sounds to like something let loose in the flywheel area in relationship to the generator, your getting spark but either is is mistimed or not all time ,hense the major backfire. Lots of fuel, but no spark or intermitent spark. Having it lock up while going down the road sounds like something broke. I was going to say woodruff key but guess you already checked that.

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  • yzf450jack

Posted November 12, 2005 - 09:13 PM

#11

You were saying that there was U4 in it. That stuff is temp. sensitive it boils at like 90'f and turns into green jelly under 30'f. Take the float bowl off and see if your jets got plug up

  • KX250FER

Posted November 12, 2005 - 11:58 PM

#12

Ethan- I was thinking bent valve but there is still load of compression even with a slightly bent valve the bike should still run. Yes I thougth the cdi was toast so I got an aftermarket CDI "revbox".

Chazlom- header bolts tight. But I have a Yoshimirua and it just slips onto the header and is held with springs.. thats pretty solid.

642mx- I will take the carb apart tomarrow, I really didnt wanna run U4 It just seemed like it would be the solution judging on other peoples jet sizes and the fact that thats what it ran fine on before. Should I pick up a Zip-ty or JD jet kit and give that a whirl? And we got the thing running the second time with the hotstart held in and then turned the fuel screw in from two turns out.. then I could let go of the hot start and it ran rock solid til I killed it. I just cant fathom what could have changed that makes the old jetting that worked amazing not work at all in the same weather conditions with the same fuel. Surely it has to be something God himself did because it doesnt add up.

Thanks a bunch for your replies I appretiate it... if there is any more input or speculation maybee one of you will come up with something we havent thought of.



I had this strange problem with a KX250 and the flywheel was out of spec and and kept rubbing against the stator. It got noticable when hot and it seized the engine up. when it was cold it ran like shit if at all...it got worse and worse until it would not fire or run at all then I took the cover off and there were shaving everywhere. When the componets got hot they locked themselves up. Very strange...it was about 6-7 years ago im just trying to describe it the best I could remember. I hate when dumb things like this happen. This was not like a long process the stator was out in like 2 days of testing. :applause:

  • Diode663

Posted November 13, 2005 - 01:43 AM

#13

DaveJ- As per my first thread I have checked everything you stated accept for the float level. The bike has a new cdi as per the manual. The coil was the only thing out of spec but by 100ohms witch I didnt think would be too much of a resistance. Im trying desprately to avoid playing pick a part and wasting money on a part that I do not need like I have already with a new cdi and the U4.

I dont think anything in the case broke..I just think the compression combined with my light wieght on a severly hard dirt road locked the tire. As for the flywheel I did check it and didnt find anything to be out of place. I didnt see any adjustment for timing so I doubt the stator plate got shifted.

450jack- there wasnt all that much U4 in the bike in upstate and in my garage its about 60 degrees and I also took the carb apart and cleaned the hell out of it.

kx250fer- That sounds intresting but there was no damage to the stator and all the ohms are dead on in spec. I cant even get it to start reliably and there is no pattern to its starting so im not sure if its electrical or not.

Thanks for all the replies guys, if you got anymore suggestions keep em comming.
If I could get someone who lives near me to come over and let me use thier coil for about 10 minutes that would eliminate one more possibility.

  • TD-3

Posted November 13, 2005 - 04:27 AM

#14

Just 2 weeks ago, I had a similar thing happen to my newly acquired 03 yz450.

I was on my 3rd ride, everything is fine, and suddenly the bike dies, locking the rear wheel. I'm stuck in a sandpit (forgot my cell phone) and really don't want to push the beast a freakin' half mile uphill to get to the main road. So I kick my guts out for the next hour. The bike does start a couple times, but then dies. I start pushing.

I get it home everything looks good (spark, compression, valve timing, carb) and it seems to be kicking over PLENTY HARD so I figure I have good compression, so much so that I think the decompresser isn't working, WRONG!

I tore down the engine a found that the lower connecting rod bearing is spent. All total for crank, oil pump, gaskets and misc. parts $600 and about 20 very cautious hours will a shop manual in hand and I'm back in business.

I'm not sure if this helps, but I was in the same boat as you, initially thinking it had to be something electrical and it ended up being mechanical.

  • Diode663

Posted November 13, 2005 - 05:22 AM

#15

Your reply scares me greatly. I can get parts for cheap... but having to tear the motor out of the bike already is kinda upseting. Thanks for the input.. sorry you had to push.. I luckly got a tow. Come to think of it I always thought the motor was kind of noisy as did my friend this very well may be the case.

  • biznet1

Posted November 13, 2005 - 05:44 AM

#16

Your reply scares me greatly. I can get parts for cheap... but having to tear the motor out of the bike already is kinda upseting. Thanks for the input.. sorry you had to push.. I luckly got a tow. Come to think of it I always thought the motor was kind of noisy as did my friend this very well may be the case.

If it's something major, for the Yammie 450 this would by far be the exception not the rule. I own a Honda and they are great bikes, particularly with the way they handle, but Yammies are by far the most reliable MX 4 strokes, and I repeat, BY FAR.

  • Diode663

Posted November 13, 2005 - 05:54 AM

#17

Well its an 04 and it was raced by Mark Wundrak. And I get the same reaction from the ppl who know him, "The bikes hammered". So it very well may this big end bearing witch Im no stranger to as I have done this 3 times on my 125 but I still hate taking apart the cases. Gonna dig into the motor today I suppose.

  • TD-3

Posted November 13, 2005 - 07:52 AM

#18

Come to think of it I always thought the motor was kind of noisy as did my friend this very well may be the case.


I had a a long time YZF owner (who's opinion I trust) listen to this motor before I commited to buying it. It was no more noisy than his 426. I will say that it is much less noisy with the new crank.

Yammie 450 this would by far be the exception not the rule


While I agree with this statement, I did find a lot on info on this board concerning connecting rod bearing failure. Most seemed to fail quickly with very little notice.

FWIW: OEM Yamaha cranks for the 450 were on backorder. I went with a Hot Rods crank assembly from monkeybuttparts.com

I also got all my OEM parts from crotchrocket.com, unbelievable low prices.

Good luck with what you find! Keep us updated on your progress.

  • Diode663

Posted November 13, 2005 - 04:44 PM

#19

Just curious why does everyone seem to get an oil pump when this bearing fails? Also was your bearing excessively tight or sloppy? Also the only rod bearing failures I have found for a 450 is yours. Everyone elses is the 426. Any more input? Thanks.

  • TX-SANDMAN

Posted November 13, 2005 - 04:58 PM

#20

I hate to hear your problems. Please keep us updated on what you find inside once it is apart.





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