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tech24

Help again please!!

18 posts in this topic

I have another thread open but not sure if anyone is reading it so here it goes again...

Bike: 2004 yz450f

Symptom: idles ok, at any throttle opening cracks, pops, and backfires

work already done: Cleaned carb more than once...no chance of dirt

checked entire ignition system even replaced cdi and coil with used unit

checked valve clearence and cam timing: perfect

performed cylinder leakage test: heres were my question is. It has no leakage as far as the guage goes by % however there is a fair amount of air coming through the air box. I assume here lies the problem. I don't know why the leakage guage reads perfect but I don't think I should have any air leaking past intake valves.

Any common problems to look for? I assume its gotta be a valve seat issue since clearences are in spec which means nothing is holding valve open.

Im gonna tear the head off tomorrow just to ease my curiousity just wanted to run it past you guys for any more tips or info

Thanks!

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i also have a 04 yz450f mine does backfire sometimes but i dont know anyone that has an 03,04 that doesnt. i took mine into the local dealer for a tune and they said a bit of backfiring was ok. thats all i can tell you

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I have another thread open but not sure if anyone is reading it so here it goes again...

Bike: 2004 yz450f

Symptom: idles ok, at any throttle opening cracks, pops, and backfires

work already done: Cleaned carb more than once...no chance of dirt

checked entire ignition system even replaced cdi and coil with used unit

checked valve clearence and cam timing: perfect

performed cylinder leakage test: heres were my question is. It has no leakage as far as the guage goes by % however there is a fair amount of air coming through the air box. I assume here lies the problem. I don't know why the leakage guage reads perfect but I don't think I should have any air leaking past intake valves.

Any common problems to look for? I assume its gotta be a valve seat issue since clearences are in spec which means nothing is holding valve open.

Im gonna tear the head off tomorrow just to ease my curiousity just wanted to run it past you guys for any more tips or info

Thanks!

You have a toasted/cupped intake valve.

Careful when you clean your carb. Auto carb cleaners can destroy the carb slide plate o-ring seal, and the accelerator pump diaphragm. Just use motorcycle carb cleaner.

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Thats kinda what im thinkin...getting ready to tear the head off now should know something in an hour or so. I actually completely disassembled the carb and rmoved all plastic and rubber and dipped it.

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Well took the head off, removed valves, and check margins using sharpie method and they are perfect. Valve seats are a little corroded so I lapped the valves. As soon as I get new gaskets I'll see if it made a difference but I have my doubts.

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It is impossible to have a leak down tester show zero percent leakage on any engine, much less on one that's blowing air out the intake.

When you did you test, did you hold the engine at TDC?

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It is impossible to have a leak down tester show zero percent leakage on any engine, much less on one that's blowing air out the intake.

When you did you test, did you hold the engine at TDC?

yep it was about 5% not exactly zero I dont know if something is wrong with the guage....it seems to pass everything but however didn't find any substantial valve issues, I cleaned up the seats as soon as gaskets get here I will see if it made a difference. But I had intake leakage (very little) on all strokes. Valves held liquid after lapping so we'll see.

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are stock valves Ti? The manual says to lap so I assumed they weren't. I understand not lapping Ti cause it'll remove coating. I was figuring these were SS.

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Yes, all YZF valves from '01 onward are titanium. IMO, the manual has that info in it as a carryover from the past models because none of the tech editors have caught it yet.

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Yes, all YZF valves from '01 onward are titanium. IMO, the manual has that info in it as a carryover from the past models because none of the tech editors have caught it yet.

Well thats a major bite in the ass and I believe your right about the carry over casue I have seen some other discrepencies also. Well I guess I just trashed some spensive vlaves. Might be time for bore and head work then..not that it needs any more power but it'll keep me from kicking it over in the parking lot

Well thanks for catching that!

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You haven't yet really determined that you have a problem with the head, as far as I can see. I'd advise getting a qualified pro top look at this for a fresh opinion. If you've damaged the valve faces by lapping them, that will show up eventually as rapid loss of valve clearance. Unless the valves are actually leaking now, I'd fix your problem, ride it, and see how it does.

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You haven't yet really determined that you have a problem with the head, as far as I can see. I'd advise getting a qualified pro top look at this for a fresh opinion. If you've damaged the valve faces by lapping them, that will show up eventually as rapid loss of valve clearance. Unless the valves are actually leaking now, I'd fix your problem, ride it, and see how it does.

Yeah Im sure it'll run fine for a bit minus the rapid valve wear once I find problem. I still think its something in the carb that im missing. Had a honda tech go over it as well and he didn't find anything. Im just pissed and I can't admit defeat. Haven't found nothing yet I can't fix but Im getting close to sending it to S&D performance with a blank check

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Careful when you clean your carb. Auto carb cleaners can destroy the carb slide plate o-ring seal, and the accelerator pump diaphragm.

Ive done this. I must admit that when I was running the original u4, it did the same thing.

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Ive done this. I must admit that when I was running the original u4, it did the same thing.

I dont believe I got any cleaner on slide or wear plate but its possible, when I get it back together and Im still lost maybe I'll just put a seal and plate in and see.

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I am having the exact same symptoms that you are having on my CRF450R.

I have verified it is not valves, compressions, ignition, or anything mechanical or carb settings.

I had already rebuilt the carb. prior to top end rebuild to possibly correct the problem.

Today i removed the slide from the carb and found the Slides Floating Valve seal to be bad.

i don't know how that plays into the carbs idle and off idle circuit, but i bet it is causing a lean air/fuel mixture.

The things ot note is that once you get the bike past idle and off idle it runs like a new bike.

i found out you can order just the seal from 2004 Yamaha YZ450 FCR Carb for $9.00 where as you have to order the floating valve set which includes the seal for $60 from Honda.

My seal should be here early next week, i hope this fixes my issue once and for all.

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did you check the hot start? it will cause that if it hangs up even a little. I have had to replace a few of them and the cable becuase of this issue.

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I am having the exact same symptoms that you are having on my CRF450R.

I have verified it is not valves, compressions, ignition, or anything mechanical or carb settings.

I had already rebuilt the carb. prior to top end rebuild to possibly correct the problem.

Today i removed the slide from the carb and found the Slides Floating Valve seal to be bad.

i don't know how that plays into the carbs idle and off idle circuit, but i bet it is causing a lean air/fuel mixture.

The things ot note is that once you get the bike past idle and off idle it runs like a new bike.

i found out you can order just the seal from 2004 Yamaha YZ450 FCR Carb for $9.00 where as you have to order the floating valve set which includes the seal for $60 from Honda.

My seal should be here early next week, i hope this fixes my issue once and for all.

Yeah keep me posted!

did you check the hot start? it will cause that if it hangs up even a little. I have had to replace a few of them and the cable becuase of this issue.

Yeah its free, clean, and working

Well I have decided to begin a major project. I got the bike used and with what appeared to be a new head but Im gonna freshen it up, new top end and valve job. Im gonna strip it down have the frame chromed, replace all bearings and bushings. Put all black plastics on it, new bars, and levers. And maybe and edelbrock carb, when im done it will be nearly new and all mine!

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