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YamahaWR450FMike

Need Carb Advise 2004 WR450

24 posts in this topic

I'm working on a buddies WR450F. He bought it used but runs very badly.

This is what it does. During start up with the choke on, it revs very very high rpms and the exhaust header turns bright red. If the choke is turned off, it will not idle what so ever. I thought the engine is running very lean.

I pulled the carb tonight and completely took it apart. It has a 170 main and a 50 pilot. Nothing seemed to be clogged with those two things.

I'm going to use a air hose on the carb tomorrow.

Anyone have any ideas what's causing these problems?

I'd really appreciate any feedback. Thanks.

Mike

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Check the float height. I built a carb for a buddy and screwed up the float height somehow. It was too high and dribbling fuel in so I could get it to idle but it was a rough idle, it idled high and once I leaned it down as much as possible it would turn the pipe red. Fixed the float level, retuned it and now it runs like a top.

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Set the float level, remove the pilot and leak jets and blow 60psi of air and spray brake cleaner in the passages (NO CARB CLEANER), replace the accelerator pump diaphragm, and you'll be good.

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got the carb cleaned and back on the bike tonight. Really not much difference. Still wants to die instantly after turning off the choke. I checked the intake boot for cracks but found none, so that is out of the question. It acts like there is a vacuum leak. Going to order a zip-ty fuel screw and see if that helps. I'd say the bike idles just a tiny bit better but still wants to die after turning off the choke. I cleaned the carb up really good but maybe there is something still stuck in it. The pilot jet was a mess and I got it pretty clean.

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got the carb cleaned and back on the bike tonight. Really not much difference. Still wants to die instantly after turning off the choke. I checked the intake boot for cracks but found none, so that is out of the question. It acts like there is a vacuum leak. Going to order a zip-ty fuel screw and see if that helps. I'd say the bike idles just a tiny bit better but still wants to die after turning off the choke. I cleaned the carb up really good but maybe there is something still stuck in it. The pilot jet was a mess and I got it pretty clean.

If the pilot jet was badly varnished, throw it away! You can't get it clean inside by hand.

Have you inspected your fuel screw for a good o-ring and steel washer?

Have you done a 'spray' test for air leaks (brake cleaner on header crush washer, intake boot, top of carb, etc)?

Have you inspected your hotstart to see that it is all the way closed, and not corroded?

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If the pilot jet was badly varnished, throw it away! You can't get it clean inside by hand.

Have you inspected your fuel screw for a good o-ring and steel washer?

Have you done a 'spray' test for air leaks (brake cleaner on header crush washer, intake boot, top of carb, etc)?

Have you inspected your hotstart to see that it is all the way closed, and not corroded?

yes checked all you mentioned except for spray test.

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You mentioned in your original post you cleaned the pilot and main jet.. Did you clean the start jet and pilot air jet also ?

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You mentioned in your original post you cleaned the pilot and main jet.. Did you clean the start jet and pilot air jet also ?

Yes I removed everything off the carb that I could. Used brake cleaner and a air hose to blow out any junk inside the carb. It's weird how the bike runs. It's just slightly better than before I tore into the carb. With the choke on it runs and idles fine but the second you push in the choke the engine just dies immediately.

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Yes I removed everything off the carb that I could. Used brake cleaner and a air hose to blow out any junk inside the carb. It's weird how the bike runs. It's just slightly better than before I tore into the carb. With the choke on it runs and idles fine but the second you push in the choke the engine just dies immediately.

That indicates a lean condition: choke required for running at idle.

It has to be associated with the pilot circuit.

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Something else you need to look at is the hot start. You need to clean the plunger and make sure it is bottoming out, the cable can also stop the plunger from bottoming out.

Had a similar problem on mine, a new cable sorted the problem out even though the old cable seemed serviceable, I replaced the cable from a recommendation from an independent MX shop, his words where they need replaceing every 3 years.

Did the trick for me. Maybe try diconnecting it from the handlebar lever and forcing the cable back into the sheath to fully close the plunger and see if it makes a difference.

Just something to eliminate.

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Something else you need to look at is the hot start. You need to clean the plunger and make sure it is bottoming out, the cable can also stop the plunger from bottoming out.

Had a similar problem on mine, a new cable sorted the problem out even though the old cable seemed serviceable, I replaced the cable from a recommendation from an independent MX shop, his words where they need replaceing every 3 years.

Did the trick for me. Maybe try diconnecting it from the handlebar lever and forcing the cable back into the sheath to fully close the plunger and see if it makes a difference.

Just something to eliminate.

thanks for the advise. I will check that, though I did pull the plunger completely out of the carb and it was totally clean. Will check to make sure it's bottoming out all the way. Thanks

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IM sure you checked it but i had the same problem and fixed it tonight by adjusting the fuel screw it made a world of differance. If it works im glad i could help if it doesnt throw it out.

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Try a brand new pilot jet.

Ensure the tip of the fuel screw is good and the tiny oing is serviceable as well as the washer and spring.

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Try a brand new pilot jet.

Ensure the tip of the fuel screw is good and the tiny oing is serviceable as well as the washer and spring.

I ordered the Zip Ty fuel screw so when I get it, I'll pull the old fuel screw out I'll check it and see if the o-ring is on there. Can't remember when I pulled the carb apart last week.

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Sometimes the oring can remain stuck up inside the fuel screw opening. So just because it does not come out on the screw does not mean it is not there, you have to check. With the carb on the bike, a mirror and flashlight may be needed.

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I took the bike out on a ride this morning. Runs pretty good. I installed the new zip-ty fuel screw and that seemed to fix my not idling problem. Only problem now is when the motor is coming down from a mid or high rmps, the rpms slowly go down, not fast like I know it should. Revs up fast but doesn't come down to a idle fast. I have the fuel screw turned about 2 turns out. 1 1/2 turns it wants to die. So now I have to figure out this little problem. Any ideas? Thanks

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I took the bike out on a ride this morning. Runs pretty good. I installed the new zip-ty fuel screw and that seemed to fix my not idling problem. Only problem now is when the motor is coming down from a mid or high rmps, the rpms slowly go down, not fast like I know it should. Revs up fast but doesn't come down to a idle fast. I have the fuel screw turned about 2 turns out. 1 1/2 turns it wants to die. So now I have to figure out this little problem. Any ideas? Thanks

It's an air leak issue.

Spray the carb boot and exhaust header input while running, with brake cleaner, and listen for rpm changes. Or hot start stuck. Or slide plate seal is bad.....

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It's an air leak issue.

Spray the carb boot and exhaust header input while running, with brake cleaner, and listen for rpm changes. Or hot start stuck. Or slide plate seal is bad.....

I'll have to try that. Hot start isn't stuck the the o-ring could be worn out. It does slide into the carb quite easily.

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I'll have to try that. Hot start isn't stuck the the o-ring could be worn out. It does slide into the carb quite easily.

The slide plate seal is a very common cause.

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