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Troy The Great

more '98 Yamaha YZ400 idling problems...

14 posts in this topic

Hey guys, I need help here. I'm really trying to get this beast to idle properly, but she is so damn stubborn. Midrange to top end rpm/speeds work perfectly, but the thing wont idle. I'm running a 45mm Pilot jet, which I think is standard. I just played around with the idle screw on the bottom, ranging from 3/4 to 2 turns out. Nothing. There is a lot of backfiring/popping going on if that helps. I just cleaned out the jets so I doubt they are clogged, but if need be I will check again. I was thinking about getting some different sized Pilot jets, perhaps trying a 42 and 48? Any thoughts on this, and/or suggestions here? I'm slowly dying inside every day I cant get this thing to run haha. Thanks guys

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Lacquered pilot jet

Tight intake valves

Air leak via the hot start valve.

...

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Do you think you could expand a little bit? for the sake of argument, lets say Im a beginner at bikes. How can I fix these problems?

-in what sense does it mean for the pilot jet to be lacquered? as in clogged?

-I assume the hot start valve I just check for leaks and seal them?

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The orifice in the pilot jet is so small that a thin film of the varnish or lacquer that can form when over-aged fuel dries out can reduce their effective size by 30% or more, rendering them as good as clogged, even when they appear open.

When the hot start is open it is an air bleed into the intake manifold. the quickest way to test for a leak on your bike is to pinch off the hose while it's running.

Go to the Common Threads Sticky and read through the fuel system links, then go to the bottom and download a manual.

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Changing pilot jets is NOT going to "gain" an idle when you bike is not idling. You first need to find the issue. 99% of the time a bike that does not idle is a clogged pilot jet and or passageway like GR commented. I've commented before the recent addition of E10 fuel is causing major fuel system headaches when the E10 based fuel sits in your bike for extended periods of time. I would suggest stripping your carb to the bare body, and have it "dipped" in a industrial strength carb cleaner that dealers use. The backyard "Gummout/air blast" method doesn't always work best.

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Thanks polar, Im not sure Im quite equipped at this point to give that a try, but I was also told that the popping and backfiring could be caused by my head clamp for the carb not being super tight causing air to slip in, so ill give that a try. If I do end up having to strip the carb, do you have any tips on how to not get myself in a pickle and end up not having a functional carb? Thanks again

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Read:

http://www.thumpertalk.com/forum/showthread.php?p=3710599#post3710599

http://www.thumpertalk.com/forum/showthread.php?p=3350613#post3350613

If the carb is just too old and worn, which does happen with the older FCR's, a good used carb from pretty much any YZ450 will fit with minor work. The best ones are the '05 and up units. Be sure to ask for the hot start cable and plunger along with it.

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Got it fixed, it ended up being a loose carb clamp on the head. Letting air in and causing major popping. Its idle is still a bit lower than Id like, but its livable. Thanks guys!

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somehow my idle adjust thing is too twisted and it doesnt work anymore. Unless you know another way to raise the idle im pretty much stuck

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Ok, I got the same problem... But, my pilot jet is slightly "bored" and my clamps are tight as a ticks butt !!! So, what else could it be... I purchased a "like" new carb from a guy and the bike it came off of was running GREAT !!! Put it on my bike and it wont idle... It runs for about ten seconds and dies... on choke it runs about 1-2 min... than dies !!

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Just for giggles try disconnecting the TPS.I had the carb off over five times on my '99.I cleaned and cleaned and cleaned.I replaced the main jet and it still wouldn't idle.It was good everywhere else though.I disconnected the tps and now it idles fine.Still hangs a little when I chop the throttle but I'm sure the new pilot jet will fix that once I get it in.

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Ok, I got the same problem... But, my pilot jet is slightly "bored" and my clamps are tight as a ticks butt !!! So, what else could it be... I purchased a "like" new carb from a guy and the bike it came off of was running GREAT !!! Put it on my bike and it wont idle... It runs for about ten seconds and dies... on choke it runs about 1-2 min... than dies !!

Every engine has its own unique quirks, only two brand new bikes, would idle the same on a carb swap !

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