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New Piston, Weird Running

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This week my buddy and I put a new piston in my '09 YZ250F. Took the cylinder to our local Yamaha place and they did a gorgeous hone on it, too. So, new piston and rings, fresh hone, valves checked (no need to adjust - gotta love Yamaha!)

So we get it together last night, and it's go time: we kick it about 30-40 times and it finally starts. I put this down to fresh metal internals needing to get used to each other. Finally starts, and died. Start it up, it's clearly unhappy, doesn't want to stay running, so I give it some gentle gas to keep it running. Eventually it calms down, but when I roll off the gas, it dies.

Obviously to do the piston, we had to take most of the bike apart. We had the carb lying down for about 2 days, and I wonder if maybe something got clogged in there? Also, the clamp that goes from the carb to the intake tract of the head cannot be re-used; the bolt that tightens it got stripped out before we even took it off. So we sliced off the rubber slit that keeps it from rotating, and put on a hose clamp that fit.

Up top in the RPM range, it runs great. A lot more power, a given since the blow-by on my old piston was nothing short of spectacular.

Two problems:

  1. when I roll off the gas, it hangs a good 5-7 seconds before dropping back down to idle (running a 42 pilot, stock - sadly we didn't check and clean the carb while it was out of the bike. sigh.)
  2. when it drops down to idle, it will start to run rough, and then die.

Spark plug looked fine, I have a small radiator leak, but my coolant level is good, so I don't think it's running hot. Obviously we are working on the radiator issue.

I haven't put a second heat cycle on it since I took it home last night, and was planning on it tonight.

Thoughts, anyone?

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First verify the cam timing since that is kind of a big deal and we know that was messed with. By what you are describing it sounds to be air/fuel issues. Tear down your carb and give it a good cleaning with compressed air and carb cleaner or contact cleaner. Replace the pilot jet if it looks like it is gummed up. After you get the the clean carb back on and running verify no vacuum leaks around the intake by using carb cleaner or soapy water. Spray around the area and if you hear a change in your RPMs then you have a vacuum leak. Don't forget to change the oil and filter after break in (opinions may vary)

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We will double check the timing and clean up the carb, we'll let you know how it goes

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he's the buddy. the timing was spot on after putting the cams in, and to my knowledge we checked the timing again after we put the tensioner in as well, but since its easy, worth a double check.

good call on the spray for vacuum leak. definitely will check that out this weekend, and i am picking up a 45 pilot today, so we'll see if that helps at all, even marginally.

thanks for the input, will post further progress/issues.

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Sounds like a vacuum leak to me, but when the carb was out, it may have got clogged.

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There is a 45 pilot in the carb I come to find out; I stopped by the dealer on my lunch break. I got a 48 pilot and talked to the tech, he told me how to adjust the fuel screw to adjust for the different pilot.

We sprayed the intake boot with brake clean after readjusting the clamp and holding the RPMs steady, and no change in RPM so I think we're OK on the intake. Gonna pull the carb, check it out and make sure everything is looking good then, if that doesn't help, check timing.

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Did you do anything to your carb? Did you remove the slide valve? If so pull you carburetor and check the orientation of the slide valve plate. This could explain why it will run fine in open throttle, but will not idle worth a crap. The notched "V" section should be at the bottom portion of the slide valve.

I would pick up a new 45 pilot and try that first. As I have read they tend to obtain a nice varnish glaze. Same thing happened to me when my carb sat out for a few days during the rebuild. Though I completely wrapped my carb in plastic, to keep dirt and dust out. I would also tear down your carb and clean it.

Does it run or start any better without an air filter?

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we took the carb out, but didn't do any thing other than put it off to the side.

at 5,000ft the 45 is what we run stock, and the 48 is what the tech recommended as he runs one is both his YZ250F and his YZ450F, so since I have that, I'm going to give it a shot; if not, try my best to clean the 45 really well and see how that goes.

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I would think a 45 would be too big especially at that alt. I am running a 42 at less than 1000ft. See if you can't get your hands on a 42 and a 40 and just swap them out and see if you can't find that elusive sweet spot

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I'll take out the 45 and look at it, that's what the shop here is recommending, that or a 48 with fuel screw adjustment. If I can just clean the 45 out and use it again, that would be ideal.

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Be careful cleaning those. They can be a bitch. Someone on here recommend a guitar string. It needs to be small enough to poke thru the hole but strong enough not to bend when trying to get rid of that damn varnish build up

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Actually that's perfect, I happen to play electric guitar; that high E string will be perfect!

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This bike is pissing me off, We tried a bunch of stuff on Saturday to no avail.

Swaped complete carb from a known good, well set up YZ250f

Swaped intake boots carb-cyl head

swaped plugs

Checked timing, timing is spot on

I don't know what else to check. Some have said a pinched ring, but the bike does not smoke one bit. Bike still hangs idle after rolling off the throttle. Any other ideas????

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And I put an hour at least on it on Saturday; runs super strong everywhere except at idle. No problems coming off idle, either.

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Did you try different size pilots? Adjusted idle mixture screw correctly? I know a lot of bikes have hanging idles. Mine does and is worse when warm. I wouldn't worry about it too much

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the 45 looked OK when it came out, and the 48 was obviously new and ran just like the 45, nothing different. the carb we swapped onto it had a JD jet kit and, correct me if im wrong luckyguy19, had a 45 in it as well. we played with the idle screw quite a bit to see what would work, and i can have it idle fast and be OK, or try to have it idle where it used to and have it die. that's the last problem now; plenty of power, etc. im going to ride it again today to see how it starts etc after a proper break-in and heat cycle ride. the previous ones were just buzzing up and down neighborhood streets.

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another thought:

i was playing with the auto-decompression on the exhaust cam when we had it apart. that being stuck (although i dont think it is) wouldn't let it run OK everywhere but idle, would it? i expect that if it were stuck, it would run like shit almost all the time?

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