Starting/running problems with 07 450f

I recently bought an 07 YZ450f and it is having problems. I was hoping someone could help. This is a bit of a novel, but I’d rather give too much info than too little.


The previous owner only had it a few years and didn’t know how many hours were on it, but I have a pretty good idea it had a lot. The clutch and timing covers were pretty well worn, it looked pretty used overall and it wasn’t running well. It was hard to start, dying at idle, and backfiring. Owner thought it was a carb issue. It was dusty from not being used and the previous owner said it had jumped timing a few times, but he had recently replaced the cam chain and that hadn’t happened again in the few rides he had taken since. Maybe I should have walked away, but it was a pretty good deal so I decided to get it.


As far as things that could be having an effect on this problem, the bike has an aftermarket FMF Ti Megabomb header, Powercore 4 exhaust, and an aftermarket Boyesen Quick Shot 2 accelerator pump. The jetting is as follows:


Main: 175

Pilot: 48

Leak: 45 (new, the 40 that was in it seized in the float bowl threads)

Starter: 72

Needle: NFLR-4

Fuel Screw: 2 turns out


I am in Northern California not much above sea level and it has been 80-85 degrees when I have been messing with everything.


When I got it home I completely tore down and cleaned the carb. Where fuel sits in the float bowl, on the float, and the jets had a green/yellow coating that almost looked like pollen. It was kind of powdery but I couldn’t just wipe it off. I cleaned everything out with Simple Green, then hosed everything down with carb cleaner and blew it all out with compressed air.


I also cleaned the airbox, the air filter (wasn’t terribly dirty), and oiled the filter with fresh Maxima FFT. The spark plug that had been in it was the wrong type so I replaced that with the correct one, a new NGK CR8EIX Iridium plug as well. I drained the gas tank and put new gas in. I removed the existing worn out and cracked exhaust gasket, cleaned all surfaces, put in a new gasket, and used high temp copper silicone to reassemble the header to the head.


Once I got everything reassembled I got it to start, but only with the choke on and the idle adjuster turned all the way in. Once it starts I can turn the choke off and it continues to run. If I try to lower the idle (I would guess it is at 3k-3,500 RPMs, but no way to measure) it dies. If I try to give it gas it backfires when I turn the throttle and wants to die. I thought it may be a stuck hot-start plunger, but with it running and the choke on using the hot-start lever increases the RPMs. I messed around with the fuel screw, adjusting from 1.5-2.5 turns out, but that didn’t seem to make a difference.


As I see it there are a few things I can do next. Maybe the jets still have some of that green/yellow coating in the passages making the bike run lean so I may just replace them all with new jets. Alternatively the cam timing may off, either from the work the previous owner did or it jumped timing again. As long as I’m in there it makes sense to check valve clearances and take a peek at the cylinder/piston based on what the previous owner told me anyway. Then there is Plan C, but I don’t know what the heck that is yet! :excuseme:


Before I go too far down the wrong road I thought I would see if anyone has any suggestions. Have you seen or heard of anything like this before? Is there anything I should check or do first before replacing the jets and checking the timing?


Thanks for the help!

I too bought a 07 YZ450F from the 2nd owner who knew nothing about it.  Unlike yours, mine actually ran real good.  That being said, one of the 1st things I did was check the valve clearances.

I knew I was opening myself up to some risk, but worst case I spend some time/money getting it right. There is no place close to me to ride it until October 1st anyway (CA red/green sticker rules) so I may as well learn about the bike "the hard way". :facepalm:

Ill start off by saying i own a 06 and let me just tell you they are solid bikes!!! Yes your having some issues but once you get them sorted out it should be like a 300k toyota :thumbsup:. Like stated above considering the timing chain had jumped a coulple of times prior, thats where i would start. Crack open the valve cover and inspect the valve train, take the chain tensioner and make sure its in good working order. Make sure valves clearences are in spec and all the tdc marks line up as whell as the lobes facing away from each other. Does the bike have good compression?? I installed a jet kit i belive by dynojet on mine and you eliminate the guessing game of what jets to run and what not, there only like $50. Since it has a full pipe on it i would also inspect the exhaust gasket, make sure its in good shape.

I replaced the exhaust gasket with a new one and that didn't have any effect.


Yeah, I'm getting the general impression valves are the next place to look, one more thing to add to the to-do list for the long weekend.

Check the clearances, but I think your trouble is with the carb, probably a dried film or blob of fuel varnish stuck in the passages that lead up from the pilot jet into the air stream.

I'm going to check clearances anyway, but I will dig back in to the carb and make sure the jets are completely clean. Thanks.

I've been digging in to the bike and had a few more questions. Here is what I've found so far. I've cleaned the carb about as much as I possibly can. In the interests of eliminating potential problems I ordered a new hot start cable to replace the one on there that is all messed up. I also ordered an aluminum hot start nut since the plastic one that is on there now is well on its way toward stripping.


The cam timing was right on and the cam chain tensioner worked fine so no issues there. The valve clearances were another story. What I found:


L+R Intakes: ~.065mm

Center Intake: ~.045mm

L+R Exhaust: ~.17mm


Based on the consistency between them all I can think is the last person to mess with them set them overly tight intentionally. I ordered a bunch of replacement shims to get them within range.


Since I'll be waiting a week for parts anyway I'm going to pull the head to get a look at the piston and cylinder. I may as well get rid of any carbon build up on the head and clean up the valves. I have a top end gasket kit on the way along with a new set of valve seals.


I've seen a video about cleaning valves where fuel tubing was put over the valve stem, that was then put in a drill and spun at low speed while holding scotchbrite against valve. Is there anything wrong with using that method? Is there any trick to cleaning carbon off the head besides a plastic scraper and is there any solvent that will help loosen up the crud?


Thanks for your help.

So I pulled the head and the cylinder and it doesn't look too bad. The cylinder is pretty glazed up so I'm going to hit that with a scotchbrite pad and some soapy water. Fortunately I don't see any scratches at all in the nikasil, but I don't see any cross hatch either. It turns there is a JE high compression piston in it and that looks OK as well. From the previous owner saying the timing had been jumping I was worried about a piston/valve strike, but I don't see any evidence that has occurred. There wasn't much carbon buildup in the head either so I'll probably just leave that alone.


I'm going to try just reassembling everything with new gaskets, get the valve clearances where they need to be, and put the new parts on for the hot start to see if it will run better as is. Now the parts just need to show up.

Something I do, is I'll take a wire from a wire wheel. Straiten it out, and I'll fish it thru the pilot jet. It's not a popular method. But it works. Then I'll take the straw from a can of b12, and I'll insert it into the carb where the pilot jet goes. I'll spray until it comes out of the throat of the carb in a nice stream. After that, I reassemble. MAKE SURE YOUR FLOAT HEIGHT IS CORRECT. That's a biggie. Then with the bowl still off, I'll attach the fuel line, and turn on the fuel valve. If no leak from needle and seat, continue reassembly. If it leaks, replace it. It will never run rite.

So all the parts showed up and I got everything back together today. All I really needed to do was replace the shims to get the valve clearances right and cleaned the carb about as well as I could. I appreciate the tip about using a wire from a wire wheel, but the wire wheels I have are pretty dirty. Instead I very carefully ran a sewing needle through the jets. I made sure to check the float level as well and it came up good.


End result is it fired right up after 4 kicks and settled in to a very nice idle. The backfiring when giving it gas was gone as was the very brightly glowing header. It still lights up, but it had been bright enough to see in direct sunlight before. Basically it now seems to be running great.


Since I changed multiple things I can't be sure, but I suspect you were right Gray. The more I think about it since it wouldn't idle and was exhibiting lean symptoms I'm guessing one of the idle jets was gummed up. At throttle settings and RPMs where the only the pilot circuit was being used it wouldn't run at all. It would only run once the throttle was loaded enough to get in to the needle circuit. While I'm sure all the valves being tight didn't help, I'm thinking the real culprit was the carb.


I was only able to run it on the stand today. I plan to take it out and ride next weekend to see how it does under load. I am also going to keep a close eye on the valve clearances and hope they stay put.


Thanks to all of you for your help. When I was looking to buy a bike and doing research it seemed like Yamahas have an edge in reliability. The fact that this community is so active with people who know their stuff helped as well. Looks like I made a good choice on the YZ! :D

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