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250f Starting Issues

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So recently this year I ran into some starting issues with my 250, I’m going to give a decent amount of info so bear with me but I feel the more the better.

Bike: 2009 YZ 250f

I’m the third owner, unsure of whether it’s ever been rebuilt, however it runs very well, so for now that seems to not be an issue. I ride in slow tight woods so it’s not like I’m riding the piss out of it to warrant scheduled rebuilds.

Mods: (all came with the bike on purchase)

Uni Dual Stage Airfilter (clean and in good shape)

FMF Megabomb Header with Factory 4.1 Muffler (small air leak at the midpipe connection, header also has play where it connects to the engine, unsure if these are normal or would cause issues)

DynoJet Moose Racing Stage 1 Jet Kit: Minimal description of sizes included except for a parts list.

Current Main jet installed is a 165, the rest are as follows-

Fuel Needle- DNO987

Pilot Jet- DPJM048

Nothing else was listed besides washers and stuff so I’m assuming the rest of the carb is stock.

I also received an installation sheet for an Athena Dual Spray Venturi Jet, couldn’t find much info on it nor do I know for sure if it’s even installed.

Upon purchase the bike ran excellent, no bog, fairly easy starting, took it to my shop to get checked up and they said the same thing so for the entire fall I was very pleased with my purchase.

When I took the bike out from storage this winter I noticed some minor things that needed fixing so was planning on taking it to the shop anyway. Upon a couple rides early this spring I noticed that hot starts were becoming near impossible. Eventually took it to the shop and they found the valves were tightening; they adjusted the valves, replaced the plug, and reset the fuel screw. Got the bike back, however I could now not start it for the life of me cold. Got it started a few times but touching the throttle or turning off the choke would just kill it immediately. Two days straight of trying to kick it, flooding, and waiting to try again, brought it back to the shop the next week and sure enough starts up second kick for my mechanic, then I give her a go, second kick for me too. I thought that maybe with the adjustments it just required a new starting method, however after getting it back to my house and trying the new method used at the shop it was the same story and was finicky as ever, sometimes it would start, sometimes it would sound like it’s about to turn over, but most the time there was no evidence of combustion at all. 

So these are my questions

1) Am I the problem? Could it be that I’m just used to the easy cold starting of the tight valves and now need to learn a new starting method? 

2) I live probably a couple hundred to a thousand feet higher elevation compared to the shop where it worked great, if jetting was slightly off would that make such a drastic difference in the performance and starting?

3) Lastly, could there truly be something off with the bike that maybe my mechanic isn’t noticing since starting these race bikes is like clockwork to him? Any ideas where I should begin looking?

I appreciate all feedback and would be very grateful for any help on this issue that’s been driving me nuts. Thanks everyone

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1. If it does not start it 6 or so kicks, you need to clear the cylinder before continuing (wot, kill button held on, kick thru several times)

2. You need to find out EXACTLY what was done to the valve clearances. It's your bike, take control. Shimming valves means that the valves are 'mushrooming' where they contact the valve seats, and it is a sign of needing head work. 250's need head work every 100-150 hours or so. You cannot shim it more than a couple of times without running the risk of breaking a valve. 

3. When the valves start to mushroom, they stop sealing as well, which is part of your hot starting problem, along with not adjusting the fuel screw (must be done every ride)

4. Since you listed part numbers, not jet sizes, and you did not list your actual elevation, it's hard to say if your jetting is off....but yes, absolutely, a 1000 feet can make a difference. You need to learn how to adjust the fuel screw, and you should have a good BRASS fuel screw like an R&D Racing, to make adjustments easy.  You adjust for highest idle and smoothest idle. It's very subtle.

5. You need to know exactly what your jetting is, and you should put in a NEW pilot jet after the bike sits for more than a month. Never ever leave gas in the carb when it sit. Lay the bike on the ground, both sides to drain the float bowl. Running it dry is not enough. Treat your gas every tank for contaminants like ethanol, or you will get carb corrosion.  You should be running a 42 pilot jet above 6k and a 45 below that. It's your bike, TAKE CONTROL.

6. Make sure you are cleaning and oiling your air filter correctly. This will extend the life of the valves 100-500%. You cannot tell if a filter is clean by looking at it. You clean it every 2-15 hour of riding, depending on dust levels. You must clean it in clean solution (water or solvent) to see if the solution remains perfectly clear. That is when you know it is clean enough to dry and oil. Oil it by SATURATING the filter, then wringing it out and patting it down. Add extra oil to the seating lip, or it will leak and pull in dirt.



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Seeing as the shop changed the valve clearance, thats the first place i'd look.  Measuring valve clearance is pretty easy to do and tight valves are one of the main reasons for difficult cold starting.  

In three seasons I have never once adjusted the fuel screw and my 250F starts and runs just fine.

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Thanks for the suggestions, a lot of good input here. 

Unfortunately I really wont be able to get some serious time with the bike again for a while. 

Moving forward, as a beginning point my buddy just got back into riding and hes been racing and working on 250's for most his life so hes gonna see about getting it started since he has experience with 4 stroke race bikes. If he can start it consistently and it runs as good as it did then the problem is most likely me. 

If that goes south though, then the valve clearances are the first thing I plan to check. I'm unsure if this may be it on account of how well it ran when I brought it back to the mechanic, I would expect if the valves were so bad then we would be able to notice the bike running like crap. 

I always maintain the filter as well as drain the carb after riding, but obviously with used bikes you don't know the full history. 

I did some digging and heres as much info I could gather as far as carb and conditions go:

Elevation: 2000ft (Shop where it started and ran great was around 1200)

Average Temperature: 70-80 degrees 


Main: 165

Pilot: 48

Needle: Wont be able to figure this out till I can get to the carb again 

Fuel Screw: 2-1/2 turns out

Leak Jet: 70

I'm also leaning towards ruling out the valves as they were just beginning to tighten, I'd have to get exact measurements from the mechanic, but were not talking a drastic change in clearance here, they were also loosened and even if over loosened that typically wouldn't cause major problems, I know it is still possible though that they were done wrong or reverted back to their old clearance after the work which I will still check for when I get a chance. 

However though something else I just thought, in order to shim the valves the cams have to be touched, could it be that during the valve job they were reinstalled off in timing, might that cause the starting issues Im having? 

Thanks again for the great responses guys 

Edited by DCRacer572

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Sorry to revive this topic but I ended up figuring out the problem. This past weekend I was finally able to work with the bike and check your suggestions, I pop started it initially and I noticed that laying off the throttle caused it to stall, turns out my idle was set way low, something that I didn't even think of in the first place. After adjusting it the bike now runs perfect, crisp immediate response and takes 1 or 2 kicks to start. Was pretty shocked honestly, I guess the lesson here is always start with the basics and that the smallest change can have a drastic effect. Thanks for the help everyone, safe riding! 

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