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YZ250F won't Hot Start and bogging/low power

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My first post here fellas so please take it easy on me... I bought my first dirt bike ever YAY! A  2007 YZ250F, Problem is that the Bike doesn't run great, it runs OK but not great, PLEASE help me diagnose as its driving me crazy trying to fix and I don't know if I've made any real progress in weeks now...

Issues - 

Boggs/Stalls when idling and you whack the throttle 

Low on power/Boggs from low RPM until hits about 5k-6k then takes off like a bat outa hell

Hot start nearly impossible  - 1/4 to 1/2 Throttle must be given to Hot start, Hot start lever makes no difference in this procedure 

Fuel Mix screw has to be exactly 3 turns out or engine stalls when throttle cracked (2.75 turns no go) (3.25 turns no go)

 

Positives -

Starts first kick EVERY SINGLE TIME when cold 

Higher RPM plenty power no hesitations or bogging 

 

Things Ive tried -

Thorough carb cleaning; wasn't dirty, zero improvement 

New Plug - Zero improvement 

Air filter replace fresh oil and oil change - Zero improvements

O-ring mod -  Zero improvement 

Quick shot 2 accelerator pump cover - Zero Improvement 

Went from 80 Leak Jet to 40 Leak Jet - Slight, Barley noticeable improvement (maybe)

42 to 45 Pilot Jet  - Zero Improvement 

45 to 48 Pilot Jet - Idle became very erratic and wouldn't start 

Back to 42 stock Pilot Jet - Improvement over 45 and 48 but no improvement from original issues 

178 Main Jet to 180 Main jet - Zero improvement (but not any worse either)

 

At this point I'm considering going down a size or two pilot jet and see if that improves. The strange thing is that the bike won't run with any less than 3 turns out of the fuel mix screw, so one would think that going to a slightly larger pilot jet would help or allow me to turn the fuel screw in less than 3 turns right? Wrong, in fact after going to the larger pilot jets I had to keep backing the fuel screw out more just to keep it from stalling. Anyone every had issues like this, its not behaving as it should based on everything my conventional engine wisdom is telling me...? Please help

 

 

 

 

 

 

 

Edited by xtactical

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Im in FL so basically sea level 75-90 degrees and very humid.

Stock Jets for the bike are 178, 42, 70 leak and 72start I believe , I have the needle clip in 3rd position (also stock)  

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Have you soaked the carb in yamalub carb cleaner overnight?  These carbs have many small passage ways that are difficult to realize they are actually there.

Do you have an o-ring in the fuel mixture screw?  Could you have more than one?

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Does it have an aftermarket fuel screw (aluminum) that sticks down for easy adjustment?  If so, that could be your issue.  The tip of those is not as consistent as a nice sharp point on a brass fuel screw.

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3 hours ago, freebird65 said:

Have you soaked the carb in yamalub carb cleaner overnight?  These carbs have many small passage ways that are difficult to realize they are actually there.

Do you have an o-ring in the fuel mixture screw?  Could you have more than one?

I did a good soak with the carb, I do have just one o ring with fuel mix screw and it is the stock fuel mix screw.

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If it's bogging on the low end and hard to start unless i'm holding throttle open, AND the problem got worse going to a larger pilot jet, wouldn't it seem like I'm too Rich on slow jet? I'm going to try a smaller pilot jet next see if that improves it. 

Another thought is maybe the fuel mix screw has been damaged by previous owner, which may explain why it's so  erratic, i've got a new one also in the mail i'm going to try. 

Could this all be related to a valve issue? I've got some feeler gauges in the way so I can check valve clearance. Thanks for the feedback fellas 

 

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I had similar problem. Same year too. Picked up few weeks ago. Would only be ridable if you left choke on and rolled throttle on quick when testing before buying. I figured simple carb issue so still took bike. The pilot jet was completely clogged. Couldnt even get the hole open. Bought rebuild kit and it was better but wouldnt idle well with proper adjustments.

Called guy who told be he put fresh fuel in it before i got there. He seemed like he was pulling the answer out of his ass so i dumped all the fuel out and got fresh gas. Runs perfect now.

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Final note. Im at about 50ft elevation and now have stock jets. 45 is to big for our elevation as i threw that in until i got new in the name of science.

The last thing i had to do was move needle clip down one position because i had a lot of deceleration popping when i let off the throttle from 3/4 to 1/4. That smoothed that out.

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I'm using the Pro X jets, but I'm back to stock OEMs with exception on the 180 main. The more I try and fail jetting the more I'm thinking the valves could be out. The whole top end was done 20hrs ago, but I guess for a dirt bike that's more significant than i'm used to on street bikes...

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Matthew from "How To Motorcycle Repar" has a real vent repair video on YouTube. I recently got a lie new and stock 2006 yz250f and it did the same thing. Ran great unless I stabbed the throttle. This is my first 4t so I went to YouTube. Thus guy had the fix I was looking for with a before and after video. So I did it. The jets are 180, 45, 40 and the 2008 NFPR needle. Not the difference I was looking for so I went to adjust the needle height. I read the letters on the needle and they had sent the WRONG NEEDLE! So, I ordered again and it was like a magic trick. It's the best running bike I've ever had. In 08 they went with a dual taper needle and it was good. After its warm I can't make it bog. Remember NFPR. You have to look it up in the yamaha oem list.

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3 minutes ago, BTD123 said:

Matthew from "How To Motorcycle Repar" has a real vent repair video on YouTube. I recently got a lie new and stock 2006 yz250f and it did the same thing. Ran great unless I stabbed the throttle. This is my first 4t so I went to YouTube. Thus guy had the fix I was looking for with a before and after video. So I did it. The jets are 180, 45, 40 and the 2008 NFPR needle. Not the difference I was looking for so I went to adjust the needle height. I read the letters on the needle and they had sent the WRONG NEEDLE! So, I ordered again and it was like a magic trick. It's the best running bike I've ever had. In 08 they went with a dual taper needle and it was good. After its warm I can't make it bog. Remember NFPR. You have to look it up in the yamaha oem list.

Between the 40 and 45 Jets which is the leak and which is the pilot? Thanks for the info

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1 hour ago, Monkeyshuttle said:

Final note. Im at about 50ft elevation and now have stock jets. 45 is to big for our elevation as i threw that in until i got new in the name of science.

The last thing i had to do was move needle clip down one position because i had a lot of deceleration popping when i let off the throttle from 3/4 to 1/4. That smoothed that out.
 

So are you running well with stock jets then at that elevation? Also what do you have the fuel mix screw set at? If I go any less than 3 turns out it backfires pops and stalls, which is why i've left it way out between 3-4 turns depending on pilot jets

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Final thought I'll submit tonight - I put a new plug in yesterday AM before riding for about 2 hours. Ran okay but if I stalled, I had to bump start it as it wouldn't hot start. The plug black and smells of fuel (again seems to be running rich) but this was at all variations of throttle.

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So are you running well with stock jets then at that elevation? Also what do you have the fuel mix screw set at? If I go any less than 3 turns out it backfires pops and stalls, which is why i've left it way out between 3-4 turns depending on pilot jets

Yeah broseph, stock jets that came in carb rebuild kit. 42 pilot and 178 main. I may order a better needle and some more jets to experiment with. At this time i am stock and smooth. First time i have ever needed to adjust needle position on my bikes though. I picked up my first 250Fs (kx250 and yz250) in past month and they have both been the most sensitive bikes jet and carb wise ever.

Have yz125, ktm150, crf70 and 80, pw80. Had a honda crf150 awhile back that i got cheap cause guy couldn't figure out why it bogged down at full throttle. That was easy carb easy. Guy had the biggest jets he could find thinking that meant more power.

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The 07 I've had experience with seemed more sensitive to carb adjustments.  We finally got ours to settle down after we lowered the float height. Have you checked float height or adjusted it yet?  A high float will give some of the symptoms you are describing.  

Also, popping on deceleration is acceptable and expected with these bikes.  The 07, they opened the carb up another mm or two.  Just my observations but that seems to make it harder to find a happy medium.  Also, once you lower float height, you might try dropping the needle height 1 clip position.

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