'08 450f bike bogs in corners

I was at a local race over the weekend and was having some issues with my bike. When I was coming out of corners the bike was bogging regardless of what gear I was in. On several occassions I stalled the bike coming into corners also. When I would get on the gas hard out of a corner the bike almost shut off. One of the mechanics at the race told me it was probably my accelerator pump in the carb and that they come from the factory with this problem. He had me start the bike and hit the gas hard when at idle. The bike almost shut off each time. I tried riding with the idle up and keeping the throttle slightly turned but it didn't help and I couldn't ride normally. The bike ran fine last week and I haven't done anything other than an oil and air filter change. I've only had this bike for 2 months. The mechanic said he charges $100 to re-wire the accelerator pump. If this were the problem, wouldn't this have been the issue since day one? Any advice?

Check out the O-Ring mod in the jetting section, takes a couple of minutes.

I was at a local race over the weekend and was having some issues with my bike. When I was coming out of corners the bike was bogging regardless of what gear I was in. On several occassions I stalled the bike coming into corners also. When I would get on the gas hard out of a corner the bike almost shut off. One of the mechanics at the race told me it was probably my accelerator pump in the carb and that they come from the factory with this problem. He had me start the bike and hit the gas hard when at idle. The bike almost shut off each time. I tried riding with the idle up and keeping the throttle slightly turned but it didn't help and I couldn't ride normally. The bike ran fine last week and I haven't done anything other than an oil and air filter change. I've only had this bike for 2 months. The mechanic said he charges $100 to re-wire the accelerator pump. If this were the problem, wouldn't this have been the issue since day one? Any advice?

I had the same problem with my 08. I changed the leak jet to a 50. Also make sure your pilot jet is 100% clean. I don't know how long you let your bike sit between rides, but if mine sits more than a week I just clean the pilot on general practice. I don't see you needing the o-ring mod or having the AP wired for the 08. My 08 runs perfect without any of that. I'll be interested to see how it does this weekend at the races....supposed to be around 100 degrees.

I cleaned the leak jet and I'm not sure if changing the jets would fix the problem. I would think I would have noticed a jetting issue from day 1. I started the bike and when I give it gas at idle it bogs or shuts off. I have to roll on the gas at idle to avoid those symptoms. I noticed this problem all of the sudden while riding.

I cleaned the leak jet and I'm not sure if changing the jets would fix the problem. I would think I would have noticed a jetting issue from day 1. I started the bike and when I give it gas at idle it bogs or shuts off. I have to roll on the gas at idle to avoid those symptoms. I noticed this problem all of the sudden while riding.

Did you make 100% sure your PILOT jet is clean. People have brought me there bikes saying "They cleaned the carb" only to find out the pilot jet was completely or partially clogged. Clean pilot jet....problem solved.

I took the carb completely apart and cleaned all the jets including the pilot jet. I also changed the spark plug. The plug was black around the thread and the center was dark tan. I put everything back together and the bike is better than before but the problem is still there. When the bike idles and I hit the gas hard it bogs and shut off a few times. Should I change the leak jet next?

Was the weather the same both times? It could be just a matter of adjusting your fuel screw. I have an '07 with a Dr. D pipe and as long as I tune the fuel screw to the conditions of the day, I have perfect jetting.

Go to a smaller leak jet. Try a 60 or smaller, as much as a 40. Or, get a adjustable leak jet (will save you a lot of time). I run a R&D, bought from James Dean. Most 4S will die if you nail the throttle right from idle. You are simply asking for too much. The real question is, does it bog when riding, if so, that is what you want to fix.

Fuel screw is only for idle.

The weather is not the issue. What is the stock Leak Jet for a 2008 yz450f? I'll pick up a new one tonight.

Guys, the bike shutting off when you whack it hard is a lean condition as the leak jet stock 55 is not enough fuel to compensate for the extreme change in throttle position. Put a 50 leak in it, and its not gonna be completely fail safe when the bike is on the stand. It should still hiccup a tiny bit when you whack the throttle at lightning speeds. The fuel delivery at speed is different than on the stand.

If the bike ran fine one week and not the next without modifications, why make him swap out jets - that just makes NO sense.

when you cleaned the carb, did you take ALL the jets out? If you didn't you could still have dirt in it clogging your jets.

if it's not dirt - I'm also leaning towards an AP issue, the AP timing screw is VERY sensitive, if it moved at all, it will throw your AP timing out causing a bog on throttle - recently happened to my 400, 1/8th turn in on the screw and it was running perfect again.

If that doesn't have an effect, then adjust your pilot screw to get everything dialed in right (you can search on how to properly do this) , changing jets should really only be done if:

your temps/humidity are way off your normal riding temps/humidity

your elevation changes drastically

you make physical performance changes to the bike.

Your jets look worn - likely NOT the problem here

Thanks everyone for the advice. I did clean the individual jets taking them out. I sprayed gum out where the jets where and compressed air. I didn't soak the carb though. I was told by the owner of my local shop to re-clean the carb again.

I agree with the last post regarding changing jets. The bike was great one week, bad the next with no changes in temp/altitude. I ordered a 50 leak jet and turned out the pilot screw two turns, I think I'll try going out to three. I'm also going to check the AP screw and try to tighten it.

The spark plug was black on the threads and the center was dark tan, this is a rich condition right?

Guys, the bike shutting off when you whack it hard is a lean condition as the leak jet stock 55 is not enough fuel to compensate for the extreme change in throttle position. Put a 50 leak in it, and its not gonna be completely fail safe when the bike is on the stand. It should still hiccup a tiny bit when you whack the throttle at lightning speeds. The fuel delivery at speed is different than on the stand.

I think i already said for him to try a 50.:thumbsup: Anyway, thats whats working for me.

if you are more than 2.5 turns out on your fuel screw, just run a bigger pilot. Next bigger size.

Well I have some new developments. I was adjusting the AP screw and fuel screw to see if I could dial in the settings. I had the bike running good and when I hit the gas hard the bike didn't hesitate. On the fourth twist, the problem started up again and kept getting worse. I had to increase the idle to keep the bike running and eventually had to stay on the gas to keep the bike running. As soon as I let go of the gas the bike shut off and wouldn't start back up. I changed the spark plug which looked fouled and it still wouldn't start. I had to hold the throttle 1/4 turn to get the bike to start and when it did it ran like crap. It sputtered and backfired. I put the fuel screw back to where it was originally and it didn't change anything. When I took the spark plug out it was wet. Now I think the bike is getting too much gas. I think something is broken somewhere and it just keeps getting worse. I don't think I need an adjustment with any jets since the bike was running great last week. I was going to take the bike into a shop unless anyone has any ideas.

Definitely sounds like you got crap in the carb.

I'd disassemble it COMPLETELY (except the TPS), clean it thoroughly, check your plate on the slider for fatigue or cracks, use a tooth brush to clean the orifices in the jets, use compressed air to blow out the passages inside the carb after some soaking in brake cleaner or carb cleaner (only with all rubber/plastic parts removed and set aside).

Also, print off a microfiche exploded view from an online retailer and make sure that you have EVERY little part shown.

Is it possible that it's an ignition or throttle position sensor problem? Have you inspected the accelerator pump diaphram? If you can have the shop run it on an egr machine to check if your running rich or lean. Best of luck.

I think I got it! Last night I cleaned the carb again taking out all jets. The bike started up but still had the bog when I hit the gas hard. I figured I'd go ahead and try the AP mod since it wouldn't cost anything. I only did the wire tie part and the bike ran great. I hit the gas hard and it didn't hesitate even when I hit it in low rpm's. I went out to a practice track today and rode. The bike ran like it should. Thanks to everyone who patiently stayed with this thread. Thanks for all of the tips and advice. You guys saved me a lot of money. The race is tomorrow, hopefully I'll do well. :thumbsup:

If you used wire here is possibility for you to brake acc pump arm then you wired, when twisting to full throttle. I suggest using as small and as thick rubber O - ring as you can find instead of fire. I don`t know about US, but here I found the best O-ring of that sort to be 6mm in diameter and 2,5mm thickness. Works great!

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