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06 runs great, stumbles, then runs great again


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 I just recently got my bike all prepped after a long Az summer sitting in the garage. Like I do every Fall, I completely go through the bike, disassemble and clean the carb, drain and refill all fluids, recharge the air filter, clean the gas tank, add fresh fuel, ect, ect. The first test ride after all the work the bike ran pretty good but had a bad stumble right off throttle. After some adjusting of the fuel screw and Quick Shot 3, she was back to running like she always has. But within 5 minutes the stumble appeared. Then 5 minutes later, it's gone! (Shortly after that, the airbox caught fire, but that's a whole different thread titled " Had a bike fire today" in general discussion.) I have done all the normal mods (o-ring mod, quick shot 3, merge racing spring, ZipTy racing mod, precise adjustment of fuel screw, proper jetting) to this bike to get the best throttle response with zero bog/stumble. For three riding seasons I haven't had to deal with a bog or stumble and smoke all my buddies that have fuel injected bikes!  Now I'm confused why it would just appear then go away :banghead: .  I have checked the valves, the exhaust and decompression clearance is a little tight, but would that cause an intermitting stumble?  Could it be something electrical?  Thanks for any input!

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I just recently got my bike all prepped after a long Az summer sitting in the garage. Like I do every Fall, I completely go through the bike, disassemble and clean the carb, drain and refill all fluids, recharge the air filter, clean the gas tank, add fresh fuel, ect, ect. The first test ride after all the work the bike ran pretty good but had a bad stumble right off throttle. After some adjusting of the fuel screw and Quick Shot 3, she was back to running like she always has. But within 5 minutes the stumble appeared. Then 5 minutes later, it's gone! (Shortly after that, the airbox caught fire, but that's a whole different thread titled " Had a bike fire today" in general discussion.) I have done all the normal mods (o-ring mod, quick shot 3, merge racing spring, ZipTy racing mod, precise adjustment of fuel screw, proper jetting) to this bike to get the best throttle response with zero bog/stumble. For three riding seasons I haven't had to deal with a bog or stumble and smoke all my buddies that have fuel injected bikes! Now I'm confused why it would just appear then go away :banghead: . I have checked the valves, the exhaust and decompression clearance is a little tight, but would that cause an intermitting stumble? Could it be something electrical? Thanks for any input!

Sounds like you've got a handle on the fcr carbs

Have you ever replaced the vac release plate seal?

I've found on my friends bikes that don't go out much and get cleaned nearly as much as ridden that the brake cleaner hardens the seal and on a 250 I found a ripped one.

a new vac release plate seal always always helps the carb behave better best place to get the vac release plate seal is from any yamaha dealer tell them its for a 2008 yz450f its cheapest that way like 8 dollars. Otherwise Honda nails you like 80 bucks and it comes would the vac release plate.

I've had some accept pump diaphrams melt in our wonderful ca gasoline.

And deffo don't want that decomp mechanism on the tight side.

Adjust it to the loose spec.

Grease your carb boots to prevent leaks

Watch that you don't have a effed up hot start plunger aswell.

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Sounds like you've got a handle on the fcr carbs

Have you ever replaced the vac release plate seal?

I've found on my friends bikes that don't go out much and get cleaned nearly as much as ridden that the brake cleaner hardens the seal and on a 250 I found a ripped one.

a new vac release plate seal always always helps the carb behave better best place to get the vac release plate seal is from any yamaha dealer tell them its for a 2008 yz450f its cheapest that way like 8 dollars. Otherwise Honda nails you like 80 bucks and it comes would the vac release plate.

I've had some accept pump diaphrams melt in our wonderful ca gasoline.

And deffo don't want that decomp mechanism on the tight side.

Adjust it to the loose spec.

Grease your carb boots to prevent leaks

Watch that you don't have a effed up hot start plunger aswell.

 

 Thanks Englert, I stopped by the dealership today and picked up a vac release seal for $13! I'm gonna get the bike all back together this weekend and test her out. My old vac seal wasn't ripped or torn, but looked old. So I'm hoping that this new seal fixes my issues. I'll report back with the results.

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UPDATE:  After a bunch of cleaning and repair, I finally got the bike back together and she runs great! :banana:  I was getting frustrated and couldn't find any problems in my carb.  I think the vacuum plate seal that Englert suggested replacing was the key to my stumbling issue.  Even though my original vac seal looked good with no rips, it must have just been old and shriveled up! The bike now runs flawless with no stumble at all. I can give it gobs of throttle off idle and absolutely no bog. I hope this thread will help somebody else with a hard to fix bog problem!

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