Bog when cracking open throttle

So I just go this bike, 1998 yz400f the owner before me just did a carb swap to the 426 carb so it has the hot start. So when i was riding it when I cracked the throttle open all the way it kinda bogged and then I let off right away. What do I need to adjust to fix this issue?

So I just go this bike, 1998 yz400f the owner before me just did a carb swap to the 426 carb so it has the hot start. So when i was riding it when I cracked the throttle open all the way it kinda bogged and then I let off right away. What do I need to adjust to fix this issue?

 

1. that scenario of going to wide open throttle is a factor of the accelerator pump system

2. you can reduce the 'bog' by fixing/maintaining/updating the accelerator pump system

Ok, I'll clean the carb after school tomorrow.

It's going to be carb cleanup either way, but the source of your symptoms depend on whether the engine

 

1) initially stumbles, or bogs, then will catch and run and tolerate full throttle, or

2) won't take full throttle regardless of how quickly the throttle is opened.

 

(1) is a fault with the pilot (idle) circuit, accelerator pump, air leaks, needle selection or setting, while (2) is pretty much just either restricted intake air or a main circuit problem.

If I go a bit slower it will take full throttle and when I crack it in bogs but then goes.

If I go a bit slower it will take full throttle and when I crack it in bogs but then goes.

 

Yup, that's the apump not working right.

 

Service it and it should be fine.

So I did a little more time on it and what I nodiced was even if I held the throttle steady it was kind of stumbling and backfiring. I took the valve cover off to find them extremely tight!

Both EX: 0.08MM Spec : 0.25MM-0.30MM

In: one 0.06MM one 0.05MM and one 0.03MM

Spec: 0.15MM-0.20MM

Do you guys think this could be a factor in my issue?

So I did a little more time on it and what I nodiced was even if I held the throttle steady it was kind of stumbling and backfiring. I took the valve cover off to find them extremely tight!

Both EX: 0.08MM Spec : 0.25MM-0.30MM

In: one 0.06MM one 0.05MM and one 0.03MM

Spec: 0.15MM-0.20MM

Do you guys think this could be a factor in my issue?

 

Yes

Ok, I'm gonna shim them when I can. I also took the accelerator pump apart to to find the diaphragm destroyed. So I think both of those are my issue.

Shimmed the valves and replaced the diaphragm. Runs better now but from 3/4 to full throttle it bogs really bad and tries to unless I ease off. Is this a Rick or lean condition? The plug had carbon build up on it and I just replaced it with a new one.

Shimmed the valves and replaced the diaphragm. Runs better now but from 3/4 to full throttle it bogs really bad and tries to unless I ease off. Is this a Rick or lean condition? The plug had carbon build up on it and I just replaced it with a new one.

 

You have fixed one part of the apump system, the diaphram.

You still have to verify the passages are clear, the check ball is not corroded, etc.

 

The fastest way to check the apump is the verify the squirt quality.

Upon WOT held open (engine off) the squirt should shoot all the way into the head, for approx 1 full sec, then diminish.

 

Typcially, the leak jet is too small (not enough fuel), the diaphram is 'short' (they make short, medium and long throw diaphrams and push rods), the check ball is stuck partially from corrosion, and the nozzle inside the carb is partially corroded, and the stream is weak or blocked or shooting the wrong direction.

 

Verify you squirt with a mirror inside the boot, or lift up the subframe and watch it squirt (the best way).

 

They you can let us know what you find.

 

If you want, you can read a few threads on the fine art of the apump and fuel screw... not all will apply to your earlier version of the FCR carb, but you will get the idea.

 

http://www.thumpertalk.com/topic/1053317-06-crf450r-still-bogs/?hl=%2Baccelerator+%2Bpump#entry11143305

Ok, I'll do that this weekend or during the week of it isn't too dark.

The symptoms you describe (bogs beyond 3/4 throttle) are unrelated to the accelerator pump system, and indicate one or more of:

  • Clogged air filter
  • Partially blocked main jet
  • Restricted fuel flow into the carb

...particularly if the symptoms get worse as RPM increases.

Air filter was just cleaned, haven't checked the main jet and the fuel flow shouldn't be restricted.

I'll ride again this weekend and see if it changes when it gets warmed up more. It was still kinda cold so I'll try again.

It could also be that the carb is jetted too lean for your engine. The smaller cc engine may have less vaccum draw to pull in fuel so you may need to raise the needle (lower the clip) and increase the main jet size.  First make sure it is clean like others said.

Ok, I've noticed it does get pretty hot. How hot do these engines usually get?

The header will normally get hot enough to glow a dull red visible in twilight or deep shade within 2 minutes.  Means nothing.  The lack of air circulation and low water circulation rates at idle will cause a normal YZ400 to boil coolant out the overflow in the same amount of time if left at a standstill.  Under way, coolant temps run all over the map, up and down from 120-270 ℉

The carb being lean will also make it run hot.

Ok, is the acc pump needs some work. I took the sub frame off and cracked the throttle like mentioned above and it ither shot out, or misted out but most of the time just dribbled out. I sprayed the passages with brake clean and they made it through but when I put it back together it did the same thing. Do you think maybe the o ring by the diaphragm is done and maybe it's leaking past it and going back to the diaphragm?

Also when it did squirt good it shot for a good 3 seconds.

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