yz400f throttle bog

Yamaha YZ400F 1998

28 replies to this topic
  • yz400fmaniac

Posted March 02, 2016 - 06:10 PM


there is a bog when i blip the throttle on my bike i know it has to do with the acc pump i watched a video on youtube on how to do the o ring mod but when i took the cover off my 98 yz400f carb has a different mechanism so i am stumped on how to get rid of the lag in the acc pump i will post a pic of what the mechanism looks like so maybe u could give me feedback on how to do a mod on it and if i even can. Also i had all new stock jets put in it and the whole acc pump was rebuilt. 2 rides agoIMG_1821.JPG

  • grayracer513

Posted March 03, 2016 - 07:14 AM




Ultimately, the best fix for an old FCR is to replace it with an FCR-MX from an '05 -'09 YZ450F.

  • yz400fmaniac

Posted March 03, 2016 - 06:24 PM


okay i looked at the mechanism today for a while and the black arm connected to the throttle arm takes about 6 secounds to bottom out after the throttle is pinned. and i cant seem to find a fix anywhere i look, its really pissing me off because i almost wiped out last weekend when i went to hit the throttle and it boged than hit. im not going to put a new carb on because i payed 1300 for the bike and a new yz450 carb is around 400$ theres gotta be something that can be done since the problem has been there since 98. and on the Taffy mod the pump arm looks way different than on mine the plinger arm on mine is skinnier and has a rubber boot on it there is knowwhere to put the mod. 

or am i missing something ?????????????? :banghead:







that is the pic from the taffy mod link not my bike

  • yz250f 78

Posted March 03, 2016 - 07:00 PM


Tie them together! Use grip wire

  • yz400fmaniac

Posted March 03, 2016 - 07:40 PM


ok i will try tying them togeather also i was looking at boyesen quick shot, is it worth it? and does it work because some of the reviews said it was great and others said it dident do anything

  • yz400fmaniac

Posted March 04, 2016 - 04:40 AM


also what is the best slip on exaust i could find for this bike under 350$

  • yz400fmaniac

Posted March 05, 2016 - 08:06 AM


ok i did what they call the eddie mod and my acc squirt went from about 7 secounds to 2 or 3 secounds it still has a pretty bad bog but not near as bad as it was, i am going to put in a #55 leak jet and hopefilly this fixes the problem :goofy:  

  • grayracer513

Posted March 05, 2016 - 09:50 AM


If you have a YZ400 carb, you don't have a leak jet. 

  • yz400fmaniac

Posted March 10, 2016 - 09:50 AM


would a 2005 suzuki rmz450 carb work on my bike http://www.ebay.com/...7tWBGae&vxp=mtr

  • grayracer513

Posted March 10, 2016 - 10:11 AM


Probably would, but there will be a lot more involved.  No cables; you need three.  Hot start?  Might need that too. Then you could run into a more complicated jetting situation than normal; it might have an incompatible needle or an air jet difference.  You could end up spending another $100 to make that one work.

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  • yz400fmaniac

Posted March 10, 2016 - 03:57 PM


ok i put a new air/ fuel needle in today from moose racing the bog is still there but its good enouph for me. on another note this mother &%$#@!er started pissing coolant out the god damn  weap hole after tuning it today all i need to know is how much this will cost to fix because i am about done spending money on it its a blast but if the coolant leak cost more than 50 i will have to sell it and get a 2 smoker because this 4 stroke has been nothing but problems.

  • damnbiker94

Posted March 10, 2016 - 08:08 PM


You will need to buy a water pump seal kit. There is lots of info on how to do this simple repair yourself on this website. Also there is a pretty good YouTube video showing what needs to be done.

4 strokes are very dependable with regular maintenance and if they are not neglected or bastardized with crappy parts.

  • grayracer513

Posted March 11, 2016 - 07:24 AM


Water pump How-To:



  • yz400fmaniac

Posted March 11, 2016 - 09:31 AM


ok i ordered the repair kit but i had plans to go out riding this weekend so could i get away with putting some gasket maker on the cover to seal it and be okay riding it for about 5 hours.

  • grayracer513

Posted March 11, 2016 - 10:19 AM


No.  If you succeed in sealing the weep hole, you will not stop the leak, which is coming from the seal.  The leak, then having no way to escape, will force coolant into your engine oil, and you might loose the entire engine in the process.


You would be better off to try keeping an eye on the coolant level, but bear in mind that you only have less than a half gallon of coolant in the system to start with, and it doesn't take that much time to loose a significant amount.


Just fix it.

  • yz400fmaniac

Posted March 11, 2016 - 01:13 PM


Yah. Thanks for the help somebody told me I could seal it but I figured if that hole was ent sealed in the first place And I sealed it something bad would happen

  • yz400fmaniac

Posted March 11, 2016 - 01:33 PM


ok now im confused yesterday it was dripping from the weap hole, one drip every, about 2 secounds after i had it idleing for about 5 minutes, i shut it off & put it away now today i checked the radiator dosent look like it lost any antifreeze and i started it up today and only let it run for 2 minutes it is 50f where i am im wi and it dident leak a drop i know this model overheats fast so could it have been dripping because of that? because yesterday was the first time i let the bike idle for more than 2 min because the header starts to glow if it sits idleing 

  • grayracer513

Posted March 11, 2016 - 01:53 PM


If it was dripping coolant from the overflow, that's one thing.  That's related to overfilling the radiator, or letting the engine idle too long at a standstill.  There's only on reason for coolant to come out of the weep hole, and it has noting to do with heat.


Having said that, water pump seals can and do leak under one or more of 4 different conditions:


  • Shut off/cold
  • Shut off/hot (pressure in system)
  • Running/cold
  • Running/hot (pressure in system) (possibly only at higher RPM)

It may leak only under any one, or as many as all these conditions, and behave differently at different times.  Once they start, they don't get better.

  • yz400fmaniac

Posted March 12, 2016 - 09:42 PM


thanks Grayracer513 i apreciate it, would you happen to know the stock air/fuel screw setting?

  • grayracer513

Posted March 13, 2016 - 06:57 AM


thanks Grayracer513 i apreciate it, would you happen to know the stock air/fuel screw setting?


about two turns out, give or take a little.  Depends on weather, the individual bike, so on.

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Yamaha YZ400F 1998 by ThumperTalk

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