bottom end poke!

6 replies to this topic
  • Taffy

Posted October 17, 2000 - 03:46 AM


can you help with the following

where can i buy different throttle cams (as in the twistgrip &/or on the carb)?

has anyone run without the APJ connected?

results of experimenting with the TPS?

does the P-38 make a real difference or is it a gimmick?

did the carbon airbox have any effect on the very bottom end?


  • Hick

Posted October 17, 2000 - 10:14 AM



Boit put a P-38 on his YZ and liked it, but he may have changed jetting at the same time.

I did run my 426 with the accelerator pump disconnected. Damn thing barely ran unless you eased on the throttle and then left it alone, it took a deft touch to bring rpms up without killing the motor. I can see how someone with a clogged or sticking pump would think:

1) timing is not advancing
2) hot start is leaking
3) jets are clogged
4) gas is bad

As the bike pops, backfires and coughs like crazy unless you hold the throttle fixed for a full second and allow it to find its revs.

What are you looking for?

  • Taffy

Posted October 17, 2000 - 11:55 AM


i'm thinking aloud here but i'm just totally pissed at the way the carb is set up. basically it has a very lean needle & makes up for it by pumping neat fuel straight in regardless of how long, how -well you name it. went up the road tonight without the APJ & it went alright. i was being gentle though!

for years we euro's have cursed as we heard of the next piece of legislation was passed in a little country called california.

if a bike could pass legislation there it was ok. trouble is, everyone else has to have the same bloody bike!!!!!

hick, i've been through your APJ posts you had with JD & i presume you have a 00 yam as well. i haven't got an adjuster screw to delay the APJ starting. so your conversation was double dutch to anyone with a pre y2k.


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  • Hick

Posted October 17, 2000 - 02:33 PM


Originally posted by Taffy:
hick, i've been through your APJ posts you had with JD & i presume you have a 00 yam as well. i haven't got an adjuster screw to delay the APJ starting. so your conversation was double dutch to anyone with a pre y2k.


Yeah, I realize that (now). I wonder why they changed to an adjustable pump timing or starting point? There must be some reason, but was it Yamaha’s idea or Keihin’s? The same reason they are changing whatever they are changing on the pump for ’01 (flat spot on tickover)? Is the Yammy the only bike made with the flat-slide FCR (I think it well may be)?

We need some street bikers to answer these questions as I believe this carb has been on various street bikes in one form or another and there is no doubt someone out there with many hours of experience getting the most out of this carb, which, I think, would have to include some fine tuning of the accelerator pump, especially on modified motors.

The one caveat, as I see it, with Yamaha using their (admittedly very good) street bike 4 cylinder technology on a dirt bike is the greater importance of throttle response on a competition dirt bike. I read an interview of the engineer in charge of the YZ400 project and remember him stating that getting the throttle response to be comparable to a two stroke was the biggest challenge. I imagine he would know how to make your FCR work better…

I wonder how a shorter actuating rod would work? If you were brave enough to lop off a section of the rod you would get more delay AND a shorter pump stroke (start later and finish at the same time). Or perhaps Sudco sells different lengths. I remember James said he was going to try one of the different diaphragms offered by Yamaha, perhaps this will be the ticket for your Taffy-timed WR.

If/when you (or anybody) figures out how, exactly, to get the rod out of the carb please let me know. Mine is working smoothly now but I would like to remove and polish or clean it.

  • Matt_Porritt

Posted October 17, 2000 - 03:28 PM


Originally posted by Hick:
Is the Yammy the only bike made with the flat-slide FCR (I think it well may be)?

The DRZ and KTMs (2000 models) use the same carb as the 99 Yamahas.
I think Yamaha is the only onle to use the FCR2 but I bet we'll find it on the 01 Poozukis and KTMs.

**Ride it like you Stole it!**
Matt Porritt
99 YZ400F
Vist the Rubber Chicken Racing Online Shop
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  • Taffy

Posted October 17, 2000 - 11:52 PM


if you have a 99 like me hick, i had the rod off last night. i undid the nut that holds those two springs on. by loosening this i was able to pull the black plastic rocking arm away & push the rod out the back. i pulled mine straight up & out the pump diaphram chamber no problem.

JD has given me a note on how to adjust a 99'er. have a look at one of yesterdays posts.

those diaphrams that james has coming will, i guess, have taller posts to stop the pump working so hard.

i've spent years doing bikes as a hobby & i find myself being embroiled in all this again. it's great because everyday there is someone to chat with about what you've found. but it's only since monday i've looked at the carb really. i can tell i'm going to spend a lot of dead time on this-it's not like the cam timing at all.

the cam timing change has made the bottom end seem dreadful, whereas i know it isn't really. as my post on the cam timing says
wr left for dead by KTM250
YZ left in 1st & 2nd
'Taf Timed' left ktm for dead in 2,3 & 4th gear

i just know the hours i'm going to waist thinking about the carb & it gets me down.

anyway not to worry.....


  • James_Dean

Posted October 18, 2000 - 06:19 AM


No Worries, lets get you up and going.

- The Euro version of the '99WR has a different needle than US. It has a DXM needle. The X is very lean for the clip position. This is 2 clips leaner than the US DTM (T-V-X). Your first change is to move the needle 1 clip richer to #5 (same as DTM#3). This will richen it from 1/8 to 1/2 throttle. Many '99 WR riders use this and never have a problem. Mostly at sea level to lower altitudes like you are. The M is a little rich at 1/8 throttle but will still work good, all the KTM's use it. The KTM 520EXC also uses DTM#2 (same as your DXM#4) and it has a slightly lean feeling at 1/4-1/2 throttle.

Try your #48 and pilot screw at 1 1/2 turns to start with (more or less). You would be better off with the #100 pilot air too, so might do it now. No more hesitations. :)

DVP and EKP are both good options, but from what you have said they will need to be used as DVP#5 or EKP#4. :D


[This message has been edited by James Dean (edited 10-18-2000).]


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