Bottom end dead spot?



11 replies to this topic
  • RodH

Posted July 01, 2000 - 11:56 PM

#1

I have just increased the number of discs on my E-Series hawkins to 8, and put in a 170 main jet. It's a 00 WR, and I still have the 42 pilot jet, and it was backfiring when backed off when the pilot screw was set at 1-1/2, and it had a big dead spot right off the bottom, when I turned the screw out to 2-1/4 turns it now runs perfect everywhere and the dead spot is not as bad, but it's still there. I am going to change the pilot to a 45 since richening it helped it before. I am just wondering if anyone thinks that I am not going in the right direction ? I am thinking that the dead spot is caused by the pilot jet being too small ? I am also putting in a carbon fibre air box at the same time, and it should flow a little better, so the 45 pilot shouldn't be to big.

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RodH Canberra, Australia<A HREF="http://www.400thumpers.oz-au.com" TARGET=_blank>
400 Thumpers Australia</A>
2000 WR400F See Photo's and Modifications

  • James_Dean

Posted July 02, 2000 - 06:34 PM

#2

RodH,
That sounds like the right direction. Are you running YZ cam timing? If the #45 is too rich you could also switch to a bigger air jet #100 to fine tune it.
James Dean

  • RodH

Posted July 02, 2000 - 07:14 PM

#3

James, I still have the WR timing, but I was considering YZ timing, I just need a prod in the right direction and be conviced that it's worth it. Some people say that the difference was huge, and others say that it's not noticeable, so what to do ?
Anyway, in going to YZ timing, what should I do in regards to the setup I have now? Should I run more discs and increase the any of the jets ? What about the needle jet with YZ timing ?




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RodH Canberra, Australia<A HREF="http://www.400thumpers.oz-au.com" TARGET=_blank>
400 Thumpers Australia</A>
2000 WR400F See Photo's and Modifications

  • Taffy

Posted July 02, 2000 - 11:40 PM

#4

Rod

you could be the perfect guy to try out my theory.

when you get ride of restrictions on the inlet side the air pressure loss on wacking the throttle open isn't as great. i.e the air gets there earlier. the result is not as much suck at the carb & so it doesn't get the fuel it wants. do you follow. we used to run bellmouths & take .5mm off the bottom of the slide. with that the low speed circuit would be jetted down (yes down) to suit. air speed through the smaller cutaway would speed back up, then when you open the throttle-bam!

i'm not sure which year WR your running but if it's a 98/99 you could find an old throttle valve, take off .25mm & see how it goes. it used to be a standard mod on roadracers.

  • James_Dean

Posted July 03, 2000 - 03:08 PM

#5

RodH,
After just having changed my exhaust cam to YZ timing, I can give you another perspective.
-The jetting needed to be leaned out a little down low to get good response. (needle clip and pilot scew) This is typical comparing stock WR to YZ jetting.
-Performance wise it feels like the WR timing had better low end pull with the stock muffler and baffle out. The YZ timing with the same exhaust had better top end power and was softer on the bottom. Then the YZ timing was tried with a YZ tailpipe and muffler. The low end power felt stronger and it boosted the top end more yet.
-The best cam timing probably depends on how much back pressure the exhaust has and whether you want a stronger or softer bottom end power. Riding in second or third gear mostly would make WR timing more appealing. If you can hold it open alot and can use the top end power, then YZ timing. More open exhaust makes the YZ cam timing best overall. It mostly depends on experience and where you ride.
-Either cam timing is better than a DRZ or KTM 400SX from my perspective.
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Taffy,
Good theory, I like the mix of less cutaway and jetting down the low speed. The circuits overlap making for an unusual mix that way. The dirt bike tuners rarely go to less cutaway and it shows their lack of knowledge when jetting. We have much more flexibilty with needle dameters, clip, and tapers now. This makes isolating changes easier. It's hard to tell if Rod will need to richen with the air box change down low or not.
James Dean

  • RodH

Posted July 03, 2000 - 04:07 PM

#6

Thanks James, you have convinced me that the WR timing may in fact be better for me. One thing though, I understand that with a tapered header pipe and E-Series the bottom end doesn't suffer when YZ timed as it would with the standard exhaust, anyone like to verify this ?
This weekend I will be putting the air box on, and changing the pilot jet from a 42 to 45, and then see how I go with the dead spot.

Also I am putting on a works connection pro-perch with the quick adjuster and de-comp lever, it gets 10 out of 10 for looks, but it will be interesting to see if there is any beniefit with the new clutch setup.




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RodH Canberra, Australia<A HREF="http://www.400thumpers.oz-au.com" TARGET=_blank>
400 Thumpers Australia</A>
2000 WR400F See Photo's and Modifications

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  • Mitch_from_Oz

Posted July 03, 2000 - 04:10 PM

#7

Hey Rod,

What altitude are you typically riding? Also, you mentioned airbox, are you refering to the DSP Doug henry unit with velocity stack????

If so, then here are the settings I am running:

180
45
2.25 turns out.

I hope you like the new airbox. It is specifically designed to improve the bottom end. The velocity stack itself is tappered.

Mitch

  • RodH

Posted July 03, 2000 - 05:25 PM

#8

It's a DSP CF air box, but it uses the original boot, and only costs $700aud. If I bought the other one, I might have risked divorce as the cost of aftermarket parts has now reached $3,200, and I still haven't got the Magura tapered bars, scotts damper and quick pull throttle.

I usually ride between 500 and 1500 metres, I think ? I will see how the jetting goes, as I am going the next size up with the pilot, and also I have the E-Series hawkins, which doesn't flow like the DSP pipe. Is your DSP the SS or TI ?


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RodH Canberra, Australia<A HREF="http://www.400thumpers.oz-au.com" TARGET=_blank>
400 Thumpers Australia</A>
2000 WR400F See Photo's and Modifications

  • Taffy

Posted July 03, 2000 - 10:09 PM

#9

Rod

if you are smoothing out the air at low speed you will be slowing it down. you'll need to go richer(modified text).

i always look at mixture as a rush of people past a doorway running for a bus.

they have all been held back around the corner by a restriction & now those that can get through can run past the door very fast! that means lot's of vacuum =pulls more fuel

if you smooth it out & there is no restriction loads of people qeue past the door in a slow orderly file = low vacuum = pulls less fuel, slow to respond due to slow air speed. so my guess is that you will need to jet up (modified text) & thats when your, pilot, accelerator, idle, cutaway the lot need playing with. have fun!

how come you didn't take the bait rod! surely someone in oz replaced the slide as a precaution & you can try the mod for $5aus?

good luck.

[This message has been edited by Taffy (edited 07-04-2000).]

[This message has been edited by Taffy (edited 07-04-2000).]

  • RodH

Posted July 04, 2000 - 03:59 AM

#10

Thanks Taffy, I will fit the air box first, and then start palying with the jetting.

I wish you didn't tell me that I will have to play with the lot :), as I was hoping that I nearly had it right, it's running 99.5% right, except for a very small dead spot right off idle

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RodH Canberra, Australia<A HREF="http://www.400thumpers.oz-au.com" TARGET=_blank>
400 Thumpers Australia</A>
2000 WR400F See Photo's and Modifications

  • James_Dean

Posted July 04, 2000 - 11:25 AM

#11

Taffy,
Something was lost in your analogy. When there are lots of people there is lots of air(more dense air) and therefore you would want more fuel to match it. The lack of vacuum would also encourage you to jet up. The decrease in cutaway is jetting up not down. Do you agree? The fact you were jetting down on the pilot with a bellmouth was probably offsetting the richening of the cutaway at idle when the air was moving so slow you didn't want to change richer.
James Dean

  • Taffy

Posted July 04, 2000 - 12:06 PM

#12

Hi james.

no, the fuel is stood back in the doorway & is only drawn out by the speed of the air-not how much. the engine says gimmy it's the boss. air must always come before the fuel your job is to jet to the air that goes through.

a big cutaway (larger hole-air slow) means a slow lazy reaction. splutter,glitch etc.

the manufacturer knows that what air can get through those 80's small filter boxes will be going like the clappers for the cutaway so it slowed it down by making the cutaway bigger. it's still relevant today. we're all running hi-flo air filters, snorkel off, baffle out. where's the vacuum gone that's going to draw this fuel up?

small carbs work well because the air is travelling through the venturi that much faster. a fast airspeed will always get a better snappy reaction. agreed?

imagine now, how quickly that air is now going through that new piddly little cutaway, it gets what it wants & more!

it over compensates.

the important point here is to get your air speed backup. once you've done that you always jet down because the carb gets what it wants by going through a smaller hole quicker.

i know what your thinking, it's pro rata it is'nt. i don't read the books but i know that the mixture gets richer as the speed of air goes up on a steepening scale.

every other part of your bike should be set by the book, there's only one place you listen to the engine & not the book & that's carburation.

come back at me james, i know this is something you've spent a great deal of time on & i enjoy reading of the hard work you've put in.

oops james i've just spotted that my reply to rod is wrong, smoothening out the airflow means more will get there right? so it will slow down & therefore rod needs to jet up to make up for the lack of vacuum. sorry rod

[This message has been edited by Taffy (edited 07-04-2000).]




 
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