DRZ400SM has a serious flat spot at mid-RPM

[osteper]

Hey guys, I need some help with my DRZ400SM 2008 model.

 

It has a serious flat spot at mid RPM, where it doesnt accelerate at all. I need to gear down to get over the flat spot, if not it will just stay there and run shit.

I have tried to ride with the airfilter cover off, which made it run much worse. Then I tried to tape 1/3 of the 3x3 hole, which made little to no difference. I then taped the whole 3x3 hole, but that didnt make much difference either.

I just can't figure what the problem is, so this is where I hope you guys have any tips! ?

 

Mods done on the bike: 

-155 Main jet and 4th clip on the needle. (I do believe this is from a JD jetting kit from the previous owner.)

-MRD ProComp exhaust

-3x3 mod.

 

 

Any help is appreciated! ?

[Stuwy123]

Isn't the suggest main jet a 160 for the MRD?

What elevation are you at?

[Rock Monster]

If your running a stock carb, try running a 150 main

 

Im running 150 main and have SSW/MRD Full system, 3x3, Cw 440 big bore kit. Runs great. I had the same problem you have with the 155.

[osteper]

Im right above sealevel Stuwy123.

Thanks for the tip Rock Monster, will try that!

[Jason.]

Is it corked? Mine bike's set up real nice when the pipes open, but introduce a DB reducer for the MOT and there's a noticeable flat spot in the middle... Remove it and the powers back

[Rock Monster]

Im right above sealevel Stuwy123.

Thanks for the tip Rock Monster, will try that!

Yeah I am running the 150 main here at 1500 to 6000ft elevation.

[carlspeaky]

mine was flat with the 155 MJ on the mikuni jD kit with open drz E exhaust but it was fine when i swapped it for 160

[Craigo]

Stock bore ? with open pipe use 160 main jet and Blue needle clip 4

[osteper]

Stock bore ? with open pipe use 160 main jet and Blue needle clip 4

Yeah stock bore and open pipe.

 

Allright, will try a 160 jet first, if that doesn't work Ill try 150. ?

 

Just sucks that no dealer around here has the jet I need so I gotta order one, takes forever to ship from USA to Norway. :/

 

Anyone know where I can buy a 150 and a 160 jet for my Mikuni CV 36 carb? Thumpertalk's store just seem to have 4-packs. 

Edited August 29, 2014 by osteper

[osteper]

Think I might found one, is this the right one for a Mikuni CV 36 carb? 

https://www.thumpertalk.com/shop/Cycle-Craft-Main-Jet-160-20722-p4573764.html

 

 

Edit: Nevermind, found on eBay. ?

Edited August 29, 2014 by osteper

[hinksy]

Osteper , when you change those jets , just check the diaphragm is seated properly and it isnt perished/cracked . Also make sure the little o-ring is located properly . As the carb works on air pressure , it sounds like you are loosing the pressure to lift the slide when you need it ( vacuum ) You may have an airleak somewhere , greg

[BLKRAWB]

Mine had a stupid flat spot at highway speeds, i only had to relocate my breather hose out of the wind to solve it.

 

434cc hotcam intake, mrd ssw, 3x3 158 main jet blue needle on 3rd clip i believe. 900ft above sea level   It even runs a little rich on a hot day at WOT under in the early half of 5th gear.

[osteper]

Osteper , when you change those jets , just check the diaphragm is seated properly and it isnt perished/cracked . Also make sure the little o-ring is located properly . As the carb works on air pressure , it sounds like you are loosing the pressure to lift the slide when you need it ( vacuum ) You may have an airleak somewhere , greg

Thanks for your reply, I have already checked the diaphragm several times, aswell as the O-ring. It looks perfectly fine. I also checked the vacum hoses, which were in good shape aswell. ?

 

Mine had a stupid flat spot at highway speeds, i only had to relocate my breather hose out of the wind to solve it.

 

434cc hotcam intake, mrd ssw, 3x3 158 main jet blue needle on 3rd clip i believe. 900ft above sea level   It even runs a little rich on a hot day at WOT under in the early half of 5th gear.

This can't be my problem, as it happends at any speed. ?

[BLKRAWB]

perhaps try a different needle profile? it could be fuel curve transitions from low to high causing you issue. shame, drz really shines mid range.

[Craigo]

Yes, what Osteper is describing in his opening post sounds needle related more so than main jet..

Also check the slide diaphragm and vacuum lines are in good shape (as Greg mentioned)

Edited August 31, 2014 by Craig-o

[osteper]

Yes, what Osteper is describing in his opening post sounds needle related more so than main jet..

Also check the slide diaphragm and vacuum lines are in good shape (as Greg mentioned)

Thanks for your reply! I forgot to mention that I have already tried all possible needle settings, I noticed a slight difference, but there is still a serious flat spot there which it cant even come over, need to gear down to ger over it.

Allmost like a rev limiter, just more smoother, there are no strange sounds when it happends.

Since the needle adjustment made a slight difference I think its worth trying a couple of other jets. Got a 150 and a 160 jet coming in the mail in a couple days. ?

Edit: The diaphragm and vacum hoses are in good shape, as stated in an earlier post. ?

Edited August 31, 2014 by osteper