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Big flat spot mid-throttle

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Hey guys, I recently purchased a used '07 DRZ-400SM with ~14k miles on the engine. It has the following:

  • 3x3 mod + the PO cut some smaller holes (3/4" x 2") in each side of the airbox covers
  • DynoJet (DJ) jet kit: 150 main, needle @ clip #2, 25 pilot jet
  • Extended fuel screw
  • Yoshimura RS-3 slip-on with no baffling (way too loud, re-packing kit and quiet insert on the way)
  • Aftermarket manual (non-vacuum) petcock
  • K&N air filter
  • lots of other mods, but I think those are all that might affect the fueling

When I test rode the bike, I noticed a very obvious flat/boggy spot in the throttle response - I'd guess it's around 1/3 - 2/3 open throttle. According to the TrailTech tachometer it was occurring between 5200-6500 RPMs. You could continue opening the throttle and it would get a lot of power past that flat spot. Also, the bike would pop/backfire quite a lot when downshifting or quickly letting off the throttle. From everything I read (and what other more-experienced people told me) my bike was running lean across the throttle range, with it being especially lean in the flat spot zone. Also of note: the flat spot was much more pronounced at lower speeds/lower gears, and wasn't very noticeable in 5th gear.

I've now taken the carb off twice, and have made the following changes:

  • taped up the side holes in the airbox (left 3x3 hole as-is)
  • sprayed all jets and passages I could find in carb with carb cleaner - none of the jets appeared to be obviously clogged
  • bumped main jet to DJ 155, as recommended by the DJ instructions for high-performance aftermarket exhausts
  • bumped needle to clip #3 to increase richness
  • dialed in fuel screw to (I think) correct setting
  • also replaced spark plug and changed oil & oil filter, cleaned & re-lubed air filter

From my limited testing since making these changes, I believe the flat spot is better but not quite gone, and interestingly it seems to happen slightly higher in the RPM range (maybe from 6000-6700 RPMs now).

Now I'm looking for advice on how to proceed. I'm tempted to buy a JD kit (seems preferred over the DJ kit) and use their differently-tapered needle and see if that improves anything. But spending $80 on a single needle seems a bit steep, and I'm worried it won't magically fix my flat spot. Is there anything else I could do to possibly resolve it with what I already have, or at least diagnose the issue more specifically than I have already? Is it worth toying with higher needle clip positions, or different main jet sizes? Does it sound like the carb still has a clogged-up circuit somewhere?

Edited by tr1z
photos are fun

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 Is it still back-firing? If so, try tightening the exhaust clamp where the headpipe meets the muffler

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Dmouse, I believe it's still backfiring/popping in some situations, though not as much as before. I think the fuel screw adjustment helped alleviate some of this, and possibly the main jet & needle tweaks did too. I'll make sure the clamp is on good and tight when I take the slip-on off to install the new packing and quiet insert I just received from Yoshi. Thanks for the tip!

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 Yeah, virtually every used bike I've bought needed that clamp tightened up, if it's loose the bike will pop and backfire, especially on deceleration

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I'm not a carb guru by any stretch, but mid throttle is need height territory.

I'm at 500ft and my needle id in the 4th down. Seems to pull smooth.

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Thanks for the input Joel - is that 4th clip with the DJ needle, or are you using the JD one?

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Thanks for the input Joel - is that 4th clip with the DJ needle, or are you using the JD one?

DJ

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Ok Joel - I hadn't seen anyone mention using the 4th clip position with the DJ needle. I'll give that a shot if it's still hesitating mid-throttle after my exhaust tweaking.

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