Suzuki DR-Z400SM rough at high speed

[McKonker]

Hi, I was just out riding my DR-Z400 on the highway yesterday when I noticed something odd. If I let off the throttle at 100kph or so and then open up quickly the bike seems to flood and falter a bit. If I lay on the throttle slowly its fine but I found I could reproduce the problem by opening the throttle quickly. The first time I did it, it also let out a loud backfire. Any ideas on what could cause this? The bike runs great otherwise. Cheers, McKonker

[FastDoc]

Sounds like too rich a main jet, but I'm not sure. Perform a plug chop and take a look at the plug. Is the bike stock? Pipe/jet changed by PO?

A plugged air filter could cause similar symptoms as could a maladjusted float.

How's your gas mileage?

[McKonker]

The bike is stock, at least I am pretty sure it is. My gas mileage is pretty good. I went 65 miles on a gallon that day....whew! maths are not my strong point had to convert 3.5 liters/100km. 8O I made a point of filling it up then stopping after exactly 100km specifically to check the fuel consumption. It's amazing how important that can be when riding out to the mountains (as I am sure you are all aware, as a rookie I am learning). :wink: Forgive my ignorance but what is a "plug chop"? Should I get a new plug and put it in and check the color?

[Weldangrind]

If that's a stock main jet, my guess would be that it's too lean. I just had a backfiring experience yesterday on a scooter I was fixing, and it was from the AIR (air injection reaction) device that draws fresh air into the exhaust stream.

Do you have an AIR device? It would be in the form of a tube that likely connects to the cylinder head. That tube goes to a rectangular block that has a reed valve inside, and it draws fresh air from the airbox. It might also be vacuum controlled.

Whether or not you have an AIR device, you could drop the clip one notch on the needle and see if the problem goes away.

[Weldangrind]

In case it helps, there's a pic of an AIR device that I just removed here: http://www.chinariders.net/modules.php?name=Forums&file=viewtopic&t=12294

[FastDoc]

If it's stock with that kind of mileage it is unlikely I was right about an overly rich mixture. A plug chop will help determine. I don't have time to Google it for you now but if you do you'll see. basically run bike flat out for 30 sec or more then pull clutch kill motor and coast to a stop then remove and examine, PHOTOGRAPH the plug and interpert the results.