M750 Pop/Backfire From Intake

[Rebel7]

Hey all,

I was recently working on my '99 carbed M750 last week and noticed a backfire was occurring when I would blip the throttle. I have never had a backfire from the intake on this bike before, and all I had done that day was replaced the ignition switch, fuel filter and put stock cored cans on it. If I roll on the throttle at a normal pace, it doesn't backfire, but if I blip it quickly, while it's revving back down, it does. Is my air/fuel ratio off possibly? Is this damaging the bike/a problem? I don't know if it's really considered a backfire, it's more of a popping noise, but it's definitely coming from the intake and not the exhaust. I currently have a stock airbox on the bike, but the previous owner took the top section off, so it is just an exposed air filter underneath the tank. Thanks for your help in advance.

[Speeddog]

Most likely the cored cans have leaned it out a bit.

[Rebel7]

That's what I was thinking, the cans were also chopped a few inches too. Could a low idle also possibly be a culprit here? I noticed after the throttle cables were re-routed it was idling warm at ~1000rpms. I looked at DucatiSuite and he had mentioned that to enrich the mixture, I could, "simply back out the fuel mixture screws slightly". Is this all it would take?

[koko64]

Popping worse when the motor is cold?

[Howie]

1000 is a little low.  up it to 11-1200 RPM.  No, this will not help with the popping, though a good throttle synch might.  Set idle and synch first.  Then you might try 1/4 turn on the mixture screws and see what happens.

[Rebel7]

Thanks for the input, koko65, it's popping from a cold start and typically warm. Howie, I realized that I could definitely be running lean since the previous owner removed the airbox lid, so I'm running with an open airbox and cored/chopped cans. Where do I find the mixture screws on the carbs? Do I have to remove anything to get to them easily?

Thanks again!

[Howie]

If you have an open air box and jetting is stock you need to either re jet or get a replacement air box lid.  In my experience the mufflers, as long as the exhaust manifolds are stock, don't make a lot of difference.  The open air box does though, and idle mixture will not compesate for lean running at speed.  Anyway, the mixture screws?  https://www.bing.com/images/search?q=mixture+screws+ducati+monster&view=detailv2&&id=58352A3B4D115B7C7E44B86A3397107CE82C4C52&selectedIndex=7&ccid=dRKayzYC&simid=608024540908815230&thid=OIP.M75129acb36023f7bd7fc00e55893a1c8o0&ajaxhist=0

[koko64]

Howie is spot on. You gotta add more fuel = jet kit, or less air = stock airbox lid.

[Rebel7]

Ahh, okay, honestly, I think I'm just going to try to locate a lid online.

[koko64]

Ás Howie said if the jetting is standard then the lid will work . However if there is a jet kit fitted it will depend on the type of kit. Some kits are calibrated for a stock airbox and some for modified.

[Rebel7]

This may be a dumb question, but how would I know if the previous owner installed a jet kit? Would I have to take apart the carbs?

[Howie]

Unfortunately, yep.

[koko64]

Just a thought, double check the ignition switch wires and connections.

If you are feeling lazy, just put on a stock airbox lid and monitor plug colour.

[Rebel7]

Hey guys, I bought a lid and the popping went away, I was hoping this would help with the jerkiness, but I am wondering if that may be from me being a newer rider. I am getting a lot of jerkiness in lower gear especially when I let on and off the throttle, is this just a Ducati thing or could there be a bigger issue?

[koko64]

Big twins are lumpy compared to an inline 4, but incorrect chain tension and not being smooth on the controls are factors for sure. State of tune is also a factor idle mixture, carb synch, etc.