Back Fire

[sandman]

The plug color is consistant with a lean mix and that could be a vacuum leak. I would change plugs for 1 that plug shields spark. I would recommend a std style. However you can put it in and start bike. Turn idle mix screw on #2 all the way in and see if idle circuit is operating. Engine should lug down. Open screw until rpms peak and a 1/4 to 1/2 turn farther or until rpms just start to come down. I see my std ngk,s shield mix by being a nice color everywhere but behind gnd electrode, is white. Good luck

[Donvito]

Checked the sliders and they are all moving. I also let the bike idle for about 5 minutes, turned it off and immediately pulled #2 plug and it was dry. Still searching for a vacuum leak but can't locate it. I guess I'll pull the vacuum hose from #2 and plug the opening and see what happens. At least that should tell me if the hose is the problem.

[Donvito]

Here are the plugs I pulled out on Saturday after about 500 miles. Three look the same and one is darker. Unfortunately, I can't identify which one came from where. However, this is before I adjusted the fuel/air mixture.

[Beau-Kat]

Who did your valve adjustments?

 

Just wondering, after all the things you have thoroughly checked, sounds like the #2 exhaust valve could possibly be slightly too tight and not seating. This could allow fuel drawn in on the intake stroke to seep into the exhaust on the compression stroke before combustion. Then, that seeping fuel will ignite in the hot exhaust. Just a theory.

 

I've seen a synch look ok while there still existed a slight vacuum leak.

[VentureYZ]

Correct me if I am wrong but the vacuum line going to #2 carb also goes to the pressure sensor. If it is bad you could be sucking air. You can check it with a meter and a vacuum pump and looking at the Yamaha service manual

[Donvito]

Who did your valve adjustments?

 

Just wondering, after all the things you have thoroughly checked, sounds like the #2 exhaust valve could possibly be slightly too tight and not seating. This could allow fuel drawn in on the intake stroke to seep into the exhaust on the compression stroke before combustion. Then, that seeping fuel will ignite in the hot exhaust. Just a theory.

 

I've seen a synch look ok while there still existed a slight vacuum leak.

 

I did the valve adjustment but the backfire was happening prior to the adjustment also.

[Donvito]

Correct me if I am wrong but the vacuum line going to #2 carb also goes to the pressure sensor. If it is bad you could be sucking air. You can check it with a meter and a vacuum pump and looking at the Yamaha service manual

 

That makes sense. The only test I can find in the manual is to test the output voltage. Is that what you meant or is the test elsewhere? If not, can you explain how to test the pressure sensor system to make sure it's not sucking air?

 

Does it matter which cylinder the vacuum hose is attached to? If not, I can switch the vacuum hose to #1 and put the plug on #2 to see what happens.

[Huggy]

That makes sense. The only test I can find in the manual is to test the output voltage. Is that what you meant or is the test elsewhere? If not, can you explain how to test the pressure sensor system to make sure it's not sucking air?

 

Does it matter which cylinder the vacuum hose is attached to? If not, I can switch the vacuum hose to #1 and put the plug on #2 to see what happens.

 

Vacuum hose can be connected to any plug. Also tight Intake Valves can cause the symptoms you are having.

 

 

Edited July 30, 2013 by Huggy