Author Topic: B12 Backfire problem (Read 7678 times)

89hawk · « **on:** July 28, 2006, 03:09:41 PM »

I just checked the valve clearance and started getting backfires.

It is backfiring thru the carb, very lttle ones. Sounds like someone spiting. They happen when giving the motor some gas and only under 2000 rpms. Other then that the bike runs good.

When doing the valves i found all the exhaust one to be tight. I set them to spec and now the bike idles at 800rpms instead of 1000.

Everything is stock on it. It is a 2000.

Thanks

rider123 · « **Reply #1 on:** July 28, 2006, 06:34:42 PM »

The little "spits" or "sneezes" through the carbs is usually a condition of the mixture being too lean. Do you have the mixture screw plugs drilled out? You could open up the mixture screws a bit then adjust the idle to where it should be.

The only theory I can come up with regarding why this is happening now after a valve adjustment is that you said all were too tight except an exhaust valve. If the valves are too tight they don't lift as much and only suck in enough fuel/air mixture as the tight valves will allow, robbing you of power because of the less volume of fuel/air mixture in the cylinders. When you open up the valves to their proper spec the engine will try and suck more fuel/air mixture into the cylinders on the intake stroke than when the valves are tight. If the jetting was borderline lean, like all Bandit 1200's stock jetting is, than it might not get enough fuel/air mixture to consistantly have enough fuel to air mixture ratio to consistantly detonate on the power stoke. Hence richening up the mixture a tad may compensate. They really shouldn't be affected much though, weird. Anyone else have a theory?

BTW the bike should idle at 1200-1300 when fully warm. If you run it any lower there is not enough top end oilflow at idle.

tannerismyhero · « **Reply #2 on:** July 29, 2006, 01:57:46 AM »

Any other mods on the bike such as in exhaust and air filter?

chupacabra · « **Reply #3 on:** July 29, 2006, 07:41:34 AM »

I don't want to shoot down your theory 123, but I beleive tighter would mean more lift. And too tight and they may not seat and you end up with burnt valves.

89hawk · « **Reply #4 on:** July 31, 2006, 11:25:46 AM »

No mods to the engine. Bone stock. Only the exhaust valves were tight. Bike has 32,000 miles on it.

I will drill out the mixture screws this week and adjust them. I was planing on getting the jet kit from Hole Shot. Sounds like it is a good time to do that.

I will also rase the idle to 1200.

Thanks

Bazza · « **Reply #5 on:** September 10, 2006, 04:14:31 AM »

Did you Sync the carbs after the valve adjust?

Out of sync carbs will also cause the spitting you are hearing, and it is recomended to sync carbs after avery valve adjust.

If you have set the tollerances to spec, this is the only thing I can think of that would cause your spitting, other than a vacuum line that has fallen off.

Rocketjock · « **Reply #6 on:** September 13, 2006, 11:45:50 AM »

And did you recheck the tappets after retightening? A lot of the time they can go out of adjustment when tightening down without the right tool.

kedeg_97B12 · « **Reply #7 on:** September 13, 2006, 12:26:17 PM »

Did you take the carbs off to gain a little more clearance for the valve cover?
If so then one of the boot clamps may be loose.

Yes they are totally right about needing a sync afterwards.