engine back fire

M

mark1

New member
engine seems to back fire at end of long straight pavement needle set second notch from top . Is this sign of too rich or lean ? power seams ok rpm at 5900
 
I've seen it happen if you had to let off the throttle going into the corner. I'm pretty sure this is a sign of being, if not too rich, overly rich. The idea about the exhaust pipe could be the reason too.
 
yes only at end of straight when he lifts . lock collar is tight . I will try and adjust needle and see if that helps. Thx guys
 
Guys, the sound you are hearing is simply a function of maximum manifold vacuum under deceleration. Fuel will continue to flow, if even for an instant, as you lift on the throttle, so the sound you are hearing is that fuel being introduced to oxygen in the header. There's no reason to adjust anything because of that "backfiring", save those adjustments for acceleration "fine-tuning" to best tune that motor.
 
Last edited:
Flash-o-matic said:
Al, if this is a LO206 running Briggs LO206 rules there is no way it can be rich.
Click to expand...
I don't have a rulebook to tell me why that is. Can you not change the Jets in the L0206? Maybe you're not aware of the fact that if the air density goes down, the engine will get richer. Too rich? I don't know, but it's possible.

A swing of only 5 points in the air density would call for a .001" change in jetting. .038"-.039" is a 5.379% change in fuel flow.
 
Flash-o-matic said:
No jet changes allowed in LO206. Al, I'm 68, lifetime drag racer, well aware of air density.
Click to expand...

If everybody is stuck with the same rule, I guess it doesn't make any difference. What about float height's and metering needle heights?

I can see somebody with 10 or 20 fresh carbs in their toolbox.
 
I can see a day when 2 or more crbs are part of your race program its a bit quicker to change carbs vs float height gosh there are so many variables to deal with iff you race multi track series I live and race at aprox 2300 ft above sea level we rAced at a track 100 ft above sea level as the day progressed the day got hot and dry as a rasult the corrected air density was 2000 ft worked well for us but others were having issues .
 
alvin l nunley said:
If everybody is stuck with the same rule, I guess it doesn't make any difference. What about float height's and metering needle heights?

I can see somebody with 10 or 20 fresh carbs in their toolbox.
Click to expand...

You can change float height and needle height. That said they are not particularly sensitive to small air density changes when it comes to real world on track performance.
As for 20 carbs, you know what they say about fools and money :)

Mark1: Basically, what Gonzari said. If the motor is otherwise pulling strongly and is backfiring on the overrrun I wouldn't be concerned, unless its a BIG bang. It would do no harm to check for leaks in various places while it's mentioned.
 
I agree with Flash, sounds like an exhaust leak somewhere. Check for leaks at #1 the header gasket, #2 at the exhaust header flange and pipe where it's welded and #3 I have seen the pipe crack 4 or 5 inches up from the weld and under the exhaust wrapping. Then after you checked all those areas do a leak down test to see if the exhaust valve is staying open. The valve guides in these motors are ......
 
You must log in or register to reply here.
Back
Top