Bogging under certain conditions (gx390)

D

Dsx_fab

Member
Hey guys, I’m having an issue occasionally.
my setup,
Gx390, arc rod, 30lb springs, intake and 1.5” header, and .042 main. All of this was done a few months ago and no issues since. Until lately.
If I’m maintaining a speed of 4400-4600 range for a while, maybe a minute or so, then go to wot it bogs out and won’t go past about 5100. But then next time I take off, if I just go wide open right away, I can go up to 6200, which is my normal top out.
one thing I’ve noticed is my exhaust, and block, seem to get very hot, hotter than I remember it getting. Could I be over heating? Maybe I need to do a valve adjust? Though that doesn’t seem likely.
I thought I was running lean, but that seemed unlikely with a .042 and my relatively stock setup.
One interesting note, while the bike is on and oxygen can’t get into the exhaust, it doesn’t do anything special, but right after I shut it off you can actually hear it ignite inside the muffler and a ball of flame comes out. THIS leads me to believe it’s rich but idk, that’s why I’m here asking you guys .
 
Check your fuel delivery to the carb and also the shut off rubber under the shutoff. Ive seen those holes in it shrink up from certain fuels.
 
Sounds like you're guessing if it's rich or lean? How much time on the engine since the last looksee? Things like leak down, compression pressure, float height etc. etc. Hot doesn't necessarily mean lean, but it's a good indication. What does the plug look like? What plug heat range? Ambient temperature. You probably don't know, but I have to ask, what about air density? That backfire cells normal, raw fuel, hot pipe, boom!
 
I have seen/driven with a bogging out problem like that not long ago with the same type of engine. Just does not power above 5200. Exhaust was an inferno. I think Alvin is on the right path.

My thinking only is either an exhaust leak or the expansion chamber on the pipe is not big enough.

In building a new one now, I am looking at exhaust with wider pipes and trumpet like expansion chambers . Remember the exhaust stroke has to push all the heat combustion out of the pipe, any restriction will burn it down. As you know heat expands an causes pressure and vacuums as well.
If you are racing it, its worse.

Here is my suggestion . Throw someone a few dollars in an automotive garage and have someone stick an exhaust probe put in your exhaust , and rev it up to 5200 . You should get an idea of what is going on. combustion wise.

Good luck and Race on
Edward
 
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