GX390 carb issue-VIDEO

astrowrench

Member
It surely appears to be lean with that exhaust fire and intake popping, gonna have to think about this one, cus your there and i'm here
make sure no bowl vent is blocked on carb try revving it holding your hand partially over velocity stack by varying amounts and see what happens
or use the choke if your jumpy!!!!!!!!!!!!!!!!
 
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2kdime

New member
Hey Astro

No bowl on this carb

If i hold my hand over it it just draws in even more fuel. I put the filter on trying to do the same thing. I could look down and see the air filter glowing.

Plug was black when i pulled it

 

Kart43

Member
Air filter glowing? Yes putting your hand over the carb should cause it to draw more fuel. You should be able to turn the high speed adjustment screw in or out to make a difference, then report results. You really haven't given much information and the video only scans around your garage with the sound of an engine that stalls, we cannot tell if you were using the throttle, we cannot see if there is an air filter etc. This is like diagnosing heart failure by text messaging.
 

2kdime

New member
Yeah it pulls so much fuel in that it backfires in a sense and it's burning fuel inside the air filter.

I can run the high side all the way in or out with the main jet in the video and it doesn't make a difference. The main was a 250

You can see in the video we tried giving it some gas and it stalls.

I can ease into it really really slow and get it revved up, but it was weak and would die if I let off.

No air filter in the video, I have to add gas to the carb to get it going. Once it's going I can slap it on.
 

astrowrench

Member
Didn't realize it was a tilly........internal fuel filter restricted due it dried residue from ethanol in our great gas?....... atmospheric vent in diaphragm obstructed... Diaphragm upside down, needle arm bent or set wrong...... needle sticking.Take that carb off put an s/a on and get it running!!!!! and have someone do a pop off and or leak-down on that carb and check your valve settings again and i think it will run Also how is fuel returning?? Good carbs but only for VERY Experienced Tuners IMO
 

2kdime

New member
It's actually a Mikuni Super BN 38mm

No alky just yet, on pump gas. But the internal filter is still in there. I plan on running an external and getting that out of there.

Pop off is set around 4-5

Valve settings as in valve lash?
 

astrowrench

Member
Yes valve lash; and I meant to say diaphragm carb sorry, but where did you get it I'll almost guarantee you it is setup for alky---- dump some in see how it runs you got to tell the WHOLE story....and pop off is OK @4-5 These are great carbs BUT there are so many tiny passages that alcohol reacts w/ alum forming alum.oxide that i have seen plug these orifices it will drive you crazy trying to figure it out
 

2kdime

New member
Carb is brand new from Niche Cycle, it's a watercraft carb

It's setup for gas

I got pop off down to 4-5 myself

There are places that convert them over, for around 70-80 bucks.
 

astrowrench

Member
I must admit the super BN is new to me but over the years did a lot of bn's and fuel flow& pressure (for lack of a better term) was controlled by either the banjo fitting or orifice in return line And I did some quick reading after goggling BN tuning I think you should check out those posts I sure wish you were closer I've got a bunch of stuff to do But I Will CK on YOU later And i can't see the return line being plugged completely!?!?
 
Just asking, Did you degree cam properly?
Will it run with a small factory carb?
I was unsure what you was saying in first post. When you said timing were you sayin ignition or cam?
 
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