Fuel flow problems with Mikuni pump. a/f ratio is Never balanced.

#1
i was going to post in tech section but i think all engines and tanks are different.
Parts: gx390 carburetor and 35lph mikuni fuel pump. 1/4"inch "T" fitting and line.

So i've been working on my engine for many months trying to pinpoint why the engine either wont start ,wont stay started, would only idle if choke is off but only runs with choke open.

Well i've figured out after all these months it is a fuel problem. At first i thought it was valve lash but it isn't. i do NOT recommend going to DIYGK for advanced engine tech answers it took me since december to figure several things out on my own.
```````` Complications #Trial and Error. #Testing so you don't have to.
So i have the mikuni fuel pump now which pumps 35LPH and at first it would flood the carb so i "T" fitted a line back to the gas tank with a 1/4"fuel line. That leaned it out a lot but it could idle but with a really lean mix (engine got warm/hot fast).
````` Tried inserting a .078 Jet in the 1/4" hose but still to no luck(too much fuel). Even tried the jet where the pulse fitting area of the vacuum line is located but it doesn't even start when trying that.
````So i squeezed the inlet on top of the gas tank so it got smaller BUT still had the 1/4 inch line still on. It started and idled but if i want to "run" it i need to open the choke at the same time. So THAT worked but not good enough.

-- So maybe i am answering my own question now by saying i just need a smaller hose somehow coming from the "T" to the gas tank. But that wont work cause i need a 1/4 inch hose for the fitting. Unless there is a restrictor at autozone or a temporary DIY method with PVC glue and drill a hole in small increments.(shouldn't hurt any cause it would just degrade in very minimal increments?)

How did everyone else do it? i haven't seen a thread how to do it. Have seen video with a thread showing it has been done.
Thanks for viewing! This will not only help me but MANY others along their build. I WANT TO GET RIDING! lol
 
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#2
What carb?? It sounds like the fuel pump is controlling the fuel mixture which is a no no. If you have a float carb and you had to install a T fitting to bypass some fuel back to the tank then you have too much fuel pressure which is overcoming the needle seat assembly. Mikuni sells different diameter seats for this issue. get the smaller diameter seat so the float can shut off the fuel against the pressure of the pump. I am just guessing here as I have no experience with a gx390 carb. I have read a lot about issues with flat spots running a gx390 carb on gas but again I have no experience with them. I would not use any jet on the return just a size smaller on the return tubing. If you can get the tank above the carb get rid of the pump.
 
#4
If you ran a return line back to the tank and you're still having an issue then that sounds like a carb problem to me. Once you have a return line the float bowl should stop flooding and of your still having problems it's likely in the jetting of the carb.

Take the fuel pump out and put a temporary tank to gravity feed the carb and see if you still have the same problem.
 
#5
If you ran a return line back to the tank and you're still having an issue then that sounds like a carb problem to me. Once you have a return line the float bowl should stop flooding and of your still having problems it's likely in the jetting of the carb.
That hose was relieving too much of the pressure.
Either need a smaller hose or something to regulate pressure.

i'm going with a fuel pressure regulator,(the inline on/off switch). It would be easier on me at this point.
 
#6
I'm afraid to post a picture cause the engine looks disorganized but here's my setup with the valve.

inline valve
downsized_0617061059.jpg

Attached
downsized_0617061117.jpg
i think the pump would look best on the side of the flywheel cover.
 

CarPlayLB

Well-Known Member
#7
Sounds like you have a jetting problem, amongst others! Have you tried a smaller main jet? Sounds like it is running way rich and probably magnified with fuel pump!
 
#9
Sounds like you have a jetting problem, amongst others! Have you tried a smaller main jet?
Sounds like it is running way rich and probably magnified with fuel pump!
It is a stock jet on the gx390 carb and it runs GREAT when fuel gets to it. Doesn't even need choke open all the way.
Brand new carburetor. Gas flows out of the tank, i've tested it by draining into a clean milk jug.

When i was gravity fed i took it on a speed run and it ran out of fuel 2times which is not fun at all having to blow into the gas tank to force fuel into the carb.

Does anyone know about the 390 carb? i'm going to look into high speed jetting. And if this helps rule anything out I run my kart over 55MPH but i need to be able to idle without moving. As you can plainly tell i'm still a newbie to this.


Why are you running a pump on a gravity fed system????
cause of the lean condition i suffer from.
 
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#10
So without the pump it runs good until it runs out of fuel? This is on a kart correct? Is it the stock tank? What engine is this? When it runs out of fuel if you take the cap off the tank and wait a minute will it again start and run OK? Could the vent be blocked? If the tank is above the carb then get rid of the fuel pump and trouble shoot the original problem. You are just adding another problem on top of your original one. If the gas cap vent is OK I would suspect a clogged fuel screen or obstruction at the carb. Does it still have the gas shut off?
 
#11
So without the pump it runs good until it runs out of fuel? This is on a kart correct? Is it the stock tank? What engine is this? When it runs out of fuel if you take the cap off the tank and wait a minute will it again start and run OK? Could the vent be blocked? If the tank is above the carb then get rid of the fuel pump and trouble shoot the original problem. You are just adding another problem on top of your original one. If the gas cap vent is OK I would suspect a clogged fuel screen or obstruction at the carb. Does it still have the gas shut off?
-Without the pump it does run good temporarily i'll say up to 2 minutes at a time. Tank drains freely and i can see the carb is getting fuel.
-Yes it's on a gokart and right now it has the Predator 212 Hemi.
I've tried with gas tank cap off before and nothing changed. Free flowing tank outlet. The vent flows very freely. <Tested all of that today.

i will put it back to gravity fed. i may just need to open up the Pilot jet it is? The one that loads the carb with fuel? Google isn't what it use to be for me and i'm having a hard time searching relevant results. i want to get a video of this thing going. ..off to gravity feed testing.. maybe this hot Texas heat will help fuel flow.
 

Twid

Active Member
#13
It seems to me the 390 carb is to much of a hassle. For $125 on eBay you can get A 22mm Mikuni setup, manifold, carb, throttle cable, extra jets.
 
#14
Just did new carb gaskets and cleaned the carb needle and float and STILL leaks out the overflow. EDIT: while in the off position.
 
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#15
Boy something is wrong if the fuel is off no fuel should get into the float bowl. If it is leaking out the overflow then fuel has to be getting past the fuel shut off. plus even it it was, the needle and seat should stop the flow when float bowl fills up. Maybe the float tang got bent. Check float height.
 
#16
That hose was relieving too much of the pressure.
Either need a smaller hose or something to regulate pressure.

i'm going with a fuel pressure regulator,(the inline on/off switch). It would be easier on me at this point.
You don't need pressure, this isn't a car with fuel injectors. All the pump needs to do is bring gas to the carb and then gravity will do the rest.

By my the sound of things it seems like your having other issues. Ditch the fuel pump and fix your carb.

Where did you buy the carb from and how do you know it's jetted properly?
 
#20
So i sprayed the carburetor insides along with the float with carb cleaner ,dried it off and re assembled it and it isnt flooding anymore. At least not for now i guess. Ethanol gas, eh? Gummy. Even had yellow residue from where the gas dried on the clear fuel line.

I have it in the garage right now with glue behind the choke valve drying. When i let it run lean the valve popped off like a cork on champagne.
That was a learning experience for me and i knew i shouldn't've done that but thankfully it wasn't on race fuel.

For now i conclude i need a slightly longer fuel hose for gravity feed cause this one barely reaches the carburetor. It's basically a straight shot from the tank to the carb and even with 1/3 gallon of gasoline the weight couldn't push the fuel to through the line unless i pushed on the lines. I think it may need a slight "S" curve;cause when i had the pump attached The fuel would go nearly half a foot up the return fuel line when turned off.

Interesting stuff.
 
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