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Issues with gx390 carb

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Boostedego

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Im running a 390 carb with arc adapter on my predator billet fly wheel, rod, mod 2 cam 14 cc head with 26lb springs and 1.3 rocker on the intake im having a problem with the carb dying once i apply the throttle i have switched jets from 41 main to 37 main to see if that was the problem bc it was running rich, but still having the same issue im running a 29 low speed jet. I thought these carbs liked the larger jets ? Anyway i cant seem to figure out the issue anyone have any ideas ?
 
Its fine on partial throttle but when i try to give it full throttle it craps out is it not getting enough gas with a 38 jet or a 41 jet do i need to throw more fuel at it ?
 
Is it on gas???
If so it is very difficult to nail the throttle with the 390 carb on gas... Alky is a different story...
On Alky and a 390 carb. We run a .040 low jet and a .067 main jet...
 
Also with the ARC adapter... make sure it is sealing good... at both the adapter to head and carb to adapter...
 
If it stumbles off idle, try enriching idle mixture. Needs extra fuel at transition to main jet. Hope that helps. @
 
37 - 41 = 23% increase in area.
24 - 29 = 46.2% increase in area.
And if you drilled them to size, it doesn't really tell you what they're flowing. Drilling jets to size is a crude way of doing it.
14cc head doesn't tell you the combustion chamber CC's, and that's important.

Still, that doesn't sound like your problem, not if you've tried everything in-between.
 
Is this a genuine Honda carb or a Clone carb? As 95 Shaw stated, you need a bigger low speed. If you are at a .029 now then go up to a .031-.032 or bigger if you need to. Get the low speed jet close meaning it idles well and accelerates well then work on the main. A .040-.044 is usually where we wind up depending on the head work, valve size, total valve lift and ignition timing.
 
Thanks yes its a honda carb the .029 seems to be ok but it doesnt like the .046 jet as it feels like its not getting fuel on full throttle so switched the low speed to .022 and left the .046 in the main and it seemed to run ok but not to its potential, i have a pin gauge set to measure the jets and so far so good they seem to be spot on. Were can i find larger main and low speed jets or will i have to drill them out? I definetely feel i need a bigger main
 
The .029 seems to be working well ideling nice but when i get to half throttle it dies out like the main is not feeding enough fuel.
 
Yes im on gas i know these carbs have some issues on it but i figured id give it a shot to see if i can find a sweet spot.
 
Go back to .041 on the main and drill the low to .032, that will get you closer to where you need to be. Are you using the stock 390 etube or a different one? You should not need anything bigger than a .044 main jet for a 390 on gas, if you do you have problems and are pulling in air from somewhere with an air leak, most likely around the ARC adapter like someone else said. I always had to put silicone around both of the studs that bolt the adapter to the head, plus put silicone around the gasket between the adapter and the head for it to seal correctly with that adapter. I prefer the 390 adapter from NR racing instead, no need to silicone everything for them to work right and not leak air
 
Weddle there is no need to silicone our adapter either, but I'm sure NR appreciates the plug. Just like any parts made they have to be installed correctly. Gaskets are all that are needed to install these, I do recommend two of the D-shaped gaskets between the adapter and the head.
 
If it is a Genuine Honda carb...
the oring on the low jet should be at the very bottom below the centre hole in the low jet...
Move it above the centre hole...
 
Im using stock 390 e tube will and yes i have moved o ring above the hole will put .041 back in and drill low to .032 also to see how that works out but as of now it seems to be running pretty damn good but always open to suggestions.
 
Another question i must have a newer predator it came without lash caps so i bought some but they will not fit on the valves almost as if there to tall or sit too high does anyone have any experience with this issue ?
 
If you are using a genuine Honda carb, do not use clone pilot jets.
29-31 pilot, 41-43 main with stock E-tube is working on my predator and clone both with similar build.
 
JPR57 said:
Weddle there is no need to silicone our adapter either, but I'm sure NR appreciates the plug. Just like any parts made they have to be installed correctly. Gaskets are all that are needed to install these, I do recommend two of the D-shaped gaskets between the adapter and the head.
Click to expand...

You might want to ask around Jody, quite a few people have to use silicone to get a good seal with the ARC 390 adapter, it is a common problem, mainly on the right adapter stud/bolt to the head
 
Boostedego said:
Another question i must have a newer predator it came without lash caps so i bought some but they will not fit on the valves almost as if there to tall or sit too high does anyone have any experience with this issue ?
Click to expand...

Cant use regular lash caps with the non hemi valves, these valve have a smaller stem than clone or hemi predators do, and dont need lash caps
 
So i dont need lash caps ok thats good and why cant i use the low speed jet ? I got the choose your size ones from arc ?
 
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