Gx390 on gx200 emulsion tube?

knighty

Member
Hi Guys. I have a 23mm bored GX390 carb fitted to my GX200, currently its around 18hp at 8500rpm on pump gas on my dyno on a 1.1mm main jet, or 0.043" to you guys, standard air jet.

What emulsion tubes are you guys running in 390 carbs?.....the one currently fitted to my bored carb looks very similar to a gx140 emulsion tube, but its a bit longer and the small upper side holes are slightly different position. What type is this???.....does anyone have a part number as I want to order some more.
 
I built a 160, standard bore, mod2 cam, factory rotating assembly, pvl, stock honda coil, 26#spring (if i remember right its been 2 years since i ordered them). standard bore 390 carb, port matched and blended for filter adaptor, removed governor. Head is factory valves and ports, but i softened up shortsides, and blended intake with epoxy, also cut off around .020, deck is zero.

I tried the factory e tube, i tried a factory e tube with modified nozzle length, and i tried a gx160 etube. Made multiple main and pilot jet changes for each tube. Ended up making the most power with a 160 e tube, made 16hp at 7400 on my dyno, modified dynomite water brake w/ performance trends software. But I had to drill out all the holes to get to where it is, and its way more balanced that it was but its still off. The fuel signal Is super lean up front and back, while 4600-5600rpm is still too rich. I want to try some more testing with a different 160 etube. i need to lean out the center, or richen the low and high so i can balance the run. I tried a ton of different combinations with the stock 390 etube, and I heavily modified one and it had worse issues than i faced with the 160 tube, it seemed to respond better to each modification i made. its a PITA to tune on gas. But that little quad still hauls lol.

.042 main
.031 pilot

is where i am now @.84 avg lambda
 
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Everything i have seen , was the 390 carb was super hard too tune on gas .
You have apparently done well with it .
The low at . O31 seems lean .
The metering holes can be opened up .
The next step might be the air bleeds in the face of the carb .
Emulsion tubes more holes , bigger holes leaner mixture is't that right ?
Its possible the the location vertically could have an effect at different vacum/ throttle position.
 
Everything i have seen , was the 390 carb was super hard too tune on gas .
You have apparently done well with it .
The low at . O31 seems lean .
The metering holes can be opened up .
The next step might be the air bleeds in the face of the carb .
Emulsion tubes more holes , bigger holes leaner mixture is't that right ?
Its possible the the location vertically could have an effect at different vacum/ throttle position.
it definitely is tricky. I build a good amount of quarter midget motors, and jr sprint motors so I don't get alot of time to goof around with carb modifications and the effects of the changes so I was having alot of fun really. I read a thread Jody from arc responded to and I think he said he was at a .041/.026 on a factory 390 tube if I remember correctly, which was was actually one of the better runs, but the fuel was still off.

I left the 160 main nozzle same dimension, and drilled all the smaller holes out, I did it several times and ended at .030 for all. the power over the 390 tube was significant, and the balance was much better. it did lean out center substantially drilling out the smaller air holes, but still have a really fat pocket 4600-5600 @.65-.79 Lambda. if I could get that pocket to lean out w/o affecting the rest of the run, or vice versa then I think I could get closer to 17.

I was going to try a 160 tube and open up the main nozzle alot more to get more fuel to the top, then slowly open up the smaller holes to see if that helped balance it better, but there could be a better way like you mentioned.

one occurrence I noticed people having that I also experienced at first , was a low rpm stumble when throttle was applied, that went away with the 160 tube. I attributed that to the enormous hole in the base of the 390 tube which is like .100(guessing) but close, which feeds the pilot jet if I'm not mistaken, that's why I chose the 160 tube it's a .024 factory size hole and opened it up from there. I think with more time balance could be found. I put this motor on a 50cc quad for the kids to terrorize, and it's pretty sketchy lol.
 

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Correct me if i am wrong .
A smaller air bleed passage will create a richer mixture
I couldn't answer that entirely based off the little amount I've tested, but from what I've seen yes that's correct, but my issue was targeting certain areas that were getting too much fuel and I couldn't figure out why only those spots were getting overly rich while the rest of the run was getting leaner, and couldn't figure out how to target those specific ranges without affecting the rest of the run. when I screwed around with the 390 etubes I couldn't get the run balanced, even drilled some extra holes in the main body and made them bigger too and I couldn't get rid of that low rpm stumble when you hit gas from stand still. with 160 tube I didn't have that problem and the fuel data, even though it was off too, it had way better balance through the ranges. My tests with 390 tube made 15.5hp best, I had several 15.8-16 with the 160 tube.
 
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Many thanks for the replies guys, so you seem to have indicated its a PITA to tune, which supports my experiences!!!!.........I have just taken the plunge and bought a 28mm flat-slide knock-off item, in an attempt to do the job properly! am on a push for 20hp, and I dont think the 23mm venturi will ever allow that to happen.
 
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