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Methanol carb tuning help

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MrDeakle93

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I need help tuning my methanol carb on my GX200 clone. I have a GX390 carb with a .29 low speed jet, .57 main jet, and stock GX390 emulsion tube. I could get it to crank and idle at 2000-2200 rpms but as soon as I touch the throttle it dies. I even tried slowly getting into the throttle and it dies as soon as the lever is touched. Do I need to lower or up the main jet size? What's going on here?
 
Take the lower oring off the pilot.. see how it does then.. if it still hesitates..increase the pilot size... make sure the gaskets are sealed good as well.
 
From what I've seen other people using, I think your jets are big enough, can't be sure, but I think they're fine. I would be checking for an air leak. Just another possibility. It could be a lot of things, but it sounds like a lack of fuel could be the culprit. If you have a second carburetor, what about trying that? Or just a very thorough inspection of the present carburetor and the fuel delivery system. Air leaks are sometimes hard to find, and they can drive you crazy.
 
I'll check for an air leak. I'm using the ARC gx200 to gx390 adapter and I'm pulsing the fuel pump from that. I also tried pulsing from valve cover. Same issue. The carb is new as well as all new gaskets. I do have another carb but it's a gx200 carb. I'll try it too. I can say that it idled fine and when I went to check the carb and when I removed the fuel line from the fuel shutoff switch, it made a hissing sound like the carb was pressurized. I don't know if that had anything to do with it.
 
One of the bolts that hold the ARC adapter to the head is real close to the port (right side when looking at the port bolted on).. I recommended putting silicone in the head of the bolt there (or epoxy). I have seen several leak at that spot. As far as jetting, a .029 with the oring still on is not near enough with methanol.
 
I was basing that off of the nrracing.com gx390 carb alky conversion kit. It includes a .29 pilot jet with 160 main jet. I drilled my jet to 1.6mm (0.060) but the pull rope broke before I could get it to fire. I will eventually upgrade to electric box starter but ordered a new pull for now as the other one was old and rotted.
 
MrDeakle93 said:
I drilled my jet to 1.6mm (0.060) but the pull rope broke before I could get it to fire. QUOTE]
"Drilling" jets is a very imprecise method of making jets bigger. When you consider that a liquid is something like 700 times denser than air, the slightest imperfection in the jet can cause a significant difference in the flow of fuel. EC Bert always recommended that you use a reamer for sizing jets. Even with the reamer, there is some chance of a surface flaw in the jet bore. One of the big carburetor manufacturers in Europe designates their jets with a flow number.
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The thing is, Al, I have yet to run it on my drilled jet. I was only able to get the motor to run on my .57 jet I bought from ARC. I drilled the jet in hopes that would fix my problem but the starter broke on the first pull, which didn't allow me to even test the .060. I also drilled the jet by hand. Looks perfect but I don't know.
 
Also you may need to put two D-shaped gaskets between the head and adapter. These heads are rarely cut square. The black plastic insulator will bend and shape to the uneven mating surface, but the more dence billet will not and could leak. Check what youngengines said also about the stud bottoming out.
 
You dont need it, the Billet piece replaces the black insulator. I was just giving you an example of why you need to run two of the D-shaped gaskets. The Billet adapter will not flex and seal up the uneven areas of the mating surface. The plastic insulator in most cases would.
 
Like stated above , Your jetting choice should B close enough Not to have those issues . IMO , We R dealing with a "venting" or "air leak" problem .
 
I would drill or ream the pilot jet to at least .034 if you are sure you dont have any air leaks around the intake or carb. Hesitation off idle usually means you are lean on the low speed circuit
 
W5R said:
I would drill or ream the pilot jet to at least .034 if you are sure you dont have any air leaks around the intake or carb. Hesitation off idle usually means you are lean on the low speed circuit
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I agree , After rereading the #'s , A .028" pilot is gas jetting #'s . Your right , A .032-.034" would B a better choice for alky and would help with a hesitation ..
 
sneaks12303 said:
I agree , After rereading the #'s , A .028" pilot is gas jetting #'s . Your right , A .032-.034" would B a better choice for alky and would help with a hesitation ..
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Not to question your recommendation, I'm sure you know as much as anybody about this stuff, but just for my own information, if .028" is a good gas jet on the low-end, wouldn't a .039" or .040" (about 100% increase) be the right jet for alky. On the high-speed, going from .038" to .054" is a 102% increase, so I'm thinking, if you're going to increase the high-speed 100% why not the same for the low-speed?
 
alvin l nunley said:
Not to question your recommendation, I'm sure you know as much as anybody about this stuff, but just for my own information, if .028" is a good gas jet on the low-end, wouldn't a .039" or .040" (about 100% increase) be the right jet for alky. On the high-speed, going from .038" to .054" is a 102% increase, so I'm thinking, if you're going to increase the high-speed 100% why not the same for the low-speed?
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I agree.. I would have the pilot .040 with no oring on the bottom, ( I shipped five 390 carbs out last week... I have some knowledge in my statement)
 
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