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Help me tune my POS 26mm MOLKT Carb (GPX MOTO 150)

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23K views 72 replies 20 participants last post by  mattmoto441  
#1 ·
Here's the deal. I finally got my GPX Moto 150 running in my CRF50 but now I'm trying to get it tuned right. It kicks over in 1-2 kicks and seems to idle decent, but if I let it idle for about 2-3 minutes the pipe starts to glow red. Does this mean that the Pilot jet is too small (lean)?



It seems to rev nice through 1/2 throttle but when I get passed that it stumbles and has very loud backfires on the deceleration. I've tried these main jets; 100, 110, 120, 130, and 150. I used the Keihin slotted round jets cause those are the only ones that work on this carb and out of all of them the 120 seem to work the best but you can still feel a power loss around 3/4 throttle and up and still loud ass backfires on decel.



I have not touched the pilot jet, so I'm leaning (no pun intended) towards putting in a bigger pilot jet to richen it up. From what I understand these 4 strokes like to run rich on the low end and lean on the top, does that sound right?



As far as the backfires are concerned I installed a gasket for the header at the head and tightened it down real nice and sprayed wd-40 around the intake and carb to check for air leaks and there was no fluctuation in the idle, so that leads me to believe that there isn't any air leaks. I'm assuming that it's backfiring because of the potential lean condition from the pilot jet. Can anybody confirm this?



Thanx in advance for the feedback!!!.....Manuel
 
#5 ·
HACKMUNCH said:
i have had a few phone calls about the 150 and from what i can tell the pilot is 2 sizes to lean.


what Kehlin or Mikuni model copy of a carb is a MOLKT..?



I took one apart at one of the MoM series races and haven't seen a pilot like the one in this carb...the mains looked like short Hex mikuni or Kehlin



and the Main had no markings on it for a size...what comes stock?
 
#6 ·
Pimpindahoes,



I'll do that, thanx!



HACKMUNCH,



Can u tell me what the actual number size it is and what it needs to be from the calls you've gotten? Niether the pilot nor the main jet have any numbers stamped on them whatsoever. Thanx



OLDSCHOOL,



It is a Mikuni 26mm copy, or an attempt to copy because it sure is built ****ty!



People have found that the Mikuni pilot jets fit on these carbs and the round slotted Keihin main jets fit on it also.



Yeah the size is nowhere to be found on the jets but from eyeballing it, it looks like the Molkt stock main jet is comparable to a Keihin 90 or 95 main jet.....Manuel
 
#7 ·
65fsbk said:
Pimpindahoes,



I'll do that, thanx!



HACKMUNCH,



Can u tell me what the actual number size it is and what it needs to be from the calls you've gotten? Niether the pilot nor the main jet have any numbers stamped on them whatsoever. Thanx



OLDSCHOOL,



It is a Mikuni 26mm copy, or an attempt to copy because it sure is built ****ty!



People have found that the Mikuni pilot jets fit on these carbs and the round slotted Keihin main jets fit on it also.



Yeah the size is nowhere to be found on the jets but from eyeballing it, it looks like the Molkt stock main jet is comparable to a Keihin 90 or 95 main jet.....Manuel




what model 26mm Mikuni....?
 
#8 ·
OLDSCHOOL,



Honestly I don't know for sure. I was told it's a Mikuni copy by some of the more experienced guys on here and it looks very similar to the 26mm Mikunis I've seen on some of guys bikes on this board. Maybe somebody that knows more on this can chime in...Manuel
 
#9 ·
Well from what I have found out so far I think the pilot that you need will be around a 25 these are the Vm 26 style pilots. you are going to have to get the pilot close before you can really work with the rest of the jetting I think the problem stems from the lack of draw at low rpm's and strong draw at high rpms this is why we have to run the pilots so rich to get it on the needle and then the velocity increases and the main jet is more efficient I think this is why the jetting seems odd big pilots small mains Hope this helps

Ps my VM 26 on the 150 is at 30 pilot 160 Main
 
#12 ·
65fsbk said:
OLDSCHOOL,



Honestly I don't know for sure. I was told it's a Mikuni copy by some of the more experienced guys on here and it looks very similar to the 26mm Mikunis I've seen on some of guys bikes on this board. Maybe somebody that knows more on this can chime in...Manuel


If its a copy of a VM....the pilot "Kind" of looks like a VM pilot..?



but none of the Jets have any numbers on them...



How about it Dom,

whats a good starting point with the Correct jet Types and sizes.
 
#13 ·
filthyboatguy,



I hear ya man, but I don't mind messing with it. After spending $600 on a motor if I get this thing running decent I'll be satisfied. I can't justify throwing down another $200 for a carb when I will eventually get this one working fine. Maybe if I was a serious racer or something but I'm that guy that needs something to jump on and get scared for a moment to get the blood flowing, that's the only reason i built this thing and also to have an excuse to be in the garage all the time! If it proves not to be tuneable I'll just have to call Pitster and have them take this poor excuse for a carb back in exchange for a good deal on a Mikuni VM26.....Manuel
 
#15 ·
65fsbk said:
filthyboatguy,



I hear ya man, but I don't mind messing with it. After spending $600 on a motor if I get this thing running decent I'll be satisfied. I can't justify throwing down another $200 for a carb when I will eventually get this one working fine. Maybe if I was a serious racer or something but I'm that guy that needs something to jump on and get scared for a moment to get the blood flowing, that's the only reason i built this thing and also to have an excuse to be in the garage all the time! If it proves not to be tuneable I'll just have to call Pitster and have them take this poor excuse for a carb back in exchange for a good deal on a Mikuni VM26.....Manuel


I hear ya, thank for being the test mule again. The mods and baselines you establish will benefit many for sure!



Jeff
 
#16 ·
micro75 said:
Oldschool the pilots are Vm the main jet is treaded Keihin the round Keihin jet will work or the oko jets but they are shorter and my require the float level to be adjusted do not use the long Keihin jets they are to long the pilot looks to be about a 17 on the one I have here
The carb is a hybrid, little bit of VM26, little bit of Keihin, little bit of TB type Shengway with some OKO thrown in...
 
#17 ·
MOLKT: a mix of a Mikuni, Oko, Lectron, Keihen, and Tillotson.... it has a little of all the carbs. The pilot from a Keihin, main from OKO, slide from a mikuni, Air bleed curcuits from a Lectron and needle from Tillotson!









_______________________________________________________

Caution: Anything above this ^ line may be subjected to sarcasm!
 
#18 ·
micro75,



One last question for ya, do the pilot jets increment by 1, for example 17, 18, 19 , and 20? Or do they increment by 2, for example 17, 19, 21, 23, 25? The reason I ask is because if u say the stock one looks like a 17 doesn't your recomendation to start at a 25 seem to be pretty big? I'm not questioning your knowledge because u can oviously wipe the floors with me with how much u know, but I guess I'm just trying to understand your recomendation. Thanx again!...Manuel
 
#19 ·
filthyboatguy & xr1million,



LOL!!!!!!!!! If that was the case i think we would have a super carb not a super POS! I don't know maybe this might turn out to be a good carb but it sure is hard to see that happening with how many issues everybody is having with them and the fact that Pitster (Gary) hasn't chimed in once to any of my threads to clear anything up or make suggestions. My .02....Manuel
 
#22 ·
micro75,



I guess I'm assuming it is easier or a better carb because of so many of people's responses to my questions in the past about this carb have been, "Buy a real Mikuni VM26". I guess until i actually use a real Mikuni i should keep my mouth shut. Can u answer the question I asked u a couple of posts ago? Thanx Bro!