Carby Mikuni VM 18mm

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Tyler6357

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Hello everyone
Running a PK80, 40 stroke, short connecting rod, low wrist pin piston, with a 6cc FredHead clone and a CDHPower G2 4 peddle reed valve/windowed piston. I currently have a Dellorto Clone(RT)Carb. It seems to run pretty good but damn, it really has trouble moving into low idle, sometimes I have to slowly engage the clutch slightly just to get it to slow down. I was thinking of upgrading my carb. I was looking at the Mikuni VM but interested in what you guys think of this carb or if you would recommend a different one to use with this set up?
Mikuni VM 18mm-----> https://www.ebay.com/itm/Mikuni-Gen...h=item4d2f826c76:g:AKQAAOSw~4taWxlQ:rk:2:pf:0
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With my 48mm engine it was running too lean if you do get one I’d buy a jet kit. They come with a 70 jet but I bought a 90.
 
With my 48mm engine it was running too lean if you do get one I’d buy a jet kit. They come with a 70 jet but I bought a 90.
Okay, that's good to know, I'm using a stock 70 right now, So how does the carb perform with the 90? Are you using a G-2 reed valve with windowed piston?
 
I ordered a 90 jet a few days ago haven’t tried it yet I’m using an OZ reed valve with a windowed piston. Basically with the 70 jet it had little to no power and wouldn’t take the throttle and if there was even a tiny hill the bike would slow down until it stalled. 48mm bore and 38mm stroke is apparently 68.76cc but the 40mm engines are about the same. I don’t know if there’s an advantage of a 1mm larger bore but it is iron sleeved.
 
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I ordered a 90 jet a few days ago haven’t tried it yet I’m using an OZ reed valve with a windowed piston. Basically with the 70 jet it had little to no power and wouldn’t take the throttle and if there was even a tiny hill the bike would slow down until it stalled. 48mm bore and 38mm stroke is apparently 68.76cc but the 40mm engines are about the same. I don’t know if there’s an advantage of a 1mm larger bore but it is iron sleeved.
I see, thanks for responding, let me know how it works with the 90. Mine just wants to rev on idle as if it has an air leak, but there is no air leak and it runs good up and down the throttle range.
 
Might be carb angle try to get the carb level as possible. I had a similar issue it would rev uncontrollably until I backed out the idle screw no hand on the throttle- it was weird might be loading the crankcase with gas as well.
 
Might be carb angle try to get the carb level as possible. I had a similar issue it would rev uncontrollably until I backed out the idle screw no hand on the throttle- it was weird might be loading the crankcase with gas as well.
No, when I back out the idle screw it just dies, but one thing is interesting is that it only revs like that when its warmed up, it will idle good when it's cold, I mean, for a few mins, revving higher and higher as it warms up. Sometimes I can get it to idle if I do a quick twist on the throttle and let it go down but it won't stay low.
 
My neighbor said something similar with the revving when its warmed up. I used to go through my yard and at the top by the garden it would start revving high when I pulled the clutch in. I would have to killswitch it or back out the screw to get it to stop revving I don't know if its just a weak spring in the stock carb or what but it was at a slight angle.
 
Well, I think what I'm going to do is try another HT carb, mostly because I have a spare new one already, so at least I can try it, maybe the carb is leaking air somewhere. I'll do that before I pull the trigger on an upgrade.
 
Well, I think what I'm going to do is try another HT carb, mostly because I have a spare new one already, so at least I can try it, maybe the carb is leaking air somewhere. I'll do that before I pull the trigger on an upgrade.
The carb is leaning out at idle mostly due to it's size.When you can put some throttle input and let it snap back to idle then it climbs is lean.As is a 70 jet for that setup you have this with an MZ pipe no?
 
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