Mikuni VM18 tuning questions

[Will'smotobikes19]

I started my big bore engine the first time a few minutes ago 48mm big bore cylinder, windowed piston, Oz reed valve and banana pipe- (and before you tell me to ditch the banana pipe this one is different and doesn't have baffles it has far less restriction than a stock muffler I tested blowing through each). and VM18 carb. Now my tank has 16-1 with synthetic motul oil. I think it is running way too rich. Lots of smoke and wants to move at idle but if I give it more than an 1/8 of throttle it starts bogging and not firing. Then when I pull in the clutch it starts revving high just like my silver engine, could it be the carb angle is bad? if the carb is slightly angled forward does that cause more gas to enter whenever throttle isn't pulled? I am pretty sure I need a leaner jet and if so does anyone know who sells them? may just run a stock carb for now because I have a smaller intake that will bolt onto the reed valve either that or the runtong flat slide because my neighbor has an extra. The good part is it's a brand new engine and it started right up when I released the clutch. The guy at zeda motorsports recommended a VM18 so that's why I got one.

 

[gary55]

First try it with 6 ounces per gallon. 8 ounces per gallon is to much oil. Remember when you use to much oil it has less gas per charge which makes it lean. Then if you still can't get it to idle down you will need to check for air leaks in the reed and reed block and seals. It will probably just need larger jets, but you can't tell which one is correct till you are sure your not sucking air somewhere.

 

[Will'smotobikes19]

6 ounces wouldn't that be 25:1? I know 6.4 ounces is 20:1. I heard from somewhere that the stock mikuni jet was already too rich for motorized bike engines and needs a smaller jet but idk that's just what I heard. I adjusted the idle screw and it didn't change much on the rpm but the choke was still on. Found this

 

[gary55]

128 div. 6 is 21.3 to 1. The vid. sounds like good advice. He is starting form a rich fuel mix and working his way leaner. You are alreasdy to lean. When I say lean I am talking air fuel. Not oil fuel. So your rich on oil fuel. Use less oil. You are lean on air fuel. So you either need a larger jet or you are sucking air somewhere or both. You can as he suggests go to a couple of sizes larger jet and work your way back down. If their are no air leaks you will gat there. if there are you won't get there till they are remedied.

 

[gary55]

Hopefully someone on here is using the VM18 with a oz reed and will pipe in to give you some better info on jet sizes. If not you could hit up StreetRyderz. He's pretty smart on this kinda stuff, or try Kurt over on the Zeda facebook. I'm just trying with my understanding. Both of these guys have more knowledge in the problem you are facing.

 

[Will'smotobikes19]

I opened the carb and the c clip was on the second to richest setting I brought it up 2 spots and am going to try it.

 

[gary55]

I opened the carb and the c clip was on the second to richest setting I brought it up 2 spots and am going to try it.

Sometimes it's the simplest solutions that plum evade me, but if you are already running lean that will only make it leaner. Might want to go the other way.

 

[Will'smotobikes19]

Same problem, the bike moves at idle but whenever throttle is given it doesn’t make power. If I try to pull the clutch in it starts revving high and the rpm won’t go down. Man this is frustrating

 

[Will'smotobikes19]

Ok got the thing to idle and rev with a stock carb and it revs good but it is so strange whenever I pop the clutch and try to takeoff it doesn’t take the throttle, I do believe it may be a gas issue but it runs with the clutch in very weird. The clutch seems to grab and doesn’t feel like it’s slipping either. Was going to post a video clip but it said it wasn’t an “allowed extension”

 

[Will'smotobikes19]

I think I'm about done with this hobby at this point me and my neighbor are both frustrated at motors not running like they should. His also does the excessive revving thing even with no hand on the throttle. I don't know what causes this except gas in the crankcase that isn't burned that circulates into the combustion chamber and revs crazy high. The Chinese need to rethink their manufacturing industry and improve instead of copying everything in mass quantities. My bike is very loud with that banana muffler and it will go normal speed down a hill but if there's any incline at all the motor doesn't burn the gas. I even put some 20:1 in that my neighbor mixed. I might try checking spark plug gap clutch and compression. The piston did come with scratches on the sides as it was shoved in the cylinder during shipping without oil.