Engine dies when chokes down >.<

[Latent]

Hey guys I am having a slight problem with my mini bike. The engine starts in 1-3 pulls with the choke up, but once the engine is warm and I put the choke down it dies. Also the idle screw seems to do nothing whether its screwed all the way in or out and the engine idles around 3600 rpm, clutch engages around 3500, but I'm going to adjust the springs and change engagement to around 3800-4100. also the fuel/air screw seems to have no effect on the idle. Sometimes when I put the choke down it stumbles then picks back up and is ride-able, but thats rare. I can ride the mini bike with the choke up with no problem it doesn't bog or sputter. I'm thinking maybe my pilot jet is too lean? My pilot jet is at a 30 and my main jet is a 190, but the engine is modified with flat top piston, arc rod, arc flywheel, 30 thou milled, 28/25mm valves, 26mm mikuni vm26 + few other goodies. Any advice or suggestions will be greatly appreciated. Threw in a few pictures, well because who doesn't like pictures? :thumbsup:

free screen capture software

 

[ole4]

What carb is that? Looks like a real VM series Mikuni. What size is it. So a couple things to check. First off there are two adjustments for idle, slide height and idle air bleed that adjusts idle mixture. First off they had a fuel enrichment device, no choke (you may already know that but just in case) And it works by unsealing a air path between air horn to a large hole behind the slide it picks up a lot of fuel for a rich mixture. Now you said when the turn off (plunger down) the enrichment device it dies, but later you say its idling at 3600 rpm? Did you mean it runs that fast with the enrichment enabled? If the enrichment is off and it is idling at 3600 then you probably have either the slide too high or a air leak. If the slide is too high it will not pull idle mix correctly and it could be why idle air mix does not seem to work. The way to adjust it, is first set the initial slide height.
back off slide adjuster till slide is all the way down, then turn it so it just raises a slight bit set the idle air adjustment to 1.5 turns out. Use enrichment to start, and it may well idle at 3600 rpm. Make sure chain is off so bike does not take off. after a minute turn off the idle enrichment device and see if it tries to idle. If it runs real slow turn in slide height adjuster in a half turn and see if it makes any difference. if it never makes a difference then I suspect your idle system is somehow not working. Take off carb unscrew idle air mix and with some carb cleaner with the tube attached insert it into the hole where adjuster was and watching the idle hole (tiny hole on bottom behind the slide and see if you see cleaner coming out. IF it does then idle passage is clear. Check pilot jet to see it is not clogged.


Here is link to adjustment manual
http://www.3cyl.com/mraxl/manuals/carbtune/carbtune.htm#circuits

 

[Latent]

It's a vm26-8074, it idles around 3600 with the enrichment device on, I have just got a new intake manifold with pulse fitting and new gaskets sealed with permatax #2. How do you go about the slide adjustment? Air adjustment is at 1.5 turns out. Carb has been cleaned thoroughly and the pilot jet is brand new. I'm thinking the pilot jet might be too lean because it dies when the enrichment device is off. I'll try your suggestions tomorrow and thank you for your input, greatly appreciated. :thumbsup:

 

[ole4]

WSlide adjustment is usually a fairly large screw with spring that is on the side of carb. It has a dull rounded end that as you screw it in it wedges up the slide. On your top picture is is on the left side.

 

[Latent]

You mean the idle adjustment? The screw with the spring around it? If it's screwed all the way in it has no change in idle rpm and if it screwed all the way out it has no change in the idle rpm. I am not sure why... I fee like the pilot screw is too lean that's why when the enrichment lever is off the engine dies. I'm not sure if a richer pilot would help but I will try a 32.5 and a 35 pilot hopefully that will help. I might buy a 32.5 35 37.5 and 40 pilot just to see if that effects the choke.

 

[danbur55]

you might make sure the small o-ring is in place on the air/fuel mixture screw and be sure to put air filter on to ensure proper air flow thru carb as free flow will affect the siphon effect of fuel. might also double check what needle to make sure it is correct for carb/main jet