VM20 Jetting for 73cc

I recently aquired a Honda express that had been sitting since 1988, and I got it fired up and put a few miles on it, but the top end let go so I decided to go with a 73cc top end with Boyesen 2 stage reeds, Proma pipe I modified, and last but not least a Mikuni VM20 Carburetor. I've got it running and took it out for a spin with the stock jets (22.5 idle, and 180 main)

It has trouble getting up to speed from a dead stop. Really sluggish. But then when it starts going the powerband will hit and launch to 45 mph no problem. Then after that it it falls on its face. If you try to cruise at half throttle it will cough and choke. Checked the Spark plug, it's dark grey.

I know A lot of people ask jetting questions and I've searched the whole website but cannot find a straight answer

I want to get a good idea of the jet sizes I should be getting before I drop too much $$$ in jets I don't need. From what my motor runs like, I should bump down the idle jet size and bump up the main jet size? Am I on the right track? Or maybe c-clip on the needle and slide if there is one?

Re: VM20 Jetting for 73cc

Check the tuning spreadsheet and compare your setup and location to others on there.

https://docs.google.com/spreadsheet/ccc?key=0AseU2kk9COgXdEZEaUE2VlNrcWhhSTZ1MGJRbDVEcVE&authkey=CJ-GnFI&hl=en&authkey=CJ-GnFI#gid=8

I run a 120 main, 17.5 pilot and needle at 2nd leanest setting on a 70cc piston port kit, but I am at high altitude so my parameters are going to be much different than yours. I'd guess you'd need around a 140 main. Play with the needle clip setting too, it's so easy to get to on the VM20. You should also tune the pilot circuit, check page 10 on the link below.

http://www.mikuni.com/pdf/vmmanual.pdf

Re: VM20 Jetting for 73cc

Depending on the head you used you may have too much compression. Check it with a compression tester

Re: VM20 Jetting for 73cc

I'm using the stock head for now. The altitude I'm riding at will be between sea level and 1000 feet. I'll check the charts and try out some of the recomendations.

Re: VM20 Jetting for 73cc

I bumped the pilot jet down to a 17.5 and that helped a lot with the bottom end power. I tried every jet from 180 to a 230 and I got the same result every time:

From half throttle to full throttle is chokes and coughs and cuts out like it's starving for fuel. I know its getting a steady stream of fuel from the gas tank.

It ran best with a 210 size jet and the c clip on the richest setting, but when I jet up to make it richer it would bog more at mid throttle and still cut out around full throttle. I'm running out of patience at this point.

Re: VM20 Jetting for 73cc

Even the 180 is HUGE. Go down to ~130 and work DOWN from there. I suspect you will find yourself at 110-120 or smaller. Don't forget to reset your needle first and tune that after you find a good main jet.

If it bogs more and mid and cuts out at full throttle, how is that BETTER?

Re: VM20 Jetting for 73cc

I went down to to a 140 and the same thing happened. I cant cruise mid throttle without it cutting out on my.

Re: VM20 Jetting for 73cc

matt boda1nz /

130 seems huge...

Re: VM20 Jetting for 73cc

matt boda1nz /

Well.. in a vm18 it would be.. sorry, not sure on a vm20. Vm18 you'd be around 90

Re: VM20 Jetting for 73cc

It's defineatly starving for fuel. Could the float be incorrectly set and not letting enough fuel into the bowl? I'm leaning towards that being the problem. If I hit a bump in the road it cuts out pretty bad like its out of gas. That might be the float closing up the needle seat and cutting off the fuel supply.

What's the best way to set the float height?

Re: VM20 Jetting for 73cc

Dirty30 Dillon /

I run a 175 in my PP 70 za50/VM20, despite being told that's way too high, that's what it needs.

Have you rebuilt the bottom end?

Re: VM20 Jetting for 73cc

I haven't rebuilt the bottom end. It was solid when i checked clearances while the top end was off

Re: VM20 Jetting for 73cc

Here's a video of exactly what my moped is doing from mid throttle to full throttle. It gets worse the more throttle you give it

Re: VM20 Jetting for 73cc

#CrazyWayne™ rocks. #CrazyWayne /

Your link is not working?

(edited)

Re: VM20 Jetting for 73cc

Let's not be saying my link isn't working. And let's not link it the the meatspin website. Appreciate it, thanks. To everyone else, my link is fine, don't click on Crazy Waynes link.

Re: VM20 Jetting for 73cc

Change your plug. That sounds like fouling and\or a misfire. A lean bog doesn't cut sharply or as fast as that.

Re: VM20 Jetting for 73cc

That was a brand new plug.....

Re: VM20 Jetting for 73cc

#CrazyWayne™ rocks. #CrazyWayne /

What did you mod on the pipe?

Re: VM20 Jetting for 73cc

Right now I'm leaning towards the idea of it being a problem with the ignition system. I'm going to switch out the coil with a new one and see if it makes a difference. The condenser might be going bad.

Re: VM20 Jetting for 73cc

That is why I mention the plug. You could have weak spark, but I encountered a VERY similar issue out of the blue. I hunted it for days, cleaning my plugs (both used) over and over. I was ready to tear my carb apart (2 months old) and pull my hair out... then I changed the plug. What you have is ignition breakup due to weak or fouled spark, or so much fuel that it simply will not spark.

Now a 140 is still way rich for you so don't count it out. MY 24mm is super happy at a 120 with the Athena WITH a filter, and you should be lower. I can get away with a 155 badly (rich) if I pull the filter. I moved the kit to my express and run a PHBN (12mm) with the jet drilled to ~110-115 with no filter and it's still very rich. Temps wont break 350 and its soggy until 25mph.

Points ignition throws a variable into the mix for sure, sort it out and you may be done. (edited)

Re: VM20 Jetting for 73cc

#CrazyWayne™ rocks. #CrazyWayne /

Your a full retard if you don't install new points and condenser when you upgrade to a kit & pipe.

Re: VM20 Jetting for 73cc

So much love and help on the forums...

Re: VM20 Jetting for 73cc

Well, the new coil didn't do anything. I'm back to where I started. Now it won't even cruise without cutting out and bucking like a horse.

Re: VM20 Jetting for 73cc

did you ever figure this out?

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