Running on half choke

frenchman159

New Member
Apr 19, 2008
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Winnipeg, Manitoba, Canada
Hi everybody

First post and i hope i'm in the right area of the forums. I just purchased a 70cc engine and am loving it. Call it pushing 40 and loving the assitance. Love the fact of being able to get back on a bike, But this time when i get tired I just "pop the clutch" and enjoy the scenery! My question is, I have to run the engine with the choke 1/2 to 3/4 on all the time. At this choke setting the engine runs very smooth. I must be missing something for it to need the extra fuel. I checked all gaskets for leaks and cleaned the carb. The float arms are sitting parallel. Any help would be great. Also any tips on vibration ?? I still can't feel A#$ !!! Thank You !!!
 

Norman

LORD VADER Moderator
Staff member
Jan 16, 2008
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pampa texas
did you raise the needle a notch?and try it or all the way qnd try it?
mike the needle at the top by the grooves, around 0.070 and at the bottom farthest away from the grooves it should be around 0.065. this is not a hard and fast rule but I've found that some of the needles are not tapered right or not tapered at all.
If they are not tapered right the fuel will be too lean wide open and that is why you need to run with the choke on.
 

frenchman159

New Member
Apr 19, 2008
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Winnipeg, Manitoba, Canada
Thanks, i'll try that. The needle is 3rd from the top. I will set the clip all the way down an see if that helps. if i shut the choke off with a warm engine it just wants to stall out. So, thats how i came to think of it starving for fuel. Thank you !!
 

deacon

minor bike philosopher
Jan 15, 2008
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north carolina
You do know almost all of these engines leak air at the carb to manifold connection. Plus the needle works best for MOST of us in the second from the top location. It most likely is one of those two things or even both.
 

Skyliner70cc

Member
Mar 8, 2008
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If you aren't running an inline filter, you may have clogged your carb main jet or other fuel passages. As a minimum run a small fine wire through the main jet and install an inline fuel filter. There is a lot of crud in the fuel tank that shouldn't be there on a new kit.
 

sisdavid

New Member
Mar 31, 2008
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Daytona Beach
I would try the little clip on the needle first. Second from top sounds like a good place to start. Just do not do it in the grass or you might loose that little clip. Maybe get an old sheet and take it apart over that. I used an automotive heater hose on the engine mounts, and muffler bracket helps with the vibration.
 
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Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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So, did you get it figured out frenchman?

I'm guessing your bicycle motor has an air leak at the manifold or where the carb clamps on the manifold....don't over tighten either. Take the manifold off the engine, smear on a THIN layer of any RTV sealant on both sides of the gasket and reinstall.

You guys are cleaning the tanks before you install them on your bikes, right?
That little screen inside the petcock will fill up with crud and clog up in 20 miles if you don't. (that is if your tank is as bad as some have claimed)
 

frenchman159

New Member
Apr 19, 2008
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Winnipeg, Manitoba, Canada
Yes on the air leak !! And i raised the needle too. I'm very happy with the way it's running right now. I'm off to get better tires tomorrow as the ones I have on now are not the greatest. I live in the country and i'm dying to take it to the city, for a nice ride through the park !!
 

paul

Well-Known Member
Dec 23, 2007
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5555555555555555555 thats a high five. good job frenchman. feels good when you get it figured out how to make your bicycle motor run good dosn't it. (^)
 

Skyliner70cc

Member
Mar 8, 2008
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Great to hear you got it all worked out! Intake leaks are almost guaranteed if you use the stock gasket. I don't bother with it anymore and cut my own.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Los Angeles, CA.
I just had this same problem with one of my new bikes... It would only run with 1/2 choke!
I sealed up all the air leaks and checked everything I could think of, but had very little change. :-||
Then I got the idea to drill the jet a little bigger.
I went to the LHWS and bought 6 tiny drill bits in the next sizes up.
Every time I drilled it a little bigger, I would test ride it & every time it rode a little better! (^)
I stopped drilling when it was running good.

I thought I would add this to this thread incase anyone ever has this problem in the future. :ride2:
 

paul

Well-Known Member
Dec 23, 2007
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55555555555555555555555 high five venice! nice job. nothing feels as good as when we figure out what is making our bicycle motors run bad and fix it. i imagine you are all smiles.
 

Norman

LORD VADER Moderator
Staff member
Jan 16, 2008
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pampa texas
Pablo had a problem kind of like yours he sent me the carb and I found that the needle was not tapered right. and it was off by only 0.005.
I compaired his to my carbs needle with a mic. I reground it and the carb would then work properly. Before it would start and idle but would not rev up if you put on the choke it would try but would not work very well at all. after I tapered the needle it ran fine. at the top the needle miced 0.070 and at the bottom it miced 0.065 so its not a lot of taper and very hard to see the difference.
check out my carb post for the main jet size as I don't remember it right off
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,609
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Los Angeles, CA.
Also... I don't know about Pablo's carb, but mine ran great as long as the choke was 1/2 on!
Put the choke "off" at full speed, & it would die.
What exactly was Pablo's doing?
 

Pablo

Master Bike Builder & Forum Sponsor
Dec 28, 2007
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Mine didn't run for crap since new, but in some general terms ran like yours, with choke I could and did ride the bike.....but it actually got worse. I almost started to drill the jet, but I would have measured it first (and found it fine!). But I was getting tired of messing with it and coolio Norm volunteered to take a crack at it. Sure enough - he spotted the needle right away. I had no frame of reference...now I know what the proper needle taper is.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
235
63
up north now
I haven't drilled any jets yet, but I have "re-tapered" all the needles.

It's worth the effort to "fine tune" the carbs by this means. One bicycle motor I even got to run very well all the way through the rpm range in this manner.