Backfiring flames out exhaust

Brandonmcgier192

Brandonmcgier192

New Member
#1
#1
Hey, new to this forum.

My son asked me to remove the governor on his Coleman CT200u mini bike. After some Youtube videos, I was able to get it out and get the bike back together. I also installed a new carburetor, the one they use on dirtbike. A new upgraded flywheel and 18lb valve springs are on the way. The issue is that when my son or I rides the bike at full throttle and let go of the throttle, the bike backfires blue fames out of the exhaust. The flames can be pretty big in size and I’m not sure what is causing this. I watched another Youtube video on how to adjust the air fuel mixture screw, yet even if I tighten or loosen the screw, the result is the same. Also, the bike is only topping out at around 37 mph with the new carburetor and torque converter, when it used to reach 40 mph with the stock carburetor. Could this be due to the cam timing off (I think I may have reinstalled it a gear or two off) or valve lash. Any help would be greatly appreciated.

- Brandon
 
Strigoi

Strigoi

#3
#3
You need to install a billet rod as well. The stock rod is the weak link of the engine.

Are the dots on the crank and cam lined up? Pull the side cover and check. If you're two teeth off, the bike would barely run.
 
SAS289

SAS289

Well-Known Member
#5
#5
Aerindel said:
Floating valves. Thats why you have to upgrade the springs if you remove the governor.
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All we know is carb and torque converter. If that's all that has been added the engine RPM will be nowhere near valve float at that top speed. Would take quite a gearing change or a Juggernaut driver to get to valve float.
 
A

Aerindel

Member
#7
#7
SAS289 said:
All we know is carb and torque converter. If that's all that has been added the engine RPM will be nowhere near valve float at that top speed. Would take quite a gearing change or a Juggernaut driver to get to valve float.
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I got it with just a zip tie on the throttle linkage and stock carb. Same symptoms. Backfiring when coming off throttle and dancing flame around the muffler. And that was with the zip tie not even fully tightened, just a moderate RPM boost. I'm not at all surprised with a full governor delete he is floating valves.

When I was getting flames I was at about 20 MPH (with 60 tooth sprocket, modded the linkage because I was down to 10 after the sprocket swap)
 
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creia

creia

#9
#9
Aerindel said:
I got it with just a zip tie on the throttle linkage and everything else stock. Same symptoms. Backfiring when coming off throttle and dancing flame around the muffler. And that was with the zip tie not even fully tightened, just a moderate RPM boost. I'm not at all surprised with a full governor delete he is floating valves.
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Scroll up and (re) read Strigoi's reply- specifically the first sentence.
Michael
 
Brandonmcgier192

Brandonmcgier192

New Member
#10
#10
Aerindel said:
I got it with just a zip tie on the throttle linkage and everything else stock. Same symptoms. Backfiring when coming off throttle and dancing flame around the muffler. And that was with the zip tie not even fully tightened, just a moderate RPM boost. I'm not at all surprised with a full governor delete he is floating valves.

When I was getting flames I was at about 20 MPH (with 60 tooth sprocket)
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Was your issue fixed with 18lb springs? I get flames at around 30-35mph when I let off throttle.
 
A

Aerindel

Member
#12
#12
Brandonmcgier192 said:
Do you think this exhaust gasket is worn out and causing the backfire issue?
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Brandonmcgier192 said:
Was your issue fixed with 18lb springs? I get flames at around 30-35mph when I let off throttle.
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I didn't replace springs....I just backed off the zip-tie and let the governor work and accepted slower speeds. For my purposes I don't need speed. I was just curious what the engine could do.

The take away is that even with a stock carb, just increasing the RPM alone is enough to get this issue. The governor is there for a reason, that reason being that the stock engine is not designed for higher RPMs. It can of course go much higher than it comes stock, but you have to set it up for that, you can't just delete the governor.
 
SAS289

SAS289

Well-Known Member
#16
#16
You would think the slide carb would at least get the same or slightly better top end than the stock carb? My guess is that the gains may start coming on at a higher RPM than you are currently running.

The slide carb guys should know more.
 
Minimichael

Minimichael

Well-Known Member
#17
#17
Brandonmcgier192 said:
I screwed the fuel mixture screw 3 1/2 turns out and the flame/backfiring really calmed down. I could still only top out at 38 mph.
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3-1/2 turns out is also the point where you should go to a smaller pilot jet. And if you think about it, your problem would seem to include unburnt charge ...continuing to burn even after it's left the combustion chamber. In other words you may be running rich and can't open your air screw far enough to accommodate the high fuel flow. You might try and swap to a smaller pilot jet.
 
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Minimichael

Minimichael

Well-Known Member
#18
#18
You might also try dropping your needle a notch further into the hole. Might work to slow the flow a bit more by delaying it until further into your throttle twist.
 
Minimichael

Minimichael

Well-Known Member
#19
#19
This is, of course, assuming you really are running rich. And now I'm wondering,...

Those vm22 clones are known to ship with lower jet sizes than what performance engines really want. And you also said it didn't matter which way you turned the air screw, so we're not even sure you should be opening that screw or closing it...

And let's face it you've got a 200, with a new header pipe and mikuni...

So maybe you're actually running lean, and too hot. I have heard that lean conditions can cause backfire because everything gets too hot especially right at the exhaust pipe. Your slide carb and header are allowing a lot of flow, which should only be making things hotter.

So now I'm thinking you should be jetting up. If there's a 15 in there, get a 17.5 or 20 for it. You might want to wrap the header, too, to try and keep things cooler near the exits.

Can you tell us more about the carb, and how the bike accelerates, and sounds?

I'm curious about the float idea tho. Seems that sure could allow overflow of the charge, too...
 
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