Gas spitting/leaking out of carburator air filter with high RPM vibration

[Evan Frucht]

Hey I have a 66cc motorized bicycle engine. I've done/added
- mild port work slightly enlarged parts by 1 to 2 mm all around. Exhaust port slightly more and polished.
- puch 70cc hi compression head old style fins
- custom made long offset intake manifold from 7/8 steal tube (about 6 inches long with 17mm inner diameter)
- custom expansion chamber exhaust I made
- old NOS Bing 14mm moped carb with #68 jet (extremly similar in design to stock nt carb just better quality german made carb)
- custom velocity stack I made

I have the exhaust and intake clamped/reinforced to the frame to prevent vibration but when I give the bike full throttle on its stand the carb vibrates and spits gas into the velocity stack I made. The carb visually vibrates a little even with my clamp. I suspect the vibration maybe be causing the float valve needle to lose its seating temporarily. But I'm also thinking maybe I just have it jetted to rich. Would rich jetting cause gas to spit from the air filter side of carb at full throttle ? Any tips or pointers please!

 

[Street Ryderz]

Hey I have a 66cc motorized bicycle engine. I've done/added
- mild port work slightly enlarged parts by 1 to 2 mm all around. Exhaust port slightly more and polished.
- puch 70cc hi compression head old style fins
- custom made long offset intake manifold from 7/8 steal tube (about 6 inches long with 17mm inner diameter)
- custom expansion chamber exhaust I made
- old NOS Bing 14mm moped carb with #68 jet (extremly similar in design to stock nt carb just better quality german made carb)
- custom velocity stack I made

I have the exhaust and intake clamped/reinforced to the frame to prevent vibration but when I give the bike full throttle on its stand the carb vibrates and spits gas into the velocity stack I made. The carb visually vibrates a little even with my clamp. I suspect the vibration maybe be causing the float valve needle to lose its seating temporarily. But I'm also thinking maybe I just have it jetted to rich. Would rich jetting cause gas to spit from the air filter side of carb at full throttle ? Any tips or pointers please!

With no reeds blow back through the carb is normal this occures with all piston port engines and will be exagerated if you opened all the ports dia by that much your blow down will now be altered as well as the durations.Opening the ports width (the sides) not the roof or floors is commen practice on these engines.

 

[Evan Frucht]

I may have exagerrated my porting. I raised the intake port by maybe 1mm and widened it 1-2mm. Exhaust was made wider by 1mm on each side and top and bottom maybe .5 mm. Alot of my work was cleaning up rough edges and beveling them slightly. I was under the impression that using my intake port advanced my timing so as to help benefit my expansion chamber. I'm no expert tho, just trying to learn

Another reason I think it may be the float losing it seat or gas foaming due to vibration is that at WOT the float bowl leaks at its gasket. Not normally though. Only at full throttle.

 

[Street Ryderz]

I may have exagerrated my porting. I raised the intake port by maybe 1mm and widened it 1-2mm. Exhaust was made wider by 1mm on each side and top and bottom maybe .5 mm. Alot of my work was cleaning up rough edges and beveling them slightly. I was under the impression that using my intake port advanced my timing so as to help benefit my expansion chamber. I'm no expert tho, just trying to learn

Another reason I think it may be the float losing it seat or gas foaming due to vibration is that at WOT the float bowl leaks at its gasket. Not normally though. Only at full throttle.

vibrations could cause that I would set the float just a bit lower and see if that helps.Advancing the intake timming would require the intake window bottom and floor of the track to be lowered not raised,raising the intake port at the window and track increases the duration.

 

[Evan Frucht]

Ok I also did the piston modification where I cut out a round section a few milimeters high on the intake side. Not sure how that plays into all of this.

I'm assuming to long of a 'duration' could cause exessive blow back ? Maybe that is a factor in my problem ... I really didn't do much to the posts tho. Just a bit of opening and smoothing with dremel

 

[Street Ryderz]

Ok I also did the piston modification where I cut out a round section a few milimeters high on the intake side. Not sure how that plays into all of this.

I'm assuming to long of a 'duration' could cause exessive blow back ? Maybe that is a factor in my problem ... I really didn't do much to the posts tho. Just a bit of opening and smoothing with dremel

The cut out on the piston skirt would do both advance the intake timming some and increase the duration.As mentioned above some blow back through the carb is normal for piston port engines and longer durations will have more volume being blown through the carb.

 

[Evan Frucht]

Ok it honestly isnt much of a problem. It only spits gas into the air filter at full throttle. And even then it isn't much + I have a decent tight seal on my velocity stack and it doesnt spit it far enough to come out. Main problem is that the float bowl leaks at full throttle, I thought the two were related.

 

[FurryOnTheInside]

Hey I have a 66cc motorized bicycle engine. I've done/added
- mild port work slightly enlarged parts by 1 to 2 mm all around. Exhaust port slightly more and polished.
- puch 70cc hi compression head old style fins
- custom made long offset intake manifold from 7/8 steal tube (about 6 inches long with 17mm inner diameter)
- custom expansion chamber exhaust I made
- old NOS Bing 14mm moped carb with #68 jet (extremly similar in design to stock nt carb just better quality german made carb)
- custom velocity stack I made

I have the exhaust and intake clamped/reinforced to the frame to prevent vibration but when I give the bike full throttle on its stand the carb vibrates and spits gas into the velocity stack I made. The carb visually vibrates a little even with my clamp. I suspect the vibration maybe be causing the float valve needle to lose its seating temporarily. But I'm also thinking maybe I just have it jetted to rich. Would rich jetting cause gas to spit from the air filter side of carb at full throttle ? Any tips or pointers please!

I would (did) retard the ignition timing by filing the magnet key. That's supposed to reduce vibration so it could help.
(I'm running an extended intake, high compression head, carb cable tied to the frame, widened (at the bottom) intake port window to 25mm, but with my air filter I wouldn't notice a small amount of fuel spitting out of the carb if it's happening.)

 

[Street Ryderz]

I would (did) retard the ignition timing by filing the magnet key. That's supposed to reduce vibration so it could help.
(I'm running an extended intake, high compression head, carb cable tied to the frame, widened (at the bottom) intake port window to 25mm, but with my air filter I wouldn't notice a small amount of fuel spitting out of the carb if it's happening.)

I find that my air filters have to be checked and cleaned regularly because the blow back causes the filter to trap fine particles in the oil residue left behind in the mesh,when I clean it I submerge it in fuel in the bottom of a coke bottle cut off at about 1/3rd and after just a couple of dunks the particulate is seen in the container and for how often I do it there is always a fair bit LOL.

 

[FurryOnTheInside]

I find that my air filters have to be checked and cleaned regularly because the blow back causes the filter to trap fine particles in the oil residue left behind in the mesh,when I clean it I submerge it in fuel in the bottom of a coke bottle cut off at about 1/3rd and after just a couple of dunks the particulate is seen in the container and for how often I do it there is always a fair bit LOL.

I just saw the OP says he notched the piston. I didn't do that, and I have the 45° filter angled upward a bit too.... but I will be getting the reed valve on after Christmas anyway so I shouldn't get any significant amount of fuel spitting out then.
The timing change should still help with fuel foaming if it does what it is supposed to (I didn't even bother trying stock timing after reading others' experiences).