Had a good idea yesterday. I decided to remove the choke ony stock carb on my 80cc kit and i filled the mouth of the carb up with JB weld up to the edge of the port opening to create a small velocity stac. The results?? ..........4MPH! I went from a max speed of 26 to 30.9!! great little cheap mod! I will post pics tonight but it is self explanitary.
quick carb mod!!
- Thread starter Whiplash
- Start date
Actually I did another better version of this mod and used a filter and velocity stack for a pocket bike and gained another 1.4MPH! for a total now of 31.4! Here is a pic of the mod. I am certain this mod will get flow close to the same as the 18mm carbs some people use for 15 bucks and a little time!
I drilled and tapped the carb with the velocity stack rotated so I had a good amount of clearance and then drilled and tapped the carb with a 1/4-20 tap, bolted it in place with the single fastener since there was no place for the other one. Next I coated JB weld liberally around the stack to seal it to the carb and help hold it in place. Finally I drilled the little breather port for the float bowl through the stack and filled in the gap between the stack and the carb face. Once it set up, I drilled through the JB weld so the breather for the float worked again. VOILA! Race carb on the cheap! This mod is all about airflow and it does not like to turn sharp corners. the velocity stack allows the air to make a gradual turn instead of an abrupt corner upon entering the carb inlet, the result is you actually get the full flow potential of the inlet diameter, not 1/3 of it as you did before. This can also be applied to any carb, so if you have a good one that still does not have a velocity stack like inlet, YOU ARE LEAVING POWER ON THE TABLE!
Believe it or not, this mod actually went from a top speed on the flat of 26mph (stock but tuned) to a new top speed of 31.4!! that's a gain of almost 6 MPH!! Not to mention I had it tuned nice and clean before this mod, the plug was a nice tan color and no 4 stroking, now if anything, its a bit rich! I think the added airflow is causing the carb to pull more fuel through it, so I will now be filling up the jet and re drilling it smaller, I just need to get the drill bit I need to do it. The next order of business is building a pipe, I have a bunch of 18 guage sheet metal laying around, so I plan on rolling it into two cones on my slip roller, and trying to see if I can get a decent pipe built up! I will likely do this before the jetting, because it will most likely need more fuel after the pipe. I will take good pics during the pipe build to document it.
I drilled and tapped the carb with the velocity stack rotated so I had a good amount of clearance and then drilled and tapped the carb with a 1/4-20 tap, bolted it in place with the single fastener since there was no place for the other one. Next I coated JB weld liberally around the stack to seal it to the carb and help hold it in place. Finally I drilled the little breather port for the float bowl through the stack and filled in the gap between the stack and the carb face. Once it set up, I drilled through the JB weld so the breather for the float worked again. VOILA! Race carb on the cheap! This mod is all about airflow and it does not like to turn sharp corners. the velocity stack allows the air to make a gradual turn instead of an abrupt corner upon entering the carb inlet, the result is you actually get the full flow potential of the inlet diameter, not 1/3 of it as you did before. This can also be applied to any carb, so if you have a good one that still does not have a velocity stack like inlet, YOU ARE LEAVING POWER ON THE TABLE!
Believe it or not, this mod actually went from a top speed on the flat of 26mph (stock but tuned) to a new top speed of 31.4!! that's a gain of almost 6 MPH!! Not to mention I had it tuned nice and clean before this mod, the plug was a nice tan color and no 4 stroking, now if anything, its a bit rich! I think the added airflow is causing the carb to pull more fuel through it, so I will now be filling up the jet and re drilling it smaller, I just need to get the drill bit I need to do it. The next order of business is building a pipe, I have a bunch of 18 guage sheet metal laying around, so I plan on rolling it into two cones on my slip roller, and trying to see if I can get a decent pipe built up! I will likely do this before the jetting, because it will most likely need more fuel after the pipe. I will take good pics during the pipe build to document it.
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suite dude i wish you would have taken a pic before the pocket bike add on soi could see how the stock mod looked like but thats ok it seens simple enough i think ill give it a try how much eepxoy did it take and what type of air filture did you use regular i use a green scratchy pad
i just gps my top speed yesterday 26.6 i feel i could go faster but am being starved for fuel a wider ful etcock and a home made velocity stack might do the trick
i just gps my top speed yesterday 26.6 i feel i could go faster but am being starved for fuel a wider ful etcock and a home made velocity stack might do the trick
Yeah I am sure it will! I basically filled the "depressed" area around the inlet up just above flush with it and tried to form a nice radius around it. Air likes smooth transitions when it has to enter tubes and believe it or not, just a nice radius around a pipe inlet can easily DOUBLE airflow, and I have PROVEN this with my flowbench in the past. One cool side effect of doing the new velocity stack, is the engine is MUCH quieter! I would attest that to the air flowing in better and having a very slight ramming effect that is helping to fight the reversion that makes the typical carb chugging sounds 
good luck with it and post pics after your done! I used the stock filter first but drilled holes in it and i also cut the bars out of the screen thing that holds the filter in place....
good luck with it and post pics after your done! I used the stock filter first but drilled holes in it and i also cut the bars out of the screen thing that holds the filter in place....
i will give it a try after i get some cash for expoxy all i have is water proof epoxy and im saving that for a special use
i am having problems attatching picture first they are to large then after i correct size it says file is to large i dont know how to shrink the file
i am having problems attatching picture first they are to large then after i correct size it says file is to large i dont know how to shrink the file
wow never even thought about the restriction at the face of the carb....i guess ive always thought about all the other restrictions in the intake and exhaust....it gets cold around here and the choke is kinda nessassary....i gotta look into a solution....thanks for sharing this idea....i got myself a pretty nice shop at home and i work in a very well equiped shop but do not have access to a flow bench....are you an engine builder or something im abit envious....thanks for the advise
Iused to have a really well equipped speed shop with full NASCAR quality machining tools flow bench dyno the whole bit. I tested TONS of different things and have a VERY good understanding of airflow dynamics and can easily look at the shape of a port and know what it will take to get 90% of its max possible flow potential without needing the flow bench. I literally have 1000's of hours porting and flow testing under my belt. The only thing I have not done much of is two cycle engines as fas as setting port timing for high performance use. I have been studying port timing though quite a bit and I plan on doing a really nice write up on porting for power once I get a good understanding of thetwo cycle timing thing, but I am fairly confident of my knowledge at this point I just need to prove my theories by actually testing them on my cylinder!
wow man....its great to meet you....ill be looking for what you come up with...it sounds like ill have to be buying bits for my friend's mill soon....ive allways been certain that the port timing could be better....it seems that 68cc on a 2stroke should produce allot more power than what we have been getting
yes i also believe we should be getting better range of torq out of these motors
when you figure out the timim=njg and such could you please write up a tutorial about the effects of widening openings rasing or lowerin g them and such also if you figure out where the best low to mid range torq is at please let us know im going for low to mid power as i am going to be running a shift kit on my next bike but after i build that i will want to mod my current single speed to the highest top speed forgweing about low end power and focosing on high end power
most mods are better for high end but robb the low end rpms of torqe
when you figure out the timim=njg and such could you please write up a tutorial about the effects of widening openings rasing or lowerin g them and such also if you figure out where the best low to mid range torq is at please let us know im going for low to mid power as i am going to be running a shift kit on my next bike but after i build that i will want to mod my current single speed to the highest top speed forgweing about low end power and focosing on high end power
most mods are better for high end but robb the low end rpms of torqe
Yeah, I woud assume added torque is going to be gained by simply maximising the current port design by way of blueprinting the current configuration thus not raising the RPM band the engine was designed for, simply make it as it should be and improve flow in the ports themselves. The other MAJOR factor for torque I believe is going to be in a decently designed reed assembly and properly tuned expansion chamber. Everything I have read thus far states that without these two items, a two stroke basically CANNOT be run efficiently. They are a necessity. I am poking around for a nice small but high flow reed cage that I can build up a manifold to fit. I have read a few write ups where others have done this with great success! This along with a good port blueprint and cleanup and a good pipe, should net some impressive gains! I will be getting to this as soon as I can. It will be fun to see what I can do with very little money invested, just a bit of work! I also hope to improve the head's combustion chamber and I read where someone else has installed a PUCH head with success for improved squish and higher compression. This is also an area where these little motors could use a boost for the torque we are looking for!
whip lash i hope your your going to try some less costly add ons i certanly would like to get more umhf but at to high of cost we could buy a cheap dirt bike but dont leat me stop you really even if these mods are out of my price i still am interested maybein the futur ill have more cash
but any way i am ordering a new 2010 grubbe shy hawk comes with a cns carb then as add on im also ordering a high compression billit aluminum cylinder head , a ported billet air intake ,and a sbp expansion chamber muffler, i beleive these bolt ons will greatly increase power and with intake and exaust porting i plan to do once i figure out how i should make them wider or taller lower or both ports to do it with from what ive heard lower the intake and raise the exaust it puts the icing on the cake unfortunitly its gonna cost about 400 bucks with shipping not including the shift kit i plan on ordering from sick bike parts andd the bike frame i have to create into a strdy ride with high quality bearings and derailers and etc etc either way its gonna cost a pretty penny when its completeed
oh and to make it more expensive i want toccreate a reliable tough lighting system head lights tail lights brake lights turn signals horn all the works to try make it road legal
but any way i am ordering a new 2010 grubbe shy hawk comes with a cns carb then as add on im also ordering a high compression billit aluminum cylinder head , a ported billet air intake ,and a sbp expansion chamber muffler, i beleive these bolt ons will greatly increase power and with intake and exaust porting i plan to do once i figure out how i should make them wider or taller lower or both ports to do it with from what ive heard lower the intake and raise the exaust it puts the icing on the cake unfortunitly its gonna cost about 400 bucks with shipping not including the shift kit i plan on ordering from sick bike parts andd the bike frame i have to create into a strdy ride with high quality bearings and derailers and etc etc either way its gonna cost a pretty penny when its completeed
oh and to make it more expensive i want toccreate a reliable tough lighting system head lights tail lights brake lights turn signals horn all the works to try make it road legal
oh im getting the kit and bolt ons from pirate cycles justin has bean patient and answered all my questions oh and as a plus hes shipping seems to be the cheapest i have found even though i live in the northwest and hea located on the east coast others who are located closer arizona washington have more expensive shipping rates sunds like a mark up to me correst me if i am wronge
Yeah they like to do that it seems. You never get to pay the actual shipping...
Sound like you are going more expensive than I am! Yours should run really good setup like that just take the time to do it right and you will enjoy it!
Sound like you are going more expensive than I am! Yours should run really good setup like that just take the time to do it right and you will enjoy it!
well whiplash this is my second build and you know how it goes you move up not down i have a semi relaible motor bike that i use for my main transportation so this next build i will take my sweet old time and put the best workmanship into after all my first bikt i was so excited to get it toghether i didnt even know about even simple gasket matching now i have a little better understanding that its all about the air flow and i am going to do my best to have goood flow