|
|
Post by snakeplissken on Sept 21, 2017 10:07:29 GMT -5
I'm boring over my Polini BBK .4 I found they make an oversized piston for it. Since im doing this what size carb should i use. I feel 19mm might be too little. was thinking 24mm carb would be good. any suggestions. I don't want to burn it up. I rode the scooter 5500km with the polini til it got a little tired. Since then i burnt up to Naraku 70cc kits using 19mm chinese carb with 105 main jet. Like i said i don't want to burn this one up.
|
|
|
|
Post by ThaiGyro on Sept 21, 2017 19:45:02 GMT -5
Depends a bit on what king of performance tuning you have. Mild or wild Polini BBK? (Modified or stock?) I would think your best choice would be a 21mm, but a 24mm would be right if you are doing some porting and such, with a good pipe.
|
|
|
|
Post by gsx600racer on Sept 21, 2017 20:44:05 GMT -5
Burning up kits can be caused by several things. Improper jetting, incorrect oil ratio, too high of compression, case, crank seal/s, intake leaks.
A bbk can/will run fine with stock carb and exhaust as long as above mentioned are not issues.
Now on the performance end of things, if you are running a "better" exhaust I would suggest going with a larger carb. Larger carb and jetting will help in keeping engine temps in check.
If you are burning up these cylinder like candy, Id highly suggest running a CHT gauge so you can monitor your engine temps. Lack of oil and high engine temps are your worst enemies.
|
|
|
|
Post by snakeplissken on Sept 21, 2017 21:49:53 GMT -5
Depends a bit on what king of performance tuning you have. Mild or wild Polini BBK? (Modified or stock?) I would think your best choice would be a 21mm, but a 24mm would be right if you are doing some porting and such, with a good pipe. Nothing crazy. mild setup. Just increasing the piston size. I already have stage 6 replica pipe. About burning the two other kits. The carb isn't the stock. I had to replace it and bought the partsforscooter 19mm replacement. I believe its not 19mm. The stock one is 19mm. the bowl was leaking gas so i had to change it. |
|
|
|
Post by ThaiGyro on Sept 22, 2017 7:44:41 GMT -5
I'm with gsx600racer...something is wrong. A 19 to 21mm carb should be easily within tuning range. I run a 19mm carb on a 50cc, with good pipe and other tuning...a 21mm on a 70 no issues!
Leaks somewhere? You should do a leak-down test.
|
|
|
|
Post by snakeplissken on Sept 22, 2017 14:52:24 GMT -5
When i used the carb that came with scooter i never had a problem. The problems of seizing is only started with the replacement carb www.partsforscooters.com/Carburetor_50cc-2?sc=34&category=133078it says 19mm but i doubt it. I will go over the entire scooter this weekend. I checked it for leaks in June when i replaced the crank but will check it again |
|
|
|
Post by 190mech on Sept 22, 2017 16:08:09 GMT -5
An over bore that small will straighten a worn cylinder,but a performance gain will be verry minor..Its a good way to keep a good old cylinder fresh for a lot more miles,but thats about all you'll gain..  |
|
|
|
Post by gsx600racer on Sept 22, 2017 17:49:57 GMT -5
An over bore that small will straighten a worn cylinder,but a performance gain will be verry minor..Its a good way to keep a good old cylinder fresh for a lot more miles,but thats about all you'll gain.. I agree. .4mm = .018"(.0157" rounded up) or .008" per cylinder wall. |
|
|
|
Post by gsx600racer on Sept 22, 2017 18:53:25 GMT -5
I have several bbk'ed scooters. 2001 Honda Elite,Airsal 46mm aluminum kit w/mild compression, stock carb(14-16mm) jetted fat, stock exhaust from a Kymco Zx50, stock airbox, ZX trans swap w/stock gearing. This scooter runs like a top and runs cool all day long.(high 200's to low 300's) 2003 Kymco ZX50, 72cc Polini Corsa/case matched to cyl w/mild compression, PG race crank with stuffers, 21mm carb jetted a tad rich w/pod style filter, Tecngas Next-R exhaust, a little variator tuning(sliders/clutch/contra spring), stock gears. WOT temps on decent stretches 300-360 and cruising high 200's to low 300's. Although its a Polini Corsa kit, its a very mild build in my opinion. This has been my scooter of choice riding this summer, great low end takeoff power, solid 50-55mph with my 245lb butt in the seat. 2005 Kymco ZX50,72cc Polini Corsa/case matched to cyl, stock crank, 28mm carb jetted a tad rich w/pod style filter,PG Short exhaust,16/32 gears,a little variator tuning(sliders/clutch/contra spring). This was built with a tighter squish and higher compression. Iv had some issues, PG exhaust like to see revs, anything under 65-7000 rpm and its a turtle taking off. Iv had issues with temps until I bumped up to a 28mm carb, but I am considering going a size smaller to improve all around drive-ability.(always have to rev high to go) and the PG pipe is loud. With the taller gears my take-offs suffer but once I hit 20mph its pull nice 55-60mph. I'm getting close to my 65mph goal but Iv been frustrated getting there. So here are 3 build with different results.(all running premix 34:1) Iv toasted a Malossi 72cc sport kit and 2 pistons with the cleaned up cylinder(stock carb & exhaust, stock oil injection and adding a ounce of oil to the tank). I thought having high compression was cool, LOL My thoughts are like ThaiGyro when it comes to carb size. A huge carb on a mild build is overkill. If you have underlying issues(leaks/ect) a larger carb will just cover them up. On a 70cc sport or mild build, a 21-24 should be a place to be. I would consider getting a name brand carb. Polini, Mikuni, Dellorto, Arreche. Clones are a coin toss and sometimes the root of tuning issues. |
|
|
|
Post by honda4life on Sept 23, 2017 7:11:22 GMT -5
Trying not to hijack. What is a good cht meter?
I like the trailtech's.
|
|
|
|
Post by ThaiGyro on Sept 23, 2017 7:13:51 GMT -5
snakeman...another thing that I like to do, once you have discovered the true problem: Document everything you can regarding to your total setup. What I am saying is this: Record and chart all tune worthy components. For example, you carb...easy to look at needle taper, main sizing and pilot numbers...
But what are they really? We have played with diaphragm carbs lately. Expensive and clones. The variation of stated diameters is quite eye opening. Float carbs as well. My latest PBHG 19mm, actually has a venturi that measures 18.7, however the engine side, or throttle is actually bigger than reported. Starting off with a differential that unless measured, is unknown. Bad casting is common, inducing more deviation. Items built from CNC machining...little to no deviation. More expensive, but do they really work better? Still a matter of engine condition and your tuning. That is why a cheap clone can work well on one engine and total crap on another. Luck or good knowledge?
When you put in any part, measure it and its component parts as closely as you can and chart everything for before tuning attempt. We have done this for dyno runs for many years...more efficiency. More knowledge. Less $$ spent on dyno runs. Butt dyno runs can be costly also. Beer time!
|
|
|
|
Post by snakeplissken on Sept 25, 2017 6:21:32 GMT -5
|
|
