lost at carb tuning...

[rosswell10]

hi there, any idea of what might cause this issue of mine? i have a honda dio, i installed a bigger carburetor and i cant get it perfectly tuned, when im trying to install bigger jets because the plug burn is white/gray it doesnt wanna go WOT, it has symptoms of being rich despite the plug color. when i try to rejet it into a smaller jet it runs fine up to WOT but the plug burn is so scary lean.. i dont know how to do it anymore it defies all knowledge i learned in carb tuning a 2 stroke scooter... pleas help thanks

[oldgeek]

Greetings, tell us your kit size, old carb size, and new carb size. Also the type of airbox or filter.

[ryan_ott]

I would try a leak test first. No amount of jet changes will get a leaking motor running good.

49ccscoot.proboards.com/thread/506/make-2t-leak-tester-ways

[spaz12]

Sept 22, 2015 10:44:56 GMT -5 ryan_ott said:

I would try a leak test first. No amount of jet changes will get a leaking motor running good.

49ccscoot.proboards.com/thread/506/make-2t-leak-tester-ways

+1 on what Ryan said

What you described does indeed sound like a an air leak and if the case hasn't been split it should be a fairly easy fix. Look for  signs of oil leaking from the main bearing seals. But since you recently installed a bigger carb, it's probably your intake that is leaking.

[katastroff]

If your case hasnt been split, its a gasket thing.

Rule number one with gaskets: A gasket is like a condom. Always use one, and never use the same one twice.

[paulmohr]

My first question is why the larger carb? Did you do something drastic to the engine that it needed a larger carb, or did you just think it would make it go faster? Chances are the carb might just too large for your engine and you can't tune it. Try putting the stock carb on it and tuning it.

Also reading the plugs on a tiny 4 stroke engine with a cvt is kind of hard, mine likes to lie to me. I can run it so rich it will barely run and the plug still looks lean. I tuned mine by performance. Start with what should be a fairly stock main jet 78 to 85 depending on your set up. Then make a series of timed runs, change your jet and do it again. Do this until performance increases, then starts to drop off again. Once it starts to drop off go back one jet size.

Even with bbk a stock 18 or 19 millimeter carb should provide plenty of gas with some main jetting changes. There really isn't a need for a larger carb unless you have done a LOT to the engine or replaced it with a 125 or 150.

It could also be a vacuum or air box leak too.

[rosswell10]

i have upgraded most of the parts, installed a full vlume crankshaft, changed the block from 39mm to 47mm, i already checked for air leaks especially on the racing manifold that i used, both sides of the crankcase has no leaks i already replaced both oil seals, head and block are sealed, the casing has no leaks also as to im very careful when cracking it and putting it back together, i have tuned a number of Dio engines before and sime minarellis but this one im so lost, its like it has some stupid ghostly problem, the engine runs fine and the plug burn is fine with a stock carb and manifold (19mm keihin carb) but when i use a big carb like keihin PE series 26mm or an OKO, it has that problem, its like only in 3/4 to WOT the problem comes out, its like a sputter and the rpm on that range goes down of some sort, (130main jet) plug color is dark gray.
but when i try to change it out to 120 mj, the 3/4 to WOT is clean and rpm goes up but it displays a very light burn on the plugs and its very scary... .. i have tried every diffrent settings in between including pj circuits and needle clip but no to avail, only in the mainjet can cure the WOT problem but the burn is making me back off

[paulmohr]

Sept 22, 2015 17:05:10 GMT -5 rosswell10 said:

the engine runs fine and the plug burn is fine with a stock carb and manifold (19mm keihin carb) but when i use a big carb like keihin PE series 26mm or an OKO, it has that problem

DR. It hurts when I do this. "Well don't do that!"

If it ran good with the stock carb and runs like crap with a 26mm carb (by the way that is a freakin' huge carb for a scooter) then it is probably the carb. You wouldn't put a holley 750 double pumper on a 4 cyl. engine would you? My suggestion is go back to the 19mm carb and start around 85 for a main jet and work your way up from there.

With that big of a carb and the pilot jet that comes with it can you even adjust the air fuel mixture?

[rosswell10]

IMO i think the carb size is ok, considering that its a 70cc 2 stroke, i can even make a 28mm carb run on a 50cc stock bore with porting jobs, but this time a bit lost, those malossi minarelli kits require more carb size as much as 30mm and those are 70cc's. its just that theres somethings wring and i cant seem to figure it out, it has no leaks, not overported, it has a 70cc pipe and it should not have this problem, i just took the chance of posting here since i follow the forum every now and then... thanks for all of your replies, i appreciate it

[frank50e]

Did you do a leak down test?

[spaz12]

What cylinder and pipe are you running? 

And you have to know that as you increase venturi size, you are decreasing vacuum, requiring bigger and biger jets. At some point your 70 cc motor will not provide enough vacuum for the larger carb. 

Here is a list of cylinders, pipes and carbs broken into categories. It's actually pretty comprehensive in scope and should help you in determining what is an appropriate size carb for your cylinder and pipe. 

BTW, The Stage6 RT/70 cylinder, an extreme cylinder by any definition, requires a 28mm at the most highest tune. This should tell you something...

Anyway, here is the breakdown: 

How does it work?



Below you’ll find a list of tuning parts, each divided into profiles and categories (like cylinders, exhausts, etc.). The profiles help you to check which cylinders and exhausts match and what size carburetor is suited for those combinations. When you ride a cylinder that is found in Profile 2 you should stick with a carburetor and exhaust from the exact same category. Using a part from another category can perform worse and might even damage your engine (too small carburetor, e.g.).

Please note that we are talking about stock (unmodified) tuning cylinders. Porting (modifying) a cylinder from Profile 4, might improve its performance to a Profile 5 cylinder and therefore making it suitable for Profile 5 exhausts and carburetors.

Everything below should just be used as a guideline, therefore keep in mind that other combinations might work too.





Profile 1



Cylinders

Original 50cc

Polini 50cc cast iron

Malossi 50cc cast iron

Hebo 50cc cast iron

TopPerformances 50cc cast iron (Minarelli)

DR 50cc cast iron (Minarelli)



Exhausts

Original

Sito +

Giannelli Nardo / Shot

Giannelli Nr-X (Minarelli)

Giannelli Pro 6 / Pro Cee

Giannelli Reverse

Polini For Race

Polini Funky

Doppler

Yamashi S2

Yasuni Z

Yasuni R

Leo Vince Sprinter

Leo Vince ZX

Bidalot S2

Hebo Gara / 104

Tecnigas Next / R

Tecnigas Basic

Kundo Kinetic

Metra Kit / Metra Kit Series II

Arrow Basic

Arrow Performer

Samba

R-Force

Stage6 Pro Replica



Carburetors

Original (12mm / 14mm)

17.5mm Dell’Orto



Profile 2



Cylinders

Polini Evolution 50cc alu

Malossi MHR 50cc alu

Malossi MHR Team 50cc alu

Hebo Alu NG 50cc alu

Stage6 50cc alu



Exhausts

DPR (50cc spec)

Yasuni Carrera 20

Bidalot Carrera 20

Polini Evolution 50

Malossi MHR Team 50



Carburetors

17.5mm Dell’Orto

19mm Dell’Orto

21mm Dell’Orto

24mm Mikuni





Profile 3



Cylinders

Polini 70cc cast iron

Polini 70cc Corsa cast iron

Malossi Sport 70cc cast iron

Hebo 70cc cast iron

Airsal 70cc alu

TopPerformances Trophy 70cc cast iron

DR 70cc cast iron

Metra Kit 70cc cast iron

Stage6 Street Race 70cc

Stage6 Sport Pro 70cc



Exhausts

Polini For Race

Yasuni R

Giannelli Nardo / Shot

Hebo Gara / 104

Metra Kit Sport Production

Hebo 70cc Competition

Bidalot S2

Stage6 Pro

Tecnigas RS70

Laser X-pro



Carburetors

Original (12mm / 14mm)

17.5mm Dell’Orto

19mm Dell’Orto

21mm Dell’Orto





Profile 4



Cylinders

Malossi MHR Replica 70cc alu

Hebo Alu NG 70cc alu

Hebo Alu / Athena / Eurocilindro 70cc alu

Gilardoni 70cc alu

Polini Alu W 70cc alu (Piaggio)

TopPerformances Due Plus 70cc cast iron (Minarelli)

Athena (T-port) 70cc alu

Hebo (T-port, old type) 70cc alu

Metra Kit Sport Production 70cc alu

Airsal T6 70cc alu

Stage6 Racing 70cc (MKII)



Exhausts

Polini For Race

Polini Evo Cross

Polini Evo 5 / 6 / 7 / 8-serie

Giannelli Nardo / Shot

Yasuni R

Yasuni / Bidalot Carrera 16

Hebo Racing I

Jolly Moto

Jolly Moto SP

Arrow Limited Series

Stage6 R1200

TJT Racing (Piaggio)

Tecnigas RS70



Carburetors

17.5mm Dell’Orto

19mm Dell’Orto

21mm Dell’Orto

24mm Mikuni / TopPerformances





Profile 5



Cylinders

Metra Kit Pro Race

Metra Kit Pro Race 2

Malossi MHR 70cc

Malossi MHR Team 70cc

Malossi MHR Team 7T 70cc

Malossi MHR Team 7T – Factory Tuned 70cc

Malossi MHR Speed Replica 70cc (Piaggio)

Malossi MHR Cross 70cc

Polini Evolution 70cc

Polini Evolution 2 70cc

Polini Evolution 3 70cc

Polini BIG Evolution (85mm conrod)

Hebo Manston 70cc

Hebo Manston 2 70cc

Stage6 R/T 70cc

TJT Racing (Minarelli)



Exhausts

Malossi MHR Team 70

Malossi MHR Team 2000

Malossi MHR Team 2004 (Piaggio)

Malossi MHR Team 2006 (Minarelli)

Polini Evolution 5-serie

Polini Evolution 6-serie

Yasuni / Bidalot Carrera 16

Yasuni / Bidalot Carrera 20

Yasuni / Bidalot Carrera 21

Yasuni / Bidalot Carrera 30

Hebo Racing II

Hebo Racing IV

Metra Kit Pro Race

Metra Kit Pro Race 2

DPR (70cc editions)

Stage6 R1400

XSH ROOST 70cc exhaust



Carburetors

19mm Dell’Orto [Black Edition]

21mm Dell’Orto [Black Edition]

25mm Dell’Orto

26mm Dell’Orto

28mm Dell’Orto

24mm TopPerformances / Mikuni Flatslide

28mm Keihin / Hebo

28mm OKO / Koso

Ibea (Kart carb, different sizes)

HHC (Kart carb, different sizes)

Tillotson (Kart carb, different sizes)

Baroni (Kart carb, different sizes)

JHC (Kart carb, different sizes)





Profile 6



Cylinders

2Fast4Race (78cc; 47,6mm x 44mm)*

2Fast4Race (86cc; 50mm x 44mm)

TCR; Team Cristofolini Racing (80cc; 47,6 x 45mm)

TCR; Team Cristofolini Racing (90cc; 50mm x 46mm)*

Malossi Big Bore Kit (77cc; 50mm x 39,3 / 39,2mm)*

Malossi Big Bore Kit (86cc; 50mm x 44mm)*

Polini Big Bore Kit (83cc; 52mm x 39,2mm / 39,3mm)*

Polini Big Bore Kit (93cc; 52mm x 44mm)*



Exhausts

Malossi MHR Team 2004 / 2006 (Piaggio)

Malossi MHR Team 2006 (Minarelli)

Malossi MHR Team Big Bore

Yasuni / Bidalot Carrera 21

Yasuni / Bidalot Carrera 30

JS DPR

TRT 2Fast

Team Cristofolini

XSH ROOST 77cc / 86cc

Polini BIG Evolution



Carburetors

28mm Dell’Orto

28mm Keihin / Hebo

28mm OKO / Koso

30mm OKO / Koso

32mm OKO / Koso

33mm Keihin PJ

34mm OKO / Koso

Ibea (Kart carb, different sizes)

HHC (Kart carb, different sizes)

Tillotson (Kart carb, different sizes)

Baroni (Kart carb, different sizes)

JHC (Kart carb, different sizes)


Source: www.tuningmatters.com/how-tos/advanced-knowledge/50cc-scooter-tuning-guide/

[rosswell10]

the profile will be a great help.. thanks for the trouble of posting it... just got back from testing it, a while ago i check every single bit of passageway for air, i checked again the racing manifold for leaks, it turns out that the hole of the hose fitting of the 2t oil hose is screwed tight but not well placed because the thread is no good! (china products zzzz) anyways i have managed to seal it using silicone sealant and re positioning the thread of the fitting and now i have brown plugs... i cant imagine how small the culprit can cause weeks of wasted time in tinkering... thank y all for replying to this post!