Uni filter jet increase

[maroonls1z]

About how much do i need to jump up on jet sizes when running a free flowing uni filter? Main and idle. I was going to start with 10 and move my way back

[ryan_ott]

Which carb? Many carbs have different numbering systems for their jets. Why would you not want to keep the stock unrestricted airbox?

[oldgeek]

Uni filters can be a bear to tune properly, and they only add noise and less fuel economy. The stock airbox is sufficient for use on 70cc and below, unless you are race or hyper race 70cc

But 10 sizes up should be a good starting point if you go uni. The pilot jet may or may not need to be changed. Stable air flow is critical whichever you decide to use.

[maroonls1z]

Its a dellorto 19mm. I can seem to get the stock airbox to fit right. Its for a 103cc scoot. Was a 49cc minerelli but i increased the stroke and bore

[maroonls1z]

I read through this guys thread and he used 40 pilot and 95 main for his so i was thinking ill start 45 pilot(what it came with) and 104 main

49ccscoot.proboards.com/thread/1712/project-90-detailed-performance-upgrages?page=4

[90GTVert]

Most of mine have been 90-105 main jets, usually with a 21mm PHBG and a modified stock airbox. I quit using pod filters years ago after a 103cc that was revving 10,500RPM or more with a 25mm carb had no loss in performance when I switched back to a modified stock airbox. Only difference I could tell was that it was less picky about the tune than with the UNI, so I can't tell you what is good jetting with a UNI. Starting over 100 main is a good idea. Safer to start rich. At least a little rich doesn't seize engines.

[maroonls1z]

Thank you for replying. Based on your response i decided to give the air box one more try and fabricate up a new mount for it because i simply could bot get it to bolt up correctly.

[ThaiGyro]

I agree with 90GTVert above...try the air box to make your life easier. The extra displacement would enjoy a bigger gulp of air, but the box will help smooth out/regulate the flow. You might consider adding a larger diameter or longer snorkel going into the box...after you try running it the way it is.

An FYI for brain food: When I was racing cars, (with 1000cc Yamaha and Suzuki engines) we found very conclusive evidence that foam element pod filters are inferior to K&N filters. My Yamaha R1 engine with carbs made 172 hp with foam pods...177 with air box and K&N.  On my Suzuki engine, much better design with fuel injection, we saw 194-196 with foam and hit 199-200 a few times with K&N air box.

Convert that to your engine...we are looking for more, right?  4 hp per liter of power loss on foam...times 5% (50cc vs 1000) equals a potential loss of 2%.  I actually believe that there are more issues with sub ram effect speeds/low velocity air boxes, but a very complicated story. (I think the loss is 6% or more.). Now factor in huge jet sizes, meaning worse fuel economy...possible other tuning effects and you have a cool looking, hard to tune scooter.

[maroonls1z]

Thanks for all your input guys. I managed to make a modified air box work. I have a dellorto PHVA 19mm with auto choke and was going to start with a 98main jet and 45 idle. Both of which will probably be on the rich side. Ive read some different posts on tuning this carb and it seems the amount of options available might make it tedious. I waitched a video on tuning this carb and it talked about main jets and needle but did not talk about idle jets and mixture screws. Does anyone know a good thorough write up on these carbs?

[ThaiGyro]

Many, many folks toss the PBHA carbs with "auto" choke in favor of a PBHG or similar in MIkuni or clones. I use a PBHG and have no tuning issues with my air box at all...but have been doing it for many years.

[tsimi]

I think Raizer used a PHVA carb on his Suzuki build. He might have some info on it.

[Raizer]

Yeah I run a 17.5mm PHVA and uni style filter on my 70cc Suzuki, it ended up wanting way bigger jetting than I ever would have thought!
From memory it settled on a 117mj.

[maroonls1z]

This was on a 70cc? Ugh, im going to need much bigger jets then for my 103cc with a 19mm carb

[Raizer]

Yeah Malossi sport 70cc kit.
But it's weird, even with a stock airbox that Suzuki loves bigger jets than I ever would have expected

[maroonls1z]

Do you think its the carb or the bike? This carb has so many tuning options I think I almost need to get a wide band clamp mount for my exhaust and just pull the wideband off my car to tune it. It really seems like its going to be a pain to tune.