Low profile air filter

[Gordy]

I've owned a lot of small Japanese trail bikes, many Trail 50's and Trail 90's, even one '71 Trail 90 that had a Powroll 125cc stroker kit, that one was scary and had to be run on 110 octane aviation fuel.

Also a few Yamaha 50's, 80's and 100's, their step-throughs and Trailmasters, a couple TS90 Suzukis and a Hodaka RoadToad. Every one of these bikes ran better with the stock intake and exhaust systems, and it's not like I didn't try. Having the air from the filter enter the carb as undisturbed state as possible, ( no turbulence..) helps. I believe the pod filters create too much turbulence.

If you have seen an older pushrod OHV Honda 50 they have this slightly "S" shaped rubber tube intake from the filter's airbox to the carb, if you take that off and put a pod filter on, you will lose the ability to rev the engine fast enough to break 25 mph. Those early Hondas made 4.4 HP at 8500 rpm and could return nearly 140 mpg if you didn't pin the throttle 90% of the time. More HP per cc than most HD's and British twins.

And they were all cast iron cylinder and heads with pushrods, not overhead cams, and their intakes and exhausts were best left alone.

Yeah, I know what your talking. I have 1969 honda ct90 that I'm restoring

 

[LewieBike]

I am aware of the filter he is running because I run the same one on my Phantom 85's Bofeng carburetor. It's better than the stock open cell foam block for filtering, and in terms of useable area, it has more than the stock filter too. The element is a washable and reusable cotton element that can also be oiled if desired. That engine has zero issues with kissing 9000rpm and doing 50mph through a skinny little 14mm carburetor and OZ Reed block that came with the engine.

They flow more than enough air, and the performance of the bike changed not at all. Well, other than not ingesting sand and grit through a bit of sponge with holes you can see through.

Ingesting grit through that sponge isn't ideal, don't you think?

I have a $100 48cc bike engine kit on a old JC Higgins 3 speed frame, converted to narrow MTB wheels and tires @ 1.5" I bought the PW50 carb for my engine after spending a few weeks trying to tune that tin whistle carb.

Give me something with a real enrichment circuit, a separate idle mixture screw and metered/main jet with a real emulsion tube. And a Nylon float that will never go out of adjustment.

I made my own filter box for that carb from grey PU packing foam, an old CRT monitor came packed in. It's fine grade, oil resistant and filters out everything. Used a cottage cheese container and lid for the airbox. It runs great you can actually hear the intake hitting it's resonance harmonics at about 20 to 27 mph where the engine perks up a little. It's not a world beater but it will hold a steady 18 to 20 mph while climbing a 5%~7% grade and the engine runs smoothly with little 4 stroking.

That's all I want, smooth and efficient.

The stock carb would barely break 20 mph and despite rejetting, playing with the needle clip position, spluttering, flat spots, lack of low end and an idle I couldn't trust. And the float going out of trim every 100 miles.

 

[Gordy]

Ingesting grit through that sponge isn't ideal, don't you think?

I have a $100 48cc bike engine kit on a old JC Higgins 3 speed frame, converted to narrow MTB wheels and tires @ 1.5" I bought the PW50 carb for my engine after spending a few weeks trying to tune that tin whistle carb.

Give me something with a real enrichment circuit, a separate idle mixture screw and metered/main jet with a real emulsion tube. And a Nylon float that will never go out of adjustment.

I made my own filter box for that carb from grey PU packing foam, an old CRT monitor came packed in. It's fine grade, oil resistant and filters out everything. Used a cottage cheese container and lid for the airbox. It runs great you can actually hear the intake hitting it's resonance harmonics at about 20 to 27 mph where the engine perks up a little. It's not a world beater but it will hold a steady 18 to 20 mph while climbing a 5%~7% grade and the engine runs smoothly with little 4 stroking.

That's all I want, smooth and efficient.

The stock carb would barely break 20 mph and despite rejetting, playing with the needle clip position, spluttering, flat spots, lack of low end and an idle I couldn't trust. And the float going out of trim every 100 miles.

Order a BoFeng carb. They have an emulsion tube and a somewhat good way to tune, no idle adjustment screw thoug, just the slide clip.

 

[ImpulseRocket]

Order a BoFeng carb. They have an emulsion tube and a somewhat good way to tune, no idle adjustment screw thoug, just the slide clip.

The jrl Bofeng is definitely a step up, but I still agree with their carb preference. Idle, low speed, needle, and main circuits all independently tunable is so much nicer. Why I am a fan of the vm-18 carb on the motors. Chainsaw carbs also being up there, but needing a bit more work to fit.

 

[DAMIEN1307]

no idle adjustment screw thoug, just the slide clip.

Whtcha mean, "no idle adjustment screw" ???

It sure does have an idle adjustment screw on it...I should know since that is what I am using...lol...See it on the side, right under the word BoFeng???...YEP, that's an idle adjustment screw alright...lol.

 

[Gordy]

Whatcha mean, "no idle adjustment screw" ???

It sure does have an idle adjustment screw on it...I should know since that is what I am using...lol...See it on the side, right under the word BoFeng???...YEP, that's an idle adjustment screw alright...lol.

View attachment 191998

I meant fuel mixture screw, sorry for the confusion

 

[LewieBike]

Whatcha mean, "no idle adjustment screw" ???

It sure does have an idle adjustment screw on it...I should know since that is what I am using...lol...See it on the side, right under the word BoFeng???...YEP, that's an idle adjustment screw alright...lol.

View attachment 191998

yes they certainly have an idle volume screw, it's shame about the lack of an actual idle mixture circuit with a mixture adjust screw. Plus I have my "choke" control on my handlebar and it works a separate enrichment circuit. The carb that came with my engine had a strangler choke that the lever control was poorly peened onto and it flopped around and vibrated.

 

[weefek]

Thank you everyone for the suggestions.

Finally had some time yesterday to play around.

First, I got the engine warmed up on the normal stock filter. Then I went no filter. Then I threw on the low profile.

I have no idea why but the engine seems to love it now. I didn't have to touch a thing.

 

[Gordy]

Thank you everyone for the suggestions.

Finally had some time yesterday to play around.

First, I got the engine warmed up on the normal stock filter. Then I went no filter. Then I threw on the low profile.

I have no idea why but the engine seems to love it now. I didn't have to touch a thing.

What temperature is it where your at? Two strokes can be very temperamental in the cold.

 

[Chainlube]

Thank you everyone for the suggestions.

Finally had some time yesterday to play around.

First, I got the engine warmed up on the normal stock filter. Then I went no filter. Then I threw on the low profile.

I have no idea why but the engine seems to love it now. I didn't have to touch a thing.

It chewed up that bit of sludge that was stuck in the jet and spit it out.

Pro tip: Drain float bowl when you're not going to use it for awhile.