Reducing midrange flat spots with pre-carb runners

[highboy_coupe]

I was reading an interest article today by someone with an inline 4 engine with CV carbs, he had fitted K&N filters and had developed a midrange flat spot due to poor vacuum at part throttle; too much air turbulence around the carb mouth.
He found that rather than fitting covers, or a rudimentary airbox around the pod filters, creating some distances (maybe 50mm) between the carbs and the filters eliminated his flat spot.
He used some factory snorkels and bodged it together, but I found some 54mm ID silicone reducers of various sizes that would be perfect.

I suppose the science is that there is now a chamber of ''smooth" air behind the air filter, and that air originally drawn in sideways around the carb mouth is now channelled uniformly through the hose before hitting the carb.

Has anyone tried it? I'm going to give it a go (when my jets are sorted)

 

[5twins]

I did try putting my K&Ns on extension tubes but it made no difference. Changing to foam UNI pods fixed all the issues. Pleated style pods just plain don't work well on CV carbs. I've also read that CV carbs don't like an angled or tapered filter, which most K&N type are. That's another strike against them. The long, straight foam UNIs are also open on the ends, not capped like most pleated pods. This probably helps too, allowing air to be drawn straight in the back.

 

[highboy_coupe]

I did try putting my K&Ns on extension tubes but it made no difference. Changing to foam UNI pods fixed all the issues. Pleated style pods just plain don't work well on CV carbs. I've also read that CV carbs don't like an angled or tapered filter, which most K&N type are. That's another strike against them. The long, straight foam UNIs are also open on the ends, not capped like most pleated pods. This probably helps too, allowing air to be drawn straight in the back.

I have foam Ramair filters (straight), and have no real issues with them, I thought it was interesting idea!

 

[5twins]

So did I, so I tried it, but it did little or nothing to reduce the stumbling from the turbulent air flow the pleated K&Ns were causing. I've used K&Ns for many years on most all my bikes and even in my cars, so when I got my 650 they were my first choice. But, I had never fitted the pod style to a CV carb before. The previous bikes either had regular slide carbs or if they were CV carb equipped, the K&N had been an airbox element replacement. Those worked OK. I know from experience now that K&N style pods on CV carbs don't work well. Sure, you can get it to run reasonably well, but you'll never get the carb tune perfect.

 

[mrtwowheel]

I've thought about this before, but I never thought of using a spacer between the filter and carb. My idea, never tried, would be to block off some of the front portion of the foam filters. Like, cut strips of oil jug plastic that could be rolled up and placed into the foam filter and somehow held at the front of the filter, inside. This would block less than half of the filter and create a space for the air to stabilize some. Wondering if this could be an improvement for the UNI foam filters? Deaden the air?

Scott

 

[NashGTI]

I don't have an carburetor based experience with this, but I will note that if you look at automotive mass air flow meters, many of them have a piece incorporated into them to smooth airflow across the element. Vacuum reference through a carb you would think would react the same way so I can see the science behind the idea.

 

[gggGary]

I've always meant to fit a velocity stack so it protruded through the air filter neck to about half the filter cavity depth.
Saw this in a YZF600 I've been working on


Note they split the air passage so the diaphragm chamber, air mix jets, and float bowl vents are in "still" air away from the main flow. As the factory always does, lots of designing to make a big air filter box.

 

[NashGTI]

That actually gets me wondering, the various pod filter types sold for motorcycle engines, do they incorporate some sort of rudimentary velocity stack design as do their larger automotive cousins? I'm thinking back to a CB550 I did some work on and the pods on it simply had right angle edges but the owner said they were Ebay specials.

 

[gggGary]

That actually gets me wondering, the various pod filter types sold for motorcycle engines, do they incorporate some sort of rudimentary velocity stack design as do their larger automotive cousins? I'm thinking back to a CB550 I did some work on and the pods on it simply had right angle edges but the owner said they were Ebay specials.

None that I've seen do.