Pod Air Filters or Aftermarket Pipes on Vintage Hondas?

I get a steady steady stream of questions about re-jetting for aftermarket pipes on the range of vintage Honda motorcycles we support.  
Pod Filters

Pod Filters?

Pipes

Aftermarket Pipes?

“Pod” filters certainly look pretty “racy.” Are they a good option on your bike?

If you have sweet Keihin CR Roundslide carbs like the ones shown above on my CB1123F, then you have race carbs that are designed to facilitate extensive tuning. Non-CV Carbs like these are almost infinitely adjustable via an extensive selection array of jets, needles and other calibration components that are easy to access and swap. Armed with this flexibility, it’s easy to optimize carburetion for any given combination of air filtration and exhaust.

But what if you’re not running carbs like these and you have the stock carbs on your bike? Then, here’s my advice:

OEM Honda constant velocity carbs manufactured by Keihin of this vintage and design are very sensitive to flow dynamics on the intake side. Here I’m referring to all carbureted GoldWings, CBXs, CX500s, DOHC-4s, FT500s, etc.

That’s why departures from the OEM air filter and air-box are not advised in most cases.  K&N makes a good product, but I don’t recommend such air filters for bikes running OEM constant velocity carbs.  The Honda engineers have already calibrated the carbs for the optimal air flow for max power and efficiency. CV carbs are theoretically “self-adjusting,”  but the reality is that they are rather sensitive and their inherent flexibility is easily exceeded.

Ditto for “pod” filters of any brand actually. In fact, poorly constructed or poorly matched “pod” filters can have so much obstruction on the inside diameter (ID) aspect that they can actually block air “bleed” jets located at the mouth of the carbs!  That’s an issue you definitely want to avoid!

Adding even more air via a “free-flow” filter does always not help performance. In fact, it can introduce chronic, undesirable leanness which is hard to overcome on carbs that were not designed to be “tuned” to any great extent. The jet and needle choices for the stock Keihin carbs are very limited!

Aftermarket filters typically make lots of “noise,”  but noise does not equal power.

I’m not alone on this point of view.  Per carb expert Mike Nixon:

“One thing folks seem to not be willing to believe is that changes to the intake side of the engine have 20 times or more the effect that changes to the exhaust side have.  Specifically, change the exhaust, no change to carburetion.  Change the air filter type however, especially going to pods, and a HUGE change requiring changes to the main jet of over 10 jet sizes!  This is likely your problem if you are using pods and the carbs have not be rejetted to them.  Fitting the stock airbox and air filter is of course the better solution.  🙂  Mainly because, although the carbs CAN be jetted to match the pods, and the engine will run right, the factory seamless throttle action will suffer, even with the proper jets.”

However, setting aside the dubious merit of more exhaust noise level, these bikes will tolerate almost any exhaust without the need to re-jet so long as the intake is the stock setup.

Keep in mind that the OEM exhausts on Hondas of this vintage are not only quiet, they flow exceptionally well and out-perform virtually all aftermarket pipes.

Again, noise does not equal power!

Comments

  1. Have a cb650sc, and the carb to air box boots, are out of shape & don’t fit well, going into the air box. I’ve ordered the dynajet jet let & uni foam air filters, next step the Mac four into 1 exhaust, due to rust holes on stock mufflers.
    So, if everything was in good workable shape, I’d keep it stock, but have to try to make my bike run better then it does now.

  2. Hi

    Thanks for a very useful info source, I was lucky enough to pick up a great condition original paintwork, low mileage ’75 GL1000 and your site is a lifesaver. Further to your post above, you might be interested in this test of K&N’s which suggests that maybe they’re not even a particularly good product http://www.nicoclub.com/archives/kn-vs-oem-filter.html, or at very least have some trade-offs that don’t seem to be worth the limited benefits. I started reading around after I bought K&N’s for a Guzzi, and was horrified to note that the filter elements are so coarse that you can seen the holes with your naked eye. Not something I’d consider risking here in dusty South Africa.

  3. So, a K&N air filter in the stock airboxon my 1976 Gl1000 LTD is not a good idea?

    Thanks for all the good info and repro parts… 🙂

    • Not a huge issue for GoldWings. The K&N air filter is an excellent product, but I prefer OEM air filters for GoldWings for the reasons stated above. A “high-flow” air filter can introduce a bit of unnecessary leanness.

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