Topic: A tale of two CB750K air boxes

[willbird]

I removed the air box from my K2 in 1990 or so, and foolishly left it in a building I sold in 2003. I have run pods since. My plan is to put an air box back on the bike. I bought one from Ebay and some brilliant fella had welded two flat washers onto the bolts that hold the lid on...welded the washers on with the box assembled by the look of it. So I grabbed another from ebay.

Now to decide which one to use, which will work better. The one with two intake ports is more "complete" and ready to use. The parts will not swap between them due to a different shape so it is not like I can try each lower once the upper is installed. One has the 3/8 (or is it 10mm?) breather hose, the other does not. The apparent size difference is not "real"...it is due to the perspective the photo was taken from, side by side they are actually the same width.

I am not going to lose sleep over which one is "correct" for the K2. Both parts have the exact same molded in part number.

[Lucien Harpress]

For what it's worth, the bottom one is correct for the K2.  The top one is a later model.  77/78, maybe?

[willbird]

For what it's worth, the bottom one is correct for the K2.  The top one is a later model.  77/78, maybe?

Just looking at them I would think the upper design is part of the overall sound reduction forced upon the mfg ?

[Don R]

 I doubt anyone forced a change based in intake noise. We were using a lot of windjammer fairings on these bikes and Honda did do some sound reduction based on feedback from customers.
  The top, later one may have bigger intake rubber holes, check that. They also have an issue with the threads sticking to each other and the captive tube nut spinning (stripping) in the plastic hex recess. It appears yours already have that issue.

[willbird]

I doubt anyone forced a change based in intake noise. We were using a lot of windjammer fairings on these bikes and Honda did do some sound reduction based on feedback from customers.
  The top, later one may have bigger intake rubber holes, check that. They also have an issue with the threads sticking to each other and the captive tube nut spinning (stripping) in the plastic hex recess.

The holes look the same, I did not put a caliper on them but the new rubber intake velocity stacks I have in hand fit fine.

Bill

[Floshenbarnical]

For what it's worth, the bottom one is correct for the K2.  The top one is a later model.  77/78, maybe?

Correct. The top one is in my 77’s box of parts

[HondaMan]

I doubt anyone forced a change based in intake noise. We were using a lot of windjammer fairings on these bikes and Honda did do some sound reduction based on feedback from customers.
 

That was my take on the change, too, but I did discover the rear-facing vent improved the airbox stability (air pressure inside) in strong crosswinds - like in New Mexico...long story. It performed about the same as the K2 airbox when enclosed with Vetter Lowers, which makes perfect sense. The more still the air, the higher pressure it is.

[Tracksnblades1]

The top looks the same as a 750F1 with a single large bottom air duct…
The F1 had no trouble in stiff crosswinds either…

[willbird]

I doubt anyone forced a change based in intake noise. We were using a lot of windjammer fairings on these bikes and Honda did do some sound reduction based on feedback from customers.
  The top, later one may have bigger intake rubber holes, check that. They also have an issue with the threads sticking to each other and the captive tube nut spinning (stripping) in the plastic hex recess. It appears yours already have that issue.

Good call about the hole dia....the later model indeed does have bigger holes that do not properly fit the new rubber velocity stacks I have. That makes my decision for me I suppose. Would have sucked to have cleaned up the one with bigger holes and made or found new devices to tighten the two halves together only to find that it did not work well. :-).

Bill


Bill