Topic: airbox comparison 750F to 750K

[evinrude7]

is there much difference in these two?  bikes around 76

[evinrude7]

newer boots from 4 into 1 if memory serves.  a stock airbox i got off ebay awhile back.  guy said it was from a 76 cb750k.  saw a photo of the parts bike and looked like a mid 70s 750.  on the side of the airbox it says HM CB750 - F .  is the F for four or for the super sport f model?  anyhow i'm having a hell of time getting the boots to seal.  not on the carb side where the clamps go but on the airbox side where there are no clamps.  the diameter of the boots seem too small to fill the holes in the airbox.  i loosened all the clamps and sort of massaged the carbs and boots around until i got it as close as i could but it's not sealing well.  i thought maybe i could get some u-channel and line the holes in the airbox to make the holes smaller and thus the boots fitting snug.  anyone else had this issue?  solutions or ideas appreciated.

[pjlogue]

CB750 F is on the side of my air box too.  It is a 76" K 750 so you have the correct air box if you have a 750K.  The proper air box - carb rubbers should have a groove that engages the air box to form the seal. 

-P.

[evinrude7]

CB750 F is on the side of my air box too.  It is a 76" K 750 so you have the correct air box if you have a 750K.  The proper air box - carb rubbers should have a groove that engages the air box to form the seal. 

-P.

yeah but the grooves are way back on the rubbers and end up inside the airbox. 

[evinrude7]

holy #$%* do i have the boots on backwards?  is the fat side supposed to be inside the box?

[kerryb]

That depends on what you consider the "fat" side!  I think this would be a good time for the pictures to appear...yours or microfiche.

[evinrude7]

That depends on what you consider the "fat" side!  I think this would be a good time for the pictures to appear...yours or microfiche.

figured it out.  once i heard someone on a youtube video refer to the boots as velocity stacks the light bulb went on.  i've had them in backwards for 4 months chasing my tail for answers to fluctuating idle, back firing and the occasional flat spot while throttling.  feel like a fool but happy to have them correct now.  thanks for the input pj and kerry. 

[pjlogue]

Glad you have t resolved.  Carb boots are one of the many items on the bike that take some figuring out and tweaking to get right.  Many things seem trivial until you go to reassemble, then it's head scratching time.  Front axle orientation was another for me. 

-P.

[evinrude7]

Glad you have t resolved.  Carb boots are one of the many items on the bike that take some figuring out and tweaking to get right.  Many things seem trivial until you go to reassemble, then it's head scratching time.  Front axle orientation was another for me. 

-P.

for four months i'm thinking what a terrible design this isn't like honda.  no, no, dumbass here. 

[HughL]

Also seem to have an F airbox on my K model (76) - the boots and such seem to fit just fine but curious if it is the same or not since my bike seems to run rich no matter what I do.

Edit: The airbox says CB750F-F, I have round top 067A carbs and the airbox seems to fit fine, but unsure how it affect performance.

[HondaMan]

The lettering on the sides of the airboxes can be misleading: some have "CB750", some have "CB750F" and some have "CB750F-F" on them. The best-performing ones, IMHO, are the ones with the single wide vent across the bottom, facing backward: that one was the last design and makes for a quiet intake with an excellent air supply.

[HughL]

The lettering on the sides of the airboxes can be misleading: some have "CB750", some have "CB750F" and some have "CB750F-F" on them. The best-performing ones, IMHO, are the ones with the single wide vent across the bottom, facing backward: that one was the last design and makes for a quiet intake with an excellent air supply.

Mine has a single rear facing vent and is held in by the bolts vs wing nuts - additionally it has the vent tube on the top. Unsure if it matters or not for my bike though.

[scottly]

Also seem to have an F airbox on my K model (76) - the boots and such seem to fit just fine but curious if it is the same or not since my bike seems to run rich no matter what I do.

Edit: The airbox says CB750F-F, I have round top 067A carbs and the airbox seems to fit fine, but unsure how it affect performance.

The airbox is not causing the engine to run rich. What symptoms lead you to believe you have an overly rich condition? Carbon fouled spark plugs? Are you still using the Power-Arc ignition system?
http://forums.sohc4.net/index.php/topic,189984.msg2208732.html#msg2208732

[HughL]

Also seem to have an F airbox on my K model (76) - the boots and such seem to fit just fine but curious if it is the same or not since my bike seems to run rich no matter what I do.

Edit: The airbox says CB750F-F, I have round top 067A carbs and the airbox seems to fit fine, but unsure how it affect performance.

The airbox is not causing the engine to run rich. What symptoms lead you to believe you have an overly rich condition? Carbon fouled spark plugs? Are you still using the Power-Arc ignition system?
http://forums.sohc4.net/index.php/topic,189984.msg2208732.html#msg2208732

Yea the plugs come out black after a few hundred miles or less. Smells rich and sometimes shoots black smoke on acceleration, also some pops on decel.

It still has that power arc yea, I've been able to adjust the tensioner and valves no problem though since I figured it out.

[HondaMan]

If the spark advance comes on too soon it makes the mixture rich from carb spitback: make sure your advance does not hit full until at LEAST 2000 RPM: 2500-3000 RPM is far better with today's gas.

Mine doesn't hit full advance until 3300 RPM (cold) or 3500 RPM (hot). I'm considering making it slower yet.

[scottly]

Hugh, you should start your own thread about your problem.

[Rookster]

Like Mark said the CB750F cast into the plastic does not mean the air box was for a Super Sport.  The first of the black air boxes in 1971 did not have any lettering cast into the plastic.  They are unique to the K1. The lettering started with the K2.  This was all before the CB750F model ever came out to cause confusion...

Scott