Topic: Hondaman's airbox mods on CB750 F and late K models

[drumstyx]

Doing my best to tune the carbs with a 4-into-1 (Delkevic) exhaust, and this thread (http://forums.sohc4.net/index.php/topic,129596.0.html) has been a fantastic help with the jetting spreadsheet -- I'm now, with a K&N filter, running 42 pilots, and 110 (stock) mains on my 1978 CB750K, 0.75 turns out on the idle mix screw. This is fantastic, if a *hair* rich at a dead idle (around 11.9-12 AFR) but pretty much perfect (14-15.5 AFR) at 1/8-1/4 throttle, which is what I use cruising around town.

Now, the problem! At high RPM, under high load (WOT down a straight), it's going rich...way rich...9-10.5 AFR rich. Well okay, the exhaust doesn't have nearly as much back pressure as the stock pipes, so it'll be pulling more air, and at a certain point, the airbox just can't give enough I guess!

Here's the question though, specifically @ Hondaman! In reply #12 and #21 of the above thread, you say the main opening of these airboxes is plenty big enough for these (PD) carbs, and what needs relief is the INSIDE where the air turns 90-degrees upwards. Looking at the cross-sectional area of the carbs vs the airbox intake though, it seems the airbox intake area is FAR smaller than the carb area of all 4. Indeed, the 2 windows in the baffle look about right on (if not a little BIG) for the carb area. Of course, this doesn't take into account the proposed pressure differences (rear facing intake could and probably does have higher pressure than the moving air inside the airbox) but does it really make THAT big of a difference? I mean, I'll believe you if you say it does, but I just wanted to double check, because the opening INTO the airbox seems about half as big as the 2 windows in the baffle, which would lead me to consider drilling those 4 1-inch holes in the airbox lower, like the OP did in that thread.

Anyway, question is, I've got a spare airbox, where should I start? Part of me just wants to open that lower right up with the 4 big holes, but I can't really afford to get any leaner in the small-throttle-opening department.

[seanbarney41]

If it's too rich on the main jet, use a smaller main jet...don't make the mistake of equateing big main jet with big performance.

[drumstyx]

If it's too rich on the main jet, use a smaller main jet...don't make the mistake of equateing big main jet with big performance.

Certainly! Just that this is the stock, 110 main jet. A smaller main would help, but doesn't solve the fact that it IS choking at high rpm WOT...technically that means the higher the rpm, the more choke, which means a 105 would just move the point where it gets so rich it stumbles up into a slightly higher rpm range.

[david 750f]

You’ve got a four stroke motorcycle, only one cylinder is on an intake stroke at any given time. The inlet area of the air box only has to be slightly larger than the inlet of one carb, not all four.

[drumstyx]

You’ve got a four stroke motorcycle, only one cylinder is on an intake stroke at any given time. The inlet area of the air box only has to be slightly larger than the inlet of one carb, not all four.

Well damn, that's a really good point. Changes my thoughts completely, and I can definitely see the airflow changing direction causing enough restriction/turbulence to warrant cutting away on the inside.

I feel silly for not realizing that actually 😂 I'd still love to hear from Mark about it, since I'm measuring with an O2 sensor, so I can theoretically see minor changes as they happen. The other thread had OP opening big holes in the airbox, is that truly entirely unnecessary?

[Rosinante]

The other thread had OP opening big holes in the airbox, is that truly entirely unnecessary?

Yes. 

[Don R]

 More airflow in some cases can carry more fuel.
 When we began to use a .563 stroker crank in the jr dragster motors, they suddenly required a smaller jet in the carb. No changes other than the stroke and rod length. 
  I wonder what effect a stock air filter would have? It could defy logic.

[Rosinante]

It is common for folks to believe their stock engines can pump more air and make more power if the intake system were modified to allow greater air flow.  Generally, engine air flow is limited by the size of the intake valve.  Intake valves are not very big.  Air filters have considerably more surface area than is needed.  Improvements in power output are not so easy as changing filters or drilling holes in air boxes.  Those improvements nearly always require hardware changes.  Tuned exhausts can be effective, even very effective, depending on the engine design.  Tuned exhausts take advantage of the velocity of exhaust gasses to create a vacuum when exhaust valves open.  Intake runner geometry can also be important, and this is not always a matter of enlarging them.  Smaller intake runners can increase air velocity and 'pack' the cylinder better.  It is a mixture of art and science.  Mostly science.  If it were simple to outsmart the design engineers, then using a different air filter would get you more power, but it's not simple to outsmart design engineers.

All generalizations are false, BTW.

[drumstyx]

More airflow in some cases can carry more fuel.
 When we began to use a .563 stroker crank in the jr dragster motors, they suddenly required a smaller jet in the carb. No changes other than the stroke and rod length. 
  I wonder what effect a stock air filter would have? It could defy logic.

I was running an EMGO filter that, apparently, was supposed to have stock-level airflow. I don't know if OEM is still available. When I was running the EMGO though, I had to drop down the main to a 105, and still ran rich like this (I don't remember the exact numbers). I could drop down to a 105 again, but the chart in the other thread (http://forums.sohc4.net/index.php/topic,129596.0.html) points to ~12 AFR at WOT being perfectly possible. Granted, that's a Mac exhaust, where mine's a Delkevic, but he's using 112.5 mains, AND has a shim under the needle.

I think I'll take the spare airbox I have and drill those holes to test -- easy enough to close the holes back up. If they make things better, I'll close them up and relieve the baffle per Hondaman's recommendations, and compare the results. I guess I was just hesitant to cut into any stock parts (even spares) because of my fear that maybe some day these parts simply won't be available on ebay, but that's the whole point of spare parts right? Experimentation time!

Still glad I started this thread, such good points here

[drumstyx]

Alright, I didn't mod the airbox yet, but I dropped down to 40 pilots and 105 mains, 1.5 turns out on the mix screws, and 1 shim under each needle.

Results seem great mostly -- 14 Afr at idle, 15ish off idle and 1/8 throttle, maybe as much as 16-17 depending on conditions and exact throttle position, but cruising always sits around 14-16.

Took it on the highway though, and while performance was fine, no hiccups to speak of, I was reading 10-11 AFR the whole time, riding around 120km/h (75mph) with, I'd say maybe 1/4 -1/2 throttle....just whatever was needed to keep it at speed.

It performed VERY well getting up to speed, and I was at 160km/h (100mph) before I knew it while merging on. Still puzzled by running so rich under load, but it had plenty of power and didn't even burble when I whacked open to WOT to give it a test with the accelerator pump.

I'm thinking I'll actually just try running like this for a while, and disconnect the AFR meter so I can just enjoy it for a bit. I've never had it dialed in even this nicely, so if I just run for a while, I can have a look at the plugs in 100km or so and see how it's going.

[Bankerdanny]

If it's too rich on the main jet, use a smaller main jet...don't make the mistake of equateing big main jet with big performance.

Certainly! Just that this is the stock, 110 main jet. A smaller main would help, but doesn't solve the fact that it IS choking at high rpm WOT...technically that means the higher the rpm, the more choke, which means a 105 would just move the point where it gets so rich it stumbles up into a slightly higher rpm range.

The stock main jet size is 105. 110 is already oversized.

[HondaMan]

If you have both a #110 mainjet and an EMGO air filter, you've found the troubles. Go to a #105 mainjet and try it with simply no air filter in the box.
Like David 750F said, the total air inlet size needs to be no more than 1.25 X (the cross-section of one venturi of one carb). In the airboxes like you have, with the 'quieting' vent on the bottom facing backward, this particular one can benefit from adding 2 holes of 1/2" size thru the floor of the box and NOT thru the vent to the outside. This reduces the tumbling of the air at higher velocities (bike speed down the road) inside the box by breaking up an effect we used to call "drumming". The only way I know to describe this that others may have experienced is: at 50-60 MPH in a 4-door sedan, with only one rear window rolled about halfway down, the interior air of the cabin will alternately drop-rise-drop-rise when at the right speed, causing a drumming sensation in our ears. This, in this particular airbox, was demonstrated by a rider who 'straight-raced' on a flat, straight road in New Mexico on a 1977 K7 model with a cloth streamer hanging out of the airbox (on purpose): it alternately flapped out and back in, as observed by his buddy on a Harley next to him, circa 1980. He was the guy (that I know of) who drilled first 1, then 2, then 3 (which he later plugged, back to 2) 1/2" holes in the upper floor of the bottom vent. The bike picked up almost 10 MPH top speed for it, outrunning his buddy in the end.

This 'drumming' effect (my term, not his) was likely not even considered by Honda in 1977, as the major customer (USA) was sporting a blazing 55 MPH speed limit at the time. Honda was under the DOT's thumb then to make the bikes quieter in a full-throttle, 2nd gear pass to a microphone, recording the sound levels and being required to keep it below 78 dB. And, ONLY Honda was forced to do this as "the example to all importers", because Carter was pissed off at them...

[drumstyx]

If you have both a #110 mainjet and an EMGO air filter, you've found the troubles. Go to a #105 mainjet and try it with simply no air filter in the box.
Like David 750F said, the total air inlet size needs to be no more than 1.25 X (the cross-section of one venturi of one carb). In the airboxes like you have, with the 'quieting' vent on the bottom facing backward, this particular one can benefit from adding 2 holes of 1/2" size thru the floor of the box and NOT thru the vent to the outside. This reduces the tumbling of the air at higher velocities (bike speed down the road) inside the box by breaking up an effect we used to call "drumming". The only way I know to describe this that others may have experienced is: at 50-60 MPH in a 4-door sedan, with only one rear window rolled about halfway down, the interior air of the cabin will alternately drop-rise-drop-rise when at the right speed, causing a drumming sensation in our ears. This, in this particular airbox, was demonstrated by a rider who 'straight-raced' on a flat, straight road in New Mexico on a 1977 K7 model with a cloth streamer hanging out of the airbox (on purpose): it alternately flapped out and back in, as observed by his buddy on a Harley next to him, circa 1980. He was the guy (that I know of) who drilled first 1, then 2, then 3 (which he later plugged, back to 2) 1/2" holes in the upper floor of the bottom vent. The bike picked up almost 10 MPH top speed for it, outrunning his buddy in the end.

This 'drumming' effect (my term, not his) was likely not even considered by Honda in 1977, as the major customer (USA) was sporting a blazing 55 MPH speed limit at the time. Honda was under the DOT's thumb then to make the bikes quieter in a full-throttle, 2nd gear pass to a microphone, recording the sound levels and being required to keep it below 78 dB. And, ONLY Honda was forced to do this as "the example to all importers", because Carter was pissed off at them...

I figured the EMGO was too restrictive last year, so I'm currently running a K&N filter. It helped, but under load it was still rich with the (stock in 1978 K models, as far as I know) 110 mains. I've got 105's in there now, and while it's better, it's still rich to the tune of 10-11.5AFR under load (tested on the freeway at 120km/h to hopefully eliminate the accelerator pump as a "culprit"). That said, under MOST conditions, it's now pretty much bang on 13-16AFR. How much of an effect might I see from eliminating that "drumming" effect? As I see it, I need to move that 10-11.5 AFR at speed up by 1-2 points, which seems like a lot to accomplish from just changing the baffle situation. Heck, I don't even know if 10-11.5AFR at speed is even a problem -- revs are up in the 6k area, so it could just be that's what it needs for good power up there?

[goodtryer]

The stock main jet size is 105. 110 is already oversized.

110 is in the K8 supplement.

[HondaMan]

One other thing I might mention: I have ridden since 1973 with the Vetter Lowers on my fairing when on hiway trips. With them fitted, the top speed rises as compared to running without them, the MPG rises and the throttle response at speed rises. This isn't due to streamlining: it is due to much quieter air around the airbox. On my brother's CB500-4 (we used to tour the US West together back in the 1980s) his MPG went DOWN when fitted with the lowers, due to the superior airbox arrangement in the smaller Four, but higher wind resistance. My 750 will top out about 6 MPH higher without the Lowers fitted while his is less, so it's not the aerodynamics, but the air mix at the carbs.

And, I, also have that 'supplement' for the CB750K7/8-F2/3 and it's important to note that the F2/3 versions of the PD42 carbs have the #105 mainjet, while the '77K7 with the PD41 carbs have the #110 mainjet, and many of the K8 have this same setup, while the California version had #105 mainjets and #37.5 pilots instead of #35 like most of the others. So, Honda did a lot of switching around in the '77-78 era bikes.