Cognito Moto CB750 AIR INTAKE BOX SOHC AND DOHC

[pipbernadotte]

Just wondering if anyone has experience using the Cognito Moto Custom Air Intake Box?

I just got a '80 and '81 CB750 DOHC and they came with pod filters... but after searching this forum and a few others nearly everyone says to drop the pods for the stock airbox or I'll never get it running right (without dropping the money on a day at the Dyno)

In my search for an "in-between" (decent looking, but connected flow) airbox, I found the CM box. But, there aren't very many reviews of it's performance and I can't seem to find anyone who's posted their experiences using it.

As such, what're y'alls experiences with it if you have one?

OR

To those who're more knowledgeable than I on the subject: will this airbox work as well, or nearly as well as the stock airbox? or at least better (how much better?) than the pod filters?

Thanks in advance for anyone replying to my question~

 

[EaZ8]

is this really the only option?
i cant believe that the pods are this bad...but obviously there is a lot of negative info on them so...ya...

 

[amc49]

Here's the deal............................the CV carbs rely on the restriction of the stock PAPER filter to open the slides more. As well, all FOUR intake ducts must share the pulsing back and forth in the airbox to keep the slides lifting correctly too. When you add any type of filtration that is much lower in restriction the semi-vacuum produced in the box by the shared pulsing then drops way off, the UNI type in that post #1 filter is of that type as well as the K&N gauze filter type. That semi-vacuum allows the air pressure under the slides to drop more and faster too to activate the slides better.

When the slides drop you get the same tuning difficulties you get when a stock motor falls off in compression, the slides also lift in relation to the engine ring and valve seal and vacuum and they then drop to make for sluggish action. Richening thinking you need more fuel is a common mistake, it again makes the slides drop and slow in action and why CV carbs like slightly leaner mixture than direct lift type carbs do. When the slide drops to be artificially slightly low then it automatically richens just because of that and it can get to be a vicious cycle.

Combine bad airbox/pods and engine not brand new and trying to 'richen it up' due to old school thinking and you have what makes for all the problems. The OEM airbox is also a certain volume inside past the filter to best allow that sharing, the volume being like the length effect on a header pipe or intake passage. I've had pods running at least as well as anybody else on a healthy engine and they ran really well but you can still yank them and install the OEM airbox and then immediately feel the low/mid rpm torque bump that airbox tuning volume adds that the pods take away instantly, others who ran the pods have done the same when I dared them and they noticed the difference too. On the 750 since it has less torque than the bigger motors, the OEM airbox will pull out more rpm at absolute top end too, the pods will let the slides not quite open 100% and then you may only pull out say 8000 rpm in top gear where the airbox would pull to 9000. Not much difference and how much total time does one spend at 120+ mph?

CV carbs are very susceptible to engine draw (suck, if you will) and these are even worse than ones with rubber diaphragms as those are 100% sealed, these are labyrinth mechanical seal type and they leak even when brand new thus wasting some of the slide vacuum to begin with. Anybody who has ever run CVs with rubber diaphragms will attest to how the engine performance goes to crap with even a pinhole in the diaphragm, these come with that in effect from the factory. Why they have so much more slide hole under the bottom, it takes that much to even begin to lift them. Why too the jet kits tell you to drill the slide holes bigger when you buy their pod kit, it helps but only a partial fix of the problem.

Add to that that MANY people are misinformed and think they have very good running engines and have never checked the compression to see just how lousy it can be on a nearly 40 year old engine. These DOHC commonly had valve sealing issues even brand new when people refuse to check the valve clearances because they had to buy shims and then the slides again begin to drop in response to that. The most common thing that gets done then is that the carbs then get blamed (it's a male testosterone thing) as the problem and then how you find your 'dream bike', usually with carb issues and sitting for years. And with pods on it, you'll never find anything more calculated to drive you nuts than trying to rejet a podded older DOHC with these CV carbs, it's an exercise in futility for the vast majority of people.

Believe it, the brick walls are waiting for the heads of those that don't, and they are already well bloodied. The physics will not be denied there.

 

[naturals]

Resurrecting an old thread as I'm tempted to install a CM airbox - has anyone tried one? Is the performance essentially the same as the stock airbox, in a tidier package? Are there any alternatives?

 

[nico1ae]

Hi everyone, after 2 years of beating my heads against the wall and sending my carbs to almost every "vintage Honda guru" in LA, I finally got my CB750C DOHC to run with pods (and CM air box). I have been through all the pains that you are familiar with in getting this setup to run with the CV carbs, but it's very much doable, and don't be discouraged by what everyone tells you as far as "CV carbs will never run right with pods, or anything that's not the stock airbox". Those are just the lazy guys that gave up on their build for something easier.

As far as using the Cognito Moto air box as a replacement for the stock air box, it won't work. I purchased it, tried it, and essentially it gives you the same result as running pods because you are not fixing the underlying issue which is that the CV carbs need a certain amount of vacuum to operate properly (which the stock air box does by restricting the amount of air that goes in due to its design). When running pods or the CM air box, you still have free flowing air all around and therefore you don't have the vacuum on the carbs. To get past this vacuum issue, you need a different jetting kit (DynoJet is what worked for me and many other builders with the same bike). Then you can run pods, CM air box, or whatever free air flowing intake you want.

What you need:
-Make sure all your electrical, ignition, spark plugs, etc are on point.
-Make sure your carbs are immaculately cleaned and nothing is clogged, jammed or rusted.
-Swap the jets with the DynoJet jetting kit
-Get to a quality mechanic with a dyno and have the carbs synced

I got lucky in the fact that the same mechanic that installed my DynoJet kit also did the dyno, and synched the carbs, but if you don't have a dyno or mechanic you trust in your area, the DynoJet kit should still get you pretty far if the rest of your carbs are in order.

Also, I still have the CM air box which I'm not using and would be willing to sell it, it's got maybe 5 miles on it.

 

[Wez_]

Thanks so much for the insight. I am going to take the "AMC" challenge and swap out my pods back for the stock air box and do some testing. I also have new rubber boots. But basically, after rebuilding my heads with new shims, cleaning and valve work, the bike seems to max out around 7200 RPM. It pulls really hard like a SOB, but AMC is dead on, there is a slight dead spot coming in around 3500 RPM and the Max RPM is limited. I cant wait to do the swap to stock and get back to you guys.

 

[Wookie]

Thanks so much for the insight. I am going to take the "AMC" challenge and swap out my pods back for the stock air box and do some testing. I also have new rubber boots. But basically, after rebuilding my heads with new shims, cleaning and valve work, the bike seems to max out around 7200 RPM. It pulls really hard like a SOB, but AMC is dead on, there is a slight dead spot coming in around 3500 RPM and the Max RPM is limited. I cant wait to do the swap to stock and get back to you guys.

So what was the final result of switching back to the stock airbox?

 

[Wez_]

The first time I got everything all back together and took the bike for some hard ripping, on the first 1-2 shift going WOT, the bike almost jumped out of my hands and left me in the road. That might be an exaggeration, but not really. I vividly remember the extra torque needing an adjustment to how I handling it previously. Also, the bike now pulls to 10,000 + RPM. The needle will just max out and still climb. My timing needs to also be advanced so there is still even more power to be gained. Overall, I would never go back to the PODS. I also taught myself how to install the air box and boots in under 1 hour. This was a requirement as I am committed to the factory air box now and removing/installing these components require a procedure. I can make a write up if needed as I took pictures on how to install. On humid days, I get a surge around 8K RPM, but once I climb through it and reach WOT, 10K RPM, the surge goes away and the motor pulls hard to 10K+. Cool crisp days it runs mint. Also, the carbs will need to be synchronized for maximum performance. Bench synch OK, but not ideal. I need to advance timing but overall, I would never go back to PODS due to forcing too much air onto the slides and pushing them down and limiting the overall performance.

 

[pidjones]

A photo instructional on reinstall would be great! I'm not so worried about getting the carbs on as I am about getting the four boots from the carbs to the stock airbox on right on my '79 CB750F.