1981 CB750C Carbs Pods vs Airbox

[Barnacle442]

Hi, all. I have a 1981 CB750C that I am just making run again after 5 years in pieces. I have read the arguments for and against using Pods instead of the stock Airbox. I hate my airbox. I don't like the way it looks. I don't like having to screw with it to get my carbs in and out (which seems to be alot, lately). But the benefit to perfomance is definitely a plus in its favor. My question is this: if I made one big air filter pod that mounted to all four intakes like the airbox does, would there be sufficient constant vacuum across the carbs to maintain the performance? Does the size and shape of the airbox have anything to do with perfomance? I have looked but not seen anything like this air cleaner pod anywhere. Any thoughts or advice before I start using up favors would be appreciated.

Thanks,
Barnacle

[FastCletus]

I dont know how it works (ie good or bad) but I saw someone on a thread who built such a device. Although I dont rememebr where.  I think he made it out of sheet metal and rivets

[Johnnylevi]

I saw a guy who made one from acrylic ( I think ). Its basically all for looks. I'd get some CR's and put pods on. They work much better. As much as the airbox is a hassle I think its worth keeping it on.

[sgtslag]

My guess is that the new air box, being attached to four carbs, will pose the same issues when disconnecting it, as it will pretty much be the same thing as the OEM.  I don't see how this will be an advantage.  The vacuum will need to be roughly the same, which means the internal volume of the box will need to be identical, for the carbs to function properly (to create the same vacuum levels).

Each piston creates its own vacuum; the air box blends all four pistons' vacuum into one, average level.  With pods, each has its own, unique vacuum pattern (rises with intake stroke/falls after that), and level (constantly varying).  With one air box, each carb is subjected to a more consistent level of vacuum, all of the time, which varies with acceleration/deceleration, but they all have the same average level.  These carbs were designed for that constant, averaging vacuum level.

Change the size of the air box, and you change the vacuum/air flow levels per RPM.  I suspect you would be right back to the pod jetting issue, trying to find the 'right' jets to use.  My thoughts are that Honda engineers struggled with the carbs when they designed the engine, and they chose the happiest balance for these Constant Velocity carbs.  YMMV, of course.  Cheers!

[Barnacle442]

Thanks for the input, guys! I think I'll stick with the air box, cumbersome as it is, until this becomes strictly a project bike. Then I'll experiment.

[Truck]

This was in anouther thread on this site:
http://honda-cb750-s.456789.n3.nabble.com/I-GOT-PODS-TO-WORK-td4039223.html

I have pods on my 900 from the PO, he had rejetted it and the bike runs fine and I love the look and room to work in. Hope this helps you out.

[seestheday]

If you're going to go to that much trouble I would just mess with it until you got pods to work as well as the stock airbox or get a different set of carbs.

[Barnacle442]

OK, so, I can't make the bike idle. I checked in the other forums for a solution. I checked the timing, set the static timing per the Chlymers 't manual cuz I can't make it idle. I checked for vacuum leaks, no leaks. I can almost get the proper idle by using the choke. All I can think is that the carbs are clogged somewhere and she's starving for fuel. So, I'm gonna pull the carbs and clean and rebuild them. All that being said, as long as the carbs are apart, when I rebuilt the bike I replaced the exhaust with a MAC 4-into-1 setup. I know I need to replace the jets but what do I go to? I'm gonna stick with the stock airbox.

[Lucky 1]

People keep getting mixed up.

The vacuum in the carbs is created when the pistons are on their downward stroke.
So there will always be vacuum. If you do not believe that, put your hand flat over
one of the carb intakes and turn the engine over.

All the air box does is to limit the amount of air coming into the engine.
If you actually LOOK carefully at the air box you will see that it has a hole in it
that is about 1/3 the volume of all of the carb intakes total area.

Then the air filter is also a restriction.

All the air box, air filter, and exhaust do, is limit air going into and out of the engine.

Starting to get the picture.

[Lucky 1]

 The reason it will run with the choke on is because it is getting
less air in relationship to the gas.
It is a richer mixture.

IF you have pods and the right jet sizes and slide needle position it will run correctly.
But why not go to a DOHC forum and find out from someone
who has already done all the work for you?

[Lucky 1]

Then why does the Honda racing team bikes use velocity stacks?

[seestheday]

I'm not sure what the year is of the racing bike you posted, but the RSC tuned DOHC honda race bikes did use velocity stacks, BUT they also heavily modified the CV carbs.  They blocked off the air cutoffs, blocked one of the two main jets, put larger single main jets in the outer two carbs and even larger main jets into the outer two carbs.

This is detailed in the RSC manuals for anyone that wants to read them.

[Barnacle442]

I don't have pods. I am using the stock airbox. I replaced the exhaust with a 4-into-1 system. According to the Honda manual this will require re-jetting. I'm not a mechanic and I know that somebody here has to know what size jets to get for this setup or can direct me to the information I seek. Can someone here direct me or inform me so when the time comes to rebuild my carbs I can speak intelligently?

Also, the engine only produces vacuum on the intake stroke. For three strokes each cylinder produces no vacuum. The airbox is designed to distribute the vacuum from each cylider to the other three carbs while their cylinders are in the compression, power and exhaust strokes. I won't pretend to know how CV carbs work but I know a thing or two about plenums.

[Lucky 1]

Plenums are used with fuel injection systems.

With carbs it is just a box to hold the air filter.
Two completely diifferent sytems.