[bmartin]

I just got done 86'ing my K&N's PODs on my cb750 and put the stock airbox back on it. The motor is stock with the exception of a Kerker header exhaust. With the pods on I had jetting and performance issues with the bike. I also had problems with airflow to the carbs at various speeds. Dialing in the carbs was not the issue, all round performance was. I guess my biggest problem was with airflow around my legs and possible problems if I happen to get caught in the rain with them. Gas mileage also seem to suffer.

After installing the stock airbox, I noticed a much better mid range performance with the engine. I still have to re-jet my mains - the current mains size are 120's and I plan to reduce them by 1 or 2 jet size. (stock is 110 mains, but I still have to jet for the headers.)

If the bike was a racer or some show bike - then it might make sense to keep them on - as an ordinary street bike, it made sense to me to return to the stock setup. Besides - Honda has a much bigger engineering budget than I do...

So good-by pods - I will not miss you guys too much!!!

 

[pampadori]

i've been going thru this for a while myself. I aint puttin no stock box on my bike! mostly cause i don't have one. but i did just buy some velocity stacks and i'm hoping this will fix mine. mine is real bad at speeds above 50. the only way to get it to run smooth at that speed or faster is to give 3/4 throttle or more. I got my carbs dialed in near perfect and have a smooth great power band all the way up. but not at speeds above 50 no matter what rpm or gear i'm in, it just hates to run smooth. I've talked to a lot of cb750 guys and they all told me about this problem with pods. they said put the box back on or get stacks.
Its nice to know we aren't crazy.

 

[2smokeTyler]

damn these finiky bikes!! you know..my sweet little two stroke doesnt seem to have a problem with pods!! LOL!! i just had to rub it in!!

 

[HackAsaw]

I think it may not be an issue of "crazy"

but perhaps more the inability to fit adequately sized filters and then maybe actually jet the carbs

jetting involves much more than just screwing in the hot ticket main and even pilot jets

I long ago became bored with "jetting" threads on the net that never discussed changing needle jets, tapers on needles, air bleeds, emulsion tube hole sizes and/or adding/removing such holes, etc........

it's really a ton of very tedious work to set up a rack of 4 carbs well

and the stock air box is indeed restrictive but at the same time it does provide useful resonance

it's been proven 100% of the time that less restriction in the air intake side will always make more power throughout an engines entire operating range, but perhaps not as easy as simply clamping on a pancake filter to the bellmouth of the carbs

 

[pampadori]

i agree. and i think the cb750 with the early carbs are some of the harder to set up carbs because of the air turbulance issue that plagues them.

 

[bmartin]

I've often said - there is a big difference between a street and track bike. Track bikes are setup to run well at top speed and don't have to worry too much about riding in any kind of wet weather. Track bikes do not make good street bikes - they are way too finicky for that kind of riding.

Street bikes have to be tuned to run well at all speed conditions and in all kinds of crazy weather. I returned to the airbox to make my bike a better street bike. The difference in horse power is mice nuts and I gained a ton more mid range by returning to the airbox. I don't know if stacks are the trick setup - one thing for sure, your engine will be eating alot of road dirt with the stacks...

 

[HackAsaw]

you are telling this stuff about turbulence and rain to a guy that runs this setup.......... and it's very happy rain or shine

perspective reference

 

[bmartin]

I bet that setup will ram some air into the firebox...

 

[kenessex]

I don't believe that the stock airbox is the only solution to jetting difficulties. I also disagree about the need for clean carburetion on a race bike vs. a street bike. Either carburetion is correct or it is not. Mid-range can be crucial on a racebike on some turns at some tracks depending on gearing. It is sometimes possible to ride around those issues, but it is better to have the carburetion spot on instead. On my 350 which is notorious for poor jetting with other than stock air filters, I run the stock velocity stacks inside of foam uni-filters. It is not vey sexy looking,but it revs clean, makes good power, and keeps the crap out of the intakes.
Proper jetting is not always easy, but it is logical and if approached in a step by step manner can be done.

Ken

 

[pampadori]

so i rode the bike with the bad turbulance (said it again) issue to work today. 60mph spitting and sputtering unless i floor it, just like normal. i noticed a big rig in the right lane doing about the same speed. I got in behind him and closed the gap to about 2 bike lengths. bike starts running perfect, just like it does at speeds under 50mph. but i didn't feel real safe tucked behind him so i eased off and started to run poorly again. i'll update when i get my stacks in and put on.

 

[bmartin]

Ken - Not to say racers don't need mid-range, my comments were in a general statement form. In general, racer bikes don't make great street bikes and vs versa. Of course there is always an exception to the rule. Pampadori's last posting kind of talks about the issues with the cb750 carbs - mainly air turbulance with pods. It will be interesting to see how the stacks fair on his bike - I'm sure he will post something. In the mean time, I just picked up my new main jets and they will be going in the bike tonight...

 

[HackAsaw]

I would suggest one of two likely and very logical suspects

1) the CB750's float bowls are very inadequately vented

2) your pod filters are too small and/or not deep enough


and then with either culprit, you can forget being able to "jet" with consistent results


I've ridden and tuned far too many inlines with "pod" filters, CB750 hondas included, to ever believe it cannot be done and done very well


note that the vent hose is not shown on my carb in either picture

it won't harley run at all that way but once the hose is connected, and it terminates just behind the steering crown nut, it performs flawlessly and responds very consistently to any and all jetting changes

doesn't care if drafting behind a semi, riding the open road during a monsoon, wot at 20mp, wot at 90 mph

it's all good

wonder why?

 

[bmartin]

Hack - you might have something with the carb vent hoses - I all my hoses connected but they terminate behind my carbs, maybe they need to be terminated up under the gas tank???? I'm sure all my carb vent hoses are not clogged. As far as riding in the rain - I find it hard to believe that the K&N's wouldn't suck a little water in vs the stock airbox. Anyways - the K&N's will more than likely end-up on the 836 motor project bike -

thanks for the input...

 

[Geeto67]

on theing to consider on the cb750 airbox....in addition to the fact that it is an airbox, the inlet for fresh air is concealed well behind the sidecovers and out of the roiled air. If you could somehow create a larger enclosure (maybe from different sidecovers) that pulls the pods out of the roiled air, then maybe you can smooth out the airflow issues.

I have a couple of the old custom british style airboxes and I never had a problem with them but that is because: 1) they use the stock honda airbox rubbers, and 2) they are partially blocked on the sides by the side covers. Actually I don't know why they work as well as they do, it just sounded good to me...

 

[kenessex]

I had forgotten about the float bowl venting issue. As I recall, Honda had a service bulletin out in the early 80's about that issue on some models. I think it was the Magna or Nighthawk S and the fix was to shorten the vent hoses where they stuck out below the engine by a few inches to get them out of the turbulent air. A simple snip with the scissors cured a severe mid - range stumble.

Ken

 

[bmartin]

MMMmm - float vents in airflow.... I'll have to do some testing - Hacksaw may have hit on the solution...

 

[Diamondj]

This may or may not have any bearing on your problem - on the GS1000 Suzukis there are three vent lines from the carbs. The center is for petcock vacuum and must be connected. The outer vent lines are for the float bowls and run to the airbox when it is in place. If you attempt to leave the lines connected to the carbs once the airbox comes off, the bike will never run right. You've got to remove these vent lines and leave the open nipples on the carbs.

Also bear in mind that all pods are not created equal. Genuine K&Ns flow a certain way and need a bit of filter oil to work their best. You can over-oil them reducing airflow. Some of the K&N clones never flow well regardless of what you do with them. I've heard that the Uni foam filter flow better or worse depending on who you are talking to. They do seem to work okay but they are vulnerable to rain and hold water rather than repelling it as a correctly oiled K&N will.

It took me a while to get my carbs tuned correctly on my GS but I eventually got them working right. I have NEVER experienced varying running conditions due to airflow - sounds like something else is wrong here....

Jim

 

[75SevenFityFo]

I was wondering what kind of effect cutting the all excess of what holds/seals the air filter in place from the bottom half of stock airbox w/ the wing nuts and bolts still on and connected everything normaly. I think this would look pretty trick, the air filter doesnt look horrible imho. Would it still have the effect it needs to run smoothly in a stock condition?

 

[HackAsaw]

it is always VERY SAFE to enlarge/increase the float bowl venting

I had to carefully hand broach the vent on mine and it really did help quite a bit

the "tickler" amals are a bit more painful to increase bowl venting

I'd almost bet just nobody here has a concentric series carb like that one which has a bona-fide enrichener for cold starting

if you do and would like to sell it, I'd like to buy it and hope it is NOS like mine used to be before I bolted it on my ride


my forward facing carb with open velocity stack really will run VERY well in a down pour

the only thing it likes better is running below 40 F degrees

and the water manages to clean the carbon from the chamber nearly to the point of being shiny like when it first went together

to date, it's yet to blow a head gasket or make any attempt to hydro-lock

and I've ridden it 80+ on the superslab in rains heavy enough to have cars seeking shelter under overpasses

that probably sounds like an embellished tall tale but it's true


I don't mean to sound condescending in the least bit...... and sorry if anyone has been rubbed the wrong way

anyhow, vent hose is cheap and fortunately pretty darned easy to remove and replace

by all means, open those vents up and try moving their terminated ends

 

[pampadori]

i'm going to try that right now. mine are hanging down about 4 inches past the frame. i'll make them a good bit shorter.
thanks for the thought hack!