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Discussion Starter #1
hi i recently got a 1978 cb750k and the air filter box was broke so i put air pods on and cleaned the carbs and it runs good untill a burst of wind hit or i get going fast (60mph and up) it bogs down little

any help
 

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Hi,
Welcome to the board. I suspect that you will find the search function to be your friend on this particular question, although the short answer will be to run a stock airbox.

Ken
 

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search float bowl vent(s) venting etc

a cb or most any engine can be made to run, and run very well, with "pod" type filters
 

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and just one thing about 'seeing a lot of or always seeing this or that'

plenty of the that folks run goofy or ill engineered modifications will be the last to tell you how miserable such mods are to live with

or how well they don't work....... and maybe many don't even know better

pod type filters aren't one of those mods

they work and can indeed be set up to offer performance gains....... some better than others

size and shape does matter

but then again, just because a man can plunk down the plastic for some filters and even an exhaust doesn't mean he has a clue about how to go about making everything run well together
 

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this issue with the cb750 has been discussed to death here because a couple of members have had the same problem, myself included.
the carb bowl vent tubes need to be pulled up out of the air passing under the bike.
those are the fours hoses that attach to the bottom of each carb. make sure they are all hanging down the same length only hanging out of the frame about an inch. much more then that and the bike will run crappy at 50+mph.

after this is done, the bike will run fine but still may be effected by cross winds. this is where quality pods of velocity stacks shine.

i happen to have a factory airbox with stock rubber stacks in it if you are interested.
 

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Discussion Starter #7
velocity stacks

can i get velocity stacks for a 1978 cb750k and what else would i have to do to my bike if i had some
 

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do your carb throats have 40mm openings of 39mm? if they are 40mm, i have a set of gold anodixed ones NOS. i could part with them for a sum of money.
you would still need to pull the vent tubes up out of the "bad" air and rejet for the lack of vacuum inside the carbs from the less restrictive stacks.
 

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if anyone is trying to make those cheap Emgo pieces of junk work

you may as well run nothing at all

K&N (and 4 of them aren't inexpensive) or Uni is the way to go and don't get the shorter barrel or pancake shaped ones

or they will have the reversion pulse colliding with the filter and be all manners of ornery to never get jetted right

and you will likely have to rejet regardless if running filters or stacks

unless you are willing to settle for 'runs ok' instead of 'runs damned near perfect'

and I still am reasonably sure the bowl vents on those carbs are just too damned small

might be easier just to add another hole and stuff another nipple in each one of them

cross winds and moving your legs around won't affect an inline or triple or twin if set up correctly

I've done far too many to pay any credence to those who claim otherwise

one other clue........ those vent hoses do not necessarily have to go down under the carbs

that's nothing but a Nader thing and if your float needles stick causing overflow

the excess will drain out the pods or stacks just fine
 

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Hacksaw

"K&N (and 4 of them aren't inexpensive) or Uni is the way to go and don't get the shorter barrel or pancake shaped ones
or they will have the reversion pulse colliding with the filter and be all manners of ornery to never get jetted right"

Short barrel K&N's??? Are you saying the correct K&N's to use (on cb's) are the ones shaped like a cone??
 

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No sir, I am saying if one tries to use too shallow of a filter that it will not be fun to attempt to jet since the reversion slug will not have adequate room to expand and then be drawn back into the engine being enriched for the third time.

Velocity stacks, regardless of their tuned or un-tuned length, do not burden a tuner with this same obstacle which is why I suspect some people form the notion pods are hard to tune and stacks are not.

Have you seen some of my bikes fitted with intake runners tuned to the 2nd, 3rd, and 4th intake pulse?

I have a dual plane runner on an ironhead sportster that has virtually zero intake reversion

tuning it is quick, easy, and very predictable

it does indeed have open stacks

however those setups are run open only because that is the least restrictive and the intake is located in higher pressure clean (ram) air

not because I am concerned with trying to tame an intake reversion slug
 

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Just to add on, On my CB350 race bike, I have velocity stacks with big green uni-filters on them. It is easy to jet the CV carbs and they are really good. But it is an ugly looking setup.

Ken
 

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Discussion Starter #13
pampadori
repying to your message from yesterday

the vent tubes should be all the same length or hang down and stop at the same spot

and do i need bigger main jets would that help


thanks shannon
7/21/08


the carb bowl vent tubes need to be pulled up out of the air passing under the bike.
those are the fours hoses that attach to the bottom of each carb. make sure they are all hanging down the same length only hanging out of the frame about an inch. much more then that and the bike will run crappy at 50+mph.
 

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Shannon, the vent tubes more importantly can't hang so low that the hit turbulant air.

example, my bike had these issues even though all 4 tubes were the same length. but i took scissors and cut 2" off of each vent tube so they didn't end as close to the ground and the bike was cured.

i've had to jet bigger for pods on every bike i've iver owned. usually on a stock CB750 i've gone from 105 mains to 110 or 112. but yours may like something different. elevation here is anywhere from 1000-4000" for reference.
 

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I've been kicking about a few designs of velocity stacks that do not prevent ram air (if facing forward) yet provide K&N level of filtration

as in a funnel shaped filter inverted into or just plain covering the opening of the stack

anyone know of such a setup?

screened stacks are better than a poke in the eye with a stick but I do tire of counting on having to bore cylinders every two years
 

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Hackasaw;

I'm wondering if checking out the newer sport bike designs where they are using ram air to pressurize the airboxes and still provide filtering of the air would be any help for a 4 cylinder type bikes. For twin types, I've seen on some custom HD's they use sort of a pod pointed into the wind that gives the ram air effect with filtration. (I'm sure you've seen this set up also)

One of the coolest setups I've seen to day is the guy that used the top-end of an airbox with the K&N's bolted into the bottom half of the air box. How well that performs is another discussion.

From what I've learned from all the discussions (and minor beatings) is ensuring the carb overflow tubes are not in a direct air stream under the bike and using a good quality Pod Filter with proper jetting.
 

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well the ram air setups on the newer bikes utilize huge (typically ten times the displacement of the engine.... where do they hide that huge box??? ) resonating plenums and often combined with variable length stacks

plenty of room to filter using such a setup but for custom builds it would likely require something resembling a large fake fuel tank with an intake pickup mounted around the faired headlight area

the "forcewinder" HD setup really does not allow unimpeded forced induction and the goofy hypercharger starts closing off it's butterflies when it should be opening them wide open

not that either one sticks out far enough to be grabbing clean air, much less high pressure air

but to answer your question........ a huge plenum having it's intake located in a high pressure area

will indeed help out any engine

even if the resonance tuning isn't spot on


I think CW magazine recently did some dyno testing on some modern sport bikes showing the difference in power with the bike having no high speed air flowing past it

and then with high speed (like over or around 135 mph) air being blown both over it as well as into it's intake's pickup

definitely a difference in output numbers
 

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gee...... enough to make a body wonder if somebody on this board really knows something about tuning carbs ;)
 

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Hey hack about the filtered stacks. Not sure if anyone's doing anything like this in the states but it should'nt be to hard to make something like these out of any old velocity stack.

http://www.webike.net/sd/370893/100011601170/

I have a pair of these that will be going on my W650 if I can ever get the mufflers I ordered (another subject) but they have two screens, a course mesh and a very fine mesh screen. I plan on putting some type of foam filter material between the two that I can oil like a K&N and give me the same protection as one.
 
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