Topic: CB750k7 Air Help

[bigeye135]

So I have an odd issue that I've haven't been able to come across on this forum or any others.

I would like to preface this by saying that I rebuilt the carbs, then had them synced and tuned by a mechanic that specializes in UJM bikes.

Basically, with the airbox on (the one that came on the bike from the PO, not the stock one), the bike will rev all the way to redline in neutral, and feels quite twitchy when getting on and the throttle while riding.  I thought this was normal as this is the first carbureted vehicle I've ever had. It also gets really rackety and broke up at the top end.  I googled "cb750 breaks up" and it sounded like a rich condition, so I just went out and took the airbox off and holy #$%*!!!! The bike runs nearly perfect now.  Slight hesitation if you try and full throttle is around 3k, but if you roll into it rather nicely, the bike just takes off and keeps on going and building power until redline.  It feels like a completely different bike, it feels like how I expected it to feel. So my question is, with my rather limited knowledge, should I just go out and buy some pods?  I figure since it seems to run almost perfectly and doesn't smoke out the exhuasts (2 -1 with not mufflers on either side), pods seem to be a good fit for filtering. Or maybe velocity stacks?  Sorry if these are poor suggestions, just going off of butt-feel.

[flybox1]

Emphatically...NO! 
Dont EFF with it!  Your mech just tuned it for its current setup.

Once you add pods or stacks, you change the air/fuel mixture....then you'll have to begin the tuning process to counteract the lean-ness you've created.....AND...you'll get no more 'performance' out of it for your efforts.
Well...maybe your mechanic WANTS you to mess with it as you are funding his retirement 

Put down the tools, grab your keys and ride/enjoy it like it is.

[bigeye135]

Emphatically...NO! 
Dont EFF with it!  Your mech just tuned it for its current setup.

Once you add pods or stacks, you change the air/fuel mixture....then you'll have to begin the tuning process to counteract the lean-ness you've created.....AND...you'll get no more 'performance' out of it for your efforts.
Well...maybe your mechanic WANTS you to mess with it as you are funding his retirement 

Put down the tools, grab your keys and ride/enjoy it like it is.

My concern though is that with the airbox on, with how the mechanic tuned it, it ran like poo. Now, with the airbox off, it runs way better.  My only concern is that unfiltered air is and possible particulates are getting into my engine (only took it for 1 ride without the box).

[flybox1]

Take it back to him if its not right.   The 'cleaning and tuning' he did is a pretty easy thing to do.
If he is a so-called expert in UJM's it should be a no-brainer....

[bigeye135]

Take it back to him if its not right.   The 'cleaning and tuning' he did is a pretty easy thing to do.
If he is a so-called expert in UJM's it should be a no-brainer....

Fair enough, thank you.

[scottly]

Was the air filter new? Reinstall the airbox, but leave the filter element out. If the bike runs much better, the filter was plugged up. There is a drop-in free-flowing K&N filter element for 750s.

[bigeye135]

Was the air filter new? Reinstall the airbox, but leave the filter element out. If the bike runs much better, the filter was plugged up. There is a drop-in free-flowing K&N filter element for 750s.

I did this and the bike runs fine without the filter.  However, the airbox I have is not the stock airbox.  It looks like a generic one, and the filer was just this slice of foam running around the the outside circumference.

[flybox1]

airboxes are cheap and plentiful, and are the easiest to tune your bike to 

[bigeye135]

airboxes are cheap and plentiful, and are the easiest to tune your bike to 

Makes sense. Can't believe that didn't even cross my mind 

[flybox1]

I'm curious as to what he has in your carbs....main and pilot jet size?  IMS setting? and if they were vacuum sync'd?
Several here have run breadbox intakes, but not on stock jetting....
The fact that it runs better without intake pressure makes me think he's got you jetted rich.
What do your plug tips look like?

[seanbarney41]

Those breadbox filters can run quite well.  UNI sells filter foam in different densities, available as a sheet that can be cut to fit.  Green is denser and thicker for more restriction, charcoal gray/black is less dense and thinner.  Also many of those filter boxes can accept the stock intake boots (which are actually tuned velocity stacks)and perform even better.  Whenever I have used the breadboxes, tuning required only a mainjet upsize resulting in perfect idle and excellent part throttle response but they do not have the large volume of still air that the stock airbox offers, so full throttle/high rpm performance suffers slightly compared to the stock airbox.  Also, I have not tried using a breadbox with the PD series carbs your bike is equipped with.

[bigeye135]

Just wanted to let anyone seeing this know that I cannot see what size my jets are.  However, the previous owner told me that back when the bike was running 100% with the breadbox he did have the main jet sized up (he doesn't remember to what).  Anyways, my buddy had some pods laying and let me put them on my 750.  The bike now runs absolutely perfectly. 

[Bankerdanny]

The jet size is etched into the jet, you should be able to remove one of the main jets from an outside carb without needing to pull the rack and then assume (hope) that they all match.

750's seem to handle pods better than the smaller SOHC4's. If it seems to be running properly then I would leave it be. Way to much riding time gets wasted chasing perfection.

[Tracksnblades1]

750's seem to handle pods better than the smaller SOHC4's. If it seems to be running properly then I would leave it be. Way to much riding time gets wasted chasing perfection.

You still may have to squeeze that tank with your knees in them strong crosswinds.