Rejetting w/ airbox

[alehman]

Hey all,

In the interest of having my 79K run well and not foul plugs or burn valves over years of use, I'm switching back to the factory airbox after giving pods a shot. They work decently well, but have a few problems--- throttle response is a little sluggish when I flick the throttle (even though carbs are clean as a whistle, rebuilt, and the accel pump is squirting fine) and I don't think it's going past 6-7k rpm (I'm uncertain, though, because the tach needle is warped and sticks so I can't tell).

As everyone says Honda jetted the carbs lean from the factory due to EPA restrictions, I want to step up my jets a little. For the needle jet I was gonna go from the factory 102 to a 105 and for the primary I was gonna go up to a 75 from the stock 68. For those of you familiar with the CV carbs, does that sound like a good change for both? Is 68 ---> 75 too big a leap?

for the record, I have stock 4-4 exhaust and no changes to stock needle clip position.

[Lucky 1]

Throttle response will NEVER be good until you deal with the idle jet.
Work on ONE problem at a time.

Having clean carbs has NOTHING to do with good throttle response.
Of course the carbs (idle jets) should be clean.

The idle and throttle response up to 1/4 throttle is controlled by idle jet and mixture screw ONLY.

1/4 - 3/4 throttle is controlled by the needle position on the slide.
It would be better to richen the needle on the slide before you change main jets.
But if you are going to use the stock air box you do not need to do anything.

Tuning for pods or stacks is the same.

If you put a stock air box on the bike then you will NOT need to re jet.
Only a mixture screw adjustment would be needed.

BTW...IF your tach is broken how can you tell what RPM the engine is going?

[alehman]

Lucky---

let me rephrase. When I talk about throttle response, I am talking about throwing the throttle wide open. When I do that, the response is not necessarily sluggish, but compared to my friend's 81c which is running with an airbox it seems slightly slower to shoot up into high RPMs, but doesn't really bog or starve. Lower end is perfectly responsive. It is almost as though my slides are reluctant to rise as readily as those in my buddy's bike, which is probably the vacuum issue from podding everyone warned me about.

In answer to your question about the tach--- the needle moves freely between 1k-5k rpm but is clearly catching on the plastic face of the gauges around 5k, and I can tell that the engine is revving higher even once the needle's stuck at 5k. I am guessing it's hitting 6k or 7k tops.

I am not asking for advice about cleaning my carbs or changing needle position. They're fine other than the usual BS I was warned about with pods... even so it runs pretty okay.  I am saying that I'm going back to a stock airbox setup and that I've heard it's good to increase the jet size a step or so for the primary and secondary mains because of how lean the bikes were jetted from the factory.

Have you or anyone on the forum heard of or tried this?

[rrgunslinger]

If it runs perty ok then I would leave it alone.  Just saying...

[seestheday]

I have an 81 K.

I went from a 68 to a 75 and it made a huge positive difference.  Via a PO I was already running 105's.  That is my current set-up.

That said, my bike is probably set up differently than yours.  I have a 4-1 exhaust, stock airbox and regular air filter.  Are you running the stock 4-4's?  4-2? straight pipes?  K&N air filter or regular air filter?  All of this will make a difference.

When I was running the 68's I needed to have my air/fuel screw out 5(!) turns for it to run reasonably well.  Anything less and it was popping bad on decel and the plugs were reading lean.  With the 75's I'm now at 1 turn out which is much more reasonable.  My plugs read great (except for one that is rich, but that is due to a known vacuum leak I have to fix) and I don't have any popping issues.

If you're already at 1-2.5 turns out and it's not running too lean then I think stepping up to a 75 is too big a jump.

[Lucky 1]

The vacuum the vacuum operated carbs slide piston work on has nothing to do with
your type of air filtration or air box.

The vacuum is created by the suction of the engines pistons going down.
Thats all.
Try putting your hand over a carb when you turn the engine over. See what I mean?

The REASON that Honda went to vacuum operated carb slides is so that when a motorcycle is at different altitudes or it is a rainy day or a bright sunny day (inches of mercury) that the carbs will work correctly.

Example:

If you lived in Tucson, Arizona which is 2000 ft. above sea level and you went on a Sunday ride
up to the top of Mt. Lemon 8,000 Ft, that when you got to the top of the mountain on a bike with cable operated slides, the mixture would get very rich and it would not want to idle. Because the air is thinner at the top of the mountain.
When you come down from the mountain it runs just fine.

With a vacuum operated carb you could ride to the top of that mountain and the bike would still be running perfect because the vacuum slide piston operating in thinner air will not have as much suction and will not rise as far giving a better mixture.
To do that The vacuum operated version has two main jets that are progressive.

[Lucky 1]

That is very valuable information for the other owners!

[Lucky 1]

I do agree about the bikes being lean from the factory.

[alehman]

Thanks for the info guys.

I never knew the reason for the shift to CV from cable-actuated slide carbs, lucky, but that makes a lot of sense.

Seestheday, the airbox setup I'll have will be with a paper filter and stock 4-4 exhaust. Will a 4-4 provide more or less back pressure than a 4-1? with the pods on, I have to have the adjustment screws at 3.5 turns out--- I will have to back them way in I imagine once the airbox goes on, so I'll be tinkering with the lower RPM range a bit once I'm at that point.

[seestheday]

Oh, you haven't put the airbox back in yet?

It sounds like you're returning everything to stock. Stock jets make sense as well. Maybe some very minor changes, but as lucky said - work on one thing at a time :-).

Sent from my BlackBerry device on the Rogers Wireless Network

---

From: "alehman [via Honda CB750'S]" <[hidden email]>

Date: Tue, 23 Oct 2012 08:00:45 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: Rejetting w/ airbox

Thanks for the info guys.

I never knew the reason for the shift to CV from cable-actuated slide carbs, lucky, but that makes a lot of sense.

Seestheday, the airbox setup I'll have will be with a paper filter and stock 4-4 exhaust. Will a 4-4 provide more or less back pressure than a 4-1? with the pods on, I have to have the adjustment screws at 3.5 turns out--- I will have to back them way in I imagine once the airbox goes on, so I'll be tinkering with the lower RPM range a bit once I'm at that point.

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/Rejetting-w-airbox-tp4031069p4031093.html

To unsubscribe from Rejetting w/ airbox, click here.
NAML

[Lucky 1]

Any time you have the mixture screw out 5 full turns, you need a jet change.
It also means you are in danger of having the mixture screw fall out into the road.
Good luck finding another one. You would have to buy a set of 4.

[Lucky 1]

Mixture screw controls the IDLE mixture.... along with the idle jet.

[alehman]

Ok, I am starting to have some suspicions about my 79k's carbs. Also, beginning to lose my mind a little. I suspect that a PO put some sort of kit into it, mayber a dynojet stage 3. Why?

Firstly: the bike (79K) runs decently well with pods.
Secondly, when I rebuilt the carbs, there was evidence someone had been in there before me. There were washer shims the slide needles of carbs 2+3 --- are there supposed to be shims under the slide needles? ALSO: the needles had grooves with an adjustable clip on them. Did the CV carbs have adjustable needles stock?
Third, the jets I took out during the rebuild don't have sizes stamped on them. is that normal?

Tomorrow I am going to check to see if my slide lift holes have been drilled out.

[alehman]

Son of a #$@$#@! the secondary mains DO have stamps on the side, well-worn though, which say 126!

KAHHHHNNNNN!!!!