Topic: Rejet or re-stock stock air box?

[ron.cieri.313]

I have 4 K&N pod air filters on my Honda CB750 K (1976) which I'm starting to think are the culprit of my inconsistent idle setting...i.e., I'm constantly fiddling with the idle setting screw after my bike warms up and during 5-6 mile runs...she wants to punk out at stop signs and such; or revs too high at stops cause i set the screw too high in order to stop it from stalling.

I started the new topic because I posted under fuel line setup thinking that maybe it was they way my fuel lines where setup that caused uneven gas flow but doesn't look like thats the cause.

I know it can be alot of things but just as an fyi I've done timing, valve adjustment, carb sync (twice using a morgan tool), cleaned and re-oiled the pod filters, cleaned the carbs...she runs great except for this problem.

So, if this is the problem...do I try to go back to stock...which means finding an air box (are these interchangeable for my '76 with other 750 or the same era?) or rejet the carbs with I imagine larger jets?...do I need to rejet slow and main jets?

...thanks all!...ron.

[TwoTired]

Have you tried turning the air bleed screws in?

Pods generally reduce vacuum levels at idle, which causes less pull on the jet tubes/less fuel being drawn, even though the same amount of air is being drawn.  Result, lean mixture.  You may be able to compensate the idle mixture by closing down the air bleeds for the idle/slow/pilot system in the carbs.

Then there is the midrange, and W.O.T. mixtures to scrutinize as well.  If you use stock carb settings/tunings, you usually need the stock components, too.

Cheers,

[masonryman]

I was having a similar problem on a bike I just got and I noticed the throttle don't always return all the way. As I was messing with the idol screw the linkage was not even touching it.

 See if you get any fluctuation or inconsistency if you manually return the throttle and then if you don't.

One of the cables my be frayed or dragging slightly, start in plan sight and work in to the more difficult

Mark

[ron.cieri.313]

New report...tried turning the air screws in as suggested and it definitely helped...specifically I didn't have to fiddle with the idle screw for the first 8 miles of my run but!....then she acted like she was running out of gas again until she shuts off.  It always seem to be between 5-10 although, as I say, turning in helped the idle issue...this so seems to be fuel flow or mixture issue.  I left the screws about a 1/4 of a turn out...maybe I should turn them in all the way?

[masonryman]

differant years have differant settings, but it seems most call for somewhere between 1-1/4 ans 2 turns out from bottom.

It sounds like a fuel delivery problem, a sticky float or improper level. A plugged orifice in the carb not  letting the fuel flow where it should.


I would loosen the plugs on the float bowels one at a time and let the fuel flow through watching for a consistant stream from each, if you have good flow I would say something in the carbs is sticking or plugged

Mark

[ron.cieri.313]

Probably could be any of those things and I will check fuel flow again...if it was a dirty/plugged carb problem though, why would it only show up 8 miles into the run?

I'm wondering if the bike goes that far until the carb bowls are empty and then its rest, fills, and works again?...wouldn't that suggest a jetting size problem per TwoTiered above?

[masonryman]

A jetting issue would be more related to engine RPM or load, more so than distance traveled, IMO.
 Now if the bike is running lean enough it could be getting to hot and that opens a whole new can of worms

 If you are getting good flow and your plugs are showing lean you could lower the clips on the needles a notch or two and see if that helps, this will give you some idea where to go with the jets with before changing jets


Mark

[ron.cieri.313]

So I should definitely check for flow...and pull the plugs and take a look...but I don't understand lower the clips on the needles...the needles inside the carb?...could you explain?...thx.

[masonryman]

When you pull the top off the carbs and look in to the round slide there is a long needle in it that goes through the carb in to the main jet, there are 5 slots for the clip, if you lower the clip it raises the needle out of the jet sooner allowing more gas sooner but I don't think you will gain much if any at wide open throttle, just low and mid range.

This is a very basic description and I am sure someone will say some part of it is wrong

Mark

[Spanner 1]

Bike runs o.k. for first 6 to 8 miles?...hmmm, I say poor fuel delivery from tank to carbs...first thing, check the gas-cap vent is clear as the fuel flowing by gravity down to your float bowls will create a gradually increasing vacuum in the tank that could easily take 6 or 8 miles to build-up enough vacuum to stop the flow to the carbs, which is minimal anyway, but just enough to work with everything being correct ! Tank vent, then pet-cock filter need to be looked at, good luck. I would do this before working with jets, needles etc......

BTW...pulling the fuel-line off the carbs and opening the petcock to observe fuel flow is NO GOOD for test, fooled me many times !, you would need to keep it flowing for 30 mins. without opening gas cap for proper test of vacuum build-up !!

[TwoTired]

Too Rich, or too lean, or fuel starvation would show definite deposit patterns on the spark plug tips.

Depending on outside air temperature, the engine might not get to full operating temperature until 8 miles out.  Here it might be jetted too lean, which is highly suspect if you've changed to pods filters without changing the carb jetting to compensate for engine breathing ability.

If you want to use stock carb settings, use the stock components, both air filter and exhaust.  Otherwise re-tune the carbs for the changes made.

Cheers,

[Spanner 1]

' Depending on outside air temperature, the engine might not get to full operating temperature until 8 miles out. Here it might be jetted too lean '
TT, Really !!!!

[TwoTired]

' Depending on outside air temperature, the engine might not get to full operating temperature until 8 miles out. Here it might be jetted too lean '
TT, Really !!!!

Yes.  I used to commute year round on my bikes.  Puttering through the city lights and traffic on cold winter mornings, would take 5 miles to get the choke off completely.  If I could get to the freeway quickly and have the bike making some decent power, it would warm up much quicker.  This was with the 550s.  750s have far more mass to heat up and larger cooling fins to wick away heat.

Further, he has pods which are known to make all carb settings need a richer setting, and this hasn't been done.  The simple fact the low end improved by turning in the idle air bleeds is a big statement that the pods need carb compensation to provide more fuel than it is delivering now.

Cold engines don't atomize fuel very well, hot ones do.  And, better atomization makes available oxygen pair better with available fuel, if it IS available.

I'd like to see spark plug deposits before making a final determination.  But so far, the symptoms fit.

Cheers,

[ron.cieri.313]

I will pull the plugs soon and post findings, hopefully tonight if i don't get home from work to late.

I also lined the tank over the winter and the fuel tank cap seal is in horrible shape cracked and sealent residue stuck to it so, me thinks so it sounds like this venting issue could be a factor...I will check gas cap but can you tell me where said vent specifically is so I don't miss it?

On rejetting, if I get to that, do I replace slow jet and main jets with bigger ones?...what do the numbers refer to?...thanks everyone!...ron

[Spanner 1]

The gas-cap is a one-way valve, if you take the tank off and turn it upside-down it should not leak out the cap...it comes apart..just undo the single screw under the cap and clean the pieces ..take care to note how it comes apart to get it put together correctly..my money is still on this being your 'dies at 6 to 8 miles ' problem...the vent hole is that little pressed-out triangle in the round plate an inch from the screw head. Tank lining on any of these parts is a no-no.....

[ron.cieri.313]

I always wondered how this vented. 

I know for fact that mine leaks when turned upside...the seal is bad but I couldn't find a replacement...any ideas?

Will take apart asap...thanks!

[TwoTired]

A leaking gas cap seal is equivalent to an extra vent.

Some where inside the seal you should find a hole in the cap.  There is no one way valve in there.  The vent has to work in both directions for expansion and contract of the atmosphere/contents in the tank.

Re: Re-jetting here are lots of post about this topic.  To accommodate PODs, usually have to increase the main size, and/or raise the slide needles.  Maybe the slow jet, but usually the air screw has enough range to compensate.

Jet numbers for these carbs are millimeter specifications.  #100 = 1 mm.  #38 = 0.38 mm,   #125 - 1.26 mm.  This is the diameter of the metering orifice.  The main throttle valve has both needle and orifice.  The needle is tapered and moves with the slide valve.  Raising the needle in the slide valve open more of the throttle valve orifice.

PODs change the throat vacuum in the carb throat.  It is this vacuum that pulls fuel through the jets that are within a tube and whose other end is immersed in fuel.  Just like sucking on a straw in a beverage, a bigger diameter straw can deliver more liquid up the straw for the same level of vacuum applied to one end.

Since PODs have reduced the vacuum, it now needs bigger orifice diameters to deliver the same amount of fuel as before.  It's getting the same amount of air through the carb bore as before, but the PODs have changed fuel mixture.

Why did you change from stock air box to PODs?  And, were the bike's current symptoms present with the stock air box?

Cheers,

[Spanner 1]

TT... tank rightside-up will vent in and out, but turn tank upside-down and gas will not leak out due to construction of the gas cap..just observed this yesterday while cleaning my tank, K1 same gas cap.....

[Henrik Vik]

I just wanted to give a reply to this post, since it seems to be an exact description of my own bike! This was how my bike  behaved. Mine had pods and had not been rejetted, and the symptoms was inconsistent idling, and stuttering and stopping after 15 to 20  mins ride. I could not figure out what was going on. After some investigation and reading on this forum, I figured out that it was running lean. I then decided that going with the stock airbox would be the smartest move, all taken into consideration. This is ordered from Ebay now.

The bike came with velocity stacks when I bought it. I decided to try to put them back on while I was waiting for the airbox to arrive here in Norway, just to see if it would make any difference. And it does! The bike already behaves so much better. The idling problem is gone. The stuttering and stalling after 15 mins run seems to be gone, even if it feels a little lean still. This is strange, since the stacks probably are as open as pods.  I wonder if it has to do with the velocity stacks feeding the air in a consistent way, which makes for less turbulence in the carbs, and a more evenly pull of fuel through the jets.

Anyway, I am pretty sure it is related to the pods.

Good luck!

[Johnny5]

Sounds more like you had a fuel delivery problem. The stacks and pods should need very similar jetting.

[Henrik Vik]

Well, one would think so. But I double checked. Took the stacks off and pods back on, problem appears again.

[Johnny5]

Well, one would think so. But I double checked. Took the stacks off and pods back on, problem appears again.

Just plain weird. But would make sense if the bike was jetted for velocity stacks and not pods. Im sure there's some separate tweakage that needs to be done for both pods and stacks. So that's probably why the pods are acting up and the stacks not.

Or your house is built on a set for the Twilight Zone.

[Henrik Vik]

Haven't checked the Twilight Zone part, but who knows? The jets are stock 110's, for the K2. I'm putting my vote on the "turbulence" hypothesis. Until I have more compelling ones. But the TZ is a close second..

[ron.cieri.313]

Okay, I just cleaned the gas tank cap of all residual resin from the tank liner including the rubber seal.

Just as a little more background, the bike came with these blasted pods.

I pulled one plug (#4 cylinder) and it was pretty normal looking...a tan colored at the tip with no sign of carbon build up or anthing else...just dark grey leading up to the tip and then tan at the tip.

I haven't check the needle setting or the # of the jets currently residing in the carbs...will do later tonight or tomorrow...thanks all...ron.

[nteek754]

I didnt read all the replies to this so sorry if I repeat or step on any toes BUT if you took air box off and put pods on without jetting (thats your problem) case closed. Just got to make sure your needles are set at the right setting dont remember where that is. I am running pods and 120 jets.  If I put the stock box on I would go back to 105s. Oh I am runnig a Kerker 4 into 1 and it runs great both ways. I like the room I get to  mess around with the pods on there. And you know how much a pain in the ass installing the stock box is grrrrr good luck  JET IT TO THE PODS have fun Cowboy in Maine

[Johnny5]

Hey Ron,

Tomorrow when you find out what mains you have and if they are the stock size, go to Z1enterprises and order a couple different sets a couple sizes bigger. The main jets are rather cheap so it's wise to just buy a couple sets. I'll come down and help you tune the carbs next weekend. We can also go through the whole tuneup.

[ron.cieri.313]

Z1?...thats good cuz I've ordered from them b4

Just pulled the main jet on carb #1...survey says, "105"

I haven't read the other rejetting posts on the site but that sounds a too small for pods from what I'm hearing.

(John...also have ordered new adapters for the Morgan which should be here in a week or so)

[Johnny5]

There's your issue. Im surprised the bike ran at all. It's probably getting hot(Lean) from lack of fuel after you ride it for a few miles.

Order (4) 115 mains and (4) 120 mains. Here's the Z1 part numbers: 120 main (KL18-4524-1) and 115 main (KL18-4522-1)

Let me know when they are in and we'll get that bike roaring.

[Sporkfly]

Let's see, I'm having a similar idle problem. The difference is my bike doesn't die. '77 550

Running:

Pods
105 main jets
Idle mixture screws 2 1/4 turns out (if I recall correctly)
Needles at third clip (stock is 2 on my carbs)
Runs better when it's damp out ( )
Custom exhaust baffles

I have nothing at WOT, my idle needs adjustment at lights, and I can only get it past 75/80mph when it's damp out. My plugs were fairly white at the ceramic and the electrode was a nice tan. Thread face looked good. The way I read that is lean WOT, timing is right, idle is good. I think I may as well just go back to the stock air box though if I can get all the parts.

[TwoTired]

Let's see, I'm having a similar idle problem. The difference is my bike doesn't die. '77 550

Running:

Pods
105 main jets
Idle mixture screws 2 1/4 turns out (if I recall correctly)
Needles at third clip (stock is 2 on my carbs)
Runs better when it's damp out ( )
Custom exhaust baffles

I have nothing at WOT, my idle needs adjustment at lights, and I can only get it past 75/80mph when it's damp out. My plugs were fairly white at the ceramic and the electrode was a nice tan. Thread face looked good. The way I read that is lean WOT, timing is right, idle is good. I think I may as well just go back to the stock air box though if I can get all the parts.

Here's my armchair analysis:  Still too lean.

White at ceramic is no deposits.  The enviros will hug you.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Of course, your report is a composite reading, of idle jet, throttle valve needle, and mains mixture reading.  Could be rich on some and lean on others.

Usually, rain means a low pressure area moving through.  Low pressure = slightly less oxygen.  If you have too little fuel for standard pressure, you might have better mixture balance when a low pressure area moves through your area, hence more power.

You could duct tape half the pod outer face to prove the theory.  This should deepen the carb throat vacuum and pull more fuel from the jets at existing settings.
Or, you could try to add a little choke and see if the power gets a little better.

I like the stock air box. 

Cheers,

[ron.cieri.313]

Well, I ordered 120's and 125's...115 appear to be harder to get...out of stock at BikeBandit and Z1...anybody got some extra's lying around they want to sell or know another source?...thanks!...ron.

[ron.cieri.313]

Fyi..found some 115 on ebay...let the trial and error begin...will let everyone know how it goes...thanks....ron.

[ron.cieri.313]

Just a quick update...I reassembled the jets (the 105's I found in there) and took the bike for a ride...25 miles and no konking out but she does sputter and times and still fiddling with idle setting screw (but not as much as I mentioned before)...so, it does seem like cleaning the gas tank cap helped but gas/air mixture as others speculated still a problem.

When I returned home, I looked in my mailbox and there is my package from Z1 with some new jets...here we go again!...will let you all know...thx...ron.

[ron.cieri.313]

Okay, now I'm up to 125's (from 105 to 115 to 120) and she's never run better but still stutters just a bit when throttle is twisted but no more stalling after 6-8 miles, and idle adjustments during a trip are minimal. 

I'm going to order and try 130's.

I couldn't change my needles adjustments cause those damn little screws are rusted tight...guess I could soak, etc...if anybody has any suggestions on these little buggers let me know.

I still have to keep the air idle screws completely closed with 125's.

thanks all.