XJ650 carb tuning with the dreaded "pods"

Whoa is me.....good thing I like a challenge!

I finally started putzing around with the Maxim carbs again. I put the 118 mains back in and am still running a 41 pilot jet. Today I removed all the pilot screws and cleaned the screws and the passages to be sure they were working. I put a long length of fuel line on so I could leave the tank off, just sat it cross-wise on the frame and put it on "Prime". That almost cost me some serious issues. While I was messing with the carbs with the bike running the tank fell off and pulled the fuel line off the petcock. Since it was on prime, it spewed some gas onto the still-running bike....hot bike, hot pipes....whew! I got it shut down and wiped off quickly. Good thing I keep three fire extinguishers handy in the garage!

Anyway, I originally thought I'd set the pilot screws at about 3 to 3-1/2 turns out, but that's where they already were! So, I thought maybe turning them in would reduce the air in the mixture and richen it up, thinking that might be one problem with the larger jets. I did get the bike started and tried adjusting the screws to various settings with little success....still idling erratically and popping and spitting through the carbs. I liked someone's idea about athletic sweatbands on the pods to restrict air flow. I didn't have anything like that, so I took some plastic ziploc type bags and put them loosely around the pods thinking they'd get sucked onto the filter surface and significantly reduce air flow. Not sure how much I restricted air flow, but I did finally get some progress. By turning the screws out in 1/2 turn increments I finally got it to smooth out some. I lowered the idle and blipped the throttle. The response off idle was significantly improved and there was no hanging or erratic idle....which was making me think I had air leaks or my pilot jet was still too small.

I took the baggies off and there was no difference, so I'm not sure they were really doing anything. I shut the bike off and re-started it several times and it ran consistently and was much improved over the way it ran before, even when warmed up.

So.....I shut it off and am letting it sit so I can see if it starts any better when cold. If so, then I have definitely made some good progress and I may invest in a Gunson colortuner to get it dialed in further. Perhaps a change in jets. I took the needle apart and it does not appear adjustable, am I missing anything on that?

I'll be sure to keep track of all my settings and progress (hopefully lots of progress) and pop in here to share for anyone interested.

 

The needles are not adjustable, people have put washers under then to raise the needle though.

Good you avoided a fire!

I don't know if you have seen these links but they should help.

http://www.gassrx.com/index-12.html

http://www.dansmc.com/spark_plugs/spark ... talog.html

You need a colortune (or gas analyser) for the idle mixture unless you "plug chop".

It can be a real PITA setting up CV carbs for pods, best of luck to you.

 

willierides, if it's popping that means it's too lean. Pod filters let in more air so then you need larger pilot jets. If you had #40 stock then you should go to the next larger size like #43. Also you should read up on how the carbs operate. The mixture at idle is controlled by the screws, not air, so if you open them more it will be richer. That's probably what you want with stock jets but they probably won't open enough since they are fully open and don't do anything after about 4-5 turns. Don't worry about the main jet now since it only works at full throttle. In my case I had to increase the pilot jets one size and the main jets a size but had to Lower the tapered needles a notch since it was then too rich in the mid range. Like KiwiXJ750D says, you need an exhaust gas analyzer to check the whole range though. I installed an O2 sensor in the large part of the exhaust manifold just past where the 4 tubes join into one and put a meter on the fairing so I can monitor it while I drive since I don't have a dyno handy.

 

In short.

Pods cause the Bike to run Lean.

Too many Cubic Feet per Minute of Air is going in during Intake ...

The Airbox Boots don't just connect the Carbs to a Filter ...

The Airbox Boots are Velocity Stacks; too.

Pods = No Velocity in the Intake Airstream

Pods = Turbulence at the Intake.

Turbulence = No fast stream of Air passing over the open Emulsion Tube.

No fast stream of Air Passing over the open Emulsion Tube Opening means no lower pressure at the opening = less fuel drawn-up and in.

Less fuel = Lean Mixture

Turbulence in the Intake = Less atomization of the Fuel entering the Combustion Chamber = Bad off-idle and low performance at the bottom of the power curve ... and, poor Mid-range tuning.

Re-Jetting serves to correct the LEAN Condition ONLY!
Re-Jetting does not correct the Low Air stream Velocity.

Theoretically:
Pods will increase the Performance at the top end because the Carbs NEED to be Re-Jetted for MORE Fuel to enter the Combustion Chamber.

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In my very own personal opinion ... and NOT writing as this Forums Technical Writer ... It is my opinion that a near perfectly tuned Stock XJ-750 will outperform a Bike that has been altered to eliminate the Airbox, Rubber Boot Velocity Stacks and Engineered Cubic Feet per Minute of Air Intake with Individual Pod Air Filtering in the quarter-mile standing start and all-day long cruising down the blacktop!
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Rick, that's a great point about the airbox boots providing the "velocity stacks" effect in assisting/streamlining/speeding-up the airflow.

Theoretically (or if anyone has actually done it), would mounting the pods at the ends of the rubber boots overcome the lean problems encountered with using the pods, while also providing more top-end power (along with associated larger main fuel and/or pilot fuel jets)? I'm assuming here that you would remove the stock airbox.

And would just using a hi-flow (for example) K&N airbox filter accomplish pretty much the same thing?

 

No, it won't help unless you can make it as restrictive as the original filter assembly. Maybe if you built a 4-1 manifold and put one pod filter on the end of it?

 

Don't forget that the AirBox Intake Port regulates the Total Cubic Feet per Minute of Intake Air.

This Value is divided by four after the Incoming atmosphere passes through the Air Filter.

You have the restricted Value of Total Intake -- plus -- Air Filter Lag -- divided by four.

The Pods are virtually unrestricted!
That throws-off the CFM Value big-time!

 

Welp...hindsight's 20/20! 8O

I'm committed to getting this to start more easily. Tried this morning, still only starts HARD and, the weird part, if I shake the whole bike side to side....this works every time. Now I'm thinking I may be causing some kind of air flow change that makes the diff....dunno....

 

I'm with Rick on that air flow theory.

I figure a person would be better off to keep the stock air filter box and swap out the filter for a K&N element.

Many times auto manufacturers detune and engine by limiting air flow thru the intake....... they do this after a great deal of engineering for optimum performance at all ranges, including fuel economy.

Then we come along and "know better",

Willie if it were me, I'd go back to the stock airbox and boots. then sell those pods on fleabay to get your $ back.

Good Luck there

 

That's the thing to do ...

Join-up with the "Stock & Tweaked-good" boys.
We'll get your Carbs Cleaned ... really nice.
We'll get you Synced ... on the money ... and, then ...
We'll get your Pilot Screws just right so you can be the first guy to the next light when you are out twisting the throttle and having some FUN!

(Where are you in NY ... maybe you're close enough for your own Carb Clinic?)

 

Well....I'd really like to do that. BUT, the space where the air box used to be is now occupied with a bunch of other stuff...battery, electornics....I made my own battery tray, etc. Maybe I can find an airbox and boots on ebay and make it fit back in there, but I don't think so. I think I will keep trying to get this dialed in for now. Too bad. It really is disheartening. I really only did this for looks anyway....trying my hand at "customizing" more to experiment with my welding and wrenching than anything else. I should find some "before" pics, but here's a pic from last January. I since put the front fender on, painted to match the tank, and bigger rear turn signals. You can see that the space where the air box was has been changed quite a bit. But, if I can find one.......





I really appreciate your advice and patience. And I also really do wish I'd kept the original set up. I may have to re-fab something so I can get it back to stock form or something closer. I've studied carb theory and wrench on my own dirt bikes, one cylinder and one carb!, (relentlessly, talk about high maintenance!) and my 2004 Sportster (two cylinders, one carb!), but learned a lot here so far. I especially like Rickomatic's explanations on cfm, restricted air flow, divided over the four boots, and the velocity stack theory (venturi theory or the relationship between pressure and velocity, right?).

So....I feel ya, and I'd like to take your advice, but right now I just have to try and make my current set up work. I'm still thinking of getting some 43's and 45's for the pilot jets....but not feeling too good about my chances of success. It runs good right now once started. If I could get it to start more easily I'd call it good until I got a colortune plug, which looks like something I'm going to invest in anyway....should work in all my bikes shouldn't it? Two strokes, too?

 

Oh yeah, rick-o-matic, I'm located right on the NY/PA border about halfway between Elmira and Binghamton, NY. Where are you and where do you work? I have a friend who was a certified Yamaha wrench for years until the place he worked dropped bikes and went to boats only. But he still remembers all kinds of stuff....even the different model numbers by the year they were made (ie. XJ650G = 1981, or is it 82?, etc.)

 

I'm just outside Boston.
Near Lowell, MA.

 

check your PM box

 

Looking at the Pod on the Bike in the Photo ...

It occurs to me:

A clever person could fabricate a small diameter "Stack Insert" for the Pod and quite possible make the Intake Air have some Velocity.

A flared end tube inserted in the Pod leaving a small space at the Pod Cap to at as resistance.
If the Cap end was flared and wide and the diameter reduced at the Carb Side ... Intake air would be restricted and the restricted flow made fast enough to act upon the Emulsion Tube Opening.

Who's got the Engineering and Drawing capability to draw it up???

 

Ok, As usual I'll go totally opposite to Ricks theory. It's all very well to come up with a an excuse why pods don't work but it's just as easy to come to the right conclusion using pods if you do your research right.

Sorry Rick but IMO yur velocity theory is BS. The carb itself creates the velocity as the internal shape of the carb is it's own velocity stack. The connecting tube to the airbox is just that. A straight connecting tube to a box that also creates turbulence to the carbs. IMO more turbulence than pods as the air going through one single small intake tube is then fought for and must seperate to feed four different carbs. You can't tell me that this split doesn't create turbulence.

But, Turbulence is really irrelevant anyway. As stated, the carb itself is the velocity stack as it reduces from a 50mm opening down to less than 30 mm.

This is where the velocity is created and regardless of filter setup the carb itself creates the correct velocity over the emulsion tubes. After all, that is what it's designed for and why it's called a CONSTANT velocity carb.

Now that we've cleared that up from all those that are too scared to try, or never managed to get it right then lets deal with the original problem.

You can tune a bike perfectly with pods but there are a few small issues that need to be checked first.

The pods themselves. Remove one and make sure it does not have a lip inside that blocks the air jet intakes on the rim of the carb. This little problem has caught a lot of people out before. If these jets and intakes are blocked then the carb will not operate as designed.

I notice you have a 4 into one as well as the pods. Definately go up ONE pilot jet size. Never more than one unless your running no filters and no muffler. :wink:

Go up THREE jet sizes from the stock size. Sync and set mixtures. Take it for a ride. If it pops on deceleration from higher revs then check the plugs. If they are still clean then go up another jet size on the mains.

Resync and redo mixtures.

Then take your 750 down to see Rick and it will easily keep up with his 900. :wink:

I've been there and done it. I have 2 identical bikes. One with pods and one with stock airbox. Both sets of carbs cleaned to the same specs and both tuned by the same person (me) using the same tools. The only bike that runs perfectly is the one with pods. Stock airbox setup is giving me starting problems as it isn't allowing enough air in to make a combustable mix.

Pods are going on it as soon as I can find a set at the right price.

Do the above to the letter and let me know the result.

Cheers
HG

 

And a note on this one.

Provided the fuel jetting is correct then by your theory then wheres the problem???

More CFM (with fuel) equals more power.

At least that's what the manufacturers of Turbo's, Blowers and superchargers would like us to think. :wink:

The restricted CFM through the stock airbox feed is compensated for by small jets. Small jets, low CFM equals less power.

And Chacal.... Yes, tubes have been used with pods. To get around the restrictive lip inside some pods. No performance difference was noticed IIRC.

 

Ok, update. I replaced all four plugs, put on the choke (I know, it's an enrichment circuit) and cranked it over. It started and stalled. I put the choke at about 1/2 and hit the button. It popped right off! It warmed up quite quickly and it ran freakin' sweet. I know it's only in the garage, but I played with it for a few minutes and it was steady as a rock throughout the rpm range...no popping, no sputtering, idled smoothly. I fiddled with the idle speed and could get it to putt along very slowly but touched it up a tad just 'cuz. I will get a better feel for the power and pull over the whole range. I'll do a plug chop soon and eventually I plan on getting a Gunson Colortune plug and really dialing it in. I'm confident that I can do that.

For reference, it's a 650 so I hope Rick isn't too intimidated against his 900! (Just kidding...what kind of 900 DO you have, Rick?) Stock pilot jet was 40, I am running a 41. I found out when perusing websites (CycleReCycle, etc.) that no one seems to have anything other than 40's and 41's for these carbs anyway. Dyno Jet suggested going from the stock 110 mains to 122s. I just couldn't believe I'd need that, so I guessed and bought 118's. That's what's in there right now. I will have to count the turns out on the pilot screw, but I'm thinking they're in the 3-1/2 to 4 turns out range. I haven't re-adjusted since trying to tune it when it wasn't even running on all four cylinders. Duh. I learned.

I snuck home at lunch and threw the plugs in and started it, so I will have to do some more evaluating to fine tune. It's actually only registered and insured, NOT inspected yet. It needs a front tire and I didn't want to buy one until I knew I could get it running. So, now I need to get a front tire mounted and get it inspected so I can get out and ride it some before the snow flies. I'll probably do a little outlaw run before that because I just can't stop myself. :wink:

 

Ok, update. I replaced all four plugs, put on the choke (I know, it's an enrichment circuit) and cranked it over. It started and stalled. I put the choke at about 1/2 and hit the button. It popped right off! It warmed up quite quickly and it ran freakin' sweet. I know it's only in the garage, but I played with it for a few minutes and it was steady as a rock throughout the rpm range...no popping, no sputtering, idled smoothly. I fiddled with the idle speed and could get it to putt along very slowly but touched it up a tad just 'cuz. I will get a better feel for the power and pull over the whole range. I'll do a plug chop soon and eventually I plan on getting a Gunson Colortune plug and really dialing it in. I'm confident that I can do that.

For reference, it's a 650 so I hope Rick isn't too intimidated against his 900! (Just kidding...what kind of 900 DO you have, Rick?) Stock pilot jet was 40, I am running a 41. I found out when perusing websites (CycleReCycle, etc.) that no one seems to have anything other than 40's and 41's for these carbs anyway. Dyno Jet suggested going from the stock 110 mains to 122s. I just couldn't believe I'd need that, so I guessed and bought 118's. That's what's in there right now. I will have to count the turns out on the pilot screw, but I'm thinking they're in the 3-1/2 to 4 turns out range. I haven't re-adjusted since trying to tune it when it wasn't even running on all four cylinders. Duh. I learned.

I snuck home at lunch and threw the plugs in and started it, so I will have to do some more evaluating to fine tune. It's actually only registered and insured, NOT inspected yet. It needs a front tire and I didn't want to buy one until I knew I could get it running. So, now I need to get a front tire mounted and get it inspected so I can get out and ride it some before the snow flies. I'll probably do a little outlaw run before that because I just can't stop myself. :wink:

 

Ok, update. I replaced all four plugs, put on the choke (I know, it's an enrichment circuit) and cranked it over. It started and stalled. I put the choke at about 1/2 and hit the button. It popped right off! It warmed up quite quickly and it ran freakin' sweet. I know it's only in the garage, but I played with it for a few minutes and it was steady as a rock throughout the rpm range...no popping, no sputtering, idled smoothly. I fiddled with the idle speed and could get it to putt along very slowly but touched it up a tad just 'cuz. I will get a better feel for the power and pull over the whole range. I'll do a plug chop soon and eventually I plan on getting a Gunson Colortune plug and really dialing it in. I'm confident that I can do that.

For reference, it's a 650 so I hope Rick isn't too intimidated against his 900! (Just kidding...what kind of 900 DO you have, Rick?) Stock pilot jet was 40, I am running a 41. I found out when perusing websites (CycleReCycle, etc.) that no one seems to have anything other than 40's and 41's for these carbs anyway. Dyno Jet suggested going from the stock 110 mains to 122s. I just couldn't believe I'd need that, so I guessed and bought 118's. That's what's in there right now. I will have to count the turns out on the pilot screw, but I'm thinking they're in the 3-1/2 to 4 turns out range. I haven't re-adjusted since trying to tune it when it wasn't even running on all four cylinders. Duh. I learned.

I snuck home at lunch and threw the plugs in and started it, so I will have to do some more evaluating to fine tune. It's actually only registered and insured, NOT inspected yet. It needs a front tire and I didn't want to buy one until I knew I could get it running. So, now I need to get a front tire mounted and get it inspected so I can get out and ride it some before the snow flies. I'll probably do a little outlaw run before that because I just can't stop myself. :wink: