Restricting pods to help tuning...

Hey. Sorry for the multiple postings. This is stuff I've been thinking of all winter and now its crunch time.

My PO put on air pods. I think the PPO put on new pipes, presumably less restricted.

Now I've read all of the good work on this forum on tuning and carbs and the problems with deviating from OEM design.

My question is if you're stuck with what you got and if your not looking for max hp out of your bike (who is with a 550?) - has anyone experimented with restricting air flow on pods to closer simulate the originally designed air box flow - with the objective of getting back more reliable carburation?

I'm pondering taping sections of each pod and watching effects on performance.

 

One interesting idea that someone here had was to make a new intake manifold that pulled air through just one pod.

http://www.xjbikes.com/Forums/viewtopic ... iumph.html

Obviously you'll still have to test for the right flow rate somehow...

 

I just read that thread. It's friggin' brilliant. With all the talk of pods and rejetting lately, I don't know why this wasn't brought up before. I'm dang sure going to try this when the weather warms up. It's nothing but a few dollars in parts.

 

exactly -
a cheap fix to avoid the POD nightmare,
and done right looks ok,

 

i've been thinking about doing something like this for a while and i think i have a way you could test the air flow. if you block off 3 of the carb ports on the manifold and hook a vacuum gauge and shop vac to the 4th. do this on the factory air box and the new one and compare. the shop vac would give you an even vacuum unlike the pulsing of the running engine. if you didn't have a vacuum gauge you could even use carb sinc bottles block one side and mark how far the water goes up from even on the other side. get them back to even and test the other air box.

i'll probably give it a try next winter. i don't want to have my bike unridable for even one day right now.

 

Close, but no cigar. The PVC pipe manifold, on the surface, appears to be a solution but holds a couple of pitfalls:

With the intake off to one side (or even in the center) the flow will NOT be the same in terms of VELOCITY for each carb because the VOLUME is strictly limited by the diameter of the pipe. The same amount of air traveling a longer distance in the same amount of time will HAVE to travel faster. In the stock airbox, all four carbs are pulling from one big "bucket" not being individually fed.
This is why high-performance (carbureted) automotive intake manifolds have very complex shapes to their ports and passages, to equalize and smooth the airflow to feed all the cylinders evenly. The PVC pipes ain't gonna do that. You MAY be able to "tune" some of the imbalance out with carb adjustments but I'm afraid the resultant imbalance in cylinder tuning would rob power.
Remember the "CV" stands for "constant velocity." While the PVC manifold will physically work, it will NOT provide the SAME velocity of intake air to each carb. I suspect a tuning nightmare will ensue.

 

I'd have to agree with Bigfitz about the idea. At first I sreiously considered the idea for mine, but when you sit back and think about it, he's right. You will never get equal intake for all four carbs with that PVC setup. The stock airbox is engineered that way for a reason.
I figure being mine runs like a cat on fire now, why mess with it!

 

I've kicked around the idea of a more restrictive pod setup and I always thought that twin pods would be the way to go to provide more even volumes of air to each carb. Something like this:

 

I'm an electrical guy by trade, so I was thinking in terms of a parallel circuit, all current is the same when ignoring line loss.
So in that regard I can see that the resistance to flow will look different for each carb, but I've seen higher end V twins with very asymetrical air routes.

The picture drawn is a good idea. But it would work with PVC unless there is a 'Y' shaped piece that I'm not away of.
You could do a 90, 4 way, 4 way, 90, with two pods in the middle. That would cut the flow in half and reduce the difference in air paths.

My understanding from power plant piping is that the amount of turns is much more important the straight lengths when calculating resistance to flow. Maybe that doesn't apply with high-sensitivity applications like this.

Might still be worth a try since investment is so low...

 

YOU shouldn't mess with it. This discussion is mainly pointed at those of us who have inherited the pod nightmare and don't want to remove our engines to install an airbox.

Fitz is probably right about the whole velocity/shaping flow issue. I like the dual pod idea, also a PVC fittinig for 4 SMALLER pods could be fabricated to restrict air flow, yet keep it even for all 4 carbs.

 

Admittedly, my little design may require a bit more than just glueing some elbows together but it shouldn't be too hard to assemble with a little creativity, a hole saw and some PVC cement.

 

No offense MaximumX, but since the firing order is 1-3-4-2, then 1 would be pulling immediately after 2, then a break while 3-4 do their thing. 1 would always be pulling in the residual vacuum of 2, and 4 would also be pulling in the vac of 3. This would tend to make 1 and 4 run rich, or 2 and 3 run leaner. They would not be symmetrical IMHO.

So my suggestion would be 4 longer runners meeting in a square with one pod, if I were to do this.

 

IF, and only if, you REALLY wanted a stock airbox, it would seem not too difficult to cut one up and glue on some flanges so that you could assemble the pieces inside the airbox space in the frame. After, you could seal it up good with whatever goop floats your boat.

 

Good point, TtR.

That's beginning to sound an awful lot like the stock airbox, isn't it?

 

Just a thought...
Now how would it work with the 4-into-1 pvc idea with the airbox under the carbs? There seems to be enough room for one there. Probably use a U-shaped intake to the carbs from the airbox to keep equal pressure to each carb. I would think something along those lines should work OK.
Just thinking outloud.

 

just get out the electrical tape and have at it
i think i saw a foam sock that goes over them once

 

I'm strongly considering it. Do you think it could be done by cutting it into no more than 2 pieces?

 

I've never had the air box out of my bike, but the PO had at one time, its only cut in 2 pieces. Basically straight across between the filter box, and the longer section of the box that the intakes fit into.

I've never tried to get it out, but it looks like with the carbs off, I could remove the front of the box, then pull the filter section straight towards the motor and out the side.

Hope that's clear enough to get what I'm trying to say.

 

The problem is "sealing it up with whatever goop" afterwards. The airbox on my '81 is splitting apart at the seams, and no matter what "goop" I use it either "bounces off" immediately or fails after not long. If I could figure out an adhesive that actually BITES INTO whatever the heck bizarre plastic they made the airbox out of, it might work.

The best way would be to cut it and then glue (the challenge) flanges onto the inside so it would mate back up correctly. In MY case, if I could figure out an effective glue, I wouldnn't need to replace it.

 

lol wouldn't restricting the pods remove the effects you were seeking when you got them? why not just re jet the carbs?

 

So the plastic is slick and doesn't provide much of an interface for an adhesive. Would it help to scar up the plastic some where you would intend to put the adhesive?

 

Sushi, I'm going to try scaring the plastic and gluing a strip of rubber (an old tire tube maybe) around the seam.

 

anybody happen to know the cfm of the stock intake system?? we could used pod filters with conical restrictors in the intake after the pod to limit flow while maintaining velocity, kinda like how WRC cars have to use restrictors.

 

The CFM of the stock 750 would be- -
80% of 45 Cubic Inches X 9,500 /2 = 99 CFM
80% is the estimated volumetric efficiency at redline.

"conical restrictors in the intake after the pod to limit flow while maintaining velocity"(quoted)

That wouldn't duplicate the "shared volume" of about a gallon of airspace between the restrictor and the carb.

 

An old trick to get plastic to stick together that we use on our R/C gliders is to use either thin or medium viscosity cyanoacrolate glue. Before you apply the glue lay down a dusting of baking soda. Make sure you have the baking soda covering the area you want glued. When you drop the superglue on the soda it will IMMEDIATELY set up!


I would put the two pieces of the airbox together, place the baking soda along the seam area then cover the seam/soda with a piece of plastic that you have rubbed with soda. Place the glue at the edge of the plastic covering the seam and let it wick in.

DO NOT LET THE FUMES GET IN YOUR EYES! BURNS LIKE HADES. A tremendous amount of heat is generated almost instantaneously so be careful you don'get burned.

Enough heat is generated that it will melt most plastic.


Give it a try with a couple pieces of plastic sitting on your bench.

 

RREEEAAALLYY?? Oh, I gotta try that. You're chemically creating a heat weld. If it works I love it.

So, for a split seam: dust it with baking soda, ensuring it gets into the crack and between the two pieces, hold 'em together, and then wick in the CA?

 

That would work Fitz, but what I prefer to do if I can is to apply a cover strip of plastic over the seam. Just make sure that the side of the piece of plastic and the two sides of the seam are covered and then wick in the CA.

It will set up instantly and with copius heat. The reason I put the strip over the top is because the soda will form a crystalline structure which if covered by plastic will insure an airtight seal.

Try to find a thin plastic strip of the same material you are gluing together if possible. Plastic (saran) wrap will just melt under the heat!

Try this on your bench. The power of CA glue becomes apparent when used this way. I prefer the thin CA myself.

One last thing. I put the soda in a little container an then pinch up a bit between thumb and forefinger and. Rub together for pinpoint accuracy.

WAMAXIM

 

Tried to attach picture. No luck, sorry

 

Here is a silly question: If someone with a stock airbox were to give the air flow measurement on a bike with synced carbs, then, make donuts, kind of like giant washers, that cover the air entry into the carbs and then install the the pods over them? Start with a small hole, you could then measure the resulting flow and drill out the donuts until air flow was as it should be. Publish the information of the correct diameter of the donut ... No need for added pipes, pvc (which would look hideous) or otherwise. It will look just like any podded bike but not require any rejetting ...

Or, am i just whistling dixie here?

 

WRT sealing a crack and finding an adhesive that works, wouldn't Gorilla Tape work? Black, stronger and more adhesive than duct tape ...

 

As far as air flow rate goes, there are too many assumptions in the calculation to get it right, and even if you did, how would you know how much of the pod to tape in order to get the desired flow rate? It's just too hard of a problem.

Here's what I would do--figure out how many turns your particular model takes at your particular altitude, and then set all carbs for that level. Then, using a colortune plug, start taping up that pod until you get a good mixture. Rinse and repeat.

Once you're pretty close, then you'd follow the normal synching and tuning procedure not worrying that you've got pods on at all.

 

Why go to pods at all if you are going to restrict them anyway.
That's like going to LED's and putting resistors in line to keep the flash/cancel operating.

 

The situation being addressed is where a PO has removed the airbox ... as is the case for me and few others. Airboxes don't grow on trees, not even Chacal has those. <grin> Quite often, those who have put in pods didn't get the carbs jetted right, if they tried at all, and then they end up selling the bike because it just doesn't run right anymore ... rather than playing the dark art of rejetting and its associated costs and time.

I don't know about you, but I bought a bike because I wanted to ride. I bought an old bike because there is no way I could afford a new one. I bought an XJ because I like how it looks - a nice relaxed cruiser that doesn't sound like ... a Harley. Personally, I hate loud bikes. Enjoy your bike, fine, but don't go driving down the street making windows rattle, waking people up, and generally making a nuisance of yourself.

The guy with the Maxim-X website has something about auto-cancelling not working properly on the XJ700/750s of '85 and '86 and getting a sore thumb after a long ride and having to hit the cancel button on the flashers all the time ...

 

I did not stop to think that someone might not have an airbox to begin with.
Good point.