XJ750 Pod Filter not running well around 5-6K RPM

I've got a 1982 Yamaha XJ750 Seca (Canadian Model)

I've been building a Cafe Racer project and all that's left is the pod filters. I thought they were running good, since winter is over I went to take the bike for a ride but after around 5-6K RPM the bike just completely bogs down and won't go over 6K.

Could it be a problem with not getting enough gas and I'll have to rejet the carbs?

I plan on riding this season so hopefully it gets fixed!!

P.S. Stock airbox cannot be put back nor do I plan on putting it back.

 

cast your eyes up at the 'sticky' threads...

 

Classic - above 5k is when the airbox is needed for smooth slide actuation.
The airbox does restrict the flow beginning at 5k but that is by design.
Eliminating the airbox has altered the pressures, your slides are not rising as they should.
You've lost the positive feel between your wrist and the slides.

Aftermarket stage-3 jet kits suggest clipping a 1/2 coil out of the slide spring as a remedy but what is really need is a softer spring.

 

Actually @Simmy , I'm going to disagree with you here, what you need is a stiffer spring (to counteract the increase in inlet pressure (the bit pushing up on the slides). cutting off springs shortens the springs of course, but it also stiffens them, hence the remedy. Ideally a cut spring with a spacer would be better, but getting it right is guesswork.

 

Try this experiment, cut a bunch of coils off the spring. Is it easier to assemble the hat back on?
I would say yes, and if so then there is also less force holding the slide down.
Cutting it shorter only stiffens the spring if you stretch it back to the original length.

I believe when the airbox is eliminated there is less pressure difference between the inlet and the diaphragm chamber, requiring a weaker spring.

 

Cutting the spring shorter, whether it stiffens or weakens the resistance to the vacuum slides is an accepted practice.

We both agree you don't cure the 5-6k bog with jetting.
I forgot to mention that drilling the hole larger to the vacuum chamber is also common practice.
This is to quicken the response, also suggesting the pressure differences are less, requiring the larger hole.

 

Something else basic to think about.
@Tafshaft needs help getting past the 5-6k dead spot.
Stiffer springs will not help.
If his problem was once he got to 5-6k his bike started lurching towards redline then stiffer springs would be something to consider.

 

When you remove the airbox the inlet pressure goes up - are we agreed? This follows because the airbox and filter is a restriction (lower than atmospheric pressure), so removing it decreases the vacuum (ie increases pressure).
This pressure is then transferred to underneath the diaphragm by the passageways in the carb body.
The slide is lifted by the differential pressure from underneath the slide, transferred by the drilling in the bottom, acting on the top, against the pressure underneath.
The suction pressure, or the vacuum caused by the venturi effect of the airflow under the slide goes up by the square of the airflow. Any pressure increase at the inlet will cause the differential across the diaphragm to increase, thus lifting it sooner, or higher, depending in the spring.
Any rise in the slide which reduces the vacuum at the jet will weaken the mixture.
This is my reasoning, And how I understand it. I may be wrong, and stand to be corrected. Most people who mess around with cv carbs changing jets are guessing, bigger jets will deliver more fuel, but they also deliver it in the wrong place.

 

A simple test would be to add some spacers under the springs - say 5 to 10 mm. this would add to the static spring pressure, making the lift point higher in the rpm range, thus richening the mixture. Richer mixture is what the engine needs.
Edit to add - what adds fuel enrichment when you open the throttle? Answer is, the size of the hole in the bottom of the slide - if this is large the suction at the bottom gets to the top sooner, lifting the slide. Delay this with a smaller hole and the suction at the jet goes up, resulting inmore fuel emitting. Sure it restricts airflow as well, but the engine is in the process of accellerating, so needs added fuel more than air.

 

"I believe when the airbox is eliminated there is less pressure difference between the inlet and the diaphragm chamber, requiring a weaker spring"
Well no. Because the inlet pressure goes up, and the venturi derived vacuum stays the same. I think?

 

Yes I agree,
But with the air box gone does the air flow now slow down through the venturi? I don't know.
This would lower the venturi effect when passing over the slide hole.

 

Why would the airflow slow down? Air is drawn in by the engine past the butterfly. Open the butterfly, engine sucks air in past the slide. One gob at a time. if the engine is turning faster the gob has less time to get sucked in, hence will be travelling faster. Because there is a biggish gap under the slide it takes quite some airspeed to create a vacuum, which then lifts the slides.

 

Ok my brain hurts, I’m not going to pretend to understand what it does to the air pressures through the carbs.
My advice to @Tafshaft is to buy a commercial stage III jet kit and follow their instructions.
It will include richer jets, a drill bit for the slide hole and recommendation to clip 1/2 a coil off the springs in increments until you have a satisfactory response.

 

One more thing to consider and that is the air/fuel that gets "trapped" in the upper plenum of the airbox during the "spitback" or reversion period, this may be absorbed by the pods and alter the air/fuel ratio. I posted two videos of my XJ750 carbs in real time and one can clearly see this reversion action along with the movements of the slider pistons during a riding cycle. The link is in the "What Have You Done To Your Yamaha Today" topic, there is both a long & short video with real engine audio to correlate the throttle and speed.

 

Any commercial jets you recommend? (Canadian sites or shipping to Canada)

I'm not tryna pay a ton of money either for jets so any easier/cheaper solutions?

 

I think Sigma-6 are cheap, can’t say if they’re any good

whatever kit you buy it will never behave as good as the air box

 

https://www.6sigmajetkit.com/yamaha-xj-750-maxim-jet-kit.html

Here it is $80, if you go this route do a report for us.

XJ 750 Maxim Jetting Kit Includes

• XJ 750 Maxim Main Jets
• XJ 750 Maxim Pilot Jets
• XJ 750 Maxim Needle Adjusters
• XJ 750 Maxim Nylon Needle Spacers
• XJ 750 Maxim Slide Spring Modification
• Drill Bits for
• XJ 750 Maxim Slide Hole Modification (optional)
• XJ 750 Maxim Idle Mixture Modification
• XJ 750 Maxim Tuning Tech
• XJ 750 Maxim Bike Specs
• Carb Schematic
• Carb Synchronizer Tool (how to build for $5)
• Step by step instructions with photos for ease of installation.
• Installation Support - If you need assistance we are there for you.

Please Select the Following Information
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Exhaust Type
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Other Mods, Brand of Exh/Int

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I cannot be more overjoyed to say this... IT WORKS!!! (For now)

So no, I didn't rejet it or buy any aftermarket parts for the carbs. All I did was watch the video @a100man recommended to see the sticky threads.

And I decided to use the "velocity stack" rubber that came with the stock airbox and put on my aftermarket pod filter on the end.

What I'm guessing is that now it has a much smoother air intake, not sure but at least it works. Maybe someone more knowledgeable can answer it better.

Here is a picture of end product:

 

Haha. I know. I shot myself in the ass going for the wrong thing.

Indeed. I regained all my lost mid-range torque by adding a decent inlet short ram.

 

howdy guys, not sure if you can see what I did here but inside pods there are restrictor which totalled the same area as the opening into original air box.its not perfect but it a lot better than running pods straight on to carby

 

Dear Lord

WTAF is that thing!?

 

Like the sparkplug oil cap?

 

Mad Max lives!

 

Pods not running well with CV carbs?
Interesting.
That seems to be a theme for the last 40 years.

Sorry for the snark, but it's not like the info about this issue is hidden.

I might be in a bad mood. Forgive me.

 

howdy guys,i blame this forum for how my 83 xj 900 ended up looking.my profile picture was a buggy I made with 1990 cbr 1000 motor in it.this was my first encounter with cv carbies.i tried for weeks to get it to run properly but just couldn't.loosened all clamps and choke cable clamps as I had done countless times before,did what I thought was needed and replaced them.upon refitting I was unable to locate a screw which held choke cable in place,thought I had dropped it as I cross the lawn.3 miles out the road the screw eventually made its way past the valves and made its home on top of a piston.turned out it was cheaper to purchase another second hand motor than rebuild it.i ended up rejetting new motor after finding info on net and it run ok after that.murphys law is real