Re-jetting ‘85 xj750 maxim x

Hi all,

my maxim x came with pods (stock airbox removed) but it hasn’t been re-jetted and bogs out.

does anyone know if the xj700xn and the 750 had the same carbs? Dynajet has a kit for the 700 but not the 750.

cheers

 

Hi mate, 6 Sigma has the jet kit you need.
https://www.6sigmajetkit.com/yamaha-xj-750-maxim-jet-kit.html

 

careful - that kit is for Hitachi carbs on 750 air-cooled bikes.
I see Sigma do list a kit for the 700-X but nothing for the 750-X

I think the carbs are the same, in Chacal's information pages he does show the idle jets being different, something you don't get with the aftermarket jet kits anyway.

Main jets are the same
#105 is used on:
1986 all XJ700-X models and all XJ750-X

Therefore I think you could start with either the Sigma or Dynajet kit for the 700-X.
In my experience the 750-X will run all the way to redline with pods and no other mods but at 5,000 rpm it bogs terribly as you experienced.
If Dynajet give you replacement springs that would be the kit to get.
Sigma recommend clipping 1/2 a coil off the springs until you get a satisfactory response at 5,000 rpm, this will never work as well as softer springs (or an airbox).

 

You never came back, but my advice is to remove the pods, and run the stock jets. Idk about yours, but mine has 110's in it, runs great like that

 

Sorry to be a pedant, but clipping coils off springs makes them stiffer, not softer. Granted they may be shorter by 2/however many coils you started from times the fitted length, but stiffer they will be, stiffer still as they get compressed.
This ties in with the theory on reduced restriction at the carb meaning the slides run higher, hence weaker mixture...

 

The shorter spring might indeed be stiffer but it lessens the preload on the spring.
When the spring is shorter it is easier to assemble the carb hat.
Easier to assemble the hat means less resistance to raising the slide.

 

Yes, at fully closed. But the spring pressure at closed isn't everything, a quarter inch up is critical, and by then the spring is stiffer. I don't wish to argue this point but less air restriction at the carb mouth raises the slides, raised slides weakens the mixture, stiffer springs richen it.

 

It seems to me that a raised slide on a CV carb would increase how much fuel is being delivered since the needle is attached to it and the butterfly controls the air flow. That being said. These parts were carefully engineered. The only thing you should worry about is if they are all still the same length, or if one may have lost its spring... In which case you can stretch them a little to re-tension them just like a magazine spring.

 

On the face of it you might be correct, but this is fundamentally incorrect.
Why does fuel come out of the main jet then? Sure the needle meters the amount, but something is drawing it up from the bowl? The answer of course is vacuum. Vacuum is everything with these carbs. So the engine vacuum is allowed at the slides, controlled by the butterfly position, air is drawn under the slide, which creates a pressure imbalance on the slide, tending to raise it, but also drawing extra fuel from the jet. Eventually the slide balances the vacuum pulling it up with the spring pressure, the extra fuelling at the jet diminishes and stabilizes, and the mixture is as prescribed by the jet and needle profile.
Now the big question, this vacuum that raises the slide - where does it come from? One side is the airflow under the slide - ever wondered what the hole in the bottom of the slide does? So there now is vacuum above the diaphragm, with what underneath it? The answer is the pressure at the carb mouth - connected directly into the space under the diaphragm.
But someone just removed the carefully designed air box and stuck a relatively open pod filter on there, totally changing the conditions. But importantly, reducing any suction that existed in the air box - ie raising the pressure, the same pressure that sits under the diaphragm. Well damnit, this will raise the slide higher. Yes, it will, and you might think this will let more fuel out of the jet, but no, the first thing it does is lower the air pressure at the jet, net result, less fuel comes out, weaker mixture...
There is no magic making the slide work, it's vacuum, there is no magic making fuel come out of the main jet, it's vacuum. The partial vacuum in the air box is important, since it's been designed to be constant, damping out pulses, resonations etc. changing this pressure changes the way the carb was designed to work, hence the merry go round of jet and needles, holes under the slide, and spring changes.

 

the first thing it does is lower the air pressure at the jet

Oops, should have been lower the vacuum at the jet...
It'confusing enough without me mixing my pressures and vacuums!

 

This is a very good explanation.