jets....

well i just got a 1980 xj650... i am looking to put some air filters on it and a mac 4 in to 2 tapered exhaust on it.... just looking to see what jets i should go up to if at all it has 120 on the big side and 40 on the small side.... also what other mods would you sugest to go with them.... thanks for your time.... mark

 

A prayer, and enough 6-packs to reach the moon

The diffrences are many between bikes and their appropraite modifications on jet sizes.

There are some who can give you a ballpark on where to start, be aware I state START.

You will offically become a yamaha research and development man by undertaking this task.

Keep this in mind, "Anything is possible, it rests entirely on the drive and motivation to complete the task"
Also add money and time to the previous equation.

Good luck, and let us know how it goes. We get to hear people screaming for help after this modification, I hope you do not become another statistic too.

Good Luck

BTW I would have no clue where to start on sizes...sorry

 

BTW*

Optimal Air/Fuel Ratio is..
14.7:1

I know its not really any help but for science's sake

 

120 is already 5 jet sizes (10 numbers) larger than stock for a 650.....

 

so do you think that it could be the reasion that it was backfiring... it had to much fuel.... i think that i will just stick with this for now and see how she runs after the air and exhaust mods... will let you know what she does... thanks again...mark

 

120 is stock for the 750 and yes that would explain the backfiring. Unburned fuel spit into the exhaust and igniting in the pipes.

 

Sounds like your at a good starting point with the 120's now. Installing those air filters should lean it out a bit. From what I've been reading, +2 sizes on the main jets for pod install gets you in the ballpark, with some fine tuning to main jet needle adjustment. You may still need to downsize them.

Not sure what you'll need for the Pipe install if anything at all. It's all trial and error.

Good luck if you go there and stay with it til ya get it right !

 

thanks to everyone that has replyed to this.... soon the air filters and the exhaust will be hear and i will let you know how she runs.... the air jets are 225 and 80 on the top side how far from stock are they... thanks mark

 

That's the stock configuration of air jets for a 750. Actually, it sounds like you have a set of 750 carbs on your 650.....120 fuel jets are stock for a 750.

 

wow... is that a good thing or a bad thing... guess i have a lot of tuning.... thanks again mark

 

Pods are the deadliest trap of all ...

You play hell trying to find the right combination that will fill two holes:

Off Idle Throttle at the Low-end of the Power Curve.
Pilot Jets
Mixture Screws
You get the bike to Idle but find a Hole in the Low-end where the Bike won't respond to theh Throttles being thrown-open without a too Lean situation causing the obvious performance difficulty.

Correcting with "Hotter" Pilot Jets
Increasing Pilot Mixture Settings.
The Bike won't Idle
You become a Throttle Twister keeping the Fuel Supply Aired-up
OK Low-end and out-of-the-hole
Flat response in Mid-range and W-O-T

Increasing Main Jet Size
Wide Open Throttle OK
Idle unsat
Mids; unsat

Diaphragm Needle Adjustments:

Here it gets dicey.
You can't seen to find the right height to satisfy the Low and High End together.

Pick one and settle for losing the other.

 

My initial setup was 41 pilot, 118 main, and the screw turned out 3 1/2 turns. Then I tried to adjust using the colortune. For some reason I just couldn't get the right mixtures. There was a flat spot down low in the rpms. And it seemed like it lacked power in the upper rpms.

So this week, I cleaned the 750 carbs. I replaced the main jets to 116. I kept the stock 40 pilot jet. And did a coarse adjustmet of the screw to 4 turns.

Let me tell you this.....

... I was able to get the bike started on the second try (as opposed to over 3 minutes of attempts with the old setup). I let the motor warm up. And took it for a short drive (I needed fuel anyway). I don't have it colortuned or synced yet. But I can't believe the difference. I don't have that hole in the lower rpms anymore. And it pulls like a horse all the way to redline. I only get mild popping (backfire) on decel.

I think the combo of 750 needle and sliders with the 40 pilot and 116 mains, used with the pods and a stock exhaust on a 650 is a great setup to try. I seem to have decent luck with it so far.

Now I just need to dig out the colortune and the vacuum gauges for a little fine tuning.

 

Wink:

Make a "Tuning Log"
Jot down what you did and what you do.

It make it easy to return to a stage in tuning that was good without guessing what it was that you might have done to upset the delicate balance of fine tuning.

Read the Plugs.
Just adjust individual Pilot Mistures on holes that are too Rich or Lean.
Don't make BIG adjustments.
A couple of DEGREES (width of a Dime) is a BIG move on setting-up Mixtures.

Looks like you are definitely in the Ball Park ... bring it home!
Good Luck,

 

Ok, here's the skinny....

Today I was messing with my colortune and vacuum gauges. I finally got the idle right about where it wants to be. I had to do three sweeps with the colortune and the gauges. Each sweep of minor adjustments caused each other to slightly go out of whack. But by the third round, I've found it's sweet spot (for idling).

I took it out for a mild length ride (got her warmed up really well). Then I adjusted the idle speed and colortuned/vac gauged again (two more times). Idle now shows just a hint of orange mixed in the typical hue of blue.

After an afternoon of "tinkering", I have her purring in idle and heavy throttle in the upper rpms. There's a fat spot between 2500-4000 rpms. It looks like I have to lower the needles down just a hair. But I'll save that for another day.

 

Wink:

Mmmmmmm......you really can't "lower the needles" on Hitachi carbs, as they don't use clips and shim washers like some Mikuni carbs do, the needles are completely NON-adjustable, that is, unless.....unless.......oh no, say it ain't so Mr. Bill!..........unless someone has installed a Dyno-Jet tuning kit in those carbs, in which case, plan on spending all summer tuning that puppy up!

 

This is interesting stuff....I put pods on my '81 650 that already had a 4-into-1 exhaust (R.C. Engineering??). Dyno Jet directions recommended a 122 main jet size with those mods. I thought that was a HUGE jump. Right now I have 118 mains and 41 pilots in the carbs. I had terrible cold-starting issues and, like a dunce, tried adjusting the pilot screws to compensate on the low end for starting. Well, they are turned way out (probably maximum effect) with no difference in starting. FINALLY I got squared away and cleaned the enrichment circuit passages in the bowls. That solved my starting issue. I forgot to re-set the pilot mixture screws while I had the carbs on the bench, plus I only wanted to make one change at a time. SO, now I have to adjust the screws, then start tuning for jets. There is a slight hesitation off idle (very slight, though) and it just doesn't really seem to pull all that hard in the upper rpms. I'm not used to this bike, coming from a v-twin, but I'm thinking that I'm on the rich side. Once I adjust the pilot screws I'll get to fine-tuning. Right now it starts and idles fairly well and pulls ok, I guess, for a 650. I sold my Harley, so this is my daily rider for now. I'll spend more time working on it than I did when I could just hop on the 1200 and cruise.

 

That flat spot is because you don't have a Stock Air-box and have Pod Filters.

You should now resort to another method of Colortuning.
Plug Coloration.

Individually add or subtract Pilot Mixture to the Holes depending on what the Plugs look like after a run.

Shoot for a deep tan or chocolate brown.
But, don't do them all because you do one.
Treat each hole individually and make your adjustments in (my-nute) degrees of adding Richness or Leaning the Hole out.

 

Nope. I'm going to pull out the needles and take them to my machinist. I'm thinking about putting another groove above the stock groove. That way I can shim "up" from the new groove.

Sound like a feasible plan? Or should I just ditch the idea?

 

Wink, my goodness man, I stand in awe of you, even though my first thoughts were "gosh you are a glutton for punishment"!

But if and when you DO solve the problems, then you will have come up with a new improved method of taming the PODS MONSTER, and you will be everyone's hero!

 

Raising the Needles isn't going to solve the Problem.

The volumetric quotient of AIR surrounding the Emulsion Tubes and passing through the Metering Ports of the Emulsion Tubes isn't going to change.

You will also have to Increase the Main AIR Jet and incrementally increase the Inside Diameters to the uppermost Air Ports on the Emulsion Tubes along with installing a larger Main FUEL Jet as well.

My best guess is that YOU are going to be the first guy to get that far into it!
I don't know or have ever heard of another person who has.

With that said ... this might also be a case where the actual diameter of the Main AIR Passage might very well have to be un-blinded and drilled-out to the next oversize to provide the Emulsion Tubes with enough Air beyond their present Inside Diameter.