carb adjustmets

Hi all. I finished up cleaning/syncing the carbs on my xj750, and i must say she idles ALOT smoother, almost sounds like a car engine LOL. Anyway, I just want to verify a couple things.

The pilot mixture screws are the 4 screws that sit just by the choke plungers on the top of the carb, correct? Are these the only actual "adjustment" screws? Right now I have them 3 turns out, is this correct?

Lastly, are the pilot mixture screws adjusting for just idle - midrange? or does it affect all ranges? I assume that if you flick the throttle at idle and she bogs down its too rich? I really appreciate all the answers!

 

You are correct, those are the mixture screws, but they are not the only 'adjustment' screws on the carbs. There are also screws that adjust the butterfly valves when doing a sync. As far as 3 turns being correct, that can only be determined by running the bike and reading the plugs. Do a search for 'reading plugs' and you will get multiple hits and TONS of good information. I can almost guarantee that each carb will be a little different.

I would suspect that a quick blip of the throttle and the bike bogging down would tell me one of two possibilities; it may not be fully warmed up OR your carbs are still out of sync.

 

Is this a YICS engine? If so, you can not sync the carbs without blocking off the YICS passage ways with either a homemade tool or a factory service tool. You can build a YICS tool pretty easy and inexpesively. There are links here on the web site.

Also, you are better off bench syncing th carbs first, then fine tuning them on the bike. You will save yourself a lot of time and arrivation that way. There a lot of tech tips here on the web site on how to do it.

Trust me, it will pay off.

 

Ricks Plug Chop Instructions
Look to get thos plugs a nice tan, brown color.

 

PS Sync the carbs after doing the adjustments to the mixture screws
and of course, repeat again until it is perfect

 

A tutorial on spark plug reading

http://www.usaimports.co.uk/Mopar_Tech_ ... eading.htm

 

The precise adjustment of the Pilot Mixture Screws is vitally important to Fine-tune the Bike.
The Pilot Mixtures being right determine how well the Bike will:
Idle.

But, the Pilot Mixture being adjusted for just Idle is not enough!

You need to adjust the Pilot Mixture Screws to have Idle AND to supply an additional amount of Richness to sustain Ignition for when the Throttles are Opened (Off-Idle) ... and, the Un-fueled fresh air rushes into the Combustion Chamber for that briefest of moments until the MAIN JET Supply Fuels the Engine.

Without the additional Richness the Engine will hesitate awaiting the Main Jet Supply.

If the Pilot Mixture is too LEAN ... you have Hesitation or Backfire.
If the Pilot Mixture is too RICH ... the Mixture cannot ignite and the Engine boggs-out and begins to miss.

Your mission is to adjust the Pilot Mixture Screws to a point whare you have reached a happy medium and:
Ignition is sustained
The Engine accelerates seamlessly without hesitation or bogging-out.

The adjustment to the Pilot Mixture Screw to obtain this balance is Fine-tuning.
Fine; as in: a (my-nute) minuscule amount.

This means you have to turn the Pilot Mixture Screw only a few degrees to achieve the desired results.
Not a half a turn.
Not a quarter of a turn.
Less than an eight of a turn.
A few degrees!

As you get the Throttles to open without hesitation ... you Monitor your progress by doing a succession of Plug Reads.

Adjusting each Carb --> Individually <-- to obtain the desired coloration of the Spark Plug's Center Electrode's Ceramic surround to become colorized by the Mixture you are dialing-in.

Clean is Bad.
Too Lean.

Light Brown / Tan means you have tuned for Speed and Power.

Brown / Dk Brown means you have tuned for Cruising and Comfort.

 

wow thanks for all the info. Now this may be a dumb question, but after all this tuning is done, and the weather changes to fall....will it need to be completely retuned again??? Sheesh, I can't remember ever owning something that required so much TLC LOL.

 

Only real extremes of weather would alter performance.

 

Depending on where you live and the climate.

Throw some StaBil in the tank and run the bike for about 30 minutes to ensure you got the stablized gas in the carbs. Shut to bike off and let it cool down enough to remove the plugs. Place about 3-5 cc's of light oil each cylinder, then replace the plugs. That will keep the rings and cylinder walls from rusting. Gas can start breaking down as soon as 3 weeks. The summer blend especially. Winter blends have more additives that prevent moisture and last a little longer.

Or you can start the bike once every 3 - 4 weeks and let it run for about 30 minutes at a time, then shut it down until the next time.

Just remember to change out all the fluids come next spring and you will be in good shape.

 

Thanks again. I just took the bike for a 20 mile ride to check throttle response and acceleration. In the lower gears 1 and 2, she tends to feel lack of power. And in the higher gears, pulls a bit harder. BUT, say in 5th gear @ 50 mile per hour and 4000 RPM, if I WOT her, she just bogs......so I am thinking rich all in low and top end and just right in the mid range?

 

Don't guess.
Just pull the Plugs and LOOK!

If you are Rich; the Plugs will tell the story.
Then, you just make your adjustments accordingly.

 

Well I pulled all 4 plugs and had a look. They all look about the same. The ground strap is a dark grey/brown, the electrode is grey, the lip of the plug is black/tan, and the instulator is white at the upper 1/8", then black/tan as it goes up into the hollow cavity area of the plug.

So....now Im a bit unsure of what to do.

 

That Main Jet wouldn't start supplying the fuel around 2500-3000 rpms, would it? My Seca bogs heavily at that point, especially with the extra weight of a passenger.

 

If I understand this correctly, jets work according to throttle position, and not as much to RPM. The more you open the butterflies, the more the slides will lift, allowing more and more fuel from main jets (via the needle and the emulsion tube).

You can be turning 7K on the tach, with just 1/4 throttle (low gear, low load).. Slides will be low, allowing just a bit of fuel from the mains.

Conversely, you can be turning only 3K at WOT (high gear, accelerating from low speed), at which point your slides will be almost all the way up, and the mains will be pretty much unrestricted.


Rick, please let me know if this is BS! I'm still trying to learn this stuff! :-D

 

Sounds reasonable. I'm able to make it through that rpm range if I have the throttle only slightly open. Open it up much at all and it just sits there bogging. Perhaps that means my mixture's a bit too rich? Or would my needles be too long/short (adjustable dynojet needles)?

 

Actually, on these style carbs the jets work according to the vacuum signal (amount of vacuum) developed by each cylinder, which IS somewhat directly proportional to engine rpm and load.

Think of things from the fuel's point-of-view.....you are waiting to be sucked through a jet, and the only way for that to happen is for a strong vacuum signal to be present AT THAT JET.........ignore, for a moment, that it's not really "vacuum", but rather the difference between atmospheric pressure (14.7 psi, which the fuel in the bowls are exposed to at all times, as well as the BOTTOM of vacuum piston Diaphram) versus the air pressure created via faster air flow through the carb throats (the faster air moves, the lower pressure that it exerts on an object....that is why airplanes fly!).

That's the purposes of the vacuum piston/diaphram unit, as those two small holes in the bottom of the piston "see" the air pressure in the carb throats and pass that (reduced) air pressure onto the TOP of the rubber diaphram (remember, the BOTTOM side is exposed to atmospheric pressure of a constant 14.7 psi); and any reduction in pressure on the top side of the rubber diaphram (via those small holes in the bottom of the piston, directly related to the carb throat vacuum, directly related to the airflow speed through the carbs, directly related to the throttle rpms) forces the piston to rise, allowing more fuel through the main jets as the needle rises within the emulsion tube.

The pilot circuit is "disabled" at higher engine speeds/airflow speeds via its unique position within the cab body, exiting directly in front of and above the throttle blades, which, as the throttle blades are moved, partially closes off the small airflow sensor port directly behind the throttle blade. In reality, the pilot circuit is ALWAYS "on", it's just that it's role and amount of fuel volume through it is negligible as compared to the amount of fuel flow through the main jet once the piston starts rising........

 

The jets primarily work at the lower RPM levels, regulating the amount of fuel going into the cylinders. At higher RPM's the smaller jet are bypassed and the carbs are dumping fuel at a constant rate based on the main jet size and the needle position.

If you are bogging in the Low/Mid range like this it's not a main jet issue. I would guess it to be one of the jets under the diaphram is laquered up or your float level is low and you are starving the bike for fuel.

You could have a hole in one of the diaphrams that would also result in the bike bogging from it not creating enough vacuum to completely open the air pistons sliders.

A bad diaphram is more common then you think and unless it is a good size hole, they can be hard to find. I had one smaller then a needle prick. I finally found it by doing a air leak test with soapy water.

I made a fixture and use about 5 psig of air pressure and found one bubbling near the top of the diaphram. Lucky I had lots of spare parts on hand.

If the carbs are on the bike, you need to remove them and start doing some detective work and trouble shooting. Oh and just to let everyone know up front. The Haynes manual is wrong when it comes to carb rebuilding and syncing. Just ask Chacal.

I found some actual factory carb settings that are not in any manual the only service technicians and factory mechanics know about. I found a factory sevice manual and have some specs from an Yamaha Engineer.

If you get over your head in a carb rebuild, i'd be happy to share the info.

 

I've torn the carbs apart and cleaned them up a couple times this year already (had to get some parts from Chacal in the process). I cleaned up everything pretty good, but for a couple things: the bowl drain plugs are locked (couldn't use a screw extractor on the awkward angle, so left it be for the time being), and while the diaphram looked fine, I didn't do any real testing on it. Probably a good place to start..

Also, with the dynojet kit, I understand you need to widen one or both of those holes on the bottom of the piston. Anybody know what size it's supposed to be? I bought it as is, so I don't have the supplied drill bit, but I imagine I should be able to find one if necessary. Perhaps the holes weren't expanded, causing some problems.

Being my daily driver, I'm glad she runs ok right now. But I'd obviously like to get her as optimal as possible. And sorry for any hijacking of this thread..

 

Thanks, Len. That clears up a lot.

What I'm not certain about, though.. High RPM at low throttle opening (e.g. in neutral)... how far would the slides come up? It seems that with no (or low) load, you wouldn't need a huge amount of fuel to keep going...