Pilot Mixture Screw Adjustment, 101

1980 Xj650 Maxim
I need some clarification from the experts, please. At idle, it seems that turning the pilot screws in (CW) richens the mixture. I can confirm this by seeing black smoke when revving the engine. Back them out (CCW), the smoke goes away, and idle speeds up somewhat. And the engine stumbles a bit when revving. In my experience, this seems to be reverse order. Please confirm CW is richer mixture. Also, 1 of the screws (3# cyl) has little/no effect. I can close it all the way with no apparent change. That sounds internal to me but anyone dealt with that symptom before? #3 plug reads pretty good, light soot ring, and brown/grey insulator. Bike runs good. Any thoughts/wisdom welcomed.

 

On my xj700 in is lean and out is rich. I'm told that if the pilot screws are on the engine side of the carb in is lean and out is rich. If on the breather side of the carb it is just the opposite. Not sure I believe it, but someone here will know for sure. Getting it too lean might still result in smoke as the unburnt gas ignites on another stroke resulting in a rich situation for that ignition. Wish you had a colortune plug which makes the process a lot easier.

 

In (CW) is leaner, out (CCW) is richer. Smoke is an indication of incomplete combustion, and will occur under both rich and lean conditions.

Thoughts? Based on your description of #3; take your carbs to church.

IN THE CHURCH OF CLEAN

Everyone says their machne runs good, until they find out how well it's supposed to run.


EDIT: Please keep one thread for your bike. I went back to look at your other posts, and see that you also were complaining of a flat spot in the midrange. Adding that to the information you posted here makes me say that you absoultely must break the carb rack and do a thorough rebuild.

The fact that the screw makes no difference says that the passage is clogged. Plug color won't tell you much since you've had it running above idle before pulling the plug. When that happens, the color is going to be coming from the main fuel circuit and not the pilot.

 

Clockwise or in leans the mixture and counter clockwise or out richens the mixture.

K-moe beat me to it

 

So did tabaka...

Gary H.

 

^^^+3 clean the carbs...and your carb synch is off also

 

there's kind of a shortcut to clean out that idle circuit.
take out the drain screw, catch the gas and look for something thats not gas.
leave a rag there to catch cleaner
take the top off the carb, find the idle circuit air jet (larger hole).
take the idle mixture screw out, make sure the o-ring is out.
put the red pipe on your carb cleaner and shoot it down the air jet.
start with it in as far as it goes and spray as you pull it out.(that sounds bad)
you should get cleaner out the screw hole and back out of the jet and in your eye.
check that rag for any junk.
it's worked for me, maybe it'll fix you up

 

This ^. Good stuff sir!

 

Olala! Going to try this for sure - with safety glasses! The carbs went to Sunday school but not Church when I got the bike. It runs way to good right now to take it off line during riding season. Steady cruise speeds 55-65 MPH 35 - 38 MPG for a whole tank of gas, no surging, smooth idle, just a slow response during the @5K - 7K RPM roll on in 5th. Temporary solution - downshift to 4th! BTW, put the sync gauges on last night - 1100 RPM, seeing steady 7 - 71/2" of vacuum all 4.
Also, my apologies to all for forum thread protocol violations. I am new to this type of information sharing format.

 

That is normal, unless you are talking about the engine really bogging at that point. 5th gear has a 0.812:1 ratio (at the gearbox). I thought you were having troubles even accelerating in that rev range no matter what gear the bike was in.

 

5 to 7K isn't in the range of the idle circuit. your temporary solution might be the permanent solution

 

That is a TALL gear as I call it - and really hadn't looked at the transmission gear ratios. For sure the engine doesn't really seem happy in 5th unless above 60MPH. Guess this may fall under a very old rule of thumb - there's no replacement for displacement. Course that implies engine torque rules.

 

Indeed it might. But by nature, I feel we riders always want more.
BTW Maestro - how do you propose to remove that pilot screw o-ring with the carbs installed? Maybe some hints on a homemade tool? Tight spot being 3# .

Many thanks.

 

I used one of these:
Granted, my carbs were out, and it was still a PITA. I imagine it could be done with them installed, just make sure there are no nuns or children nearby. Those o rings are TINY , so I would probably try to put something brightly colored under the carbs, like a white sheet, to catch it should one fall.

 

Sewing needle. Heat the tip, bend a tiny hook into it (a 90º bend is plenty). Perfect tool either on or off the bike (thanks RickComatic)