Quick pilot screw question.


XS400 Enthusiast
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I'm coming from a long line of thumpers and this is my first twin. It's a 360. Bone stock except the ,idlers have been repacked and don't appear stock.

So, reading good old post #34 on the great idle thread, it says to adjust the screws until highest idle is achieved, the back them out half a turn. Are we chasing that elusive high point in the idle by adjust the carbs separately and dialling in or are we adjusting the pilot screws the same increments equally on both cylinders?

I seem to get a bit more idle from one cylinder with half a turn more.

Tips on setting idle screw correctly? - post #34
Each carb is adjusted on it's own to achieve the best idle. Then slowly lean (turn in) the mixture screws until the idle speed drops, then a 1/2 turn out from there. That should make the idle mixture slightly richer than needed, but not enough to cause problems. It should also allow the bike to idle on a hot day in traffic without getting too hot, and to idle without the choke when the engine is just starting to warm up.

Syncing the carbs afterwards is important as well.
I figured it should be by cylinder. I got it idling great but since it's stock, I'm puzzling over a full throttle 6k and up hesitation or misfire. That's next on my list. It almost feels like a flameout where it's just not igniting the fuel.

Thanks for the quick reply Dave.
The high RPM issues could be the ignition crapping out, or the carbs. Specifically the float levels, needles, or main jets. Have you torn down the carbs? And removed the emulsion tubes? They often get plugged with corrosion or fuel deposits if the bike wasn't stored properly.
With the xs360 carbs make sure to replace the emulsion tube o-rings. Make sure to sync the carbs with a manometer. The bike should be fully warmed up when you do this.
Well I had them totally apart a few days ago in the ultrasonic cleaner. I did not replace the emulsion tube o-rings however. If I take them apart and replace the o-rings and it still feels like traction control at 8000rpm, I'll go electrical.
From what I can see of your bike it looks like you have an aftermarket exhaust in there. Post some better pics so we can see.
Yes, some sort of aftermarket repackage popes. That I packed last week.

So, I live at sea level. Bike is stock save the pipes. Pilot screw 2 turns out. H pipe has no leaks. New air filters. New clamps all around. New BP6ES plugs. New o-rings on the emulsion tunes. Carbs are clean, clean, clean. Diaphragms are perfect. Floats are dead on. Rebuilt and full functional peacock set to prime for testing. Isle os 1200 rpm hot. Stock jets are 135. I've tried 137.5 and 140 and it still flames out, albeit in a different manner at 7-9000 rpm. This bike has a cam sensor, no points.

At this point I'm thinking it's the ignition wires or the coil(s). It needs a bit of throttle to start it. On the pilot circuit around town, taking it easy, it feels hesitant compared to last week.

I have a 400 on the bench I can pinch the coils from and I'm going to buy some ignition wire today.
"Packed" mufflers? Most aftermarket mufflers with real baffles need no packing. If your have a plate at the end with a small hole in it and you can't look straight through the muffler it needs no packing. My guess is your choking out the motor. What do the plugs look like? My guess is the bike is running too rich. I run xs360 carbs on three of my xs400s' and use stock 17.5 pilot jets, third clip on the slide needle, 135 main jet and 1-1 1/2 turns out on the idle mix screws. One of them has K&N air filters and a 2-1 free flow through exhaust. One stock and one with stock exhaust and K&N air filters. All have H-pipes on the carbs.
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The pipes can have the baffle taken out. The baffles have FMF insulation wrapped around the core. Then the core is slid back in the pipe. It's impossible to choke the motor, the baffles are straight through.

The plugs are nice and grey/brown.
Hold up. I retract that plug statement.

The right plug is soot black. It wasn't a few days ago. Left plug is perfect. Both carbs are 1.5 turns out and 135 jets.

My multimeter is DOA, typical. So I may be chasing two tails here. When I had the carbs apart today I may have altered the float height on the right. I was damn careful though. I'll check my floats.

I don't usually chase stuff like this. I feel like a rookie.
Float heights are dead on. Checked the carb over three times.

I think the right side is misfiring and causing the plug soot. Of course the 400 coils just don't plug in. That's enough motorcycle for one day. I'm a bit burned out. I need a few days away from wrenches.

Time to take the Ducati out. Lovely fuel injection.
Sync carbs with manometer. Plugs porcelain should be white or a bit tan. Remember with a free flow exhaust and stock air boxes your letting more air out but the same air coming in.
Escaped wax from a coil on the right side upon inspection this morning.


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If the key is left on too long they can overheat. Test them to make sure they are in spec. You said you have changed the ignition to an electronic one?
Well. I've just about tried everything. It absolutely is not ignition. I've totally ruled that out. I even got down to swapping out the resistors inside the plug caps and swapping coils etc. I put new points in it. Different condenser. Everything. It's not electrical. Replaced my 22 year old multimeter with a new one and tested the piss put of the bike.

It's a fuel issue. Took the carbs off yet again. Went over it with a fine tooth comb, the micrometer. I swapped the float needles, the floats are 100% good. Took the choke circuits apart again. 2 cans of carb cleaner. Put it all back together. Still doing it, but it's "better"? by the butt dyno.

Still fouling the right plug.

Took the tank off. Stared at it for an hour thinking. Pulled the top off the carbs and took a closer look at the jet needles. They show wear. Wear I dismissed before. It's a 40 year old bike with 13,500 miles. Some wear is expected.

Seems that the jet needles are no longer made. NOS supply has been used up. It's the one part on the bike that is unobtanium. Sitting here at 1:35am with a micrometer trying to cross reference with SOMETHING.

I'm swapping them side for side in the morning to see if I can buy some time for this gorgeous bike. I have a spare set of xs400 carbs. Long term, I may have to use xs400 carbs. I'm trying to keep it stock, concours even. But I may have to cave on the carbs. Or spend some money one day to have some manufactured and sell the rest of the batch. I'm not even sure that's possible.

2:32am edit. Lol. Seems after a lot of digging, this following 4M1 needle has extremely similar dimensions to 4FP21-3. It may be a dead ringer or close enough. Slide (Jet) Needle - 4M1 - Fits: XS650C/D (1976-77) Stock Mikuni BS38 CV - Carbs
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Compression check on motor? You have not said that a carb sync with a manometer has been done yet. It is VERY important that you do this to balance them to get the bike to tune well. With an aftermarket exhaust I would not worry about a concourse bike. That is long gone at this point.