Idle is rich - pilot screw not responsive

[bochnak]

76' XS360
compression is good
timing is good
carbs are clean
pilot 1.5 turns out
jetting is stock
bike is stock
carb sych is good

Guys,

I have a rich idle and can turn the pilot screws all the way in and the motor will still run, and barely changes note from 1.5 out to zero. If I go 1.5 to 2.0 it starts to sputter and die.

The screw only has a spring on it, no washer and o-ring like I'm used to seeing. Maybe the seat is damaged in carb body?

Where is it pulling fuel from? Choke plunger pads are new, so that is not the source.

Thanks,
Matt

 

[bcware]

What is the float height set to?

My '78 xs400 does not have any washers or o-rings under the mixture screws. I do not believe these came into use until the 80's.

You say the jets are stock, but what exactly is the pilot size?

 

[bochnak]

What is the float height set to?

I set it up in December, about 2mm below mating surface.

My '78 xs400 does not have any washers or o-rings under the mixture screws. I do not believe these came into use until the 80's.

Your right, I also checked the fiche finder, no o-ring or washer.

You say the jets are stock, but what exactly is the pilot size?

I forgot the size, it was the size listed in the manual. Regardless of size, turning them in should kill the motor. The fuel is coming from somewhere.

 

[HoughMade]

I've never seen an XS400 run right with the screws less than 2.5 turns out. It will run at less, but it will pop and be very lean. I would set it to three turns out and then see if you can get it to idle adjusting the idle air screw.

 

[bochnak]

I've never seen an XS400 run right with the screws less than 2.5 turns out. It will run at less, but it will pop and be very lean. I would set it to three turns out and then see if you can get it to idle adjusting the idle air screw.

Yes, from what I have read that is the case.

This bike does not like choke to start. I give it a bit of throttle with no choke and it starts right up, even at 30°F. Plugs come out black when cruised around. It runs crappy at 2 turns out. It has never popped, or bogged indicating a lean mix.

 

[drewpy]

wonder if your pilot jet has worked its way out?

 

[bcware]

Any mods to the intake? Perhaps a lack of air is the issue instead of an abundance of fuel?

 

[bochnak]

Alright, I had a couple of issues:

1. The 30+yr old air cleaners was breaking up causing small junk to be sucked into carbs. I cleaned the carbs as a result.

2. The butterflies were waaay open exposing 100% of the pilot passages. The idle mix screws were lean at 1.5 turns, causing me to increase the idle to add fuel. That is also why they were not responsive. I turned the idle mix screws out 3 turns, and lowered the idle.

Runs great now!

BTW, I do have vacuum leaks around throttle shaft, even though I replaced all the seals
Anyone ever use o-rings to seal the shafts?

 

[bcware]

Do the leaks affect performance? You said the bike runs great now.

Those small leaks will probably only affect idle. Once you add throttle so much air is coming it that a tiny leak doesn't matter.

 

[bochnak]

Do the leaks affect performance? You said the bike runs great now.

Those small leaks will probably only affect idle. Once you add throttle so much air is coming it that a tiny leak doesn't matter.

It's as good as it's gonna get given what I have to work with.

The leak affects idle.

 

[bcware]

You bought new butterfly shaft seals and they leak? Not many people replace them, so there isn't a whole lot of information on troubleshooting in that regard. The few people that have mentioned changing the seals with new ones from MikesXS did not mention any problems, but maybe someone has a suggestion.

 

[bochnak]

You bought new butterfly shaft seals and they leak? Not many people replace them, so there isn't a whole lot of information on troubleshooting in that regard. The few people that have mentioned changing the seals with new ones from MikesXS did not mention any problems, but maybe someone has a suggestion.

The original seals were brittle and I had to dip the carbs. I bought mine from MikesXS.

Bike has about 5k miles, and the shafts felt pretty tight in the bore. No evidence of wear on these parts.

I can kill the engine by spraying carb clean on throttle shafts.

 

[bcware]

Have you tried using butane? Don't spray carb cleaner on those things; it can melt the seal.

 

[bochnak]

Have you tried using butane? Don't spray carb cleaner on those things; it can melt the seal.

Butane, propane, water, carb clean, my finger....end result is the same...it leaks.

I've heard that carb clean can melt rubber, although in the 15 years or so I have been playing with MC's & cars, I have never personally seen it.

 

[bcware]

It takes a while for melting to occur, but it does happen. If you spray a little it will generally vaporize before melting occurs. I soaked a few pieces in pure carb cleaner (in a bucket) for several hours and it melted the plastic cap on the choke lever pretty badly.

 

[bochnak]

It takes a while for melting to occur, but it does happen. If you spray a little it will generally vaporize before melting occurs. I soaked a few pieces in pure carb cleaner (in a bucket) for several hours and it melted the plastic cap on the choke lever pretty badly.

By all means I don't ever let solvents stay wet on rubber or paint, so maybe that is why I don't see it happen.

With the engine running, the seal will dry up quickly after a shot of carb clean.