setting/tuning air mixture screw

[unicorncomputer]

I have a 79 XS1100 Special.

I have cleaned the carbs and put the mixture screws back the way they were.

all were 1 1/8 out.

the bike is running really rough though, to the point I don't think I can ride it.

I understand the normal starting point is 1.5-2.0 turns out so I am going to change it to there.

how do I tune or dial in the mixture screw? I have vacuum gauges, but do not have colortune or any other devices.

Thank you,

Steve

[IanDMacDonald]

Mixture will be best determined by a plug chop @ idle with a fan blowing on the engine. Clean the plugs, etc. Carbs will then have to be vacuum-synced post-mixture adjustment.

[unicorncomputer]

Sorry, but I don't know what you mean by a plug chop at idle.

Can you explain more.

The plugs are new this season and gapped at .30

[BA80]

Mixture will be best determined by a plug chop @ idle with a fan blowing on the engine. Clean the plugs, etc. Carbs will then have to be vacuum-synced post-mixture adjustment.

The idle mixture can be set with the sync gauges but it's very difficult to explain.

It requires patience and the carbs need to be resynced repeatedly.

Set all cylinders to the highest vacuum with the mixture screws, resync and repeat that procedure until the carbs are synced and you can no longer increase vacuum on any cylinder with the mixture screws.

This method will generally leave you a little lean so if you experience popping from the exhaust open each screw 1/4 turn one at a time until the popping stops or changes. If you open one as I said and it doesn't change the popping turn it back where it was and go to the next carb.

Like I said, hard to explain but it's easy to show someone how to do it.

[bikerphil]

See if it is firing on all 4 at idle, spritz the headers with water.

[SFerinTEXAS]

Have you confirmed no mixture screw tips broken off? Early carbs are known for this. My 78 had all 4 broken off when I got it.

[unicorncomputer]

Yes, when I got the bike in 2012 and had to do a rebuild of the carbs, we found 2 broke off, what a pain that was for my Brother-in-law to get out.

I have gotten everything set, just before the rain started tonight.

Hopefully tomorrow will be nice so I can get on the road with it.

Steve

Have you confirmed no mixture screw tips broken off? Early carbs are known for this. My 78 had all 4 broken off when I got it.

[alaskey2]

Can I check if my mixture screws are broken without pulling the carbs?

[mack]

Yes

Yes, back them out all the way. Once removed look at the tip's. If they aren't needle sharp with a long point and the same length, then the short ones are missing tips. Now whether the tips are still in the carbs is another story. Try inserting a long one in the suspect holes and see how far they will screw in. Be gentle. if they don't go all the way down, you still have the tips in there. Once you've removed the carbs(if that is in fact the case) you will feel and see the tip sticking through the hole in the carb throats.

[alaskey2]

Awesome, Thanks!!!

[mack]

gently

Remember I said gently. If your too aggressive you will have two broken tips in there.

[CaptonZap]

Remember I said gently. If your too aggressive you will have two broken tips in there.

It might be advisable to take the spring off before you screw it in, it is easier to feel it hit something that way.
CZ

[alaskey2]

Ok, now I'm really confused. They dont't seem to go in the same amount. The inner carbs are shorter. Also, my carb3 drain bolt has old gas on it and i pulled the air filter and it smelled lime gas too. Guess that is why my MPG sucks

[mack]

Ok

so did you manage to get the #2 & #3 screws out and look at the tips ? I know they are hard to get to while the set is on the bike. Do they still have sharp points or are they blunt ends.

[TopCatGr58]

Ok, now I'm really confused. They dont't seem to go in the same amount. The inner carbs are shorter. Also, my carb3 drain bolt has old gas on it and i pulled the air filter and it smelled lime gas too. Guess that is why my MPG sucks

Hey Alaskey,

First of all, as was stated, IF the tips are broken off in the carbs, then the pilot screws will NOT screw down the same distance, because it will/may be hitting the end of the broken piece from the other screw? OR...they had gotten broken off by the PO, he/she removed the broken pieces, fixed/filed down the broken screws back to a point, but they are now shorter and so will screw in farther before seating!?

Secondly, the fuel ON the drain screw isn't necessarily indicative of a leaking drain screw. You found fuel IN the airbox/airfilter. That's an indication of a leaking carb due to a float needle valve/seat combo not sealing properly and likely flooding while riding, or worse....leaking while parked...which can be bad because the fuel can then leak forward thru the carb throat and intake manifold and into a cylinder....and if that cylinder's intake valve is even just partially open, will allow fuel to get into the cylinder where it can then easily leak down past the piston rings and into the crankcase where it then contaminates the oil, thins it out too much and can cause engine destroying bearing damage to the mains and con bearings, as well as cams!

SO...you will want to check your oil to see IF you can smell/detect fuel in it, but it's difficult when it's not much, but it only takes a little bit of fuel to contaminate the oil to the damaging level/viscosity. We used to recommend putting stick in the oil and then try to light it...but learned that it wasn't a valid test....that one could still have a damaging level of fuel contamination but would not be enough to allow it to light on the stick! SO....if fuel in the airfilter, it's best to just bite the bullet and change the oil/filter, but check and fix the leaking carb problem FIRST or you'll end up having contaminated oil again!

T.C.