81 xs650 difficult tuning

[TDodge7]

Hey everyone , I've been lurking on here for a while but now I finally have a reason to post , the 81 xs650 that I've been getting back on the road for a friend of mine is running again but something isn't quite right . Since getting the bike I've done the following (plus a bunch of non engine related stuff that isn't worth mentioning here) :

new oil filter (and pan screen)
new oil
adjusted valves
adjusted cam chain tension
rebuilt carburetors
ultrasonically cleaned carburetors
adjusted float height to factory spec
synced the carbs with my morgan carbtune

currently running
Main jet 140 (132.5 is stock)
pilot jet 42.5 (original)
Air jet 135 (original)
Mixture screw kits from mikes xs (o rings were bad on the originals ) screws are shaped differently from stock but are the same length and appear to have the same taper as original .

mods
aftermarket exhaust (unknown brand , no brand names on it anywhere , seen in the picture below)

Mikes xs pod filters
Accel coil

Timing is correct
idle speed is around 1100
Checked everything for leaks with an unlit propane torch , no leaks found .

Here is where the problem comes in , I'm absolutely horrible at setting idle mixture , I didn't have an ear for it so much that I went out and bought a gunson color tune , the spark plug with the crystal window so you can see the color of your burn . Well it seems I also don't have the eyes for it either so I just got an older inductive dwell tach that reads the correct rpm . Based off that I used the dead cylinder method to set the mixture , watching for when I got the absolute highest rpm on the dwell/tach . The smoothest running and highest idle both came close to 1.5 turns out from lightly seated on the mixture screws . The problem is that While it sounds like it's idling pretty smoothly , there is almost no throttle response . Anything more than a smooth roll on and it tries to stall out for a second before revving up . Once it does rev it returns to idle without any hanging , and doesn't pop on decel . The plugs look lean to me , or at least very clean white , but I've always been horrible at judging them . Basically with the mods/jets I've listed is 1.5 turns out a reasonable place to be ? And what is up with the throttle ? I should be able to rev it a little faster than a roll on in neutral without it trying to choke out before it finally gets around to revving . Is it possible that I need richer pilot jets to compensate for the off idle ? I have a set of richer pilot jets I can put in if need be , also adjustable needles and mains but it preforms well everywhere but off idle.

here is a picture of my friend enjoying his first ride around the block.

 

[5twins]

1.5 turns out on the mix screws is way lean for the BS34s, they usually like about 3. Yes, with your mods, I would go up one size on the pilots. Your new mix screws may be the non-E.P.A. style. I've heard that's what's coming from Mike's lately .....

If they are then they may not need 3 turns out, but I would still start probably at 2 or 2 1/2 turns out. Tuning the BS34s by ear is difficult because they're so E.P.A. restricted. Mix screw setting changes don't usually seem to have much effect unless you go way past the ideal setting (like one or 1.5 turns) in either direction. The fact that it idled fastest at 1.5 turns could have just been a "too lean" indicator. Idle speed can increase when you're too lean. Try 2.5 turns out and see if it helps with that off idle flat spot. By the way, a flat spot is also an indication of being too lean.

 

[littlebill31]

Honestly I think 1.5 is too lean, no matter what all the gadgets say. Turn it out to 3 and see what it does. It's not gonna hurt it if you do, then you go from there.


5twins beat me to it. I read too slow.

 

[pa23driver]

was beaten to it

 

[littlebill31]

I run the non EPA, which are shorter than stock, and it's still 3 turns. But every bike is different. So far with mine with pods, cut exhaust (as of the other day testing) I have 145 mains, 45 pilots, 3 turns out and it's running sweet!!! I put the 147.5 mains in and it will take off like a shot at higher throttle, but runs a little off around town. So the 145 are doing just right.

 

[TDodge7]

It would appear that I have the non epa mixture screws . I just went out and set each carb to 2.5 turns out . Now it will crack wide open in neutral without hesitation but the idle it's self seems much slower and lopey , even when set back to 1100rpm is doesn't sound like it's running as strongly. And it seems to hang at around 1400 rpm for 2-3 seconds before settling to it's idle now which totally doesn't make sense since I made it richer .

 

[5twins]

It may still not be rich enough, try 3 turns out. I use the throttle blip and watch for that hanging idle on the 34s as a mix screw tuning aid (since it's hard to hear any changes). If the idle hangs high, I richen a screws some more (usually 1/4 turn). If you increase the pilot size, you will need to fine tune the mix screws again. A larger pilot sometimes requires slightly less turns out.

Even though it's hard to hear any changes, these mix screws do have an effect. My buddy brought his '80 650 over one time and the idle was hanging. I found one of his mix screws at 3 turns and the other at 3 1/2. With some tuning and throttle blips, we ended up with each at 3 1/4 turns, idle dropped quick like it should, problem solved. So the proper setting is important, even if you don't think it's doing anything different that your previous setting.

 

[littlebill31]

+1 on the mix vs. pilot. You can use the mix to adjust instead of switching pilots, but you don't want to go past 4 - 4 1/2 turns out otherwise the mix spring stops doing it's job and the screw will back out or become useless.

 

[TDodge7]

1.5 turns = highest rpm / non hanging idle / horrible off idle flat spot
2.0 turns = slightly lower rpm / slightly hanging idle / horrible off idle flat spot
2.5 turns = low lopey idle / hanging idle / no flat spot
3.0 turns = lower lopey idle / hanging idle / no flat spot
3.5 turns = even lower lopey idle / even worse hanging idle / no flat spot
4.0 turns = ultra low lopey idle / same as 3.5 hanging / no flat spot

The more I richen it the more the idle hangs and the less of a flat spot there is ...which makes no sense at all

 

[5twins]

First of all, do you have the rubber plugs in place over the pilot jets in the float bowls? The idle circuit won't work correctly without them and you wouldn't be able to tune it.

If that's right then I would try the next size up pilots (#45) and start over with mix screw tuning. The pilots flow their mix into the main bore through 4 tiny holes. Three of these are clustered together around where the butterfly closes and are unmetered. They flow the full strength mix from the pilot jet. To change the strength of the mix here, you need to change the pilot jet size. The 4th hole comes from the mix screw (which is fed by the pilot jet). You can alter the flow and strength here by adjusting the screw. Since you only have an adjustment over one out of the 4 holes, that gives you control over about 25% of the mix. That's why mix screw adjustments can only do so much. Pilot jet size changes are often needed instead to change the mix more than the screw alone will allow.

 

[TDodge7]

yes , I forgot to mention that , I bought new rubber plugs for it because the old ones were falling apart , so they're good . I have some 45's ready to go in so I'll give that a try .

 

[retiredgentleman]

It should be easy to adjust the mixture screw when doing the "dead cylinder" method. If you are trying to adjust the mixture screw with the rpm too high, the screw will have no effect. It has no effect because the butterflys are open enough that the 3 bypass holes are bypassing the mixture screw.

You need to lower the rpm down to around 300 or 400 rpm, to close off the 3 bypass holes. Go back and forth between the idle speed screw and the mixture screw to get the rpm down until the engine almost stalls out. Lower the rpm with the idle speed screw, and peak the rpm with the mixture screw.

 

[TDodge7]

well I put in the 45 pilot jets and set the mixture screws out to 3 turns , took it for a spin to warm it up , when I got back (and when I came to stops) I noticed that after a rev the rpm's would drop below idle and then come back up and the idle just altogether felt weak , so now at least I know 3 turns out is too rich with the 45's . 2.5 seemed about the same , 2.0 the rpm was starting to climb and then it started raining like crazy so I called it a night . I should still be going for the highest rpm correct ?

 

[littlebill31]

Make sure the are synced as well.

 

[TDodge7]

I'm really starting to get fed up with this bike ....or all bikes in general when it comes to setting mixture . I've seen mechanics walk up to a bike before and then walk away 20 seconds later with perfect mixture . I've been working on bikes for almost 20 years and it still manages to baffle me . I own every tool they make for setting mixture except a 4 gas analyzer and none of them help me at all which is infuriating . I can do every single thing there is to do on a bike except mixture . I put in the 45 pilot jets but everything seems backwards . When I have it between 0.5 and 1.5 turns out it idles at the highest rpm and the idle doesn't hang when I rev it (most of the time , the bike seems to like randomness to help make me mad) . When I adjust it to 2.0-3.5 turns out the idle sounds like crap and it hangs when you rev it , which makes absolutely no sense at all since richening it is supposed to make it stop hanging at idle not cause it . Luckily if and when I figure this out I have to move on to my cb700 where you can't even reach any of the mixture screws without pulling the starter so there isn't going to be any doing it by ear on that one , it'll have to be half turn , reassemble , listen , disassemble , half turn , reassemble . I missed all of last summers riding trying to dial in my old xs1100 , this is getting ridiculous .

 

[littlebill31]

Have you checked for any air leaks? The carb holders will do this if cracked. Get the bike running and spray carb cleaner around the boots, vacuum barb plugs or any other place you think would be leaking. The rpms will rise if air is coming in.

 

[5twins]

You're right, those carbs aren't acting the way they should. BS34s with the mix screws at 1/2 or 1 turn out should barely idle and pop like crazy every time you blip the throttle. There may be something else wrong with them. You said you set the float levels to spec. What spec (in MM please)? These 34s can be very picky about that setting. And how about the o-rings on the float needle seats, are they good? Is the air passageway and the air jet that feeds the pilot circuit clear?

I should also ask about the floats themselves - plastic or brass? Originals or MikesXS replacements? The replacements from Mike's are of pretty low quality and can cause issues. They're a very loose, sloppy fit on the pivot pin and as a result, they sometimes don't meter the fuel flow very accurately.

 

[littlebill31]

Just noticed you did check for leaks. Those are not acting correct. I can always set mine to stock (45 pilots/145 mains for pods , free exhaust) and they usually need minimal tuning.

 

[TDodge7]

1/2 is poppy , by 1 turn out the engine is idling just about as fast as it will , 1.5 is the smoothest highest idle I can find , then it starts going down from there . Here is a short video (I can take more if anyone wants to hear something specific)

I did check for leaks with an unlit propane torch and none were found . The floats are the original plastic type , I forget now what I set it to but it's whatever the manual was calling for , maybe 21mm ? it's been a few weeks since I did that so it's not fresh on the brain . All the o rings in the carbs looked good except of course for the mixture screw o rings which I've replaced .

When the old jets were in I was able to get the plugs way too lean or way too rich , but not in the middle , this bike seems to take a while to color the plugs so I don't have a read on them with this setup yet . When you make it rich it gives you lean symptoms , when you make it lean it gives you rich symptoms . I ultra sonically cleaned these carbs , you could eat out of them.

One of the problems I have is that most of the bikes I get have never run right the entire time I've owned them as I usually buy them not running , so being able to pick out how it's supposed to run by ear is hard for me.

 

[5twins]

That really doesn't sound too bad. These bikes do have a rather lumpy idle, I don't know why. I'll bet those non E.P.A. mix screws are what is allowing you to run at such a low setting compared to most 34s. If you want to fine tune the idle circuit more precisely, you can start playing with air jet sizes. You can achieve a richness (or leaness) level somewhere between 2 pilot jet sizes by playing with those.

I like those pipes. They could be Bubs.