74 BS38 fuel mixture screw

[cbuckle]

Probably a premature thread but just trying to find out if my fuel mixture screw should be soo closed (TX74’ BS38 carbs). It is maybe ¼ turn open on both Carbs, but really wants to be shut fully off when performing the “dead cylinder” method to obtain the best idle. I start off at 0.75 turns open as per the carb the adjustment procedure and the idle only goes higher when closing the mixture screw. Everything is stock on the ’74, minus running air pods. Jets have been cleaned. Floats confirmed at 24mm, but now at 25.5mm as an experiment. Needle clip in stock slot 4. Pilot jet 45, assume main jet is stock (no identifiers), Enriching system clean and closed/off, gaskets looks good.

My only guess is air leak somewhere? Or where can the additional fuel be coming from?
Can someone please confirm is the fuel mixture screws should be almost closed.

Also, my carbs lack vacuum barbs but see you can tap them in above the throttle body, or in the manifold.. any advise whether I should be drilling out my carbs or manifold to install some barbs to ensure sync or am I just over thinking.

 

[fredintoon]

Hi cbuckle,
pull the power wire off the dead side's coil to kill the spark and that cylinder can't run, eh?
And yes, I'd install vacuum spigots just so I could hook up a manifold pressure differential gauge.
No XS650's garage is complete if it lacks a 5-foot tall stick with a U-loop of transparent hose 1/4-full of ATF taped to it.

 

[grizld1]

Getting best idle with mix screws fully closed can be a symptom of high fuel level, caused either by low float setting, leaking floats that have taken on gas, or defective float valves. Click the "Tech" button for a link to the Carb Guide and check your oil for fuel contamination (you shouldn't smell gas when you sniff the dipstick).

 

[TwoManyXS1Bs]

.... I start off at 0.75 turns open as per the carb the adjustment procedure and the idle only goes higher when closing the mixture screw...

That's when you back-off the idle stop screw, lower the rpms, then readjust the mix screw. If you can't get the mix screw to raise the rpms after that, then carb problems...

 

[cbuckle]

Did the sniff test in the oil and it smells like normal 10W40.. no fuel. Dry floats (no fuel inside them) It is funny Griz you mention defective float valve. If I leave the bike in the garage overnight with the fuel on the entire garage stinks like fuel (no puddle on the floor or stains). If I turn the bike off, then turn the fuel off it doesn’t stink the next day. I was thinking I had a defective float valve but didn’t think this would be why I could not get a good idle with the fuel mixture.
2M – I do back off the idle screw, then readjust the mix screw… but the RPMS only rise if I further close the mixture screw again. I’m sure I am following the procedure correctly
Think the issue is just from a defective float valve?

 

[5twins]

If your jets are old originals, it may be time for some new replacements. Jet's don't really wear out but they can be eaten away internally by gas varnish if the carbs were ever all gummed up and sat that way for a while. That would make the jets bigger and richer than stock. The fact that you can't read the numbers on the mains may indicate this.

Also make sure your pilots are the correct type (BS30/96) and the float bowls are the style that use them .....

 

[cbuckle]

Parts ordered! hope it's less than a week. Thanks everyone. I will update the thread once shes back together

 

[TwoManyXS1Bs]

If your jets are old originals, it may be time for some new replacements. Jet's don't really wear out but they can be eaten away internally by gas varnish if the carbs were ever all gummed up and sat that way for a while. That would make the jets bigger and richer than stock...

Just to reinforce what 5twins is saying, here's a pic comparing a new Mikuni idle jet to my original 46-year-old jet, same 42.5 size.

 

[Bob Kelly III]

Good info Guys ! Thanks !
Bob........

 

[cbuckle]

New gaskets, jets and float valve assembly. Before the pilot was #45 and now #42.5, main assumed #127.5 now #130. Think I found the source of the fuel smell with a bad drain plug washer in the bottom of the bowl. Air fuel mixture left at 0.75 turns (think I might just leave it here). As I was setting the carbs I found the right cylinder was not firing at all. After hours of Tshooting I determined it must have been the points. I was getting large arcing on the right contact point. Cleaned them, re-gaped Took a video, can someone tell me if this is excess arcing? Both Left and right are gapped at 0.015”, I used my dwell meter and measuring 22 degrees on the right, and 20 degrees on the left (on 4 cylinder setting). let me know if you see any issues... thanks guys
https://youtu.be/ghVCkg9BWkc

 

[retiredgentleman]

With BS38 carbs, you should always check the O-ring on the needle jet (emulsion tube). If its old, it should be replaced. When its worn, it can allow extra unmetered fuel to affect the mixture.

 

[5twins]

Hard to tell from this video but I watched your other videos too. The top points set (right cylinder) does seem to be arching too much. I'd change the condensers.

I watched your left cylinder timing video. I'm not sure you understand the method for setting that cylinder. You don't move or adjust the whole points backing plate, you only move/adjust the little half plate that the lower points set is mounted on. Yes, moving the whole backing plate will alter the timing on that lower points set, but it will throw the timing off on the upper set in the process.

The proper procedure is to set the upper points and timing (right cylinder) first by moving the whole backing plate. Then do the lower points set and timing (left cylinder) by only moving the half plate.

For points gap, you can use a feeler gauge for the initial "rough" setting but then I'd rely on the dwell meter, it's more accurate. This is especially true if you're dealing with used points. They develop pits and the spark jumps between those pits. You can't measure between the pits with a feeler gauge. The gap will actually be bigger than your feeler gauge is showing.

The dwell for these, as read on the 4 cylinder scale, is 22°-24.5°. Your right cylinder gap is perfect, your left is too big. That is partly responsible for the advanced timing reading you're getting on the left side.

 

[cbuckle]

Hey 5twin, the other videos were from other trouble shooting last year.. when I was trying to diagnosis my sticky advance assembly (which also may have returned) http://www.xs650.com/threads/cam-rod-and-timing-points.45390/#post-465638 .. timing is still between the fire markers on both cylinders

Last year I checked the condensers http://www.xs650.com/threads/condensors-capacitance-arcing-at-points.46176/ and they had a good readings. Could I cross connect the condensers (making the larger arcing go to the other cylinder/point)? Would you recommend this?
I will check to see if I can tighten the gap on the left point…

Regarding the emulsion tube, I replaced the o-ring on the one carb but unable to get the other one out…. So it has not been changed

Thanks again all… I really wish there were more 650 Guru’s closer to my town!

 

[5twins]

If by "cross connect", you mean swap connections, yes, and you probably should try that to see if the heavy arching changes to the other points set. Your condenser is actually a "pack", 2 separate condensers mounted together on one bracket. One could be going bad. Each black wire coming out of the "pack" comes from one of the condensers. Unplug and swap them between the points sets.

 

[retiredgentleman]

Hey 5twin, the other videos were from other trouble shooting last year.. when I was trying to diagnosis my sticky advance assembly (which also may have returned) http://www.xs650.com/threads/cam-rod-and-timing-points.45390/#post-465638 .. timing is still between the fire markers on both cylinders

Last year I checked the condensers http://www.xs650.com/threads/condensors-capacitance-arcing-at-points.46176/ and they had a good readings. Could I cross connect the condensers (making the larger arcing go to the other cylinder/point)? Would you recommend this?
I will check to see if I can tighten the gap on the left point…

Regarding the emulsion tube, I replaced the o-ring on the one carb but unable to get the other one out…. So it has not been changed

Thanks again all… I really wish there were more 650 Guru’s closer to my town!

You should really make an effort to replace the stuck O-ring. Stuck means its bad.

 

[cbuckle]

Update on my points arcing. Found the problem!! I guess when I pulled the tank off to better work on the carbs I knocked the condenser wire... with it practically disconnected. Cleaned the contacts on the wire and properly seated the connector and now no arcing at the points!!! So excited!!
Right cylinder is still a little advanced, and need to adjust the points slightly with dwell meter..
Thanks all (sorry to take the thread off topic)

 

[5twins]

What you have to know when using a dwell meter is that the reading it gives you is inversely related to the size of the points gap. In other words, a larger reading indicates a smaller gap, smaller reading means a larger gap. As points wear during use, the gap usually gradually gets smaller, so I try to set them at their largest in spec gap. They should remain in spec longer this way as they wear. So, using the dwell meter, you want them set closer to the 22° reading than the 24.5° reading.

A dwell meter gives you this "reverse" reading because it's not measuring how long the points are open, it's actually measuring how long they are closed. The larger the gap, the longer the points remain open and the less time they are closed, hence the smaller meter reading.