XS1100 ignition misfire, jumping tach and possible vacuum advance issues

[somesuch]

I am reviving my old thread because I finally found and corrected my problem with erratic vacuum. The short version is I had bent valves. Therefore exhaust pressure was getting into the intake. The root of the problem was the cheap eBay valve adjusting tool I used. It did not fit correctly and in the process of adjusting valves, it scored the lifter bores. This caused the lifters to bind, the valves to hang open and piston contact bent the valves. The cheapest solution was to buy an eBay head and refresh it. I now have normal idle, vacuum and vacuum advance. I rode the bike for years before I finally figured this out, although it still has less than 15K miles on it. I synced the carbs and the smoothness off idle and throttle response are amazing. I hope this helps someone else out who is also chasing a ghost.

Thank you for posting this. I am sure many feel the same.

[Bax]

Hi GM, After you corrected the valve issues did you redo the Compression/CLT and notice a difference? I suspect the CLT should have signaled leaking valves but may have confounding results if all the valves were bent and you were just looking for a difference on #2.

---Bax

[Bonz]

Excellent work on finding the issue, as has been said, must have been driving you nuts!!! I’m surprised the tool scored the bores that badly. Was it something you saw but didn’t think twice about when it happened?

[2WheelFun]

Went through the entire thread, what a fox hunt! How in the world did you notice the damage from the cheapo tool?

You must have been pulling your hair out chasing this one down.

[shutchis]

That's awesome, congrats on getting it tuned up.

[3Phase]

Right on!

It's hard to divine the root cause over the 'net so thanks for following up and letting everyone know what the real problem was!
(tosses mighty Carnak hat back on the hatstand)

[GMGalambos]

I am reviving my old thread because I finally found and corrected my problem with erratic vacuum. The short version is I had bent valves. Therefore exhaust pressure was getting into the intake. The root of the problem was the cheap eBay valve adjusting tool I used. It did not fit correctly and in the process of adjusting valves, it scored the lifter bores. This caused the lifters to bind, the valves to hang open and piston contact bent the valves. The cheapest solution was to buy an eBay head and refresh it. I now have normal idle, vacuum and vacuum advance. I rode the bike for years before I finally figured this out, although it still has less than 15K miles on it. I synced the carbs and the smoothness off idle and throttle response are amazing. I hope this helps someone else out who is also chasing a ghost.

[cywelchjr]

Step one as 3Phase says, do the checks for vacuum leaks. The dying when pulling up to a stop is one of the symptoms of a rich mixture, and both a rich and lean mixture can cause popping on decel, just a different kind, rich is kind of a deep throated pop and lean is a rather high sharp popping, almost a crackling type, and will tend (without the vacuum advance issue) to hang high on the idle, but as you can see the vacuum advance can mask a rich condition if the throttle plates are open too far by causing the same symptom. It's also possible that your low speed jets are one step rich, which you are adjusting out with the screws, but but check for leaks with carb spray or wd-40 or such first, spraying on and around the carb boots while it's running, pay attention also to around the where the throttle shafts go into the carbs, that's a potential leak source, and a leak there can cause surging. Once you find the leak if there is one, it's a matter of determining the fix.

[3Phase]

GM, have you looked for any vacuum/air leaks around the rubber boots on the engine or the air box? Checked for holes in the carburetor slide diaphragms?

Have you read Ken Talbot's tip about synchronizing the carburetors and setting the mixture screws? Try it and see if the surging goes away.

ADJUSTING PILOT SCREWS BY EAR vs. COLORTUNE


The vacuum advance will be a little jumpy because it's only connected to the #2 carburetor so it pulses a little when it's working.

You could try to smooth out the advance by drilling the other three carburetor bodies for a ported vacuum source like the #2 carburetor, press in three more vacuum nipples, then connect all four carburetors together with tees and vacuum line to make a common plenum for the vacuum advance. It's not worth it but it would eliminate the pulsing of the vacuum advance.

[GMGalambos]

update

Last night I set out to adjust idle speed and mixture. I measured engine RPMs with a Fluke 88 and inductive pickup so my readings are digital. The first thing I did was back the idle speed adjuster off so the throttle plates would be closed. My idle dropped from 1000 to 800 RPMs. I then started turning mixture screws and adjusted for highest RPM. This is where it gets weird. The engine ran best with all mixture screws about 1/2 turn from closed. In my opinion this should be too lean to run, but it made the RPMs come back to 1000 at idle. I no longer have port vacuum at idle. The vacuum actuator responds to opened throttle, but is still very jerky even with the restrictor in line. I rode it around the block and it ran better than before from a stop. Engine did not try to stall at stop signs either. I did notice some surging at a constant cruise RPM arund 3-4K. Could I have a carb issue now that I'm pretty sure my ignition system is ok?

I also checked the vacuum advance for free play. I feel it's pretty loose but I will take a video of it and you guys can tell me if it's "free enough".

Thanks again for all the help guys!

[3Phase]

GMGalambos, James,

If the vacuum advance is pulsing at idle that means the carburetors are not synchronized correctly or the idle is too high. Either way, the #2 throttle plate is too far open and it's starting to expose the vacuum advance port to manifold vacuum.


The idle in the vacuum advance video isn't high, it's right where it's supposed to be. I had a helper work the throttle while I pointed the camera at the two separate vacuum gauges to show that when the carburetors are synchronized and the idle setting is correct:

Intake manifold vacuum is high at idle and decreases as the throttle is opened

Ported vacuum for the vacuum advance is not present at idle, then it starts out low and increases as the throttle is opened

The vacuum advance does not move at idle.


I also made a short ride video showing the manifold and vacuum advance vacuum with those same two gauges. The ported ignition advance vacuum does not drop to zero when riding except at very, very, low speed if you crack the throttle wide open and lug the engine; something you are not really supposed to do but even then it recovers nicely when the engine RPMs start to pick up.

The vacuum advance starts to move with only 2" to 4" of vacuum and is fully advanced with only ~6" of vacuum so unless the cylinder head came off of the engine or the carburetor slides magically disappear there is always vacuum present at the vacuum advance port while you're riding the bike except when the throttle is completely closed.

[crazy steve]

Checked the valve lash? A tight intake valve could be 'leaking' back into the carb and fouling up the vacuum signal....

[motoman]

Unless you REALLY like chasing your tail......better go back and make DARN sure that advance assembly if TOTALLY free......with the vacuum advance can REMOVED before touching those carbs. Advane sounds like it is STILL not working correctly......Other than that, guess I can only "lead a horse to water, can't meke 'em drink".

[natemoen]

+1 on what Cy is saying.

[cywelchjr]

I'm sure the carbs are synched but not sure about the idle mixture. I researched that a bunch and most people were of the opinion that 1.5 turns out was good. Where would you start with the idle mixture screws and what is typical for newly rebuilt carbs?

Btw, thanks to everyone for the input and advice. It is all greatly appreciated.

That was the starting point 30 years ago for the gas back then. Again, that's the starting point, and you need to adjust either by ear or with a colortune to actually get the right mixture for YOUR engine and setup. If this has not been done, the carbs are not really synced as this is the first step of the sync process. It's one of the common misconceptions that 1.5 turns is THE setting, it's not and almost NEVER is it where you end up by the time it's right, frankly your more likely to get out something like 2.5 or 3, but you can't do it blind, you MUST do it properly or you would just make it worse. I'm guessing you get popping and such on deceleration as well, which can be another sign of lean idle mixture. Unfortunately, plug color is not usually much use for idle mixture, it's mainly for the mains.