Need Troubleshooting Help - Carbs, Timing?

[BPT]

Cranked up the bike for a ride last night. It started right up and sounded very nice - ba da dump, ba da dump, ba da dump.  It idled for a bit while I got situated, I left the driveway, got about 2 blocks away and noticed a change - felt and sounded rough.  I let off the throttle and the idle dropped pretty low and it died. I started it up (not as easily as the first time) and it was still noticeably rough. Decided to go back to the house and had very little power, kind of like trying to drive in too high of a gear.  I got back home, had to be somewhere so I left it to rest.

Got home from work this evening, fired it up and it started right away and idled great, with that same nice sound.  I revved it up and down, nice and smooth.  Let it idle at various rpm's, still good.  I figured I'd give it a spin around the block so went into the house to put shoes on.  Came out, started it up, still nice.  As I was sitting there, all of the sudden the sound changed and it went back to the rough running just like before.

So, the only thing I've figured out so far is that something is changing once it warms up.  At first I thought maybe one of the cylinders wasn't firing but they both are.  It feels like the right one is harder and louder than the left (or the left is quieter and softer?).  I'm pretty ignorant on this but decided to play around with the mixture screw to see if I could notice any changes and didn't sense much difference.  It did the same thing when I tried to ride where it wanted to bog down unless I gave it tons of throttle.

With decreasing sunlight, increasing mosquitoes, and visions of Shovelheads dancing in my head, I gave up for the evening.  As it is right now, I won't be able to ride it to the local shop to get their help.

Any thoughts, diagnosis?  More importantly, a particular spot to start tinkering?  Feel free to be basic, simple and detailed.

        Thanks in advance.

[Bob_Roller]

Kinda sounds like one of the carbs isn't getting fuel .

[BPT]

That thought crossed my mind. But I'm confused as to why it is normal at first, until it runs for a little bit. Is there something that changes that could have enough fuel initially but then not after it warms up?

I did check the choke to make sure it was opening/closing and they both looked to be operating correctly.

[misterpepper]

Do you have the ability to short the plugs? If so, get it to happen again and then short the good cylinder. I'm guessing the bad cylinder is not firing at all and the weak pulse you are feeling is from the exhaust crossover tubes. If it is firing, just very weak, it's probably carb related. If not I'd first make sure you are getting spark.

[BPT]

I'm familiar with the "lay the plug on the cylinder" method of checking spark and remembered something was different with these bikes and you can't do it that way.

But didn't know the proper method and I'm digging around on the internet.......

[mrclubike]

You need to make sure the spark always has a path to ground when you "short out" the cylinder
You should not just pull the plug wire off and let it hang
Removing the wire from the plug and putting another plug in it place is OK as long as it stays grounded to the head or block 

but what you need to to do is short the plug out while it is running
You have to rig up a way to expose the high tension lead and ground it out intermittently
You can make an extension out of a spoke    
 

By the way a bad coil can have symptoms you describe
It will only run one cylinder at a time and can be either cylinder

But your problem sounds more like a carb bowl draining  down as the engine runs

Also could be a valve opening up as the engine gets warmed up
Have you adjusted your valves lately

[mrclubike]

That thought crossed my mind. But I'm confused as to why it is normal at first, until it runs for a little bit. Is there something that changes that could have enough fuel initially but then not after it warms up?

I did check the choke to make sure it was opening/closing and they both looked to be operating correctly.

A restriction in the fuel supply

[BPT]

The back story: I'd been having a random wandering idle problem so I'd recently adjusted the valves in order to be able to make sure the carbs were correct.

Last week I went to my local shop to have them give it a listen and the guy there tuned my carbs for me.  It was fine when I left (about 15 minutes away with a good stretch of highway).  This was the first time I've tried to ride since then.  I've ridden several times since the valve adjustment with no problems.

The one thing he told me after adjusting the carbs was that I most likely need my timing chain replaced.  I knew that was coming but he's saying sooner rather than later.

[mrclubike]

first thing i would do is drop the float bowls one at a time and check for crap in the them and the flow out of the needles

[BPT]

Ok, I like that, nice and easy. Anything in particular I should watch or look for as far as flow?

[misterpepper]

There's usually a bit of internal thread exposed on the top of the spark plugs. I used some screws, M3 thread I think, and ground the heads of the screws to match the diameter of the spark plug nipple so the plug wire would still fit. Other people use wheel spokes because the threads are the same. Then you just need a way to ground the bolt to the head without getting shocked yourself. A screwdriver works great. That way, with the engine up to speed you can quickly "switch off" one cylinder to get an idea what the other one is doing. It is helpful when tuning/syncing carbs as well.

[BPT]

Thanks Misterpepper, I'm getting a better visual on that now.  I'm at work today and won't be home until late evening so won't be doing anything tonight.  I can pop the bowls off to look for crud though.  Something tells me I won't get lucky and have it simple and obvious though......

[BPT]

Ok, please school me on carburetor flow.  I only did a very quick check today but didn't see anything that looked unusual to me.

There was no noticeable crud in the bowl.  I had steady flow from both carbs after opening the petcock.  Moved the floats up and down and it turned the flow off and on.

So going with the theory that it's a fuel constriction problem, where is the next place to check since it looks like the bowls are being filled with no problem?

I figured I'd start with the carbs, then move to the spark if that doesn't work.

[Justin B.]

Still, next time it does this, shut off the fuel petcock, turn key off, coast to side, and check the bowls to see if one of them is almost dry...

[mrclubike]

Sounds like the fuel is good

I would check the valve lash again especially exhaust

Then test the spark
I like this one from Lisle
The tip unscrews so it will work with out style of spark plug boot
Compare the spark when it is running good to when it is running bad