My bike suddenly lost a cylinder

[Guest]

Good evening,

The #2 cylinder on my bike not firing appears to be the reason my bike has lost power.

This problem suddenly happened when I was out riding a few nights ago. I was on the freeway and I noticed it seems I had to give it more throttle than usual to hold about 70 MPH. I made it home but pulling away from the stop lights I had to give it more throttle than usual.

Tonight I finally got a chance to work on it. First I fired it up and rode it. Yep, the problem was still there.

Then I let it idle while I pulled off the plug wires one at a time and found the bike almost stalled when I pulled off every cylinder plug wire except #2. I also noticed that when I held the #2 plug wire near the block a spark would jump. Seems I was getting spark but maybe no fuel or no valve operation.
I pulled off the air pods and confirmed all cylinders were working the same: butterflies were opening and gas was squirted. So I went to look at the valves.

TomMLC told me to use a flashlight down into the cylinder but I could not see anything when turning the crankshaft except the pistons going up and down. I think I didn’t know where to look.

I pulled the gas tank and the valve cover and made sure the cams were in what appeared to be the right position, from my description to Tom, as I turned the crankshaft. Seems they are all pushing down correctly so at this point I’m not sure what to try except pull the top end off, which I’ve never done and pretty shy about.

I decided first to pull off the carbs and have a look inside the boot-end to see if maybe something had got in there. I twisted the throttle and saw from the inside the butterflies opening and the gas squirting through. They looked pretty darn clean inside.

Confused and losing light, I put it all back together – before I forgot how I took it apart. I took it for a test ride and it’s running a bit worse than before, probably because the boots are now leaking a bit as I did not give them a cleaning before putting it back together. The boots are virtually brand new, not even a year old on these racing carbs.

I did the same test with the plug wires and confirmed #2 is still not firing.

Today I went about as far as I feel comfortable pulling the bike apart without at least collecting more knowledge and/or having Tom and another friend there for support.

Please let me know if you have any suggestions.
Thank you!
-Bob

ps. I forgot to mention I did swap plugs around. The #2 plug *may* have looked a little wetter than the others - then they got mixed up and they all looked about the same!

Also I did a compreswsion test and all four were right on 170 PSI with the throttle open and the bike warm but not hot.

[Steve]

You lost a cylinder?

I have seen the work of a guy that knows what to do with extra cylinders that he has "found".











.

[littleroot]

Thanks I needed a laugh!

You lost a cylinder?

I have seen the work of a guy that knows what to do with extra cylinders that he has "found".











.

[SqDancerLynn1]

It sounds like something pluged in the carb, you could try some seafoam in the gas to clean it out..

[littleroot]

It sounds like something pluged in the carb, you could try some seafoam in the gas to clean it out..

Hi, OK, I tried this but no joy. I put in about four ounces in half a tank and did about 20 miles tonight.

I think there is gas getting through - I am getting a lot of backfire when I close the throttle so I was thinking this is the unburned gas going through cylinder #2

Is it possibly the coil not giving it enough juice? I have spark on the side of the head with the wire removed but is it enough?

Please anyone with more suggestions - I am stuck.

Many thanks

[fenderfreek]

I also noticed that when I held the #2 plug wire near the block a spark would jump.

You mention a couple times that spark goes from the plug wire to the head - have you actually observed spark on the plug itself?

I had one plug that just quit on me and gave the exact symptoms you're describing. When I pulled the plug is was extremely fouled and not sparking outside the cylinder.

I hit it with some carb cleaner and used fine grit sandpaper to cut through the crud on the tip and ground electrode. Gapped and hooked it back up and it worked like a champ.

[Texas Don]

The coil that powers cylinder #2 also powers cylinder #3. It #3 is firing, then the coil is proven good. Back to basics. You must have fuel, compression and spark to produce combustion. You've verified compression. Spark is observed at the plug when grounded against the head but you have no way of assessing whether it is of the proper amplitude to ignite the fuel. The spark can be weak due to a bad connection at the plug boot. Also if you have the type of boot that has a resistor in it, the resistor may be bad. You can replace it with a new one or just a piece of aluminum or brass rod to test it. If you replace the resistor and it fires, then you've found the problem. If the plug wires turn out to be okay, then you're down to fuel delivery. Check the wires first and post your results.
Don

[littleroot]

Hi fender,

I swapped all the plugs around and it did not make a difference.

[waterman]

Is it possible that you have a bad fuel petcock that is leaking fuel down the vacuum line into #2 and flooding it out? A wet plug may indicate too much fuel.

Check to see if the spark plug has fouled, switch them around to see if one is bad. Good spark in the air is not always good spark in the cylinder.

[littleroot]

Is it possible that you have a bad fuel petcock that is leaking fuel down the vacuum line into #2 and flooding it out? A wet plug may indicate too much fuel.

Check to see if the spark plug has fouled, switch them around to see if one is bad. Good spark in the air is not always good spark in the cylinder.

I gotta check this and the plug wire as Don said. I think I should be able to swap the plug wires around (at each end of course). I did already swap plugs around: the first time with 2 and 4, and later all four got mixed up after I did the compression test.

Is there a way to get vacuum without connection to the carb? I was skimming basscliffs megawelcome last night and when he said a bad petcock could make the #2 foul I was not sure why. I went out for a ride and moved the petcock to all three positins while riding and it did not make a difference so I gave up on that. But maybe I should go back to it. How can I test for this?

I gotta wait for a decent hour here before I wake the neighbors with my uncorked exhaust.

Thanks!

[waterman]

You should be able to check the vacuum hose from the petcock and see if it is wet with fuel. Any fuel would indicate a leaky and faulty petcock.

Likewise, you could remove vacuum hose from petcock, stick pencil in hose to block off vacuum from #2 carb and set petcock to prime and see how it runs.

I am grasping at straws but may be problem.

[rockford]

Hopeful

I'll be eager to hear how it goes. If you find your problem, I think you may also be finding mine.

[littleroot]

You should be able to check the vacuum hose from the petcock and see if it is wet with fuel. Any fuel would indicate a leaky and faulty petcock.

Likewise, you could remove vacuum hose from petcock, stick pencil in hose to block off vacuum from #2 carb and set petcock to prime and see how it runs.

I am grasping at straws but may be problem.

No joy.

I have swapped the plug wires at the coil.

I have disconnected the vacuum line and plugged it and no joy.

Also I discovered on this flat slides carb the vacuum is on carb 1. Doh!

Thanks for trying but I am stopped again.
-Bob

[littleroot]

The coil that powers cylinder #2 also powers cylinder #3. It #3 is firing, then the coil is proven good. Back to basics. You must have fuel, compression and spark to produce combustion. You've verified compression. Spark is observed at the plug when grounded against the head but you have no way of assessing whether it is of the proper amplitude to ignite the fuel. The spark can be weak due to a bad connection at the plug boot. Also if you have the type of boot that has a resistor in it, the resistor may be bad. You can replace it with a new one or just a piece of aluminum or brass rod to test it. If you replace the resistor and it fires, then you've found the problem. If the plug wires turn out to be okay, then you're down to fuel delivery. Check the wires first and post your results.
Don

Dumb question:

If I have good compression does that mean my valves are working OK?

[Texas Don]

You only need to swap #2 and #3 plug wires. When you do, see if the problem transfers with the plug wire. You can switch the petcock to prime and run the bike. You have no vacuum applied to the petcock that way. See if that makes a difference.
Don