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Weak cylinder

  • Thread starter Thread starter kjsamm
  • Start date Start date
K

kjsamm

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Just got done replacing intake boots, sealing airbox and exhaust, valve adjustment, reset timing, new spark plug boots and wires. Started right up after a month of sitting, but #2 is running weak. Exhaust gets hot( not sure if it gets as hot as the others or not.) but when pullng boots there is barely any ( if any) difference on how the motor runs. When shimming valves #2 was really tight on ex side...valve not burnt, bent or chipped. Was 5 sizes to big of shim on #2 ex . All others had clearence but barely. Just looking for some input for things to look at when I get off work. Thanks
 
Idle scew set at 2 1/4 a turn. Stock air box, after market exhaust. Carbs have been rejetted.
 
Did you completely strip and rebuild carbs with fresh o rings? Strip and dip or sonic cleaned? Not just a spray and pray? That's going to be the first question the grumpy old men (lol) are going to ask you. Are the valves all in spec now?
 
I did a complete cleaning years ago, and do quick cleanings every month. Valves are in spec now. Do i really nees to tear down the carbs again? I keep them pretty clean. But I suppose it wouldn't hurt to strip them again. Just really dont want too, since i just got bike put back together.
 
I'd check the plugs and the petcock before I stripped the carbs

If the #2 plug is significantly richer, a leaky petcock may be your problem, it's leaking gas down the vacuum line into the carb
 
Big T said:
I'd check the plugs and the petcock before I stripped the carbs

If the #2 plug is significantly richer, a leaky petcock may be your problem, it's leaking gas down the vacuum line into the carb
Click to expand...

I agree with that. That would be the first thing I checked. The next thing I would check would be the compression on #2.
 
kjsamm said:
When shimming valves #2 was really tight on ex side...valve not burnt, bent or chipped.
Click to expand...

How did you synchronise the carbs?
How do you know the valves are not burnt?
Compression check?

And how much has the engine been run lately?
 
tkent02 said:
How did you synchronise the carbs?
How do you know the valves are not burnt?
Compression check?

And how much has the engine been run lately?
Click to expand...

Carbs are bench synced

I looked at them

no compression check yet, but with that valve not being able to seat for so long I wouldn't doubt that there is a lot of carbon build up still stopping it from seating properly giving me little to no compression.

Bike has only been run for 5-10 min since i got it all put back together.

petcock is a new from a few years ago.

Just started bike for a min (as its late i didn't run it too long) checked spark at #2 and its strong. In the little bit in ran exhaust for #'s 1,3 and 4 are warm #2 is cold.
 
Big T said:
I'd check the plugs and the petcock before I stripped the carbs

If the #2 plug is significantly richer, a leaky petcock may be your problem, it's leaking gas down the vacuum line into the carb
Click to expand...

When I pulled plugs before my tear down #2 was running rich. The rest were good.
If it were leaking gas down vacuum line wouldn't #2 still fire? It has all it needs to fire...air, fuel (i'm guessing, if it was leaking) and spark.
 
Last edited:
Not a a petcock issue. After starting the bike and running for a min, (#2 still dead) I pulled off the vacuum hose to petcock and it was dry, as was the nipple it attaches to. Also I would think that if that were leaking even the slightest I would have noticed, as the tank had been sitting on a wooden bench for the past 6 weeks.

New thought, I am unsure at the moment but I believe that #2 does eventually fire. As the plug was never wet and read as a rich cylinder condition.
 
Last edited:
If #2 exhaust valve needed a shim 5 steps thinner than before, there is a very good chance it's burned.
 
I pulled exhaust on #2 and took a flexible light and mirror and looked at valve. Looked fine.
 
She lives, just took awhile for carbon to burn off that valve. But #2 cylinder is firing now.
Runs pretty good except I still have a slight hanging throttle after slow down when coming to a stop. Air leaks are fixed, what else could it be?
 
kjsamm said:
She lives, just took awhile for carbon to burn off that valve. But #2 cylinder is firing now.
Runs pretty good except I still have a slight hanging throttle after slow down when coming to a stop. Air leaks are fixed, what else could it be?
Click to expand...
Hanging idle is an air leak, keep looking.;)
 
kjsamm said:
I pulled exhaust on #2 and took a flexible light and mirror and looked at valve. Looked fine.
Click to expand...

The part that burns first is inside the combustion chamber, actually where it seals on the valve seat. You can't see that unless you pull the head. You wouldn't see a burnt valve from the port unless it was so burnt a big pie wedge was burned out of it. Same with looking through the spark plug hole.

Again, what does the compression test say?
 
tkent02 said:
The part that burns first is inside the combustion chamber, actually where it seals on the valve seat. You can't see that unless you pull the head. You wouldn't see a burnt valve from the port unless it was so burnt a big pie wedge was burned out of it. Same with looking through the spark plug hole.

Again, what does the compression test say?
Click to expand...

Haven't done a compression check yet, I am going to in the next day or two. I am waiting for cylinder 2 to burn off some of its build up before testing.
 
Also, ONLY while I'm on the throttle I have a light ticking noise coming from motor (sounds like more from top right side). Only appears when I have the throttle open, even just a hair and it doesn't get louder as i open the throttle just faster as RPM's increase. Yes, all valves are in spec and timing, no scoring on cam. Is this just normal Valve train noises?
 
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