One cylinder not firing at idle

bosco659

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I just bought a ‘76 bike and noticed that on idle, the right cylinder didn’t sound the same as the left one (dual exhaust). Being new to the bike I started checking some things out. I took my infra gun out and measured the temperatures of the header pipes. The left side measured high one hundreds to just over 200 degrees F. On the right side the pipe was barely 80 degrees.
I pulled the plugs and they looked ok. I swapped them from side to side and the same result - dead on the right side. I did a quick compression test and both cylinders were around 160 psi. The bike has a Pamco ignition system. I ran the engine for a minute more and found the right plug was a bit wet with fuel.
I’m puzzled on this one since I have compression, seem to have fuel and should have spark, otherwise the left side shouldn’t be running.
I’ll check spark plug leads tomorrow and will check carbs and the rest of the fuel system. The bike does not have stock carbs. I’ll double check the make and model tomorrow (flat slide carbs). Also noticed on a cold start with both chokes pulled, as soon as it started, if I pushed the right choke it, the bike would still idle. If I pushed the left choke in, the bike would immediately die. I tried adjusting the mixture screws but that didn’t seem to make a difference.
Throttle response seems good and on the road it seems to gave ample power. The problem appears to be at idle.
Any ideas what might be causing this? I’m a newbie so please be kind :).
 
It’s a dual output coil so could one side go bad and the other side works fine? Since it runs well above idle could the coil be getting weak but generates enough voltage at higher rpms? I’ll order new plugs and test the wires and caps tomorrow. Maybe I’ll switch the wire assembly from one side to the other to see if the problem follows the wire / cap. Thx
 
At the very least, pull the carbs and Bench-balance them. Lining up the master blade with a transfer port hole, using the idle adjust screw, and then lining up the slave blade with the balance screw.

I was fighting a similar issue earlier, it was carb balance. My vacuum gauge found it. After setting both sides the same, the blades were identical compared to the transfer holes.
 
Pretty sure it has fuel. To be sure I took apart the carbs and cleaned them in the ultrasonic cleaner. Reassembled, reinstalled, same problem. Switched ignition wires from side to side and same symptoms. Btw, the carbs are PWK Keihin 32mm.
 
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At the very least, pull the carbs and Bench-balance them. Lining up the master blade with a transfer port hole, using the idle adjust screw, and then lining up the slave blade with the balance screw.

I was fighting a similar issue earlier, it was carb balance. My vacuum gauge found it. After setting both sides the same, the blades were identical compared to the transfer holes.
Is this procedure for a stock carb? I have PWK’s.
 
Is this procedure for a stock carb? I have PWK’s.

Yeah. With the PWK or any flat-slide, you'd have to figure out how to adjust to the same height, or use a manometer to set the vacuum to be the same on both sides, that would be a good variable to eliminate before digging into more serious stuff.

Then again, I'm not familiar with those carbs and could be way off-base.
 
Squirt some starting fluid into the dead side carb at low RPM, see if it comes alive.
Thx. Good idea. Tried that and the cylinder started firing but intermittently even after the starter fluid burned off. Suspecting a bad coil plus carb issues.
 
Yeah. With the PWK or any flat-slide, you'd have to figure out how to adjust to the same height, or use a manometer to set the vacuum to be the same on both sides, that would be a good variable to eliminate before digging into more serious stuff.

Then again, I'm not familiar with those carbs and could be way off-base.
Once I get both sides firing consistently I’ll hook up vacuum gauges sync them up. I can probably set slide height with a vernier caliper with the top of the carb removed.
 
Are the carbs synched, or are they dependant?
A picture or two might help your cause
The carbs independent running off of a pair of throttle cables. I’m repeating the cleaning of the carbs tomorrow in the ultrasonic bath. I’m still suspecting a plugged idle circuit and I’ve ordered a new coil as the current one measures out of spec.
 
Don't think it is a spark problem. On a dual output coil it can't fire just one plug. Both plugs must be present and working.
And pls don't ask what I think about Keihin carbs
 
Bought a new coil just to be sure. Completely disassembled and cleaned both carbs in the ultrasonic cleaner. Both sides running now but very rich. I think I’ll replace the oring assemblies on the piece that holds the main and pilot jets. Will also swap out float needle valve. Odd but the float height was quite high. Measured about 22mm from the edge of the bowl. I couldn’t find the exact spec for the 32mm PWK but believe it’s 19mm? No way the floats can be lowered that much in these carbs because at 19mm the floats would hit the body of the carb.
 
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