another carb Q.

[avenue]

I got my fresh rebuild project running a few weeks ago. After resolving a no left cylinder spark issue the first day, the 277pamco/capacitor engine was running strong, smooth, and surprisingly responsive. I took it for some laps around the neighborhood over the next few days...everything still running strong and smooth. The last lap I made, however, the bike started to run very rough right out of first gear and I sputtered my way down the street back home. When I pulled into the driveway and throttled ever so slightly and got a strong backfire out of the left cylinder exhaust pipe.

The left side is not firing, again. The first time was a no spark issue. Now, I'm getting spark. This morning I pulled the carbs for a driveway cleaning, pulled the pilot, mains, air jets and sprayed all passages I could with carb cleaner. Still, no fire on the left cylinder.

I've read the carb guide countless times but still feel pretty lost and don't know where to look next.

The plugs were pretty fouled after only a few miles of riding. it'll kick over and idle on the right cylinder at half choke but dies quickly with any small blip of throttle.

'83 277 rephase, green monster coils, pma, capacitor, kick only, bs34. 45/140 jets. uni pods. pandemonium ya-mama straight pipes.

thanks for any input,
avenue.

 

[weekendrider]

Cam chain, valves, good gas flow to the carbs, float level and operation(not sticking).
Rubber plug and pilot jet out seeing carb cleaner coming out the metering holes in the throttle body(throttle plate propped open so you can see). Clean, clear openings in the jets. Mix screws? Synced? Just throwing out the sequence I would use. With lotsa head scratching along the way.

 

[avenue]

WER,
thanks for the tips. I just finished going through the full tune-up procedure, in correct order, as you prescribed and still no left side fire.

I even switched the float needles and reset levels, as I suspected this might be an issue. after kicking it over, I also made a temporary center stand to see if the side stand lean was an issue with the float.

still, only firing on the right cylinder.

on another note, I dropped the pilots one size as I was running quite rich when it was firing on both cylinders (was pulling strong but too rich, me thinks, from the looks of the plugs).

any other help or ideas is appreciated,
avenue.

 

[weekendrider]

New plugs?

 

[gggGary]

Vacuum petcock yet? If so plug the line and run it on prime. Float valve seat o-rings replaced? They often go bad and allow a bad gas leak around the valve. Compression test. What valve seals did you use in the rebuild.

 

[avenue]

New plugs?

thanks for the help and sorry for the delayed reply. I got sick on monday night and just now got out of bed for the first time.

the last thing I did on monday when trouble shooting the problem was change out the plugs. the bike fired right up on both cylinders. I made a manometer and dialed in the synch. it's still needs some jetting, though, as it's still not running right in the lower end.

now, I replaced the BP7ES plugs with some iridiums but I also have resistor plug caps at the moment. I hadn't changed them out because of the double resistor factor. I'm going to put the old plugs back in on opposite cylinders to confirm the left plug is faulty.

other than being fouled-up, what would cause a plug to go bad, and so quickly?

thanks for the input,
avenue.

 

[jetmechmarty]

Go back one to Gary's post. Pull the vacuum line to the petcock. Is there evidence of fuel in it. That vacuum line is supposed to be connected to the vacuum port on the left carburetor manifold. A ruptured petcock diaphragm would likely flood the left cylinder.

 

[avenue]

left plug in on the bottom.

 

[avenue]

Go back one to Gary's post. Pull the vacuum line to the petcock. Is there evidence of fuel in it. That vacuum line is supposed to be connected to the vacuum port on the left carburetor manifold. A ruptured petcock diaphragm would likely flood the left cylinder.

the bike does not have a vacuum petcock. ball valve R-off-O petcock with inline filter into the bs34 T. no leaks or overflow at the bowls.

 

[weekendrider]

The one that is bad was born on a Fri late afernoon?
Are these 34's with spigots. Confirm the fuel level with a sight gauge.

 

[gggGary]

If they are brass float BS34s a leaky float is always a possibility, shake to hear gas sloshing, dip it in hot water watch for bubbles.

 

[avenue]

they're not the brass floats. and, I checked the floats with a sight gauge before removing for the first time.

I'll be jumping back in for some jetting this weekend and will go through the motions again. any other ideas in the meantime are appreciated.

thanks,
avenue.

 

[avenue]

an update. I switched out the plugs, pulled the carbs for another quick cleaning, and changed out the pilots/mains to 45/135. also, filled up with 93 octane.

the bike fires right up on the first kick, idles smooth and the throttle is responsive. I swapped in the old left cylinder plug (the side that was giving me problems) to the right side and guess what? no fire on the right side.

so, currently I'm running Iridium R's and resistance plug caps. I should get rid of one of the R's, yes?

avenue.

 

[XSLeo]

Hey, that sounds like a bad plug. A plug can fire good outside the engine but under compression it can fail to spark.
On the R's you just need one. Unless you have a very hot coil. A coil with over say 40-50 KV will have plenty of spark to over come the resistance. With a hot coil and two R's the coil has to build to a higher voltage before the plug can fire. Opening the plug cap also requires more voltage. This higher voltage makes a hotter spark.
On the stock coil this won't work well as it doesn't have enough KV's.
Leo