93 Popping from exhaust at idle

[the496rocket]

Working on my dads 93, the bike runs great while riding, however, when its warmed up and idling, there is a constant popping or "coughing" from the exhaust,, about once every few seconds. also it decel pops a decent amount, which I read is pretty common, but the idle popping is what I am trying to solve. The carbs were already cleaned a year ago. I currently have parts on the way to remove the PAIR system, and am going to replace the carb boots, and sync the carbs with a set of gauges I ordered. I have checked for exhaust leaks and can find none.

I don't know which pilot jets are in it, but I am wondering if I should make sure it has a 40 pilot in it instead of factory 38 while the carbs are off, and if so, what mixture settings should be at with those pilots as opposed to factory for a 93 (non cali)]

As I said the bike runs great, no hiccups at all, I ride it all the time, but that popping at idle (and decel) tells me something isn't right, and being who I am, I need it fixed. Any advise/suggestions?

 

[Larry Fine]

I would check the spark plugs first, for gap and cleanliness.

 

[ddiez]

Working on my dads 93, the bike runs great while riding, however, when its warmed up and idling, there is a constant popping or "coughing" from the exhaust,, about once every few seconds. also it decel pops a decent amount, which I read is pretty common, but the idle popping is what I am trying to solve. The carbs were already cleaned a year ago. I currently have parts on the way to remove the PAIR system, and am going to replace the carb boots, and sync the carbs with a set of gauges I ordered. I have checked for exhaust leaks and can find none.

I don't know which pilot jets are in it, but I am wondering if I should make sure it has a 40 pilot in it instead of factory 38 while the carbs are off, and if so, what mixture settings should be at with those pilots as opposed to factory for a 93 (non cali)]

As I said the bike runs great, no hiccups at all, I ride it all the time, but that popping at idle (and decel) tells me something isn't right, and being who I am, I need it fixed. Any advise/suggestions?

Hello. Where did you get the plates to block the openings after pair removal ?
Thanks!

 

[Al st1100]

Working on my dads 93, the bike runs great while riding, however, when its warmed up and idling, there is a constant popping or "coughing" from the exhaust,, about once every few seconds. also it decel pops a decent amount, which I read is pretty common, but the idle popping is what I am trying to solve. The carbs were already cleaned a year ago. I currently have parts on the way to remove the PAIR system, and am going to replace the carb boots, and sync the carbs with a set of gauges I ordered. I have checked for exhaust leaks and can find none.

I don't know which pilot jets are in it, but I am wondering if I should make sure it has a 40 pilot in it instead of factory 38 while the carbs are off, and if so, what mixture settings should be at with those pilots as opposed to factory for a 93 (non cali)]

As I said the bike runs great, no hiccups at all, I ride it all the time, but that popping at idle (and decel) tells me something isn't right, and being who I am, I need it fixed. Any advise/suggestions?

were the carbs synced after rebuild? Is the fuel cut vaccuum valve good or bypassed ? How many turns are the idle mixture screws out?

 

[bstig60]

Carbs out of sync.

 

[the496rocket]

Larry I’ll get some new plugs as well. Al Not sure about the fuel cut vacuum valve, is it a good idea to bypass that? I’ll have to research it. I’ll be syncing the carbs when I get the gauges. And I’m not sure what the mixture screws are at anymore it’s been a bit too long to remember. Just need a good baseline depending on what pilot is in it.
Does I got the plates for a hayabusa

thanks for all the helps so far, hopefully can get this sorted easily.

 

[Oldbikefixr]

Larry I’ll get some new plugs as well. Al Not sure about the fuel cut vacuum valve, is it a good idea to bypass that? I’ll have to research it. I’ll be syncing the carbs when I get the gauges. And I’m not sure what the mixture screws are at anymore it’s been a bit too long to remember. Just need a good baseline depending on what pilot is in it.
Does I got the plates for a hayabusa

thanks for all the helps so far, hopefully can get this sorted easily.

I rebuilt the vacuum valve on our 1991 right after I bought it nine years ago based on suggestions from this board I had just joined. It was working fine but the diaphragm was cracking. I checked it two years back this month when I rebuilt the carbs and other things (fuel leak) and found the same condition...replaced the diaphragm again. Sooooo....I'm guessing the OEM diaphragms last about seven years on mine. I considered bypassing it but I'm a long time restorer and a fan of keeping my bikes mostly stock whenever that's possible.
Welcome aboard! most anything you would want to do is detailed somewhere/sometime on this site, just use the search function....and 'search' the titles for what you're looking for.

 

[the496rocket]

Been working with the carbs and bike for the past week as time allows, replacing every coolant line and hose, orings, stat, cap, and elbows. new carb boots are on, I ordered #40 pilots, and have a carb sync tool made up. also ordered .020mm shims for the slides, **HOWEVER, i took one of the slides out last night and it looks like there is already a steel shim down in the plastic housing, and I had to remove the inner part of the black housing in order to remove the shim. (not the white cam to get the slide itself out) and the "shim" in there was .020. The parts diagram just calls it a washer.

is there always a shim from the factory and you add a second? its #21 in this photo. or do some bikes come with a shim like this, and the rest just have the slide bottom onto the plastic itself? cant find any info online, and certainly dont want to double up if its already where it needs to be.

 

[Tdinova]

Is there an exhaust leak... exhaust leak will suck in air and backfire in the muffler

 

[Al st1100]

Been working with the carbs and bike for the past week as time allows, replacing every coolant line and hose, orings, stat, cap, and elbows. new carb boots are on, I ordered #40 pilots, and have a carb sync tool made up. also ordered .020mm shims for the slides, **HOWEVER, i took one of the slides out last night and it looks like there is already a steel shim down in the plastic housing, and I had to remove the inner part of the black housing in order to remove the shim. (not the white cam to get the slide itself out) and the "shim" in there was .020. The parts diagram just calls it a washer.

is there always a shim from the factory and you add a second? its #21 in this photo. or do some bikes come with a shim like this, and the rest just have the slide bottom onto the plastic itself? cant find any info online, and certainly dont want to double up if its already where it needs to be.

Great write up that should answer any questions you may have by one of our members. I used it a few years ago and had good results. .020 shim is good.
http://www.st-riders.net/index.php?topic=10249.0

 

[jfheath]

I have experienced popping and backfiring as a result of one of the carb balancing vacuum tubes becoming detatched. These are the ones that stick up from behind 3 of the cylinders, and are they are normally capped. (The fourth one, rear right cylinder, is attached to the petcock).

In my situation the behaviour was really bad, making it difficult to coax any speed out of the engine unless very tiny movements of the throttle were used. But my vacuum tube was completely detached. A small tear, or a split which might get sucked closed as the throttle is applied, may well behave as you describe.

Whatever. They are worth checking out.

 

[the496rocket]

Well boys she's back together, New air filter, new carb boots, carbs COMPLETELY cleaned, new 40 pilots, float valves and seats, floats height set, PAIR delete, New iridium plugs, every coolant line/hose replaced, flushed, new tstat and cap, carbs synced and idle drop test and resync, vacuum valve delete and new fuel filter, oil change, and shes running better than ever! NO more popping.

ALSO bypassed both red wires, (no melting but corrosion, and also installed an ignition bypass relay. Before, charging voltage would only get up to 13.4v at 5k rpm. After, it gets up to 14.3. Definitely an improvement! (plan to install a 40 amp this winter even tho this one is working good)

Thanks for all the good suggestions everyone! love this ol girl.