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Dead cylinder at idle

  • Thread starter Thread starter glenwill
  • Start date Start date
G

glenwill

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I noticed last season that my idle was rough. I then noticed that the exhaust pipe on cylinder 4 is cold when the bike has been idling. I am assuming from this that it is not firing. I confirmed a good spark, and figured it must be the carb.

This winter I tore down the carbs and fully cleaned them, including dipping in Berryman's, and spraying all of the key passages to ensure they are clear.

I put them back on last weekend and it started right up. With the choke pulled, it was running fine, and I checked the pipes with my infrared thermometer, and all 4 got hot. Once it warmed up, I pushed the choke back in, and let it run for a few minutes. The idle was rough again, and once again, cylinder 4 cooled down, while the others remained hot.

I am uncertain how to proceed. I'm looking at getting a colortune so I can see what is really happening with that cylinder.

I could also pull the carbs and teardown number 4 again, and see if I missed some blockage or cleaning step on that one.

What else could cause this symptom? Valve clearance? I checked and adjusted the valves when I got the bike two years ago, and have not checked since. I don't put a lot of miles on, so it's probably 1,500 or 2,000 miles since they were done.

I checked the spark plug. The gap was a bit tight, so I regapped it.

Last year I checked compression, and all 4 cylinders were close. I don't remember the numbers, but I think I posted them on here. I can look for it if need be.

Ideas?

Thanks,
Glen
 
O-rings on the carb to head boots. Replace all 4.

Did you replace the carb O-rings as well? You can strip and dip all you want, but if the O-rings are brittle, they need replaced.

Personally, I would strip and dip the carbs again. And then check to make sure you don't have any holes in the diaphragm on that carb.
 
My first thought is O rings as well.
You can check the mount O rings with a little spray of wd40 on the boots where they bolt up, if the r's pick up then you know. Doesn't always work but worth a try.

Something that I would do before pulling the carbs, is unscrew the spark plug caps and snip about 1/4" off the ends of the wires and screw the caps back in. This freshens the connections, don't forget to look into the caps for condition of the little screw that screws into the wire, it should be bright and clean.

V
 
Runs fine with choke?
Doesn't without?
Something is still clogged in the pilot circuit.
Could be o rings too.
 
Thanks to both of your for replying.

When I got the bike 2-3 years ago, I did a thorough reconditioning. On the carbs, I replaced the carb boots, including the o-rings. I cleaned the carb and replaced all of the o-rings and pilot plugs, set the float level. I replaced the 1 bottom gasket that was shot. I inspected all of the other components, and they looked good.

I'll try the spark plug wire idea first, and if it doesn't help, I'll pull the carbs and clean again. I'll inspect the diaphragm at the same time.

Anyone use a colortune? Would it help me confirm no combustion vs very little combustion at idle?

Glen
 
tkent02 said:
Runs fine with choke?
Doesn't without?
Something is still clogged in the pilot circuit.
Could be o rings too.
Click to expand...

Thanks. I'll check the pilot circuit again more carefully. Where all do I look for clogging? The cleaning guide mentions two passages to spray into, and where it should come out. Both look related to the choke, not the pilot. I did inspect/clean the pilot jet, which I can do again. Is there another passage I need to look for? I'm thinking it would be between where the pilot jet screws in, and where the pilot adjustment screw is located.
 
Could also be carb synch, if one isn't open at all it will do as you describe.
 
What about a carb sync? If number 4 carb was closed at idle as compared to 1,2 and 3 would this not be the symptom? The OP stated that he stripped and dipped but did not mention a bench or vacuum sync.
 
I did not sync them when I put them back together. Probably a mistake. Last year I bought a Motion Pro SyncPro and synced the carbs. This year when I cleaned them, I figured the positions were locked in, and I just put them together. I should have at least bench synced them before putting them on the bike, to make sure they were still good.

I may sync them before I take them off the bike, and see if that is the issue.
 
Take a propane torch, turn on gas but don't ignite. Move it around the carb to check for vacuum leak. When the leak draws in the propane the rpm's will rise.

if it is not a vacuum leak then tear into the pilot circuit.
 
Try to synch it first. It's a lot less work, and at least you could rule that out as the problem.
 
glenwill said:
I did not sync them when I put them back together. Probably a mistake. Last year I bought a Motion Pro SyncPro and synced the carbs. This year when I cleaned them, I figured the positions were locked in, and I just put them together. I should have at least bench synced them before putting them on the bike, to make sure they were still good.

I may sync them before I take them off the bike, and see if that is the issue.
Click to expand...

Yeah, what he said. You've got the tool, man! Syncs should be at least done annually, whenever you do carb work, and when you adjust the valves.
 
glenwill said:
Anyone use a colortune? Would it help me confirm no combustion vs very little combustion at idle?
Click to expand...
Yes, I have used a Colortune. Yes, it can help confirm proper (or NO) combustion at idle.

However, as others have mentioned you should use a Carbtune (or your Motion Pro) first.

If you already have a Colortune, feel free to use it, but don't count on it. :-k
I used one on a bike with VM carbs and was VERY excited about the results, so I took it home to use on my wife's bike. I saw absolutely NO change in the color throughout the adjustment range. The only thing I saw change was the frequency of the nice, blue flame. As I leaned the mixture screw (on the BS carbs), the flame would occasionally stutter, then finally go away. As I richen the mixture back up, the flame would stutter into existence, then stay steady. Richening the screw farther made no change to the color of the flame.

I posted the results here and asked about the experiences that others have had with a Color tune on BS carbs and the results were about 50/50. Some saw changes, others only saw the stuttering that I saw, so I chose to not get one for my own toolbox.

.
 
I don't post often but this subject has always interested me because I've experienced it and it's come up so many times through the years on this forum. As near as I can remember, the thread always 'dies' with the issue never being resolved. I've had several 4-carb (BS) GS's and have run a total of 5 (6?) sets of carbs on them. I rebuilt every set of carbs I ever used (some several times) on all my bikes but only experienced this issue once on #1 on a GS750. (I had 2 sets of carbs for the bike and the second set ran perfectly.) I've used Colortune (gimicky) and have a Carbtune (excellent and fun) and scads of new plugs and carb parts. I got good at taking #1 almost completely apart on the GS750 by using a mirror and I must have done it 20 times. I could solve the problem temporarily but I NEVER found a permanent fix.

My guess is that the problem is one that nobody has mentioned and I feel especially sure it has nothing to do with the carbs being out of sync. My other guess is that it's something really simple but I'll be damned if I could ever figure it out.:confused:
 
The Colortune was devised in the days of home/diy mechanics looking for a method of working on their rustbuckets at home and placing their faith in technology of the day. It works bloody well on SU carbs (which, in its market, millions of cars were fitted with) and with other fixed-jet carbs it also works, but more to give you an indication that the massive jets Benny told you would get your crappy little buzz-box flying were actually killing it.
It's not a gimmick - in the right application it works fine. That application is aimed squarely at CV carbs with adjustable jetting - SUs and Strombergs (spit).
 
I just fixed the same problem on my GS850, and it happened to be on the same # 4 cylinder. My issue was a blocked passage between the air jet in the air horn and the fuel pickup just above pilot jet circuit. Tricky circuit to clean. I accomplished the fix by ample applications of persistence, cleaner wires, lots of carb clearer, patience and more persistence, more clear wires/tools and more carb cleaner. Finally got all the gunk out after many attempts. Runs like a champ. It took about 6 months to fix this problem if you count the time from the original carburetor dipping and the initial rebuild. Things never come easy to me.
 
Well, I've got lots of ideas now! Thanks all.

I'll work on it this weekend and post the results. I'll start with the carbtune, and if that doesn't do it, I'll pull the carbs and see what I find.

Glen
 
Update:

I went to use the Motion Pro Syncpro, and discovered the blue fluid is depleted. The instructions say it is fine to store horizontally, but maybe that is not ideal. I ordered the fluid.

Meanwhile, I decided to rerun a battery of tests to confirm the problem.

I did the temperature test again. This time, even running for 5 minutes at 4K, cylinder 4 is cold enough to grab the pipe. At the very top, it gets slightly hotter (about 125), but never as hot as the other 3 pipes.

If I pull the spark plug cable from cylinder 4 while it is idling, there is zero change in RPM. If I pull it while at around 3K, there is zero change in RPM. If I pull any of the other cylinders at idle or 3k, it stumbles or the bike stalls.

I checked compression, and all 4 cylinders have similar compression, around 145.

I swapped the spark plugs on cylinders 3 and 4, to see if the plug is bad, but the problem remains on cylinder 4.

My understanding of the ignition is that since the same coil feeds cylinders 1 and 4, it is the same spark. I don't believe internally it is sending a different single to the two wires. So I swapped the cables for 1 and 4. The engine runs the same, and if I pull the plug on cylinder 4 (using wire 1), no change in idle. If I pull it on cylinder 1 (using wire 4) it stalls. This seems to confirm that spark is not my problem, as the problem remains on cylinder 4.

So I believe am in the right place looking at the carb. I forgot to shutoff my Motion Pro aux fuel tank, and came back in the garage a few hours later to a puddle of gas on the floor. So, at least one of the float valves is bad. I took off the air filter and peeked in with a mirror, and I see a drip from carb 1. I don't know if that means the problem is in carb 1, since rising fuel in any carb looks like it would flood all of them through the feeder tube.

Regarding the float valve, is it safe to just replace all 4 of them with this set? http://www.z1enterprises.com/ItemDe...Mikuni-type+float+needles+6-Pk&item=KL18-8957

It doesn't seem to be an actual Mikuni part, although it doesn't say who it is. The OEM part on partzilla includes both the valve and the seat, for $33/each.

Glen
 
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