Dead cylinder at idle

[rockford]

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.

[Grimly]

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).

[tdeglass]

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.

[glenwill]

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

[glenwill]

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/ItemDet...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

[jsandidge]

You better check to make sure you didn't get gas in your crankcase. Open your oil filler cap. If it smells of gas, change your oil and filter.

[chuck hahn]

OK..try this then. Look inside the plug cap and youll see the brass thing has a screw driver slot. Unscrew that and dump the contents into your hand. The ceramic thing is a resistor ( i am assuming they are the stock style caps ). take some 1/8 inch copper rod or brass rod ( real thick gauge house wire will work too ) and cut a piece the length of the resistor and out it in the cap..then reinstal the brass thing that grips the plug. If then resitior has failed this will again restore spark to the plug. If this fixes the problem do it to the other 3 before they fail too.

Regap the plugs to 32 to 35 as well to be sure.

[glenwill]

You better check to make sure you didn't get gas in your crankcase. Open your oil filler cap. If it smells of gas, change your oil and filter.

Good point. I was going to change the oil anyway. At least I didn't already do it.

[glenwill]

OK..try this then. Look inside the plug cap and youll see the brass thing has a screw driver slot. Unscrew that and dump the contents into your hand. The ceramic thing is a resistor ( i am assuming they are the stock style caps ). take some 1/8 inch copper rod or brass rod ( real thick gauge house wire will work too ) and cut a piece the length of the resistor and out it in the cap..then reinstal the brass thing that grips the plug. If then resitior has failed this will again restore spark to the plug. If this fixes the problem do it to the other 3 before they fail too.

Regap the plugs to 32 to 35 as well to be sure.

Thanks Chuck. I did swap the plug wires between 1 and 4, and the problem remained on cylinder 4, with cylinder 1 working strong with wire 4. I think that would confirm the resistor is good. I did take it out and look at it today, and it looked intact as well.

[chuck hahn]

Yeah..if it fired off 1 then its good still...but still a good tip for you to file away for some day down the line.

[Spyderman]

glen i would bet that you still have a plugged passageway in your carb somewhere that is causing of all this

[glenwill]

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.
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 did a search and found your post, and saw the mixed results. I'll hold off buying the Colortune until I see how I do recleaning the carbs.

What about the float valves I posted?

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Mikuni-type+float+needles+6-Pk&item=KL18-8957

Anyone have experience with them? I don't think they are Mikuni, but they are cheap.

[glenwill]

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.

Thanks for the tip. I took it apart today, and I think I found the passage you are referring to. I cleaned it with a cleaner wire. I see 3 different tiny holes in that vicinity. One on one side of the throttle, and 2 on the other side. I was able to push a wire into each, but I can't tell if I got enough. I'll work on it more this week. It's in the dip now.

Hopefully this does it.

I also found that I was missing the rubber gasket that sits in a small circle under the paper gasket. I have ordered that part to replace. I'm not sure what symptoms that would cause.

[Skinner]

What about the float valves I posted?

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Mikuni-type+float+needles+6-Pk&item=KL18-8957

Anyone have experience with them? I don't think they are Mikuni, but they are cheap.

I installed these last year and have experience no problems whatsoever.

Cheers!
K

[glenwill]

I am running on all cylinders!!

I thoroughly cleaned carb 4, again. I dipped overnight this time, and used steel cleaning wires to get into the tiny holes on either side of the throttle valve.

It started much easier, and revs so much quicker. Took it for a ride, and cannot believe the difference. I think I was on 3 cylinders all last year.

Thanks to everyone for the assistance.

Glen