Missing on cylinders after sitting

[Guest]

So strange but true. 1981 gs650gl missing on number 4 if it sits for a week or more. Last time it sat for quite a while it missed on 2 and 4 for a long while. I suppose it could be missing a little all the time, just more pronounced when it hasn't run in awhile.

Carb sync?
weak coil?

new plugs, caps, valves adjusted, compression is even across all 4.
Might be easy to just swap coils and see if the miss moves with it.
Carbs have been disassembled soaked scrubbed blown out and reassembled with new o rings. I bench synced and synced them on the bike. However I only have vacuum gauges and had to swap back and forth while I plugged the other ports. So could be off a little, but seemed to be good at the time. Course I was riding it everyday then.

Chris

[Big T]

Missing at idle?
Missing at all throttle openings?

[wishy]

All rpm. Just till its not. They do get going after a while.

Still have the atk?

[hillsy]

The fact that it missed on 2 and 4 previously could indicate a coil related issue. Have you trimmed the HT leads down a bit at the caps? The wires can corrode and cause an intermittent spark so trim 1/4 inch off the lead and screw the caps back in.

[chuck hahn]

Sounds more to me like some carb refreshing and a resync are in order. I would also revisit the valve adjustments.

[Fjbj40]

Do the simplest first.

Swap coils around see if the problem follows. If it does look at coils? If it does not follow look at power feed to affected coil. If no problem there then look at shims then carbs.

[tom203]

Trim the plug wires back 1/4 inch...then clean the 4 wire connector under seat that feeds 12v to the ignitor and connects ignitor to both coils.

[Steve]

A carb sync won't change while warming up to clear up a problem, but that doesn't mean that a carb sync is not the problem.

You really need a set of FOUR gauges to do a proper carb sync. There are those that say you can do it with one gauge and a manifold that allows you to switch from one cylinder to the next, but if you have ever seen a carb sync done with a four-gauge set, you will be amazed at how much adjusting one carb affects all the others. By switching one gauge around, you will not catch on to that, and will be chasing your tail for a LONG time.

The forum favorite is the Morgan Carbtune. It comes from the UK, so it's priced in British Pounds. The price is 73 Pounds, shipped to your door. As of today, that converts to $97.82. Use it once, you will be bragging about it, too.

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[chuck hahn]

Correct Steve. But a carb sync WILL get all 4 carbs pulling equally. And if they are CV carbs this is vital for warm up at idle range RPMs. Very often a carb with a slightly more closed butterfly will act like its "trying" but wont heat the pipes till the throttle is opened up with the grip..then fall away when returned back to idle. VMs will do this also but not as sensitive as CVs are.

[jdvorchak]

Sitting for a week on the side stand? First thing that comes to mind is that the #4 is the highest when on the side stand. Means it will dry out first as fuel is still in the lines and interconnects replacing any evaporation on the other 3. If your bike has a "Prime" selection on the petcock then I would try turning it to Prime for 10 or 15 seconds before attempting to start it. Standing it straight up while in prime might help as well.

But since it also misses while warming up and even when up to temp I suggest you pull the carbs and clean the idle jet and passages and the enrichener circuits as well. The BS3x carbs will plug up the enrichener jet, located in the float bows, pretty quickly as they are located at the bottom of the bowl.

Really sounds like you need to pull carbs and give them a good cleaning and check float levels. Don't think that just soaking the carbs will clean up those tiny passages. I use a #10 (skinniest) guitar string to make sure the idle jets and emulsifier holes are really clean. Then judicious spray with brake/carb cleaner then visual inspection of the jets. Then spray carb/brake cleaner into the pilot jet hole and make sure you see a nice stream of cleaner come out in the carb throat just below the butterfly. Same with the enrichener jets. Poke in the guitar string and spray cleaner. The straw on the spray cans fits nicely into the intake of the enrichener jet so it's easy to tell if you have the jet clean.

[OldVet66]

If you can't find the problems described below as the issue check the petcock. My last go around with a similar problem where #4 would misfire and then migrated to #4 and #3, oddly enough, turned out to be the petcock. Replaced it and all is well even after sitting a long time. In my case running it on prime did not help, both fuel feeds were bad.

[wishy]

They seem to pick up and run after warming up.

[jdvorchak]

They seem to pick up and run after warming up.

Another symptom of a partially clogged pilot (idle) jet and possibly the enrichener jet plugged.

[chuck hahn]

Poke out all the jet holes, run some carb spray thru all the passages, and lots of compressed air. Then use a FULL can of this in a full tank of gas. Ideally you really should strip, dip, and redo the entire rack. Get the Berrymans at Orielleys auto parts.

https://www.ebay.com/i/152830759115?chn=ps

[jdvorchak]

As Chuck said but I like Techron Complete Fuel system cleaner for about 1/2 that price and have had great luck.

https://www.walmart.com/ip/Chevron-Techron-Concentrate-Plus-10-oz/16647989?wmlspartner=wlpa&adid=22222222227025239943&wl0=&wl1=g&wl2=c&wl3=71545448329&wl4=aud-310687321802:pla-161071186849&wl5=9022289&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=online&wl12=16647989&wl13=&veh=sem

Either way a good fuel system cleaner just helps with the final complete cleaning of the carbs.