#3 misfire when cold

[Guest]

Hello and thanks for this great forum. My bike is a 1980 GS1000L. I have rebuilt the carbs and the airbox using the carb cleaning series and the o ring kit found here, and I have replaced the intake boots and o rings on both ends of the carbs. Also adjusted the valves and balanced the carbs. Bike now runs really well. Cylinder #3 seems to need a little time to get going when cold though. The carb has a repaired float post (also done from this forum), but I don't think that's it. As far as I know, the coils, coil wires and spark plug caps are original. Once warmed up, it runs like a champ. Any ideas? Thanks. Bike is all stock as far as I know.

[roeme]

How did you determine that #3 need some heat before doing its part?

[Nessism]

Did you use a piece of wire inside the choke tube to make sure it's open? Also, check the passage in the float bowl to make sure that flows through too?

[Redman]

..............
........................ As far as I know, the coils, coil wires and spark plug caps are original. ...............

Welcome to GSR.

Certainly sounds like you are doing a good job of it.

Good that you recognize that the sparkplug caps are separate items.
The symptoms you mention dont sound to me like what a marginal sparkplug cap might cause, but the caps are easy to check (and not expensive to replace).
If you havent checked the caps, I suggest that you do. Each should measure about 5K ohms.
Or the even easier thing to do is to measure from cap 2 to cap 3, should be about 30K (5k cap + 20K wire and coil +5k other cap). And same for 1 to 4.

Again, Welcome to GSR.
Do post pic of the bike.

[Guest]

For sure. I followed the rebuild series quite thoroughly.

[Steve]

Also need to verify that we are discussing the correct "#3".

The cylinders are numbered from left to right, as you sit on the bike.
#1 is under your clutch hand, #4 is under your throttle hand.

Are we still discussing the same #3?

Regardless of position in the rack, if it seems to clear up when warm, it appears that the starter circuit or the pilot circuit is in need of some assistance. As Nessism suggests, make sure the pickup tube for the starter circuit is clear. If that is clear, make sure the pilot circuit is set properly.

.

[Guest]

Cylinders are numbered correctly. #3 was identified as the problem by the old spit on the header technique. The one thing I'm not 100% on is the idle mix screw setting. Best I gleaned from this forum was to start with the screw backed out 2 turns, then while warm and idling, screw it in until idle begins to drop, then back it out just enough to restore idle speed. All 4 screws seemed to adjust a little differently, but all needed to be more than 2 full turns. I'm having trouble uploading a pic.

[Steve]

Yes, there are several on the forum that say "start at 2 turns out", but I have found that many bikes require just a bit more than that. An engine will run better when slightly rich than when slightly lean, so I start at THREE full turns out. Most carbs will dial-in between 2 1/4 and 2 3/4 turns, but I have had some that required about 4 turns out.

You might try turning #3 out another turn or two to see if it starts running better, then adjust as necessary.

Remember: not all the carbs in the rack will necessarily require the same setting.

.

[Guest]

Thanks Steve. My screws are in that range more or less. I'm beginning to wonder if it may be electrical. it only happens when started from dead cold, and shows the same symptoms with choke out or in. it only lasts a minute or two, with the cylinder firing first not at all, then firing here and there, then finally correct performance. The idle is then consistent but a little under 1k. After a good ride, (motor performing great) the idle also smooth and consistent a little over 1k. It's not a big concern, but I do want it to eventually run like new.IMG_1648.jpg

[Steve]

I'm beginning to wonder if it may be electrical.

Redman suggested that in post #4.

A quick check would be to pull the spark plug caps from #2 and #3, use your ohmmeter to see if you have about 30k ohms between them. If you don't, unscrew #3 cap from its wire, measure the resistance from the screw to the terminal that touches the plug. It should be 5k ohms. If it's not, unscrew the terminal, check the resistor behind it. You can replace the resistor with a piece of heavy wire (welding rod, hanger, etc.) or you can replace the cap. If it seems to measure about 5k, clip about 1/4" from the end of the plug wire and screw the cap back on. While you are at it, clip about 1/4" from ALL the wires.

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[Guest]

I'll do that, but no more wrenching for a week or two. I'll update when it's done. Thanks all.

[Steve]

What brand of tires?

They appear to be REALLY sticky.

.

[Guest]

I need to quit hanging from the rafters when I post pictures.

[mvalenti]

Must be from Australia...

[standswithabeer]

you can easily swap #2 and #3 plug leads, to see if the problem moves to #2.