Very Odd Suspected Ignition Issue

[Scoutmann]

Hey Everyone!

I recently traded one of my Sportsters for a buddies 1982 GS850. I’ve loved the bike since we helped him buy it last year. He was a new rider, so the bike went down a couple times in rookie mistakes, both he and the bike were okay, just a little worse for wear. My plans are to restore the bike, and turn it into a longer range bike than what the peanut tank on my sporty will allow.

Enough with the backstory, here’s what’s going on.

So the carbs were rebuilt and synced twice last year, once for a freshen up as the old girl had sat for 20 years, and again because of leaking fuel between the carbs. The bike started dumping fuel and stopped running well about January this year. I got it 3 weeks ago. I took the carbs off and found the floats out of spec per the service manual, so I adjusted them into 22.4mm +- 1mm, including the lip where the gasket sits. I then found the petcock had failed, and constantly dumped fuel in the vacuum line, so I blocked off the vacuum line to the carb with vacuum caps brand new from O Reillys. I adjusted the air fuel mixture by turning the mixture screws 3 turns out from seat and turning them in slowly until the idle decreased and then turning them back out 1/8 turn. This is where I started to notice something weird. Cylinders 3&4 gave a very noticeable idle decrease, while cylinders 1&2 barely took an adjustment. Then I adjusted the idle into approx 1050ish per the bikes tach. On a 3 mile ride around my neighborhood afterwards, I noticed that the motor would rev seemingly normally, but with no power and top speed in 4th and 5th was 60, per my GPS speedometer app on my phone. The bike would die if I pulled over and stopped, but would refire normally and strong. On a stupid whim, while stopped and running I slightly touched cylinder 1 and then 2 exhaust pipes, noticed they were warm, rather than hot. I could hold them without burning myself, while 3&4 seemed at normal operating scalding hot temperature. My buddy and I pulled the plugs, 3&4 had strong spark to the cylinder head, 1&2 had weak spark. 3&4 plugs looked like normal running plugs. 1&2 were wet, but not overly. Plugs are gapped to approximately .025. Performed compression tests, warm bike and wide open throttle. All cylinders were over 110 psi, with 1&4 over 150 psi. My compression tester is a cheap Oreillys special and I could not get the best seal on cylinders 2&3, but they read 110 and 120 respectively, but I believe them to be higher.

Based on the weak spark at 1&2 cylinders I’m believing to start there, but I’m at a loss.

Thank you in advanced, sorry for the information dump!

[zed1015]

Cyls 1-4 and 2-3 are paired by the same coils so if you had a single faulty coil or associated wiring the weak sparks would either be on 1-4 or 2-3 as pairs and not 1-3, 1-2 ,2-4 or 4-3 .
Apparent weak spark symptoms on 1-2 indicates a non ignition related problem which is causing plug fouling.
The wetness could be due to overfueling/ or oil fouling - badly worn stem seals - worn oil control rings or even head gasket failure between 1 and 2 but that should have shown up on compression or leakdown tests.
Compression readings should be within 15psi from highest to lowest so worth confirming the actual readings on 2-3.

[tom203]

This “while cylinders 1&2 barely took an (mixture) adjustment“ and cold exhaust pipes suggests 1 and 2 are too rich to fire consistently.
i d check 1and 2 “choke” plunger positions in “off” spot…compare to 3 and 4 positions in case something got whacked.

[zed1015]

This “while cylinders 1&2 barely took an (mixture) adjustment“ and cold exhaust pipes suggests 1 and 2 are too rich to fire consistently.
i d check 1and 2 “choke” plunger positions in “off” spot…compare to 3 and 4 positions in case something got whacked.

Really should have come up with that suggestion myself since i specialise in choke plunger repair .
The rubber seats in the end of the plungers should also be inspected as even if they are fully closing they are likely to be worn and leaking.
Not all the plungers degrade at the same rate either which can really throw a spanner in the works if you don't know what to look for.

[Gorminrider]

as above diagnosed, not spark. I suspect lack of fuel mix and especially where bike dies at low rpm-the idle passage and jet...
"no adjustment" by idle mix screws confirms ... "bad compression" may indicate valves not adjusted=too tight. You should try a more thorough job at the carbs and valves. Tear the carbs down completetly including jets....It's not unusual to do it twice if it wasn't complete ...and loosened crapola reblocks passages and jets.

...too much gas commonly fires and smokes but there's no lack of power unless it's extreme. Try pulling choke part way and see if it helps.