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GS750, cylinders 2 and 3 not running?

  • Thread starter Thread starter stucknarut
  • Start date Start date
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stucknarut

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Apologies if I'm posting something that's been asked a thousand times, the search function's not working well for me tonight.

I've got a new-to-me 1982 GS750T, bought non-running and sitting for a few years with gas in the carbs, pretty gummed up. I cleaned the carbs per the Mikuni_BS_CV guide, adjusted the valves to spec, replaced the air filter with a new (lightly oiled) K&N stock replacement, and sealed the airbox. The bike now starts with choke wide open, but only the pipes from cylinders 1 and 4 (outside 2) get hot, and it needs full choke to run. Fuel is gravity-fed with a funnel while my tank de-rusts, and I blocked off the petcock vacuum port. Spark on both 2 and 3 looks good. I also swapped the wires around and both of the coils seem to be working. Haven't checked compression yet, need an adapter for my tester.

I can pull the carbs again and see if I missed a blocked tube somewhere, but on both carbs? It's a PITA and doesn't make much sense. I didn't see anything in the guide on screw adjustments except for the mix screw, which I set to the standard 2.5 turns on all 4. I also ran in the idle adjust in to make sure they were all open, not sure how much is open enough and figured I could tune it once they were running. I'm a little concerned about the hoses from airbox to carb, but don't see any obvious gaps. Maybe will try spraying some starter fluid on them tomorrow to check for leaks.

Any other ideas?

Tim
 
Update, maybe electrical? Resistance across the coils is the same, around 4.5, but one set of wires has 11.5v, the other 10.5v, checking from positive to ground. Would that make this much difference? I will search the archives for more info.
 
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If you got decent spark on #2 and #3, I'd blame clogged idle circuits in those carbs. Did you disassemble them and soak? Many folks have had to clean their carbs more than once- it is a PITA, but gummed up idle circuits prevent the CV carbs from responding. After it runs a bit with 1 and 4 on choke, check spark plugs 2 and 3 and see if they are dry or wet.
 
I disassembled and soaked, but maybe I missed something. You're right, after running for awhile the 2 and 3 plugs look dry. Dangit. Guess I'll let them soak longer and see if that does it.
 
stucknarut said:
I disassembled and soaked, but maybe I missed something. You're right, after running for awhile the 2 and 3 plugs look dry. Dangit. Guess I'll let them soak longer and see if that does it.
Click to expand...
If it runs full "choke" with 1 and 4 , but 2 and 3 plugs look dry, you likely got blockage in the "choke" ( actually an enrichment device) in these two carbs. When you clean them again, pay attention to that little hole in bottom of fuel bowl that feeds the choke system, blow it out and also check that the passage is clear up to carb body- I find Gumout spray with the tube works well to blast things.
 
Thanks - the second cleaning did the job, all cylinders firing, and took my first test ride! For about 50 feet, only to lock up the front brake due to all the crud in the master cylinder. On to my next job, disassembling and cleaning the brake system. Probably should've done that BEFORE the test ride :).
 
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