1979 gs850 second cylinder not firing

[Guest]

Hey folks, I'm a bit stumped here so I think it's time to ask for help-

The second cylinder on my '79 gs850 won't fire. I haven't tested this at higher rpms than idle yet, but at idle the second header is icy cold.

Some background; Got the bike 6 months ago, ran great (easy one kick starts with a bit of choke), rode the hell out of it. Started running poorly when winter came, ended up lowsiding in the rain and decided to park it for the rest of the winter to clean her back up. I was pretty sure the running issues (stalling on idle, bogging down through the bottom half of the rev range) were just gummy carbs, so I popped em off and rebuilt them.

I've done/checked

-valves (they were off, but they're good now [~.09mm])

-compression is strong (all four between 130-120psi, checked after the valves were sorted)

-new plugs

-new resistor/spark plug caps, new coils 9 (4 ohms...should be the same as stock I think. The bike was upgraded by a previous owner to electronic ignition with a coil relay
mod, still figuring out all the specifics of those changes). Checked for spark (grounding old plug against frame, solid bright spark on all four. I changed the coils because I swapped the leads and it seemed like the misfiring cylinder moved with the leads (odd as it's only one misfiring though)...but I guess that wasn't it after all.

-Rebuilt carbs (the '79 uses the VM carbs, not the CV carbs). Returned everything to the same settings I found them at, testing running with the airbox on. Didn't look terrible to be honest, besides some oxidation in the pilot circuit. I followed the guide for the VM carbs on BikeCliff's site to the t. Only possible issue I saw there was that the orings I got (from the supplier gent everyone here uses) for the fuel mixture screws didn't fit quite right and were a real pain in the a$# to get the screws back in. So those probably need adjustment, but I think they're where I found them. I didn't see any notable differences between the carbs, or anything off in carb #2

I'm running old gas in it, but it does have stabilizer in it and the other three cylinders don't mind it.

What am I missing? I've heard about #2 plug fouling because of crappy petcocks, but I think that only applies to CV carbs and these are brand new plugs anyways. I feel like I've narrowed it down to something in the carbs, but I can't for the life of me think of what it is or how to troubleshoot this. The old plugs were pretty sooty, if that helps at all. They looked like it's been running rich.

I'm thinking for next steps-

open the drain screw on carb #2 and make sure it's getting gas. If yes, take carbs off and see if starter fluid makes each cylinder fire.

Any suggestions?

[Phred]

Check the float bowl. I have only cleaned one set of VM carbs but to my recollection there is a little hole that runs through the float bowl. This can easily be overlooked when cleaning the carbs. This can easily get varnished if there is old gas sitting in it for a while. I used carb cleaner first but then bought a sonic cleaner which did a really good job.

[Steve]

The second cylinder on my '79 gs850 won't fire. ... I've heard about #2 plug fouling because of crappy petcocks, but I think that only applies to CV carbs and these are brand new plugs anyways.

First, let's make sure we are talking about the correct cylinder. The cylinders are numbered from left to right, as you are sitting on the bike. #1 is under your clutch hand, #4 is under your throttle hand.
CV-type carbs have their vacuum line to the petcock on the #2 carb, but the VM carbs on your bike have it on #3 carb.

-valves (they were off, but they're good now [~.09mm])

-Rebuilt carbs (the '79 uses the VM carbs, not the CV carbs). Returned everything to the same settings I found them at, testing running with the airbox on.

Technically, your valves are out of spec, but many of us DO run them as loose as 0.10mm. The critical thing is that they should not be too tight. Minimum spec is 0.03mm.

"Returned everything to the same settings" is only good if they were the proper settings to start with. The pilot fuel screws (the ones on the bottom of the carbs) should be 3/4 to 7/8 turn out from LIGHTLY seated (for a stock intake and exhaust), the pilot air screws should be about double that, or 1 1/2 to 1 3/4 turns out. Set the fuel screws, leave them alone. Start with the air screws about 1 turn out, to have a slightly richer mixture. Warm up the bike, slowly turn the air screws out, listening for a change in engine speed. When the speed drops, turn the screw back in about 1/8 turn, move to the next one.

.

[roeme]

Ah, a fellow rider working on his VM carbs. This thread will be watched.

(...) to my recollection is a little hole that runs through the float bowl. This can easily be overlooked when cleaning the carbs. This can easily get varnished if there is old gas sitting in it for a while.

This I can confirm. What I don't yet understand is this passage's purpose. To pass fuel to the pilot fuel screw?

What I would give for someone to provide an useful VM carb schematic/cutaway drawing...

[steve murdoch]

Lots of info and drawings of the VM carb in this thread from Mikuni.

http://www.mikuni.com/pdf/vmmanual.pdf

[uk gs nut]

I believe that brass tube and the little jet at bottom of float bowl where the tube sits, is for the choke fuel pick-up.

[roeme]

Lots of info and drawings of the VM carb in this thread from Mikuni.
http://www.mikuni.com/pdf/vmmanual.pdf

I have found this before; but skipped it as it refers to a different model of VM than the ones mounted on my bike. Now looking at it again, it seems that the differences aren't huge, and consist more of layout changes rather than differing operation principles. Thanks for linking it again.

I will study this document more in-depth this evening.

As mentioned, seems it provides fuel to the choke circuit.

[Steve]

I can confirm. It is, indeed, the supply for the "choke" tube.

.

[uk gs nut]

Thanks Steve, I sent a set of those tubes to Australia for member tfb for his VM 29s.

[jdvorchak]

The enrichener (choke) is a hole cast into the float bowl. It picks up fuel from the bottom of the float bowl and routes it up to the enrichener plunger. I've never seen an old set of carbs, that have been sitting, where that passage way was NOT plugged. I squirt carb or brake cleaner into the casting from the bottom of the float bowl and then back down through the hole back into the carb bowl. In most cases I use a "E" guitar string to clean out the jet, that is in the bottom of that casting, to get it really clean. I've never seen that passage way plugged above that metering jet so once you get it clean you should have a working "choke".

[jdvorchak]

Another thought. I never set the mixture screws "back to where they were". I don't trust anyone but myself for tuning carbs and I've found that, more often than not, it's pointless to adjust them from the factory preset after you clean the carbs.

Careful bench syncing the carbs always seems to get initial sync close enough that the bike will run good without vacuum sync.

As for vacuum sync. Don't be in a hurry to perform it. Bench sync will get it very close and things tend to settle in after a few miles of riding. Especially if the bike has been sitting for a long time. If it starts easily, idles smooth on all 4 and takes the throttle without hesitation, when warm, ride it for a while. And next time you want to do service would be the time to vacuum sync.