Sync issues, erratic idle

[ChrisMA]

I've made multiple attempts to sync my carbs, but my GS1000G still isn't running right. I suspect there may be a leak somewhere, but if so, I'm not finding it. Bike is all stock, and I believe I've gone through all of the required maintenance (details below), including detailed carb cleaning, float adjustment, orings, valve adjustment, new boots & manifold pipes, fuel & air lines, air filter, sealing airbox, etc.

A few weeks back, I synced the carbs (bench first, then carbtune). I got them dialed in just right (2&3 even, with 1&4 just a little higher), and right around the middle of the scale. It was idling beautifully, had it dialed in around 1000 rpm, and throttle response was great. When I took it for a spin, I noticed some mild popping when letting off the throttle, figured I just needed to tweak the mixture. But when I got it home, I came to a stop and it stalled out. If I dialed up the idle, the rpm was way too high next time I started it. One of my air boots (#1) felt a little loose, so I ordered a full set. New boots are tight.

I re-synced the carbs again this weekend. This time, the vacuum pressure was low - off the scale (I believe <8 cm), so I had to flip the carbtune to get them synced. I finally got it so the bars were synced up, but they were all just above the bottom of the "right side up" scale. During both these tests I had the petcock vacuum line plugged. After all this, I tried adjusting the air screws again and settled around 3 turns out.

Took it out for a ride this morning (about 30 minutes), and it repeated the same behavior: sporadic popping when letting off throttle, then towards the end of the ride, it stalled out whenever I would let off the gas long enough for the idle to drop. I again tried dialing the idle adjustment screw so it would hold an idle, though when I would blip the throttle, the rpm wouldn't drop off quickly, but would "hang" for a bit. After letting it cool down for a few minutes, when I started it up the idle was up >3K. I pulled the plugs, and #s 1 and 4 were white, though I'm not sure if that's from the last ride or my previous attempt. So I'm guessing that I still have air leaks, but despite multiple attempts at spraying all around the intakes, I haven't found any.

I'll try dialing the air screws out a little farther, but if they're already at 3, I don't think I have much more room to play with. Is there anything obvious I could have overlooked?

Here's the long list of what I've done:

• Adjusted valves, replaced gaskets
• Fully stripped and cleaned carbs following both sets of instructions on Bikecliff's site (multiple times), including checking all passages
• Replaced o-rings & float gaskets
• Adjusted floats (by measurement, not yet set up to directly measure fuel levels)
• Replaced air filter (Uni filter and oil)
• Re-sealed airbox with weatherstripping
• Replaced airbox boots and carb manifold pipes (along with their o-rings)
• Tested for leaks around intake boots a couple times (sprayed with wd40 and carb cleaner)
• Replaced all fuel and air lines
• Synced carbs with Carbtune Pro
• Adjusted pilot screws, starting at 2.5 turns (all currently around 3 turns)
• Oil, filter
• New battery
• New NGK plugs (PO had recently replaced leads, also NGK)
• Cleaned tank (EvapoRust)
• Recent petcock (replaced by PO, looks new)
• Tested compression (on a cold test, all 4 are within spec, haven't tried a hot test)

(Also, FWIW, exhaust is stock, though I have a rattle in the right side pipe so I know there's some internal rust. But no obvious external rust or holes. I haven't tried replacing the exhaust seals yet, though there are no obvious signs of leaks.)

[scratch]

By what methodology are you adjusting your mixture screws? Is it just arbitrary partial turns for all four together or, are you working each carb to just back from high idle?
Have you checked the vent hoses? Two ff the carbs plus one off the fuel level sender... Venting issues can do strange things

[tom203]

Make sure the "choke" cable is fully retracting the plungers- i.e. not holding them open somehow.

[ChrisMA]

I'd tried doing the "high idle" method: for each carb, trying to find a point midway between where the idle increases and then drops off. But I thought I'd read that around 3.5 turns open was about the limit, and didn't push it beyond that. Maybe that was for different carbs, though - I'll try again.

You might be on to something with the vents. I sucked a little air through them both, but the one on the right side seemed restricted at first. Maybe it was crimped under the tank. We'll see if it makes a difference. (The third line you mentioned, from the tank sender, that one I'm pretty sure is just a safety drain to divert fuel away from the engine in case of a sender leak).

[ChrisMA]

Good idea to check the choke cable, but the plungers do appear to be fully closed when the choke lever is pushed all the way down. Thanks for the suggestion, though.

[scratch]

I'd tried doing the "high idle" method: for each carb, trying to find a point midway between where the idle increases and then drops off. But I thought I'd read that around 3.5 turns open was about the limit, and didn't push it beyond that. Maybe that was for different carbs, though - I'll try again.

You might be on to something with the vents. I sucked a little air through them both, but the one on the right side seemed restricted at first. Maybe it was crimped under the tank. We'll see if it makes a difference. (The third line you mentioned, from the tank sender, that one I'm pretty sure is just a safety drain to divert fuel away from the engine in case of a sender leak).

Safety drain, cool... did not know that. Is there any other venting for the tank? How much gas is in the tank?

Thinking about this after looking at your post again:

It ran well on a fuel bottle with a plugged vacuume line
Ran poorly on tank with vacuum line attached

Perhaps that is a good experiment to repeat. just to be sure.. if the results repeat... start looking at the #2 carb, petcock & vacuum line.

[ChrisMA]

Is there any other venting for the tank? How much gas is in the tank?

The only tank vent I'm aware of is in the cap, which I suppose could lead to a lean condition if the venting is poor (?) When I drained the tank yesterday after the bike cooled down, it had maybe 1.5 gallons in it.

It ran well on a fuel bottle with a plugged vacuume line
Ran poorly on tank with vacuum line attached

I'll certainly take a closer look at the vacuum line. I'll also look into testing the petcock (though it seems to do its primary job well: fuel only flows when there's a vacuum).

I'm just not sure that I can say for certain that it's running well on a fuel bottle, given that these issues really only seem to show up during a long-ish ride, and the significant difference in overall vacuum between two two sets of tests. When I'm syncing the carbs, I have a box fan on the bike, but I'm still worried about overheating and stop after a while to let the bike cool a bit before re-testing. So, I'm not sure that I'm seeing the same conditions when I'm testing in the garage vs on the road.

Here's a view of the plugs, for what it's worth. They only have about 20 miles on them, and I didn't do a "high rpm cutoff" type of test. But I assume that 1&4 are lean, and 3 looks rich?

IMG_20180730_080219.jpg

I re-checked the air screws, and all were between 3 - 3 1/4 turns out. When I test next I'll do a more thorough job of checking for rpm drop at both ends of the adjustment range for each cylinder.

[TxGSrider]

I'd think the blocked vent would be easy to rule out. Unhook the fuel line and put it in a gas can then switch to "PRIme" and let it run for a couple minutes. If its blocked, the flow will stop or come to a trickle. If it does, loosen the cap and see if the flow speeds up. If that doesn't make any difference then maybe your petcock screen that goes into the tank is blocked or really dirty.

[ChrisMA]

Victory!!!

After all of that stress, it was the airbox. Even though I'd already sealed it, it seems there were likely still some leaks, probably along the outer covers (which could explain 1&4 running lean). I ripped out all the foam sealing tape (along the outside edges as well as above the filter), scrubbed the surfaces, then re-taped. The one thing I did differently was to add more insulation around the inside of the side covers (a tip of BassCliff's which I'd overlooked the first time around). I then took the carbs off and bench synched them again. This time, once I had them dialed them in with the carbtune, the bars had jumped up to 1/3 of the way up the scale and stayed there.

I've just now enjoyed my first test ride without any stalling, hesitation, racing or backfiring. And when I got it home, the idle was right around where I'd set it.

Thanks, everyone, for the moral support and help. I'm amazed... I finally have a bike again!

[mvalenti]

Congrats! Got some pics?

-Fellow M*******

[ChrisMA]

Congrats! Got some pics?

-Fellow M*******

Thanks, Mark - always nice to meet fellow Massh***s here on GSR

I need to take some more pics, but here she is, both dressed and nekkid:





Paint work will be the project for next winter...

[mvalenti]

oh thats not naked.. IMG_3026.jpg thats naked..

[ChrisMA]

oh thats not naked.. [ATTACH=CONFIG]55825[/ATTACH] thats naked..

I stand corrected! Nice work!

[mvalenti]

Thank u and right back at ya.