GS1000G Backfires and dies when decelerating.

[Chris Bock]

‘80 GS1000G Dies after backfire when decelerating.
Engine runs great when on the center stand.
Dies when riding after I decelerate (engine brake) and it pops a couple backfires. After a minute or two, will start up and run enough to hobble home.
Stock everything except for the coils.
New Dyna coils, leads, and caps.
New NGK plugs gapped to 29. Look healthy, not fouled or lean.
New oil/air filters.
New Carb Airbox boots. Intake manifolds are in good shape.
Complete tear down, dip, and clean of the carbs.
Fuel mixture screws around 2.5-3 turns out after tuning.
CarbTune synced.
Floats are set at 22mm.
Rebuilt petcock.

I found a post from years back with the same issues, but there was never any real answers.

[Buffalo Bill]

Have you done a compression test?

[LAB3]

The only thing that comes to mind is a hung up carb float, a couple of taps on the bowls after it quits would be worth doing as a quick check.

[1978GS750E]

I would suspect the petcock is the problem. I am guessing, (an very uneducated guess at that) that it is related to the vacumn pull on the petcock. More info might help.

How far are you going before it stops? Does it "only" die on deceleration? Does it run good under power without any hesitation?

[Chris Bock]

-Runs great up to WOT on the stand. Feels good riding until decelerating. First time was after 3-4 miles on the first ride after test rides during the restoration. Now it’s within 1 mile, after warm up.
-Complete carb clean rebuild. Measured the floats. Tapped the float bowls after it’s died, didn’t have an effect. Still was only able to limp home. Had to keep wicking the throttle when slowing or stopped, or it would die.
-I haven’t done a compression test, but all cylinders fire, and hold balance after syncing. No smoke. Plugs look great.
-The PO installed new valves, rings, and gaskets on the petcock. He put the spring washer on the wrong side of the selector lever! I’ve gone through it, and assembled it correctly. Seems to operate properly. Tried running on Run, Reserve, and Prime, after it dies, but no difference.
-Won't start right after it dies, but starts right up after a couple minutes.

[Chris Bock]

The PO's petcock is new generic aftermarket. If it was made for a smaller bike, but fit the gs1000g, could there be a flow issue? As in, the amount of fuel being restricted when running, not keeping up, and draining the float bowls.?

[Buffalo Bill]

If there was a fuel restriction, it would bog and die during acceleration, not deceleration.
If the PO did a valve job he could have made mistakes assembling the camshaft/timing chain - timing.
Compression test will reveal problems.

[Chris Bock]

Did a compression test. Cyl's 1-3 were at 96. Cyl 4 at 92. Went WOT uphill after the backfire/die/wait/restart. It was totally bogging down, so I'm thinking it's fuel restriction.
Again, the engine runs terrific on the center stand, all way up to WOT. If it was timing, it would be pretty evident, right?
I'm going to swap out the tank from my GS850, which runs like a champ. Then I can verify if it's the petcock. Or even the tank cap valve?
I don't believe the PO messed with anything as far as valves and timing. He was way out of his league just replacing the petcock, and he never mentioned any engine work.

[Chris Bock]

Crap. Those compression numbers might not be accurate. I was at my buddy's shop, and we checked compression on a few bikes. I'm gonna check it again, and log the numbers. I do know that all four cylinders were within 5 psi, and that was a dry test. If they were 90-95 dry testing, though, would that be low? Maybe needing new rings, and honing?

[Buffalo Bill]

Those are deaths door comp numbers. Look up compression ratios here on the site for the GS. I think 110 is the bottom end, 150 is healthy.
If the engine was warmed up before the test, that's the best reading.
With only the valve cover removed you can do a valve clearance test. Could be it only needs new shims to wake up. Surely needs a few anyway.
Compression is power, low compression, low power.
Running good on the stand is only useful for carb tuning.

[Chris Bock]

Picked up a compression gauge, and rechecked.The other numbers were from my donor bike, and I spaced out in my note taking.
All cylinders are at 140 after warming it up.
Put some Marvel's in the cylinders, and got 160-160-145-145.
Dialed in the carbs, again... just to make sure that isn't the issue.
Runs great for about 6 blocks, then starts to stumble. Dies if I don't keep wicking the throttle, but bogs down if I go WOT on inclines.
I'm gonna pull the valve cover next, but thought I'd check in with y'all to see if I'm missing something right in front of me.

[salty_monk]

Have you looked at the ignition? There is a series of tests for the Igniter box in the Clymer manual. Have you checked the advancer is advancing and returning correctly?

I had a Dyna coil fail that caused something similar to this. I also had an igniter box fail that showed some similar sorts of symptoms.

Have you checked the new valves are in adjustment?

Does it have the original airbox on it & a filter in it? Is it running standard jets?

[Nessism]

Have you tried the 850 tank yet? Another thing to try is to carry a spare spark plug with you and when the bike dies pull off one of the leads, install the spare plug, lay it against the side of the engine and look for spark while cranking it over. Realize of course, that in bright daylight the spark will be very faint, so don't be fooled. One other check, back to the tank, is to pull one of the float bowl drain plugs after the bike dies to see if there is gas. Please carry a container and rag to catch any gas that may be in the bowl.

Oh, and it goes without saying that you need to check the valve clearances. It's not the cause of this problem, but that doesn't matter.

[Chris Bock]

Just installed a dyna s ignition. Was thinking about the ignitor as the problem, and figured I might as well get rid of it. And I've already installed dyna coils and leads, and a new R/R.
Static timed 1-4, but there's no marks for timing on the 2-3 side.
Any tips for dialing that in?

[Chris Bock]

Did try swapping out the tank (with a petcock that has proven to work) but that didn't have any effect.