GS1000G 1980 intermittent cylinder power loss and stalling

[bluebeam]

So this is a bike that had been sitting some six years before I bought it. I fixed it up and got it through inspection without any issues. Haven't touched the engine, other than oil and filter change, airbox cleaning and fitting a K&N filter, since it ran so very well. After that, it has been running for maybe 10 hours. The last couple of days, I've experienced the following:

1. Bike runs great for the first 20-30 minutes.
2. Then, while cruising att low speed ca. 45 kmh, some cylinders suddenly stop firing. At least two, maybe even three.
3. Approaching an intersection with less throttle, the engine stalls.
4. Engine starts properly on third try and then runs as usual.
5. The above can then repeat itself after another 10-15 min drive.

I would appriciate some tips on what to check first in this case. Someone had a similar experience? Thanks.

[Redman]

Welcome to TheGSR.

Bike sat for 6 years, and you didnt have to do a carb cleaning. Must have been put up for storage in a good manner. Otherwize you would not be saying "runs great" at any time.

When say runs for a while, then has trouble, and restarts after a short while, that makes me suspect the vent in the gas cap. GO for another ride, when it starts to stumble after a while, open the gas cap (easier if have another key) and see if symptoms clear immediately.

Other suspect, for somewhat random but more heat related, could be the sparkplug caps. They have resisitors (5K ohm) in them can go bad (higher ohms) and cause strange-intermittent ignition problems. The caps are removable from the wire. Check with ohm meter.

Does it start easy? Have you checked valve clearance?

Hey, wait, we havent seen pic of the bike. That is suppose to be required before can give any assistance. Need to know if the maroon one or the black one or if custom paint.
(Joking)
Again, Welcome to The GSR.

Other folks here will be along and have more ideas.

[oldGSfan]

Years ago I spray painted my CB750 and stuffed shop rags in the fuel filler hose. One of the rags dropped inside and would occasionally impede fuel flow. I would stop and the bowls would fill up, and off I'd go. It took me ages to figure that out, as sometimes the rag would be on the other side of the hump. Boy did I feel stupid.

Anyway, the characteristics you describe make me also suspect fuel starvation. I also once had a bike that would only run on reserve, the petcock was vacuum operated and was messed up. Maybe try it on reserve if it is a vacuum type, because reserve bypasses the vacuum need - at least it did on mine. Easy to tell if there's a rubber hose going to it, and check the hose for any problems.

[bluebeam]

Thanks for replying. Now I got some good leads to follow up;

1. Gas cap not venting
2. Bad sparkplug cap resistors
3. Valve clearance out of specs
4. Petcock-vacuum fault
5. Fuel hose clogged / gas tank debris

The bike always starts without problems (with choke when cold), even down to 2 degrees Celsius. Maybe I'll post a picture later

[allojohn]

Investigate fuel starvation first. Run it on Prime to check the petcock.

[Brendan W]

Have a look at the condition of the coil supply plugs under the tank.
They can hide a lot of corrosion inside.
You could check the voltage at the coil side of the plug.
I had a lot of trouble with misfiring in the first few months.
Only went away after I cleaned those plugs, opened and cleaned the contacts in the ignition switch and the kill switch and thoroughly cleaned the fusebox.
90% of carburettor problems are electrical

[bluebeam]

It may be too early to mark this problem as solved, but after three 30 minute runs, the bike runs flawlessly. I did the simplest thing first: setting the petcock to "Reserve".

So here it is as requested: Suzuki GS1000G 1980 with a Rickman "Polaris" fairing, complete with those Jaguar mark 10 indicators. Top pack from Denfeld. The fairing and the top pack makes it look considerably less cool, but they are a part of this bike's history, so they are staying on! And to be fair, they are extremely handy.

Suzuki GS1000G 1980.jpg

[Redman]

/////////////////QUOTE
It may be too early to mark this problem as solved, but after three 30 minute runs, the bike runs flawlessly. I did the simplest thing first: setting the petcock to "Reserve".
////////////////ENDQUOTE

Do you mean you have to leave it on REServe? Or do you mean you had it there for a while and now can us the ON position?

REServe is the same as ON, in that is vacuum operated the same as ON, just that it uses the lower portion of the filter that is in the tank. So if runs better in REServe than does in ON, I would suspect the upper part of the filter is clogged up, or are real low on gas in the tank.

[rphillips]

That don't sound right, both the "on" (long) inlet tube & the "res". (short) inlet tube are both housed inside the same filter. If upper part of filter is clogged, fuel will still flow thru lower part of filter. I don't see how fuel will flow on "res" but not on "on" unless the longer "on" tube is partially clogged & that just doesn't seem probable.

[allojohn]

Switch it back to the "ON" position - see if the problem still exists.

[Grimly]

Do you mean Prime?

[salty_monk]

If the coils aren't stock I've had it where the coil lead can be partially disconnected at the coil end causing similar issues.

I once had a CDI box go bad on a 1000G and it caused all sorts of weird problems! Good luck

[bluebeam]

I switched it back to "ON" and did 2x45 min runs. No problems. That should indicate clogged fuel conduits, as some of you suggested, that somehow got "de-clogged" when I ran it on "Reserve". So a complete fuel supply system overhaul is what has to be done now; clean the tank (it has some rust), new fuel hoses, install a fuel filter, and maybe have a look at the petcock, if it can be refurbished. Thanks for all your help!

[Cipher]

Do you mean Prime?

Yes the petcock is a tad confusing in its markings.

I vote for a bad petcock vacuum hose or petcock internals.