Fuel Starvation Symptoms - 79 GS 550 - I've Hit the Wall (pic included!)

[Guest]

Afternoon GS Gents!

I'll get straight to it: Has anyone ran across false fuel-starvation symptoms?

If interested, please read on:

I've hit the wall on this latest troubleshoot, and I'm hoping the collective wisdom here has run across something like this before. Two years ago I finished the rebuild on my 79 GS550 (see pic below, it's my baby!) and it ran like a champ. Since completing the rebuild I've rode it thousands of miles without so much as a hiccup, even xc with a passenger!

All of a sudden it dies. It acts as though there is no fuel: the sputtering, surging, if you add too much throttle it tries to die. This happens intermittently, two or three times then the problem vanishes before I can diagnose it. It's hit the wall. Now it only starts with full choke, idles for a few minutes (don't even try the throttle) and dies.

The rebuild was comprehensive, so I'll debunk a couple things here. Petcock rebuilt and tested, carbs fully disassembled and dunked as per bikecliff's recommendations and rebuilt w/ Mr. robertbarr kit (thanks guys!). The engine was rebuilt from the rings up ('new' head even), Dyna S ignition, new intake boots, o-rings, and refurbished wiring harness - the works. I even re-cleaned the carbs and petcock after the it stopped running, but I found nothing and nothing changed.

All this was completed, it ran well, and now it's dead in the water. Has anyone ran across something like this? False fuel-starvation symptoms on a rebuilt GS? Or, does anyone have any insight from the GS experts that inhabit this forum?

I'm hoping to have it ready for spring - happy riding gents!

[tkent02]

Kinked fuel line? Leaky or kinked vacuum line? How does it run on Prime?

Are you running an inline fuel filter?

[Eli69]

do the carb bowls actually become empty?

[tom203]

Yes, put petcock on "PR" and make sure flow flows at all times- once you know that fuel flows to carbs (now, not 1 year ago, cuz stuff happens), then you can blame carbs , or ignition circuit (but this is unlikely by your description)

[hjfisk]

Sounds like a faulty petcock. Does it run on prime?

[derickson104]

Fuel Starved Part 2

Howdy Gents, an update based on your suggestions:

I pulled the carbs out and the tank off, and rigged up a few fuel flow tests.

1. on Prime, full fuel flow (putting the petcock on prime never affected running quality anyway)

2. In-Line Fuel Filter: yes, I am running one and fuel flows through it like a waterfall.

3. Float-Bowl Test: I hooked the tank (w/ in-line filter) to the carbs and removed the drain plugs. When I apply vacuum to the vacuum line (petcock set to 'ON'), fuel runs out the float bowl drains. All the filters and lines were the originals from off the bike. As well, none of the lines had kinks or sharp bends when on the bike.

Thus I know fuel is getting to the float bowls. The next step, I suppose, is another full dunk and clean of the carbs? I'm thinking about the ignition circuit - could problems with it create a smooth idle but only running choked? I thought spark was a discrete kind of thing, either there or not?
I welcome any and all suggestions or speculations.

Thanks again GS community, it's always a pleasure.

(I'm just excited my GS850 is running strong - something to keep me riding during this fix!!)

[tkent02]

I'm thinking about the ignition circuit - could problems with it create a smooth idle but only running choked? I thought spark was a discrete kind of thing, either there or not?

The ignition does not know whether the choke is on or not, it's a fuel thing.

If it runs fine on choke and won't idle without it, your pilot circuit is plugged up somewhere. Strip and dip, and ensure each and every passage in each carburetor body is clear before you reassemble the carbs. Use spray carb cleaner for this, shoot it in each hole and watch the spray pattern where it comes out, it should be identical on all of the carburetors. Be especially careful about the tiny ports where the pilot circuit goes into the venturi.
You will want safety glasses for this.

[RedBaron]

Agreed with tkent02. Sad as it is to hear carbs again, the starting issue sounds like pilot circuit. What's odd is the original complaint almost seems like a seperate issue. The first issue sounded more like a fuel tank vent or carb vent issue. Forgive me, It doesn't sound ignition related. Sounds like pilot circuit, maybe tight valves but I've never had a problem on a multi cylinder bike failing to run from that, only singles. You've got a new head though so more carb fun I'd wager.

[derickson104]

Right. Strip and dip it is. I'll post a progress report here afterward. Thanks gents.

[Grimly]

Just make sure the spark is actually healthy. Common on older UJMs is a marginal spark causing fuel-failure symptoms. On mine, when a few thousand miles had passed under them, the OEM wiring started to show its shortcomings and I ran into exactly those types of symptoms a couple/few times. It got me into the habit of thoroughly checking all the low-tension side from base to apex every Autumn, before start of the Winter, when you least want it to fail on you.
I replaced nearly all of the critical connectors with some decent ones and by this time it all needs done again, really. Some of the original wiring is still there, but it's non-critical.
Symptoms of a failing LT side include having to gas it to get anywhere, misfiring, mimicking fuel starvation, popping and backfiring, and just generally lacking power.
This can be caused by a simple corroded connection or wire core that's almost vibrated through or corroded.
I will say this for the factory carbs - until they wore out they were pretty damn good and never gave me any trouble.

You can do a quick and easy check for spark health - on a points engine - simply hold the bare end of the HT lead 1/4" from the cyl head - it should leap the gap and be blue.
On an electronic ign equipped bike, it's not too recommened to do that, so just use the spark plug or get a proper spark gap device (which mimics the same idea above, but stands less chance of blowing any circuitry) In both cases, the spark should nice and fat blue - you can tell by the streaks of orange in it how far from good it is and if it's all /mostly orange and barely leaping the plug gap, never mind an external one, you need to look at things in more depth. As I say, failing coils weren't the major cause, it was usually bad connections on the LT side.
You can even feel it in the running of the engine - many times I felt it wasn't right, cleaned up and re-made the 12V and sensor connections to the factory Eleg Ign or Pirhana unit and that solved it.

[allojohn]

Have you checked how much crap you have in the tank? I know you're using a filter but some crap can get through and plug the carbs. Wouldn't want you to have to strip the carbs again next year....

[derickson104]

Progress Report

Progress Report: Yup, Occam's Razor strikes again. It was fuel. I finished the carb strip and dip, and during reassembly I noticed diminished airflow through the main fuel connector between carbs 2 and 3 (the only real piece that can't be dipped because it's coated in rubber). Air compressor, enter! Blew it out and the air flow increased. With the carbs back in, the bike runs like a champ again - that is: it starts, runs and revs w/o choke! Nonetheless, I'm still going to give the ignition circuit the once over, (Grimly's post has me thinking) while it's freezing out. As well, allojohn's correct, I'll check the tank out.

The one thing I can't solve yet is where the clog came from, ie: how it passed both a petcock and in-line fuel filter??? Perhaps broken bit of fuel line? Thoughts anyone??

Thanks again guys, and seriously, it always is the carbs!!!

[hampshirehog]

I'm with the others on this. Those in line paired pilot jets on the 550 need extra special attention when cleaning.

[Big T]

The one thing I can't solve yet is where the clog came from, ie: how it passed both a petcock and in-line fuel filter??? Perhaps broken bit of fuel line? Thoughts anyone??

Thanks again guys, and seriously, it always is the carbs!!!

Most likely that was some kind of build up from fuel evaporating while the bike sat over the last 36 years

So, it probably didn't come down the fuel line

[RedBaron]

I have had trouble with the low permeation and generic grey fuel lines with inner dark liner that come on new bikes losing their inner liner in chunks. That and the fuel cock rubbers. Shouldn't be a problem on an older unit with the standard black lines.