GS1000ST (1980) running rich

[Guest]

G'day all,

I need some sage advice. Failing that, your opinion would be of value too.

I have had my 1980 GS1000S for about 3 weeks now, and have been very busy sorting stuff out.

Most recent thing was pulling the carbies out, and giving them a good clean -- they were full of rust sludge. I have also put a POR-15 kit through the tank too, to eliminate rust probs in the future. The carbies were reassembled by following the excellent article here on the GS Resources.

Anyway, the bike is running really rich -- like about 10kms per litre (which I guess is about 25 miles per gallon). :shock: The plugs are sooty black and there's enough carbon inside the muffler to fill a toner cartridge for a photocopier, I guess.

My question is whether you folks think that this could be explained by worn needle jets alone? Or should the fact that a previous owner has put a small muffler fit only for a 250 onto a 1000cc machine, be the likely culprit?

When I had the carbies apart, I noted that the needles have been dropped by one clip position (I'm assuming that the central clip position is the standard setting), which may well have been done by a previous owner attempting to rectify the rich mixture. I have left the needle clips where I found them.

Let me add that I have a new air filter installed, new spark plugs, and I have checked the timing too. The bike idles sweetly, starts easily, and while it doesn't have great compression (100psi on one cylinder I tested) it's a very smooth engine with no rattles at all -- not even clutch rattle.

Sage advice and humble opinions welcome.

Cheers,
Mike.

[KEITH KRAUSE]

Well, you mention two things that will cause poor combustion/black sooty plugs.
Poor compression, at least at the one cylinder you tested. The others are possibly low too?
Incorrect muffler. The engine can't operate as it should if there's too much back pressure. Again, poor combustion.
I could go into a long list of things that cause sooty plugs but there's no reason to start on that list because the two things above must be corrected first. Sounds like the leaner jet needle position is an attempt to compensate. Compensating never really works, it just wastes time.
Fix those things first and test. If it still has a problem, we can figure it out.

[tfb]

Thanks Keith. I guess what I'm wondering is, What's the likely candidate if you're getting the woeful rich running I am? Could worn needle jets explain it alone? Or might the muffler be the primary (but not only) culprit?

Don't get me wrong, I fully intend to fix anything and everything on this bike in due course -- but with the limited budget most of us have, I would like to attack things in some sort of sensible order.

My own inclination after a good night's sleep, is to go for ditching that wretched muffler. (I could try running the bike without it and see what happens, I suppose, but I'm not sure the neighbours would like it.)

Time to start trawling eBay!

[KEITH KRAUSE]

If the exhaust is restricting flow, then it has to be changed. That would be a step in the right direction. As for low compression, you can't get good combustion with low compression. That will darken the plugs and give "rich" performance/symptoms. What about the other three cylinders? I know that repairing the compression problem will be more work but it all starts with correct compression. You can't tune/jet correctly with low compression.
Yes, worn needle jets would cause a rich condition.

[michael]

I had a similar situation with my GS850GL. Tha carbs were gone through by a local carb specialist who took the whole bike with him. On return the bike ran much better, but real rich. I was told the bike had poor compression and thats why it was too rich. I soon grew tired of cleaning the plugs and took a chance and bought a replacement set of carbs on eBay. Viola! rich problem gone, runs like it should with nicely colored plugs. Just my 2 cents. I will deal with the compression problem when time and $ permit.

Cheers,

Mike.

[KEITH KRAUSE]

I had a similar situation with my GS850GL. Tha carbs were gone through by a local carb specialist who took the whole bike with him. On return the bike ran much better, but real rich. I was told the bike had poor compression and thats why it was too rich. I soon grew tired of cleaning the plugs and took a chance and bought a replacement set of carbs on eBay. Viola! rich problem gone, runs like it should with nicely colored plugs. Just my 2 cents. I will deal with the compression problem when time and $ permit.

Cheers,

Mike.

Hmmm. One of two things happened here.
Either the local carb specialist is wrong about the poor compression and you found identical replacement carbs to fix the problem or... you do have poor compression and the replacement carbs are off a different model (resulting in a leaner mixture) or are identical but jetted leaner. Hopefully it's the first possibility. If not, the bike may seem OK to you but I doubt it runs to its full potential.

[tfb]

All of what you say makes good sense to me, Keith. I would like to fix the compression side of things first, but from experience on past top end rebuilds on my bikes, I know I'm not going to have much change out of AU$1000... :-|

So last night I pulled that nasty restrictive old muffler apart. Started by pulling out all the sound-proofing fibreglass wadding. Put the muffler back on and while you wouldn't think that would have made much difference, voila, about 5% more acceleration power, I would say. Also had to back off the idle speed, as that had risen by about 300 rpm.

So that's all moving in the right direction. Tonight I will do some more experimental 'surgery' on the muffler, to reduce the back pressure further, and see if we get any more gains there. I will also be interested to see whether the fuel economy improves.

Like I said, it's a nasty old muffler, so I have no reluctance at all when it comes to modifying the wretched thing.

Cheers again,
Mike.

[tfb]

Update:

After cleaning out the innards of the muffler last night, I decided to go a step further and hacksaw some slots across the bottom of the muffler. Then I took a hammer & screwdriver combo to one side of each of the slots, and bashed it in to open up some 'gills' along the bottom of the muffler (where the cops can't see them).

I have christened them "speed slots". More realistically, they could be called "noise slots". Either way, I am quite proud of my handiwork.

Now for the results of my little experiment:
Before all the surgery on the muffler, I was getting about 10kms per litre.
Now, I estimate I'm getting around 17 kms per litre -- that's more like it!

[KEITH KRAUSE]

I wonder what the plugs look like now?

[tfb]

The plugs actually look pretty good! The sooty black has gone and now the plugs are a nice grey colour. :-D

The thing I need to do now is hook my air-fuel ratio (AFR) meter up to the exhaust system, and see exactly what the AFR is at different rpm and engine loads, to double-check that it's in the right range.

But to do that I need to weld an 18mm threaded collar onto the exhaust system.

And to do that I need to bribe my neighbour Ashley across the road into doing the welding for me with his TIG.

A couple of beers should do the trick.