GS 1000 G - It mumbles from 3000 to 4000 rpm

[Alexander]

Greetings to everyone on the forum.
After having fixed a GS 550 and with only 9000 km (beautiful motorbike)
it was sold to buy its father, the gs 1000 g with only 20,000 km,
the bike is perfect in every way, clearly before putting it back on the road
I carried out some small maintenance, brakes, all seals, engine oil and cardan change,
candles.
At the first start the bike ran at three, the fourth muffler cold, I dismantled the carburettor unit
to check and the fourth carburettor was dirty and no fuel was coming in, fixed!
The first carburettor leaks fuel and I think it's the float which gets blocked when the bike is tilted, as well as the fuel tap
it seems that in the OFF (PR) position it does not close.
Ok, reassembled everything after wasting a lot of time with the air manifolds of the filter box, they are a bit ruined
but in the end I put it back, these will be the next purchase along with a complete overhaul of the carburettors.
The bike keeps idling and during acceleration it seems crazy with how much it moves, sometimes I feel some gaps during acceleration, the clutch slips a bit, this too needs to be fixed, but
the problem that I can't understand and which seems to go in threes and mumble, from 3000 to 4000 rpm, below and above is fine, I also see the first muffler shinier than the others.
I thought about carburetion, but playing with the screws, first two and a half turns, then two, then one, the bike improved
under acceleration and at low revs, it starts with a little difficulty compared to before, but the problem is still the same,
from 3000 to 4000 rpm the bike grumbles, so even if the carburettors need cleaning and overhaul, I think it's not them,
otherwise by acting on the fuel screws something would have had to change.
I understand that there are many variables, but I wouldn't want to go to the exclusion of changing everything!
I rely on someone with good experience for advice on what may have happened,
thank you for your patience and for the not exactly perfect English

[Rich82GS750TZ]

On your Fuel Tap (petcock). PR is Prime. This is the position to use when the carburetor bowls are empty and you need to fill them. It is only meant to be left on long enough to fill the bowls, then turned to ON or RES. Prime position allows fuel to flow freely from the fuel tank without the application of vacuum required to allow fuel to flow in the ON and RES (reserve) positions. Prime DOES NOT MEAN OFF)

Welcome to the forum.
It would help to have the year of your motorcycle, and whether is has CV or VM carburetors. I'm sure others will have advice for you regarding carb screw settings. Most of the advice will likely start with recommending a thorough carb cleaning.

Your location- Toscana. Tuscany, Italy?

[Alexander]

Thanks for your clarification regarding the tap, I'll try leaving it on and see if it still leaks.
The motorbike was registered in 1984, I think model 79/80, cardan shaft, headlight and round indicators.
Work on the Mikuni carburetors certainly needs to be done but I would like to enjoy the bike now that I've just got it, then do the necessary work in the winter.
It could be the carburettor leaking but it's strange, because if I move the screw to regulate the fuel flow, the performance changes but the problem always remains between 3000 and 4000 rpm,
Toscana - Montalcino - Italy
thanks

[Rich82GS750TZ]

Alexander, for those that will chime and try to help with the carb settings (screw settings are not something I'm comfortable advising) '79 or '80 is an important distinction. At least here in North America, '79 models had VM carburetors that look like this:
VM CARBS:


The '80 and later models had CV carburetors like this:
CV CARBS:


Tutorials for cleaning either can be found on BikeCliff's Website. First link in my signature if you can see it, or:

https://gsarchive.bwringer.com/

[Alexander]

thank you very much, so you recommend cleaning the carburettors, even if to produce this defect it means that the offending carburettor works well at idle and maximum, while between 3000 and 4000 it doesn't pass petrol because it seems to run at three. I'll post a couple of photos to see the bike, the non-uniform exhausts, see the first one and the carburettors, everything is original


filedata/fetch?id=1751641&d=1722958193&type=thumb

[Rich82GS750TZ]

I'm not recommending anything. It's beyond my level, as I've never had the issues you're having, and have never had to play with the screws on my carbs or work on the internals of the carbs at all. I had a local mechanic clean/rebuild by carbs some years ago and it's been running great since.

What I am saying, is that it's impossible to know if any adjustments you do make are actually helping or if they are merely compensating for dirty carbs and other maintenance that may have been neglected, like making sure the valve clearances are within tolerance.

[rphillips]

Wondering if you've checked the valve clearances? You need to know they're OK as a tight valve can cause an expensive problem if not fixed. I don't see a tight valve affecting mid range performance, it would more likely affect starting and idling. Have you ran a can of fuel system cleaner additive in a tank of gas through it? We have Seafoam over here, not saw'in it will help but sure can't hurt, and think how excited you would be if it did help. Hoping a good carb guy will chime in, If you follow their guidelines, I'm reasonably sure your problem will be fixed... Oh, almost forgot WELCOME

[KiwiAlfa156]

Hi There, really important to start with as many variables taken out of the equation before you start looking at carb settings. As folks have mentioned above, the valve clearances need to be at spec, and the ignition timing should be checked that the ignition advances smoothly from the idle timing marks to the full advance timing marks at around 2,750 rpm and that you are getting good spark on all four cylinders. The alignment of the timing marks at full advance is critical for maximum power production and engine efficiency. If you have access to a carb balancer and you are getting a reliable idle on all four cylinders, the carbs should be balanced. Float heights should be checked too.

Can't stress enough that carbs need to be surgically clean. The pilot and needle jets are 'emulsion tube types' meaning they mix air with the fuel before it enters the intake airstream under the slide. The air for the emulsion tubes enters through the small round openings at the edge of the carburetor bell mouth. So the very small holes on the sides of these jets need to be clear as does the tiny hole at the bottom of the pilot jet. Also make sure all the choke plungers are fully closing when the choke is off. You should be able to squirt aerosol carb cleaner though all the fuel and air passages. This gives you a visual confirmation that these pathways aren't blocked.

If the issues is being caused by one carb its like that something is blocked in that carb.

Also check the slide diaphragms for holes, tears, and incorrect assembly. I've done a couple of sets where a diaphragm has been folded over or pinched and so hasn't operated correctly. The slides on the BS Mikunis only start lifting after after a engine intake velocity creates enough vacuum under the slide. As you can see from this photo of the BS carbs above there is quite a gap under the slides when they are seated at the bottom of their bores, and the airflow has to be significant to start these lifting. Air flow is controlled by the butterflies, and these are connected to the throttle cable. This is important because if the engine is performing more or less normally up to 3,000 rpm and then the trouble starts, it may be a clue that the problem is related to the main fuel circuit (main air jet, needle jet, needle, and main jet) rather than the pilot circuit (pilot air jet, pilot jet, fuel screw and the pilot and by-pass orifices - the main jet should also be part of this list as the pilot circuit on the BS32 draw their fuel through the carbs main jet).

If the issues is being caused by one carb its like that something is blocked.

As your bike is stock, the jetting should be unchanged from the factory. Adding more fuel (bigger jets) without more air makes less power and wastes fuel.

If the bike is idling on four cylinders, the easiest way to set the fuel screws is to pick an initial setting across all four carbs (say 1-1/4 to 1-1/2 turns out from lightly seated) then ride the bike until its fully warmed up. Blip the throttle with a short sharp action, letting the throttle snap shut, so the revs peak at about 2,500 or thereabouts The revs should climb quickly and cleanly and drop back to idle just as quickly. If the revs drop below the idle you blipped from and then rise back to idle, or the bike stalls, its rich on the screws. Turn each screw in 1/8 turn and repeat the test until the revs climb and drop quickly back to the same idle rpm. If the engine revs, and then slowly drops back to idle, its lean on the fuel screws and the all the screws should be turned out 1/8 turn and the test repeated until you get the revs climbing and dropping quickly back to the same idle rpm.

From that point with clean carbs, you have likely solved your problem, or if it continues, it gives the best start to further trouble shooting.

[Alexander]

Thanks so much for the tips.
Now I have a bigger basic problem, I broke the #1 carburettor, the float rod was caked and a blow too hard (I'm a chicken), broke the column, so I bought a set that seems to be in better shape than mine, al cost that I would have incurred to overhaul yours, now with two sets I hope to make one that works!.
It would be interesting to know who else has the BS34SS on the GS 1000 so we can find one and refurbish the set for resale or keep as a spare!
As soon as I manage to get it running again, I will have to do these steps that WikiAlfa156 suggests, the problem will be finding a mechanic who knows this bike well.

b0c3ce24-3c4c-425d-83b5-1c46ce04b010.jpg

[KiwiAlfa156]

For the future, the safest way to remove the float pins, is to use a flat bladed screw driver in the small gap between the head of the the float pin and the post. Twisting the blade will pry the pin head away from the post without putting a side load on the post. Never try to tap the pin out using a punch, the posts are brittle and will snap under a side/shock load, as you've unfortunately found.

[dgates1]

My float pins were tight against the post, so I couldn't get a screwdriver in there. I did use an automatic center punch on the end of the pin and they popped right out. Was even able to do this to all of them without un-ganging the carbs. However, I think it did mushroom the small end a little bit, as they were a little hesitant to go back in smoothly. I just used a little sand paper and twisted them for a bit. Then went in smooth. I didn't seat them all the way against the post on reassembly, so I can use a screwdriver next time.

image.png

[Lorenzo]

Ciao Alexander, you got sound advice from the board members; if I may give my opinion, from the pic of the broken post carb. no way they are near usable with success; I mean they wanna be spic&span.
If the bike is operative, when you bleep the throttle the tacho needle (contagiri) drops down as soon you remove the gas or stays up and then slowly drops?

[Alexander]

Ciao Lorenzo,
in what sense can I no longer use the carburettors, are you telling me that they can't be replaced individually? still a bad deal. When it went, the throttle was smooth and very precise, I had no delays. One of the last tests with these carburetors was that with the screw adjusted to one turn, the acceleration was so violent that the clutch slipped, perhaps this too or just the springs needed to be redone...

[Lorenzo]

Everything can be done, even mending the broken post with epoxy; all I meant for "no way they are near usable with success" is perhaps better understood by looking at pic #9 where it can be seen there is crud on the needle valve holding plate, also the main jet appears to be powdery on the base and the pilot jet plug seems squashed, did you clean the jet inside?
Also, -it cannot be appreciated from the pic - the carb boots, the eight of them, must be free of cracks, which you can tell by having them off (expecially for #2 and 3 ones)

[chuck hahn]

Another tip for removing float pins. Just heat from a heat gun and get it pretty hot. then a shot of penetrating oil and lt it set a few minutes to soak in.I use a socket on the head side to support the posts and gently tap on the pin to remove.