GS1000G 1980 BS34 CV Carbs - Lean Running?

[Guest]

Hello,

My bike, a GS750/1000 hybrid, has 1000G CV carbs, BS34 with a stock airbox and Eagle 4:1. The jetting is all stock.

I'm currently setting it up, and with the pilot screws 4 turns out, it's still hunts/surges slightly at cruise between 60-70mph.

Am I right in thinking I should go up a size on the pilot jet, to a 42.5?

The hunting/surging is improved at 4 turns out, than when it was at, say, 2.5 turns out, but the idle is now not as fast.

I understand from the tutorial that it's common enough to want to raise the needle too?

The rest of the bike's running seems very good, anything above about 1/4 throttle, the bike pulls like a train.

TIA

P.S. Avatar pic is way out of date! In that pic the bike had a slide carb head.

[Nessism]

I'd try raising the needles to see what happens. It's unusual to have to bump the pilot jets while running the stock airbox.

BTW, a 1000G is a shaft drive bike. I've never heard of anyone installing a shaft drive engine into a chain drive chassis. Please share what you did to install the engine, or maybe the G part is a mistake?

[Big T]

Ed,

He has the G carbs on his 7/11

[gs7.11]

I'd try raising the needles to see what happens. It's unusual to have to bump the pilot jets while running the stock airbox.

BTW, a 1000G is a shaft drive bike. I've never heard of anyone installing a shaft drive engine into a chain drive chassis. Please share what you did to install the engine, or maybe the G part is a mistake?

Ed,

He has the G carbs on his 7/11

Yep, just the carbs!

[Nessism]

So is it a 7/11 or a 7/10? 10 is 8V and 11 is 16V so the G carbs may not be a proper match jetting wise for the 11.

[gs7.11]

So is it a 7/11 or a 7/10? 10 is 8V and 11 is 16V so the G carbs may not be a proper match jetting wise for the 11.

It's an 11. But it's a GS1000 engine, as I said.

The 11 comes from the 1085 Wiseco kit. Here's a more recent pic. Disregard the RS36 flatslides and Pods filters, which have now been exchanged for 1980 GS1000G carbs with a matching airbox.

[londonboards]

Presumably you have stripped and cleaned the carbs to within an inch of their life?

Starting any carb tuning without a known clean set of carbs is fruitless. They need to have been cleaned in the last 6 months.

Then all the O rings and rubber components replaced including the rubber plug in the idle jet aperture.

Then you need to replace the O rings in the inlet carb holders, the part where the carbs attaches to the engine.

Get all your O rings here: http://www.cycleorings.com/

When that is all done, set you idle mix screws at 1.5 turns out and ride the bike in second or third year as slow as you can and see how the throttle responds from fully closed to only just open. There should be no snatching or hunting.

What you describe happening is odd. You often get hunting with a lean mixuture but opening the idle screw mix should be making the off idle mixture richer - if the idle jets are clean that is.

Those idle jets are the first things to get bunged up - get them out and clean up.

Start here:

http://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf

You will also need to synchronise them.

Then check to see how it works at the 60-70 mph level and adjust the needle position if necessary.

Really doubt you will need bigger idle jets unless someone has already changed them or they are not original.

Greetings

[gs7.11]

Thanks for the input

Yes, the carbs are very clean. They were actually a very clean set that were imported from a very dry state in the US, as they have virtually no tarnishing on the plated parts. Anyway, there was debris and gumming internally, and yes the pilots were blocked. I've cleaned them thoroughly with a combination of carb cleaner, lemon juice and compressed air. I can guarantee there are no blockages.
O rings have been replaced where necessary. The float valves were toast, and have been replaced, along with the pilots, although the pilots have since come clean after further soaking.
The carb/head rubbers are recent along with the O-rings. The previous carbs had no issues in that area, so lets assume they're good. Replacement parts are Keyster and are fine. Lets not get into that one.
The rubbers bungs were in ok shape to reuse.

The float heights are 22.4 EXACTLY, both sides (i.e. both floats), and I checked the fuel level on 2 carbs (to sample) and it was 5mm, exactly right, so I'm assuming that float heights are good too.

I tried the setting in my factory manual at 1-5/8 turns out, but that was way too lean, with lots of hunting and snatching, and the idle hanging at 2k rpm. Having read the tutorial as mentioned above, it states 2.5 turns out as a starting point, so I tried that: much better. 3 turns out better still and 3.5 turns out better, but now the idle is getting a bit fluffy, whereas just above idle is still lean.

Going off what I've read therefore, this points to either needles or PJ's. I have to say, it feels more like needles to me, as it's a bit above 'off-idle' in my opinion, and although the pilot screw has helped, it feels as though I've gone past the limit of what that can achieve.

The tutorial briefly alludes to needle raising as if it's understood that it might well be necessary.....I'd like to know more!

[Big T]

The carbs are from the US?
Do they have adjustable needles?

I agree, it should be needles or jets, rather than mixture screws

[gs7.11]

The carbs are from the US?
Do they have adjustable needles?

I agree, it should be needles or jets, rather than mixture screws

No, the needles only have 1 groove.
I'd have to use washers or whatever.

[Nessism]

No, the needles only have 1 groove.
I'd have to use washers or whatever.

FYI: to lift the needles you have to reduce the thickness of the plastic spacer on top of the clip. Some people mistakenly think they are to put washers under the clip but that's incorrect.

[gs7.11]

FYI: to lift the needles you have to reduce the thickness of the plastic spacer on top of the clip. Some people mistakenly think they are to put washers under the clip but that's incorrect.

Yep, I read that, thanks. It's a great tutorial.

However, I'm not aware of the background to that. Is it common that bikes fitted with the BS34 ran lean in the midrange? Was it an emissions control thing?
Why some and not others?

[Nessism]

The 450 and 550 were famously lean from new in the later years. Most likely emissions related. In your bikes case you have some mods which don't help the situation.

[Big T]

Yes, 1980 era bikes had fixed needles for new emission laws. The taper on that needle is not ideal for a modded bike.
Dyno jet and other kits have a much more tapered needle for richer running

[limeex2]

Just throwing this out there, but with a 4 into 1 and bore kit, its going to need more fuel? I would bump up 1 on the pilot and shim the needle, which is what we used to do to bikes when installing slip on mufflers. Cleaning jets with lemon oil/juice? Sorry, but I wouldnt be chasing jetting issues after cleaning jets with it. Just my 2 cents worth