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1979 GS550 Carb Settings

B

bill_face

Forum Apprentice
Hello.

I've got a set of VM carbs on a '79 GS550, that someone else has cleaned and recalibrated for me. I've got stock airbox, and motad 4:1 exhaust (apparently).

The guy that set them up for me wasn't sure what the "factory set petrol screw" (his words) settings were, and says they may need adjusting. Anyone know what the stock setting was?

I've had a look at the carb specs sheet on Bike Cliffs website which shows a value of 1 for Pilot Screw. Is this the same as what my guy is talking about? If so does that mean one complete turn out from fully tightened?

Many thanks
 
The pilot fuel screw, on the underside of the carb, can be set to 7/8 of a turn from lightly seated, and the pilot air screw, on the side of the carb, about double that. Be careful when turning in the fuel screw because it's hard to know when it's bottomed and if you turn it in too tight, the tip will wedge into the hole in the carb and break it off. In fact, be sure to check and make sure each fuel screw has it's tip before you try to do any tuning.
 
What is the effect of taking these screws further out? At the moment my bike is very hard starting, but once it gets going it runs nicely. It takes a few minutes of holding the starter on, full choke and it firing but not running on its own before it runs very weakly, and eventually strengthens up (as it warms?)

I'm wondering if I should adjust these screws more?
 
I wouldn't tweak the screws based on how it runs cold. Set them based on a warm running configuration.

The choke pickup tubes in the carbs have a tiny orifice which easily plugs. You may want to take a strand of wire and pass it through the hole or better yet, spray carb cleaner though the pick up tube and verify fuel reaches the choke plungers. To do the spray test you have to disassemble the carbs pretty far though. Another thing to check is the passage in the float bowl. Fuel needs to freely pass through the little hole at the base of the fuel feed passage, thus filling the passage. These holes are often plugged with varnish so you need to check carefully.
 
Thanks. I'll look into it.

I've had the carbs rebuilt and thoroughly cleaned and had them tuned as mentioned above and it's had this problem since then (I rebuilt the engine at the same time).

I'm presuming the choke is working alright as when it does warm up it really starts to rev hard on full choke.
 
bill_face said:
What is the effect of taking these screws further out? At the moment my bike is very hard starting, but once it gets going it runs nicely. It takes a few minutes of holding the starter on, full choke and it firing but not running on its own before it runs very weakly, and eventually strengthens up (as it warms?)

I'm wondering if I should adjust these screws more?
Click to expand...

No, you have lots of other things to check, then you'll probably have to clean the carbs properly.

What you're describing on start up is likely multiple issues

Read the Top 10 Newbie mistakes list, then let us know your results

Your bike should start in less than one second
 
Thanks for the input. I think it is worth posting this issue to a new thread with a video, and more details of what I've done.

I've had a look at the Top 10 newbie mistakes and i feel i'm in the clear for those. Will look through the other thread. I'm guessing its probably to do with the carbs, but as I paid to have them overhauled it would mean going back to the shop. I'd like tto have checked anything else it could be, but I don't know what those things might be

P.S i've installed the SH775 you sent me Nessism, seems to be working a treat.
 
Bill, just to be clear, you did check/adjust your valve clearances?
 
I did adjust the carbs. The cold starting problem also seems to have cured itself, with nothing more than a little razz down the motorway.
 
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