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Setting Carbs to OEM Specs

tas850g

tas850g

Forum Mentor
I have questions regarding setting my rebuilt (cleaned) carbs to OEM Specs.


GS850 (77-79) the specs are for the VM's

idle r/min........... 950-1150
carb................... mik vm26ss
id no................... 45060
bore size ............ 26 (1.0)
float height ......... 23-25. (0.91" - 0.98")
fuel level ............ 3.0 - 5.0 (0.12" +/-0.20")
pilot air screw ..... 1.25 turns out
pilot screw........... 5/8 turns out
pilot air jet........... 1.2
pilot jet............... #15
cut away............. 1.5
jet needle........... 5DL36-2 (the -2 2nd notch)
needle jet .......... 0 - 6
pilot outlet........... 0.6
by pass ............. 0.8
main jet............. #102.5

I have a 1979 GS850. When i disasembled my carbs i counted the turns for my air and fuel screws. I counted the number of turns to bottom out (turned them in) and recorded this information. When i reassembled the carbs and air box, it ran much much better with the air box off versus on the bike. My plugs were white - lean. I am adjusting my valves as i write this (waiting for shims from the shim club and valve gaskets). The number of turns that i counted pre-cleaning were more than what the specs recommended.

Finally my question, because my bike did not seem to run well after cleaning of the carbs, should I re-adjust my air and fuel screw turns according to the specs? The turns out on the fuel screw (5/8) does not seem to be close to what it was set at prior to cleaning. Prior to cleaning for fuel screws were between 1 to 1 1/2 turns and for air was 1 1/2 to 2 turns.

I did not record what jets i had. My float heights where within specs and i did not check my fuel levels - not sure how to do this.

Thanks for your help,

Todd
 
Use the highest RPM method. Idle you bike, and adjust that screw in and out until you get the highest rpm out of the engine. Repeat for each carb.
 
Skyboy8950 said:
Use the highest RPM method. Idle you bike, and adjust that screw in and out until you get the highest rpm out of the engine. Repeat for each carb.
Click to expand...
That works well with the CV carbs on the '80 and newer bikes, but he has VM carbs on his '79. :-k

Unfortunately, unless you have a special tool, you will have to remove the carbs to turn the screws. About 1 full turn on the fuel screws should do it, then you can use the air screws to fine-tune when the engine is warm.

The US EPA was starting to control fuel mixtures by '79, so mixtures were a bit lean from the factory. Going out a full turn or maybe just a bit more can really help the way your bike starts and runs.

.
 
You can get at the fuel screws with a screwdriver bit but be prepared to burn your hands! In theory you should set the pilot airscrews to the book value and tune using the fuel screws (that's what they did in the factory - each carb is a tiny bit different to every other one ever made). I normally do it that way but it is a bit pointless as one turn out on the fuel screw and tweaking the airscrews to match shows the same result.

However, a lot of the 79 and earlier bikes had a dot punched on the carb next to the air screw and the fuel screw - these dots were meant to line up with the screws. They are tiny dots and many will have disappeared over time but worth checking to see if they are still there - they'll certainly give you a good starting point.
 
I agree with Steve in setting the fuel screws at 1 turn, or slightly less, as a start point. The carbs were very lean from new in order to meet emissions rules, can't see any value in returning them to the same level unless you are concern with your "carbon footprint".:rolleyes:

Another thing for the OP, check your fuel screws to make sure the tips are not broken off and stuck in the carbs. This is very easy to do when seating the screws so you can count how many turns out the screws are.

Good luck.
 
Prior to installing the air/fuel screws i did check the points and all looked good. That doesn't mean i didn't break off the points when i turned them in. I don't know how much pressure it would take but i was aware of this potential problem. When i turned in the screws pre/post cleaning i was extra catious of this.

As far as the dot goes, there is a line etched into the carb next to each fuel/air screw and none of the screw flat blades were lined up with them.

And yes i have kinda burnt my hand some trying to adjust the fuel screws. I would rather set the fuel screws and adjust air screws if possible.

Because my plugs were white (lean - i keep saying white/lean so i can remember the concept) i will tighten my fuel screws to 1 turn hoping this will create a better air/fuel ratio.

I will try these suggestions and see how it performs.

The good side of not having a bike running is i have more time working on my poor polishing technique(s). It's looking better. One day at a time.
 
Just remember that you are only playing with the pilot circuit, and that is mostly used at idle and very low throttle openings. If you want to see how your plugs are doing on other jets, you will have to do plug chops. Check for plug chop information on BassCliff's little site.

.
 
Steve,

I have your recommendations printed out on how to perform the chops and how to check the plugs at different speeds/throttle positions etc.

Still waiting for parts, although it's only been 4 days. Guess i am just anxious to try all the recommendations.

Thanks for the info.
 
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