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79 GS850 VM advice

  • Thread starter Thread starter driftcreek850
  • Start date Start date
D

driftcreek850

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Now I will be fine tuning my carburetors. I would like to know the order in which I should adjust fuel screws, air screws and slides.

Should I Sync first, then do air idle? Fuel mix is last, right?
I synced once. The only symptoms I have now is the bike seems to be starving at high rpm's. It is also very difficult to start and needs to be nursed for a minute or two. But once a little heat gets behind those carbs, She idles great. I do not seem to have any issues below 4000 RPM's.

Anything over 4000 I am getting a surging power loss. All of my screws are set to default. I am getting a colortune. A quick rundown of the order of tasks would be helpful.

:confused:
 
I'm now aware of an order per say. I'd set the fuel screws to about 7/8 turn, air screw to 1.6 or so, and sync. Tweaking the fuel screw won't upset the sync, not sure if the air screw will but I doubt it. Of course, these starting points depend on your bike being stock. If you have modified your exhaust or intake the jetting will need to change so the fuel/air screw settings will be different as well.
 
ready to give up

ready to give up

rode in to work today after spending 2 hours on carburetors last night... by the time I got to work [25 miles] bike was barely running on 2 cylinders. I gradually lost power over the course of the ride and limped to my destination. Is it time to start all over again??? I am depressed.
 
The order of adjustment is not the problem. Sounds like something more fundamental like a petcock problem, or the bike is fouling plugs.
 
I think you are right. I can let the bike sit and it runs OK again. I have definately got a fuel issue. Should I just replace it or run it on prime?
 
I had the same issue. Running on prime fixed my issue instantly. As you may know, turn the petcock to either off or on when the bike is not running so it stops fuel flowing into the carbs.

Once i figured my problem was the 30 year old petcock i bought a new OEM. Don't buy a repair kit. Not a very reliable fix.
 
Ok stole this credit to Keith K
some of this pertains to rejetting which will not be needed on stock bike..

Keith’s post
Stock carbs (U.S.) are VM26SS. I don't know if Canada bikes got 26mm or possibly 28mm? I do know 33mm are not stock anywhere. Sounds like your bike is starving for fuel above 6,000 rpm's. The jet-needles need to be raised and the main jets increased and probably the pilot jets increased too as I posted.
Your jet-needles are inside your slides.
As for the screws...the PILOT screws are reached from underneath and are engine side of the floatbowls. Use a mirror too see and adjust them. Turning them OUT (counter-clockwise) richens the mixture. They can be tight if never moved before and it's easy to strip the head off. Use a good fitting tool. They are sensitive to adjustments and even 1/8 turn can change the sparkplug color. For a starting point regarding the pilot circuit, I would adjust each pilot screw out 1/4 turn and test. When plug reading for the pilot circuit, ride the bike a few miles in 4th or 5th gear at about 40 mph. Just have the throttle open just past "idle". Do what the plugs/performance tells you. If you find yourself adjusting these screws more than a full turn out further from where they are initially, then you most likely need a step up (2.5) in pilot jet size and of course turn the pilots back in to where you started. You may even have to try larger pilots and turning the pilots out a little. It's suggested you start with fresh plugs or at least clean yours well.
For the SIDE AIR screws...adjust as follows. Put the bike on the centerstand and warm it up first. Turn the screws on both carbs facing you in 1/4 to 1/2 turn increments until you achieve the highest possible rpm's. Fine tune these two screws. Do the other two screws the same way, until you achieve the highest rpm. Now re-set the idle to 1,100 rpm with the idle screwknob. They usually end up about 1 1/2 to 1 3/4 turns out. Turning these screws OUT leans the mixture. Careful, these screws strip easily too if never moved.
Do the pilot screws first and then the side air screws. Jet needle adjustments must be followed with a vacuum tool synch. Then test.
A good carb synch is important with these carbs and re-jetting is too difficult without a good synch. Raising the jet-needles is the most work.
Test for the jet-needles at 1/2 throttle and the mains at 7/8 or wide open throttle.
I forgot to mention, remove the two float bowl vent tubes so the bowls vent better. The increased air intake creates a vortex in the tubes and this cause fuel starvation. Removing the tubes helps the jets draw fuel easier. This is recommended by jet kit makers regarding stage 3 kits, especially with VM carbs.
 
driftcreek850 said:
I think you are right. I can let the bike sit and it runs OK again. I have definately got a fuel issue. Should I just replace it or run it on prime?
Click to expand...

It could also something simple like a plugged vent in your fuel filler cap also. When the problem starts loosen the cap and if you hear a hiss or all of a sudden it runs properly it's the cap. Easy to clean.
 
SqDancerLynn1 said:
I forgot to mention, remove the two float bowl vent tubes so the bowls vent better. The increased air intake creates a vortex in the tubes and this cause fuel starvation. Removing the tubes helps the jets draw fuel easier. This is recommended by jet kit makers regarding stage 3 kits, especially with VM carbs.
Click to expand...


This was the problem!! Someone before me had these blocked off. I opened them up and all is right with the world!! THANKS
 
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