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THE ADJUSTMENT SCREW UNDER THE CARB?

  • Thread starter Thread starter Tarbash 27
  • Start date Start date
T

Tarbash 27

Guest
ON MY 79 GS750L, IT HAS VM26 CARBS. WHAT DOES THE SCREW UNDER THE CARB ADJUST. ITS RIGHT NEXT TO THE BOWL. IS THAT THE MIXTURE SCREW OR SOMETHING?
 
Set the idle speed only when the bike is warmed up to normal operating temp.

Earl
 
HUH? :-) The big single round knurled knob between the 2 and 3 carbs is the idle adjustment screw. On CV carbs, it is under, on VM carbs, its over.

The 4 small screws, one under each carb, protruding though the float bowl at the front are the pilot fuel screws. Those are found on VM mechanical slide carbs. Those carbs do not have mixture screws.

CV carbs have mixture screws, but they are located center, top on the front of the carb throats.

Earl


SqDancerLynn1 said:
The screw under the carb is the fuel mix screw
Click to expand...
 
If you're talking about the pilot fuel screw underneath and engine side of each bowl, that's the pilot fuel screw. It regulates a second mixture passage into the carb throat. It adds to what the pilot jet supplies.
Turning them clockwise, leans the mixture. Out, counter-clockwise, richens the mixture. They are sensitive to adjustments. If you move them, use a good fitting tool and be careful not to strip the head if they're tight. NEVER tighten them or the sharp tips can break off in the body. LIGHTLY seat them if you're trying to see how far out they are. Keep a record of all adjustments if you want to put them back.
Their adjustment varies bike to bike and even cylinder to cylinder. GENERALLY, a stock bike will have them anywhere from 1/2 to 1 1/4 turns out. If you think they need adjustment, try 1 turn for starters and test.
AFTER setting them, you must adjust the side air screws (on the side) using the highest rpm method. Again, use a good fitting tool. The side air screw regulates the air that joins the pilot jet/pilot fuel screw flow. These screws are sometimes very stiff because of factory sealant jamming the threads.
Highest rpm method: set air screws to 1 1/2 turns to begin. Put bike on centerstand and warm up completely. Set idle to 1,000/1,1000 rpm with the idle adjuster knob. Not higher. Starting with any carb, slowly turn a screw in either direction until you hear the rpm's max. Find the sweet spot. Don't go beyond the point the rpm's stop rising. The screws usually end up about 1 1/2 to 1 3/4 turns out on a stock bike. Then re-set the idle to 1,000/1,100 rpm by using the idle adjuster knob. Repeat this to the other carbs.
If you're talking about the single, large, idle adjuster knob, some VM carbs have it above, pointing rearward between 2/3, and some below, pointing down between 2/3.
 
KEITH KRAUSE said:
Their adjustment varies bike to bike and even cylinder to cylinder. GENERALLY, a stock bike will have them anywhere from 1/2 to 1 1/4 turns out. If you think they need adjustment, try 1 turn for starters and test.
AFTER setting them, you must adjust the side air screws (on the side) using the highest rpm method. Again, use a good fitting tool. The side air screw regulates the air that joins the pilot jet/pilot fuel screw flow. These screws are sometimes very stiff because of factory sealant jamming the threads.
Click to expand...

Is the only way to test the pilot series is by doing plug reads? How do you think the pilots were set at the factory? Will adjustment of the pilot screws effect idle speed at all?
 
renobruce said:
KEITH KRAUSE said:
Their adjustment varies bike to bike and even cylinder to cylinder. GENERALLY, a stock bike will have them anywhere from 1/2 to 1 1/4 turns out. If you think they need adjustment, try 1 turn for starters and test.
AFTER setting them, you must adjust the side air screws (on the side) using the highest rpm method. Again, use a good fitting tool. The side air screw regulates the air that joins the pilot jet/pilot fuel screw flow. These screws are sometimes very stiff because of factory sealant jamming the threads.
Click to expand...

Is the only way to test the pilot series is by doing plug reads? How do you think the pilots were set at the factory? Will adjustment of the pilot screws effect idle speed at all?
Click to expand...
Accurate pilot circuit reads are harder to get than needle and main circuit reads. Many reasons for this. Small carb synch/compression differences between cylinders, etc. I just test at minimal throttle position. Steady speed of about 35 mph in 4th gear, level or uphill. That gives you the least chance of jet needle overlap.
The pilot fuel screws are set at the factory with emissions equipment. That's why you can't put them back exactly as they were. But with patience, you can put them back good enough.
Adjusting the pilot fuel screw will effect idle quality. The rpm's can drop if you adjust them too rich. That always happens when the mixture is rich. If you're adjusting yours, you should FOLLOW any starting adjustment with the side air screw highest rpm method. If you move the pilot fuel screws more than a 1/4 turn from the initial adjustment, you should re-check and adjust the air screws again. There may be a slight change, maybe not, but you should still fine tune. The side air screws are always used for final adjustment.
You may find you have to tinker with both screws just a little with cold weather coming. Depends. If so, adjust and keep a record of the adjustment that works better with hot and cold weather. Then you can set them to a known setting that works in cold and then set them back for hot, if you're really picky.
Also, the side air screws are meant to be moved, even during a ride if you need to. They're not a "cure-all", but for example, if part of your ride takes you to significantly higher elevations, a temporary additional 1/4 or 1/2 turn out can help with hard/hot re-starts and lean the mixture out by compensating for the thinner air.
 
Is this different on 1983 models with CV carbs? I don't see that fuel mix screw on mine

SqDancerLynn1 said:
The screw under the carb is the fuel mix screw
Click to expand...
 
Thats because there is no mixture screw underneath the carbs on CV models.
:-) :-)

Earl

volks28 said:
Is this different on 1983 models with CV carbs? I don't see that fuel mix screw on mine

SqDancerLynn1 said:
The screw under the carb is the fuel mix screw
Click to expand...
Click to expand...
 
This thread is just what I was looking for. Last week I purchased a 79 GS1000E with 31,000 miles on it. Kind of a project but fairly clean and untampered with for the most part. It ran poorly due to sitting for some time so I took the carbs off and apart. I have lots of experience with cv carbs from my newer GS's, and also the older vm carbs on my GT's, but when I took the float bowls off of these carbs, and saw the pilot adjusting screw inside the float bowl, well that was a new one on me. Came to the tech section loking for adjustment figures and what do you know, I found them. I do have a couple of questions for Keith though. After I adjust the pilot fuel and air screws and then sync. the carbs, will it be necessary to go back and mess with the pilot fuel screws again? It appears that it has 95 main jets. Are these the stock size? This bike has a 4-1 Kerker on it and the stock air box. My bike is the same paint scheme as yours, although not nearly as nice. I robbed a couple of pics from your website for inspiration. Thanks for any suggestions.
 
I think 95 is the stock size, so you may need to go up 1 size for the pipe & raise the needles a notch, the pilot screw adjustment needs to be done in small steps // doing a plug read
 
OK, SO YOUR SAYING ITS THE MIXTURE SCREW. I KNOW MINE ARE 1 1/2 TURNS OUT EXACTLY ON EACH ONE. BUT MY PILOT AIR SCREWS ARE ALL THE WAY IN. I GET THE HIGHEST IDLE WHEN THEY TURN ALL THE WAY IN. IF I PUT THE AIR SCREWS AT ABOUT 1/12 TURNS OUT, THE BIKE WONT RUN AND STARVES FOR GAS. COULD I ADJUST THE MIXTURE SREWS UNDER THE CARB TO MAKE IT MORE RICH SO THEN I CAN ADJUST THE AIR SCREWS CORRECTLY. BECAUSE BASICALLY ME HAVING THE AIR SCREWS IN IS JUST LIKE A CHOKE TO THE CARBURETORS TO GET MORE GAS TO FLOW. ALSO I STILL THINK MY BIKE RUNS A BIT LEAN. I GOT IT TO RUN PERFECT NOW WITH NO HESITATION. BUT WHEN YOU GIVE IN SOME THROTTLE, YOU STILL THAT LITTLE GASP OF AIR. I MEAN IT RUNS FINE, BUT I AM SURE I COULD RICHEN IT OUT A BIT.
 
twowheelhorse, set your pilot fuel screws first and then adjust the side air screws, using the highest rpm method. Then synch.
Test for pilot circuit performance (warm ups, idling, minimal throttle performance.) According to results/plug reads, you may have to adjust the pilot fuel screws some. It does take some fine tuning. Naturally, if you move the pilot fuel screws enough, say 1/4 turn or more, you should adjust the side air screws again, in case there's an effect. No need to re-synch though, unless you adjust/disturb the jet needles on VM carbs.
For just the pipe, and if the stock airbox still has its lid, at least one full size up on the main will be required, but I'd start two full sizes up and test. That means a 105 main. (5) is a full size, 2.5 is a "step". Test and do a chop test/plug read at 1/3 throttle to get a jet needle read. Mark your throttle housing and grip to be sure.
TARBASH, no mixture screws on your bike, just pilot fuel and side air screws. I think you're mixed up. Re-read my post. If either screws must be seated to achieve best idle/performance, you have another problem somewhere. I'm out of time, will try to join in later.
 
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