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1978 GS 750 Carb Rebuild

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Anonymous

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I have all the carbs disassembled and cleaned. I have the needles out and I want to know where to set the clips. I have individual K & N filters and a 4 into 1 pipe with no baffle. I'm going to run 115 main jets, just wanna know where I should set the clips. Also the air screw on the bottom of the carb, how many turns out does it need to be set? And the air screw on the side of the carb, how many turns out for these? Thanks in advance, Ric.
 
Re: 1978 GS 750 Carb Rebuild

chockypudding said:
I have all the carbs disassembled and cleaned. I have the needles out and I want to know where to set the clips. I have individual K & N filters and a 4 into 1 pipe with no baffle. I'm going to run 115 main jets, just wanna know where I should set the clips. Also the air screw on the bottom of the carb, how many turns out does it need to be set? And the air screw on the side of the carb, how many turns out for these? Thanks in advance, Ric.
Click to expand...
I'll give her a shot. :)
If the needles are in their factory setting, raise the jet-needles 2 positions by lowering the E-clip 2 positions.
You may be able to run the stock pilot jets by adjusting the pilot fuel screws (underneath) OUT an additional 1/2 turn from where they are. These are precision and tinkering is mandatory. Then adjust your side air screws out to 1 1/2 turns for starters.
Make sure your manifold o-rings are good before re-installing. If it's been a while since they were replaced, I would just replace them. Apply some hi-temp' bearing grease to help them last and torque to 6 ft/lb by replacing those Phillips with Allens.
Oil the K&N's moderately before the vacuum tool carb synch. After the synch, oil them completely.
Next, do a carb synch by eye followed by a vacuum tool synch and get the levels within 1 cm of each other or closer if you can. I like to set my levels at 3,000 rpm's. These VM carbs are sensitive and a good synch is mandatory during re-jetting. Then adjust the side air screws to achieve the highest rpm's. On a warmed up motor and on the centerstand, adjust each side air screw until you hear the rpm's max out. Do all four and then lower the idle with the idle adjuster knob. Keep in mind the valve clearances and ignition timing must be good before synching the carbs.
Remove the 2 floatbowl vent tubes to avoid poor bowl venting and fuel starvation. Just be careful with water if you wash the bike.
I think your stock mains are 102.5? I would go with 120's but the 115's may work. It depends on how your pipe flows. Good luck! :)
 
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What's a good starting point for the pilot fuel screws, I took them out when I was cleaning the carbs. They were around 5/8 of a turn out. Thanks for the info, Keith.
 
Re: Reply

Re: Reply

chockypudding said:
What's a good starting point for the pilot fuel screws, I took them out when I was cleaning the carbs. They were around 5/8 of a turn out. Thanks for the info, Keith.
Click to expand...
For your mod's, I would start with them 1/2 turn out more than they were.
So 1 to 1 1/8 turns out to start. Seat these screws LIGHTLY and then adjust. Keep in mind they are sensitive and you may have to adjust them further, either in or out some. For example, on my '79 GS1000 with the same VM26mm carbs, they were roughly 3/4 turns out from the factory. After my pod/pipe/bore mods and a lot of tinkering, they ended up approx' 1 1/2, 1 7/8, 1 3/4, 1 3/4. I've seen many various final settings.They are the fine tuning for the pilot circuit to each cylinder and it's common to end up with them set a little different from each other.
The pilot circuit allows your bike to idle, but there is an overlap effect with the needle circuit up to about 1/5 throttle position. From there, the needle circuit takes over until about 3/4 throttle.
It can take several road tests and plug reads to get the pilot screws set. I test the pilot circuit at about 40 mph and below, just cruising around. I chop the motor off and take reads. Don't let the bike idle just before taking reads.
Remember, the needle circuit is working at these speeds too. I don't know how others do it but I test and get the needle set first. Then I fine tune the pilot screws. If you find the pilot screws don't have enough richening range, you may have to increase to a 17.5 pilot jet.
BE SURE to vacuum tool synch the carbs correctly or YOU WILL get uneven plug reads and testing/reading will cause nothing but frustration.
If you have to change the needle positions later, you must always re-synch after disturbing the needles.
 
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