Carb Help

[Mr.Ed]

Hi Guys! Before I get yelled at, I have used the search engine and came up with some useful info, but need help with some terminology with these carbs. First I am running a 78 GS 1000 with stock carbs (see pic)

The bike has a 4 into 1 V&H pipe with K&N pod Filters. What are the adjustment screws for on the rear outer side about half way up the carb, and underneath the carb? What do they adjust (pretty sure one is for air mixture and the other is fuel?). Also what should they be set at? Anyone out there running a set up like mine? I know the bike has Dyna green coils not 100% sure if it has points still. Any help would be greatly appreciated! Bike runs good, carbs 1 & 2 are rich and 3 & 4 are a bit lean.

 

[trippivot]

air box side is a-- pilot circut air mixture screw
underneath engine side is a-- pilot circut fuel mixture screw

the individual cylinders may not be set identically but a lot of people do set them identically.

a good starting point is to check and list where every screw is set at.
write it down! there will be 8 settings. these will effect the other carb settings like needle jet groove setting and necessary main jet size.

try 1 turn on the airs' and 1.5 on the fuels

going in on the air screw will richen--less air
going in on the fuel screw will lean--less fuel

you are going for a combination on each cylinder to achieve the highest idle speed without turning the idle speed knob up.

You DO want to turn the idle speed knob DOWN as you get each cylinder to run at it's best mixture setting.

just check where the screws are at BEFORE you begin to adjust so you can go back to where you started . in case it runs worse this takes practice

don't forget to have a fan running on the engine to prevent overheating
taking too long will cause negative tunning effects. Also have plenty of fuel in the tank, this too will have you chasing your tail.

 

[bwringer]

air box side is a-- pilot circut air mixture screw
underneath engine side is a-- pilot circut fuel mixture screw

the individual cylinders may not be set identically but a lot of people do set them identically.

a good starting point is to check and list where every screw is set at.
write it down! there will be 8 settings. these will effect the other carb settings like needle jet groove setting and necessary main jet size.

try 1 turn on the airs' and 1.5 on the fuels

going in on the air screw will richen--less air
going in on the fuel screw will lean--less fuel

you are going for a combination on each cylinder to achieve the highest idle speed without turning the idle speed knob up.

You DO want to turn the idle speed knob DOWN as you get each cylinder to run at it's best mixture setting.

just check where the screws are at BEFORE you begin to adjust so you can go back to where you started . in case it runs worse this takes practice

don't forget to have a fan running on the engine to prevent overheating
taking too long will cause negative tunning effects. Also have plenty of fuel in the tank, this too will have you chasing your tail.

Thanks for putting this so clearly and succinctly!

A couple of weeks ago, I had the wrong idea about what the air screws did, and drove myself batty trying to dial in a '79 GS850 with the same carbs.

I finally got it sorted out, but I wish I had seen this post first...

 

[Jaguarjoe]

you are going for a combination on each cylinder to achieve the highest idle speed without turning the idle speed knob up.

So you are saying to adjust the air screw for max RPM and also adjust the fuel screw for max RPM too?

 

[mark]

With a pod and pipes your setting won't be the same as the stock settings would be so its hard to say what they "should" be. Advice above sounds in the ballpark.........

Keep in mind that both of these screws adjust your pilot circuit, which only functions from just off idle to about 1/8 (maybe a little more) throttle openings. After that your needles take over with mains taking over approaching full throttle.

Also keep in mind that "rich and lean" may change at different throttle settings, so when checking plug color do it at different throttle settings, usually 1/8, 1/4 and full throttle (riding the bike of course). Adjustments from about 1/4 throttle on will be done with your needles and main jets.

Carb synchronization and float settings can also affect rich and lean settings, so make sure those are correct. Valves must be adjusted correctly for a good carb synch. Also make sure your manifold boot o-rings are good.

 

[Mr.Ed]

You guys are awesome! All I have to do now if get busy playing with them to straighten out. Thank you for the help!

 

[KEITH KRAUSE]

Thanks for putting this so clearly and succinctly!

A couple of weeks ago, I had the wrong idea about what the air screws did, and drove myself batty trying to dial in a '79 GS850 with the same carbs.

I finally got it sorted out, but I wish I had seen this post first...

I've been posting what the VM pilot fuel and side air screws do for about 5 years now. Just for the record of course.

 

[KEITH KRAUSE]

So you are saying to adjust the air screw for max RPM and also adjust the fuel screw for max RPM too?

When re-jetting the VM carbs, initially set the pilot fuel screws to something that makes sense, say, 1/2 additional turn richer than stock for pods/pipe. If you don't know where they were set at the factory, try 1 1/4 to 1 1/2 turns out from LIGHTLY seated as a starting point. Now adjust the side air screws using the highest rpm method. Now test the pilot circuit and get plug reads after minimal throttle position cruising. If adjustment is needed, richen or lean the pilot fuel screws according to performance/plug reads. Then re-adjust the side air screws for best idle again. You may or may not notice any significant changes to the air screw adjustments.

 

[KEITH KRAUSE]

Hi Guys! Before I get yelled at, I have used the search engine and came up with some useful info, but need help with some terminology with these carbs. First I am running a 78 GS 1000 with stock carbs (see pic)

The bike has a 4 into 1 V&H pipe with K&N pod Filters. What are the adjustment screws for on the rear outer side about half way up the carb, and underneath the carb? What do they adjust (pretty sure one is for air mixture and the other is fuel?). Also what should they be set at? Anyone out there running a set up like mine? I know the bike has Dyna green coils not 100% sure if it has points still. Any help would be greatly appreciated! Bike runs good, carbs 1 & 2 are rich and 3 & 4 are a bit lean.

By the photo (bluing header), I think you may be lean, most likely at all 3 basic jetting circuits.
Your uneven plug reads may be because the carbs need to be vacuum synched which is a very important part of jetting.
The valves and ignition timing must be correct to do an accurate synch. One thing depends on another.
I suggest checking the plugs at minimum throttle, 1/3, and full throttle to see what the pilot circuit, jet needle, and main jets are doing respectively.
Once tuned and ready for the re-jet, your specific model and mods generally call for a 130 Mikuni main jet and to raise the stock jet needle to its maximum richness (e-clip in the bottom/5th position), along with a richer 17.5 pilot jet and/or richer pilot fuel screw adjustments.
Many times the stock jet needles won't work and you need to get the stage 3 Dynojet jet kit, model 3304.
Also, remove the 2 floatbowl vent lines when running pods.
Running lean is serious.