1979 GS850 Jet question.

[Guest]

New to the forums, this is also my first bike. Purchased a 79 GS850 back in February. Have never owned a motorcycle before, but am more than capable of working on my own vehicles. VW/Audi Fan. Wrenched on every car i have owned since i was 14. So my question comes down to Jetting. When i purchased this bike i did not have a motorcycle license. I sent a work friend to check it out and he picked it up for me. I trailered it home the same weekend and have been working on it ever since. The bike runs, starts easily every time. I had to track down some minor wiring issues but seem to have most of that worked out. Issues im currenlty working on are smoothing out the idle and stopping it from stalling when coming to a stop. Previous owner had the petcock connected incorrectly. Vacuum capped and tank overflow landed on petcock vacuum nipple. I also noticed there is a fair amount of rust inside the tank. My cylinder 4 doesn't seem to be firing properly and the new spark plugs were almost completely black after installing them and starting the bike. (except cylinder 4) I changed out the stock very rusty very leaky exhaust with a 4-1 Mac system I found on Ebay. 4-2 were hard to come by. I also noticed the air filter is a K&N drop in (not pods). I pulled the carbs today but can't tell exactly which jets are in there now. They are hard to read, but I believe they are 102.5 mains because 1 is almost legible. Any suggestions and input is greatly appreciated.


Side note: I have not checked the valve clearances yet, but plan on doing that asap.

[Bortasqu]

My beginner tip? Do the valves first, and then do the carb cleaning.
It's not expensive and you can do it in the driveway with one zip tie and a feeler gauge. Check what clearances you got, and work from there.

[mattd1205]

These came stock with 102.5 jets however you will need to up jet to about 115-125 on the mains to get the air fuel mix right for your exhaust and air filters. You might need to shim your needle to richen it up too. On top of what’s suggested above by bortasqu I’d recommend replacing the orings between the intake and head. You can get them from cycleorings.com. While you’re on the site it would probably be a good time to order the vm carb rebuild kit from there as well. Valve clearances, timing, points gap, carb clean, rebuild and upjet will be your top priorities (I’m sure there’s more I’m forgetting but that should be a good starting point)

[Steve]

I changed out the stock very rusty very leaky exhaust with a 4-1 Mac system I found on Ebay. ... I also noticed the air filter is a K&N drop in (not pods).

These came stock with 102.5 jets however you will need to up jet to about 115-125 on the mains to get the air fuel mix right for your exhaust and air filters. You might need to shim your needle to richen it up too.

Let's back up and get a little more information before you order up some new jets.

Does your MAC system have the street baffle or the performance baffle? The easiest way to tell is to look into the end. The performance baffle will be straight-through, the inner core of the baffle might be 1 1/2 to 2" diameter. The street baffle will be about 1 1/2", and will block your view within about a foot. If you have the performance baffle, you will need to go up a couple sizes on the mains. With the street baffle, you might not have to make any adjustments. With your K&N filter in the stock airbox, there are no jetting adjustments necessary.

Bottom line: with the street baffle, you might need to go to 105 or 107.5 mains. With the performance baffle, maybe a 107.5 or 110. If you had individual pod filters, definitely in the 115-125 range that mattd mentioned.

Your needles are adjustable, so it will be rather easy to move them if necessary. Move the clip one notch DOWN, closer to the pointy end, you should be pretty close.

.

[Jwhit89]

Let's back up and get a little more information before you order up some new jets.

Does your MAC system have the street baffle or the performance baffle? The easiest way to tell is to look into the end. The performance baffle will be straight-through, the inner core of the baffle might be 1 1/2 to 2" diameter. The street baffle will be about 1 1/2", and will block your view within about a foot. If you have the performance baffle, you will need to go up a couple sizes on the mains. With the street baffle, you might not have to make any adjustments. With your K&N filter in the stock airbox, there are no jetting adjustments necessary.

Bottom line: with the street baffle, you might need to go to 105 or 107.5 mains. With the performance baffle, maybe a 107.5 or 110. If you had individual pod filters, definitely in the 115-125 range that mattd mentioned.

Your needles are adjustable, so it will be rather easy to move them if necessary. Move the clip one notch DOWN, closer to the pointy end, you should be pretty close.

.

Steve,
mom picking up a 4-1 from a fellow forum member this weekend. I want to order some jets so I can be prepared to toss it on next week, bone stock 79 850 would I need to just jet or would I have to move the needle too? Appreciate your time