1979 Carb Jetting

[Sammy_1101]

Hey guys,

Recently got my hands on a '79 GS850G. It did run, rather terribly, and would stall if you let in the choke. It also wouldnt rev up at all without dyeing.

Since then I've taken the carbs off to give them a good clean and to see what has been done to the jets.

The bike has pod filters and a 4 into 1 exhaust. The idle jets were still stock at 15, however 3 of the main jets have been upgraded to 120, and 1 at 122.5 (Compared to a stock main that 102.5 I think).

Is there any reason why 1 jet would be different? and also is the Jetting correct or close to overall? What is your reccomendation for jet size for these sorts of mods.

Thanks for you help

 

[Guest]

When I had pods and a pipe on my 750 I had 115 mains in it, and the needle clips in the bottom groove. The 750 and 850 were jetted very similarly, so you should be around there give or take a little. The general guideline is to go up 2 jet sizes for 4:1 exhaust, 3 sizes for pod filters; then add them all up and subtract 1. Jets go in steps of 2.5, stock jets were 102.5, or close to that, for your bike.
If I were you I would dip those carbs for 24 hours, blast out the passages with compressed air, put in new orings, and do some plug chops with your current set up to figure out main jets. You need to check your needle clip positions too, so plug chops are in order there too. There are tons of threads about this, do some searches for your bike and read up before you dive in. Also, if you have cheap pods on it may be very difficult to get it to run right. Let us know how it goes!

 

[Sammy_1101]

Thanks heaps for the info.
Does that size changing for the pods and exhaust apply for the pilot jet also?

Thanks

 

[Big T]

The pilot jet should stay at 15, since the VMs have both fuel and air adjustments for the pilot circuit. If the fuel screw gets past 1 1/2 turns, go to 17.5 pilots

 

[Sammy_1101]

Ok. That makes sense.
Thankyou very much

 

[Steve]

There should not be any need to change the pilot jets. The reason that any jetting needs to be adjusted is because you allow more air to flow by removing restrictions. At wide open throttle, the restrictions are the airbox and the exhaust. Stock GS exhaust actually flows quite well, so only minimal adjustments are necessary for a 4 into 1 pipe. The flow characteristics of a 4 into 1 pipe are somewhat directly related to how loud it is. If it's quiet, you might not need to make any adjustments. If it's loud (shame on you ), two or three jet sizes up on the main.

The airbox is the main restriction. With proper filters on the carbs, you might need to add 3 to 5 sizes to the mains. However, that extra air only flows at full throttle. If you close the throttle below wide open, the carb slides become the restriction, not the airbox, so by the time you get down to idle and low-throttle settings, there is no extra air flowing, so no extra gas is needed in the form of larger pilot jets. The needle might have to be raised a bit, just to help the transition from pilot to main.

Stock jetting is, indeed, 15 pilot and 102.5 main. Depending on the quality of your pods and the volume of your pipe, I would expect main jetting to be somewhere between 112.5 and 120, they should all be the same.

.

 

[Gumpster]

The main probably needs to be closer to 130 or more. The dynajet stage 3 kit for a GS1000 comes with 138 and 142 mains. The stock main was a #95 and the 138 dynajet was perfect with 4 to 1 exhaust and K & N pods. A 138 dynajet is equivalent to a 130 Mikuni. If you are already running 120 and 120.5, a step up to 130 to 132.5 would be close since the symptoms indicate lean condition. Same carb as a 1000, VM26. Needle clip probably needs to be in position 4 or 5 which raises the jet needle. ( This is assuming stock jet needle )