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check my rejet numbers/assumptions

  • Thread starter Thread starter Govee
  • Start date Start date
G

Govee

Guest
I have the carbs on my 77 GS750 apart for rebuild/rejet. The o-rings were dry rotted so this was needed even without rejetting.

The bike has a Kerker 4 in 1 and stock air box. On tear down I learned the mains were already at 112.5 and the jet needle lock washer is in the middle position. I had a little popping out the pipes when I let off the throttle.

I will be running:
Kerker, pods, and I am swapping in an 850 topend. I have been reading the rejet threads and I plan to start at 117.5(or 120?) mains and moving the lock washer to the 5th position (I mean the position closest to the pointy end of the needle, I get confused by references to top or bottom). I will start the fuel screw at 1.5 out and air at 2.5 out.

Please tell me where I am wrong or need to change.

Thanks
 
Stock jetting for the 750 changed a few times in the early years. I don't have numbers that are specifically for '77, but the '78 had 100 mains. Since you are going to an 850, the stock mains for the 850 were 102.5.

Depending on what baffle you have in your header, you might want main jets 4-6 sizes larger than stock. That would put the 750 at 110-115 and the 850 at 112.5-117.5.

Not sure the pilot jets need to go more than 1 turn out, maybe as much as 1 1/8, but not 1.5. Air screws typically start at double the fuel screw setting.

Since the pointy end of the needle is below the blunt end while the bike is travelling down the road, that is considered the "bottom". The grooves are typically numbered from top to bottom (blunt to pointy), just as you have done. The position for the clip (not the "lock washer") that works best will vary from bike to bike, but it's best to start in a neutral position, like #3 or #4, then tune from there. Doing plug chops will help determine what settings you might need to change, but do yourself a favor, change only ONE thing at a time.

.
 
Thanks for the responses.

I will go with 117.5 Mains. 4th position on needle clip,and fuel screw at ~1 1/8. and tune from there. :|
 
If your stock mains were 100's, then 117.5 is too lean if you run pods.
If the motor is in good condition/tune the bike will handle 125 mains or maybe 127.5's. 1 1/2 or 2 full positions richer than stock on the jet needles (you need approx' .022" thick jetting spacers to make "1/2" position changes). When using the stock jet needles, you'll want 17.5 pilot jets. Also, remove the two floatbowl vent lines for better venting/fuel flow.
If you run just the pipe, somewhere between 105 and 110's will work. The pods is what will really increase the need for more fuel. Pipe only often doesn't need jet needle adjustments. If it's needed, no more than a "1/2" position richer is needed. No pilot jet changes, maybe richen the pilot fuel screws underneath a little.
In either case, after full warm up, adjust the side air screws using the highest rpm method. Then vacuum tool synch. Whenever the jet needles are disturbed or it's been awhile, you need to vacuum synch to accurately re-jet and test performance.
Don't worry about some decel' popping. It's normal when you install freer flowing intake and exhaust that has less baffling that hides decel' pop already there. Excessive decel' pop means you're lean and the pilot circuit needs richening.
 
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