Rejetting questions (again??)

[keef]

Hi all
New to the site so please be gentle.
Just dragged my 78 GS1000 out of the garage after 11 years, gave the old girl a top end rebuild (valve seats, shims and stem seals) decided to do away with the air box as the rubbers had turned into stone. Went for a set of cone type filters (I had used these type many years ago to great effect on another GS) I have run the bike up and found it flat spots at between a quarter and a third throttle. I seem to remember using an air corrector kit so looked on ebay but no joy. A friend told me it is possible to jet round the problem. I have a 4-1 exhaust and have raised the needles 1 notch (many moons ago) to adjust for the change from stock and it worked fine. All jetting is standard (95 mains I am in the UK) and it idles well.

Any ideas would be gratefully received.

Cheers

Keef

[chef1366]

Lower the clip on the jet needle a couple of notches.

[BassCliff]

Greetings and Salutations!!

Hi Mr. keef,

You'll find links to carb tuning on my little website. Here's your "mega-welcome".

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Thank you for your indulgence,

BassCliff

[OldVet66]

I just rebuilt a new ('79, but still had the factory paint on the pilot fuel needles and air screws) set of carburetors for my '78 GS1000E lately. I had a Dynojet stage 3 jet kit in the old carburetors that I switched to the new carbs. I could not remember the setup, and Dynojet doesn't have a .pdf file for that jet kit posted. I called the tech at Dynojet and he recommended the DJ 142 jets (Mikuni 132.5), main jet needle clip third notch from the top (if I remember correctly) and the pilot fuel needles at 1/2 turn out from lightly seated. I wasn't sure he was correct, because on the old carbs I had used the Dynojet 138 jets (Mikuni 130 jets) and had it running fairly well, but could never get it just right. It didn't have any luck with the 138's, so I tried his setup, and it really ran fairly well right away. With the pilot fuel screws at 1/2 turn out, the air screws were almost closed for highest idle so I adjusted the fuel screws out between 1/2 and 3/4. That moved the air screws out close to 2 turns out for highest idle and removed most of the slight blubbering with the throttle barely cracked opened. It's running better than it ever has. Plenty of low end power and very smooth through redline. It can still use some slight tweaking to remove the last traces of blubbering at about 1,500-2,000 RPM (town traffic speeds), but I'm waiting for my angled screwdriver set from Morgan so I can do the fine adjustments to the pilot fuel screws when the bike is running. Through this rebuild I learned that my old carb bodies were all ruined. On the new carbs, with the pilot fuel screws lightly seated, the tips of the fuel needles are just visible below the hole in the throat of the carb. With my old carbs, all the fuel needles extended into the throat of the carbs, so all the carb fuel needle seats were ruined. That explained why I could never get them just right.

[BassCliff]

Hi,

I'm passing along another bit of advice from a friend:

"Set the needle in the stock (center) position and get some 112.5 or 115 main jets and finish with air and fuel screw adjustments.That is ALL that he needs to do if the carbs (pilot jets in particular) are clean."

If ALL of the other maintenance has been performed, clean carbs, valve adjust, air-tight intake system, etc, then this should get you close.


Thank you for your indulgence,

BassCliff

[OldVet66]

I believe he is jetting for pipes and pods.

[keef]

Well firstly, thank you BassCliff for a very warm welcome!
And thanks to OldVet and Chef for your pointers.
I have had a hunt around my garage and found some new 115 mains so will use these as a starting point.

My idea was to use the old girl through the summer and then over the winter give her frame a lick of paint, polish up the motor and rewire throughout.

I will keep you posted.

[MisterCinders]

Well firstly, thank you BassCliff for a very warm welcome!
And thanks to OldVet and Chef for your pointers.
I have had a hunt around my garage and found some new 115 mains so will use these as a starting point.

My idea was to use the old girl through the summer and then over the winter give her frame a lick of paint, polish up the motor and rewire throughout.

I will keep you posted.

For pods and pipe, try moving the needle to the No. 4 position (counting down), and prepared to shim it up further if that doesn't sort out your flat spots.

Also, since you are hitting the flats in the transition from pilot to needle, you should consider increasing the pilot jet one size (e.g., replace 15 with 17.5).

[OldVet66]

Just a little update, with the jetting setup I mentioned in post #4 I am getting 44 MPG.