Coast valve shimming info

[The1Chef]

A while back I had asked if anyone had info about shimming the coast valve. It had been in the tech section on this board years ago but was lost. Few remembered the mod and no one remembered the info.

Well, I took my old carbs apart and found what I was looking for. Such a simple mod which served the purpose of eliminating backfires, pops and stalls on quick strong deceleration. Mine used to stall every time I came to a quick stop. Very frustrating when you expect to takeoff and instead almost fall over. It also used to pop and farther on deceleration. This mod stopped all of that completely.

It involves adding a thick washer ( about the thickness of an American nickel), behind the spring on the coast valve. It needs to have a large enough hole to fit over the springs cradle. That's it....I can't seem to get the pics to post so here's links

https://ibb.co/9pR74L0
https://ibb.co/hXkMd1m
https://ibb.co/G719dpd
https://ibb.co/TKszVZ5
https://ibb.co/L54X1HW
https://ibb.co/k51Dhtp

[Hylander]

Hey Chef thanks for the post.

Do you know how this fix works? Like what does adding that washer do to make the bike run better?

[The1Chef]

Just puts more pressure to the spring and more effectively closes the system

[sky02]

I think that you get it wrong. By shiming it you make it harder to close and operate.

Normally the spring push the brass rod against the wall of the carb. When there is negative pressure on the membrane (during hard deacceleration) it compresses the spring. The brass rod moves away from the wall of the carb and blocks air passage. The rod is thicker at the end and that end part is closing the passage.

So shimming makes it leaner at deacceleration.

I never had any stall or excessive popping during engine braking. And my bike is fitted with kn drop ins and and non restrictive slipons. I tuned the carbs pretty rich using wideband oxygen sensor. All i have is one pop when after grabbing a full throttle I suddenly fully close it in lower gear.

[azazelakira]

I think that you get it wrong. By shiming it you make it harder to close and operate.

Normally the spring push the brass rod against the wall of the carb. When there is negative pressure on the membrane (during hard deacceleration) it compresses the spring. The brass rod moves away from the wall of the carb and blocks air passage. The rod is thicker at the end and that end part is closing the passage.

So shimming makes it leaner at deacceleration.

I never had any stall or excessive popping during engine braking. And my bike is fitted with kn drop ins and and non restrictive slipons. I tuned the carbs pretty rich using wideband oxygen sensor. All i have is one pop when after grabbing a full throttle I suddenly fully close it in lower gear.

What wideband oxygen sensor did you used? I am interested on this

[sky02]

https://www.aemelectronics.com/products ... ller-gauge

And i have two welded bungs before the h pipe to tune each cylinder. Facing inside so you cant see them.