Changing Jets?

[firefightermatt]

Hey forchetto, should I adjust my needle also when I change the main & pilot jets or is it not needed?

[forchetto]

If you're getting the carb out, you might as well. It'll be easier on the bench and it does run better. The mid-range pull is more positive and, surprisingly it starts better and the cold "driveability" is vastly improved as well. Don't know why.

[firefightermatt]

Well I finally got around to changing my jets today and it was a piece of cake until I striped out 2 screws on the needle cover on top of the carb.

Mine did in fact have the 122.5 main jet and 32.5 idle jet so that has now been increased to 125 main jet and 35 idle jet.

I moved the clip on the needle to the forth from top position, it was at the thrid from top.

My mixture screws was 1 3/4 turns out so I screwed it in and then back out 3/4 turn. I hope this is about where it needs to be.

I ended up replacing both the top cover and float bowl screws to hex head screws so I wont run into the stripping problem again.

Once the rain lets up here I am going to go ride and see if it made any difference.

[forchetto]

That's exactly how I have it now, including the idle screw at 3/4 of a turn out!!!. Mine runs fine apart from a glitch when after decelerating for a turn or bend, I blip the throttle to change down a gear. The engine sometimes gasps. This doesn't happen when the bike is at a stand still, you can blip it as many times as you like and it never gasps. Only in motion. I've tried all sorts of things, including making the slide spring 10mm longer, as adviced elsewhere, but it remains there. I do hate these CV carbs...I'm still suspecting that ACV (air cut-off valve) on the left side...

[Luis Carlos]

I confess that when I saw this picture thought very funny, plus I really think it makes sense to raise the plunger to allow a whole equate the air too.

[forchetto]

That guy probably heard about the "washer" mod. I've never heard of that one, anyway the Qingqi's Mikuni has an adjustable needle.

I have solved the problem of the gasping at last. It seems that when I fitted a larger idle jet to improve cold running and low speed behaviour I overlooked the action of the ACV device (air cut-off valve), also called variously "coasting enrichener","Transient Enrichment Circuit",anti-backfire valve, etc. It's the lump on the left of the carb, just behind the choke lever.

The idle circuit has a fuel jet and TWO air passages to feed it during normal running. As modern bikes are very lean jetted due to emissions, the high vacuum created during deceleration causes an over-lean condition that leads to backfiring in the exhaust. The air cut-off valve reacts to the high vacuum cutting off one of the air ways and so enriching the idle mixture momentarily and avoiding popping in the silencer.

As I've already made the idle circuit richer with my idle jet (nº35) the mix is now too rich during deceleration and this gives what is called a "rich stumble". This is why this only ever happened after prolonged deceleration and not during a quick down gear change or when standing still.

I've blocked off the vacuum passage from the carb to the valve by blocking the tiny hole in the valve lid with a piece of rubber. I have photos if anybody gets this problem.

[Luis Carlos]

I have this problem but now is not happening always.
Once when I got out of the avenue to enter the district and the engine died as I had not control the bike, it cost me a few centimeters of skin from my knee.

[Luis Carlos]

I've noticed that. It's probably for the Brazil market for some reason. Mine came with 122.5 main and 32.5 for the slow jet. It's not an error as I have a brand new spare carb and it also has 122.5 main.

In Brazil gasoline is mixed with alcohol. The carburetor has a cuba where gicleurs that interconnect the same passage of the air, so if you have a gicleur with 14.8 times less area that the passage of the air mixture is 14,8:1.
This would be the ideal proportion for the use of pure gasoline, the alcohol was 9,5:1 and our gasoline with 24% alcohol with which we use today is somewhere around 12,5:1.

[forchetto]

Good deduction Sherlock Holmes... I'd never have thought of that, but of course you're probably right. Brasil does use sugar alcohol in their fuel and explains the larger main and idle jets.

[forchetto]

As I pointed out on message nº10 on this thread and the solution on a later message, I'll repeat it complete with photo:

"There's just one tiny glitch in the carburetion left to tackle: after decelerating from high revs, blipping the throttle to change down makes the engine gasp. This does not happen if the deceleration is gradual and over a distance or when stationary, so it's not the idle speed mixture setting. I suspect it's that blasted ACV (Air cut-off valve, Coasting Enrichener, Transient Enrichment Circuit, anti-backfire valve...there are lots of names for them, I just call them bastards...). I had trouble with that before when a piece of foam from the air filter blocked one of the passages in that device. If you trawl various forums and sites you'll find the disabling of this emissions device a common mod. I'll report back when I get it sorted."


I have solved the problem of the gasping at last. It seems that when I fitted a larger idle jet to improve cold running and low speed behaviour I overlooked the action of the ACV device (air cut-off valve), It's the lump on the left of the carb, just behind the choke lever.

The idle circuit has a fuel jet and TWO air passages to feed it during normal running. As modern bikes are very lean jetted due to emissions, the high vacuum created during deceleration causes an over-lean condition that leads to backfiring in the exhaust. The air cut-off valve reacts to the high vacuum cutting off one of the air ways and so enriching the idle mixture momentarily and avoiding popping in the silencer.

As I've already made the idle circuit richer with my idle jet (nº 35) the mix is now too rich during deceleration and this gives what is called a "rich stumble". This is why this only ever happened after prolonged deceleration and not during a quick down gear change or when standing still.

I've blocked off the vacuum passage from the carb to the valve by blocking the tiny hole in the valve lid with a piece of rubber. See photo.

What may happen now is that the exhaust backfires a bit under deceleration. To cure this richen (turn anticlockwise) the idle screw a tiny bit or until the backfiring is acceptable.