376 carb

[Lazza]

Just introduced myself and realized that I should post my enquiry about jetting here:

The standard setup 376 carb I have has a bellmouth fitted in place of filter.
Now the valve clearances have been correctly reset the bike idles rather fast and I cannot get a reliable slower tickover.
She's running a little lean and the wisdom here seems to be to fit a bigger main jet.
Currently on 240, but what would be the consensus be? 250 or bigger?

Thanks for any help

Lazza

[Swarfcut]

 Check the throttle slide is not sticking and that the cable has just a little slack. It can be readjusted after setting the idle speed with the throttle stop screw.

 From experience a cheap foam filter is better than an open bellmouth.  Fine road grit scores throttle slides, bores and pistons, and there is a good deal more of that in Sunny Spain in the dry season.... Worth a try for starters before buying more jets.

 Swarf-eager.

[groily]

Welcome!
Main jet shouldn't have any bearing on tickover. It controls things from about 3/4 throttle.
The most relevant parts are the PILOT jet (there are several sizes) and its associated air-bleed screw which needs setting to control the mixture, the throttle cut-away (which should I think be 3 1/2 on your bike), and the slow running screw.
A  throttle cable that's poorly adjusted would cause grief as Swarfcut says, if the slide can't drop right down with a satisfying 'click'.

Best check the air slide (choke) too for correct operation, if there is one. And the fuel level in the float chamber if the tickover speed is irregular.
The bell mouth should not prevent your getting a steady tickover - but as Swarfcut also says, in dusty climates!

Good luck!

[KiwiGF]

Just introduced myself and realized that I should post my enquiry about jetting here:

The standard setup 376 carb I have has a bellmouth fitted in place of filter.
Now the valve clearances have been correctly reset the bike idles rather fast and I cannot get a reliable slower tickover.
She's running a little lean and the wisdom here seems to be to fit a bigger main jet.
Currently on 240, but what would be the consensus be? 250 or bigger?

Thanks for any help

Lazza

It would be worth checking the slide is not loose in the bore due to wear, when there is a gap some air gets sucked into the engine without going past the fuel jet, this results in an unstable idle as the idle throttle stop adjuster is not able to fully control the idle speed, and the idle mixture screw adjuster is not going to work properly either.

The slide wear can be measured by pushing the slide back and forth when it is at the lowest position it can go, as that is where the most wear occurs. A few thou may be ok but not a noticeable click or movement. Slides can be re sleeved but new Amals are not expensive.

Air leaks at the flange is also a potential cause, put tape over the joint to see if that helps. The flanges get warped due to over tightening, I use spring washers under the nuts and stop tightening when the spring is compressed, it doesn’t take much to bend the flange, the new amals are less prone to this.

[Lazza]

Slide seems fine and no obvious air leakage.
I'll head down the filter route and maybe go one size up on pilot jet.
Thanks to all for your helpful comments

Lazza

[Slymo]

I recommend replacement of the brass needle jet. It suffers a lot of wear and makes tuning on and off idle quite difficult.

[Rex]

I'll second that. Well worth replacing, along with the needle.