Choke flap spring replacement

I'm rebuilding a pair of CB360 carbs, one of which has a mangled choke flap spring. The spring must have broke at some point and someone unwrapped two coils pretty sloppily to act on the flap. I was concerned the sharp 90* angle off the wrap of the torsion spring would be a weak point to break in the future. The factory spring measures about .013" and has 10 wraps. I ordered some .015" "music wire" the measures .014" and made a new spring with 10 wraps and a little preload.

To install I drilled the pot metal out of the end of the choke flap using a 5/64" drill bit, and disassembled the choke flap. Reinstalled with the new spring and capped the end by filling the hole with JB Weld. I considered peening or crimping the boss that the pin goes through, but given how small and fragile it is a figured an epoxy cap might be better. The next guy might need to replace this spring in another 47 years, so this will give him more to work with. Always think of the next guy... Anyway, my eyes aren't what they used to be for this small stuff, so I hope it came out better than it was.

Excellent write up and now it's a sticky

Very nice. Do you moonlight as a jeweler?

Haha no, just figuring this out as I go. I used a 2mm rod to wind the spring on. The first attempt was imperfect with one loose wrap, second attempt came out nice. Once the epoxy cured I used small needle nose to move the rod back and forth to ensure it wouldn't come free on either end. All was good so I reinstalled it and used a small punch to re-deform the end of the screws so they can't fall out.

Something in that assembly (as pictured) is Wrong or backwards...The spring-loaded flaps open inwards (towards the slide or engine....)
The springs and flap hinges should not be visible from the air cleaner side when the choke is closed.....
Screws seem to have been installed from the correct side of the shaft and through the choke plate, so did you purposely flip it 180 for the picture?

I would avoid using a punch to deform the screws, easy way to bend the shaft so it binds up. Instead cut a slot in the end of the screw and use needle nose vise grip to crush them or use Red Loctite.

When the left carbs are apart the choke flap can spin nearly 180*, without the choke lever restricting the range of motion. I moved the choke all the way that direction of rotation to show the installed spring and flap, just rotate the other direction to match the carb with the choke lever and reinstall as normal.

Regarding damaging the shaft, yes be very careful if using a punch. For throttle shaft screws I use a dab of blue loctite. The choke shaft is a little less fragile, and is fine with some very gentle tapping, just enough to deform the last threads, while holding the choke so the flap helps support the shaft on the carb body. If in doubt, Loctite is the safer route. I very rarely use red and wouldn't for this application... Think of the next guy.

Just an FYI about loctite torque specs. Bear in mind these are teeny little 3mm threads, somewhere just south of 1/8”.

1nm ≈ .75 ft-lbs.

Even using blue, you’ve got quite a bit of force ahead of you on those little JIS screw heads to break them free.

I think LDR suggested using rounded head torx (or star, something…) heads to better spread the load.

Personally, I used purple and they’re hanging in there just fine.


-Ed
1972 CL350

I'll rescind my suggestion of Red and go with Ed's Purple after reading the differences. I'm old school and we used to only have choices of Red and Blue available easily.

I learned this lesson the hard way when I used orange thread locker (https://itwperformancepolymers.com/wp-content/uploads/umb/14045/25210-threadlocker-orange.pdf) on an M6 Phillips head.

Absolutely wrecked the heads trying to get them off. Had to drill one out and use an impact on the other two.


-Ed
1972 CL350

Considering the results if the screws come loose, I see nothing wrong with Red Loctite for this application. You can release it with a soldering iron easily enough. The only problem is if the future person is smart enough to apply heat when the screws are hard to turn.