Vented Carb Screw Question

[Pete483]

Anyone know how long the vented screw that goes up from the thottle body to the carb is supposed to be?

I'm rebuilding a non-USA 1977 carb (H3660 tag) and am missing the vented screw which I think is PN 21824-38010 Screw, Set, No.2(For Carburetor Body Set) and from what I can tell, is a standard part for a bunch of models and was the same on US and general market carbs. I managed to track down a used one, but it is too short and ends right at top of the insulator block. I'm trying to figure out if the screw I bought is broken off, or if I need a different part number for a longer screw.

My non-EGR insulator is 20mm thick and the screw I have is 43mm long and has 13 threads on it. Anyone have any idea either how long this screw should be on a 1975-80 carb, or how thick the insulator block is on a USA spec carb (to see if my problem is that the non-EGR insulator is thicker)? This screw is still available in Japan, but before I pay for shipping, I'd like to make sure it's the right screw. I know I don't have the pre-1975 shorter screw as I previously bought one of those, which is way too short.

 

[Pete483]

Alternatively, does anyone know how important this screw actually is? My carb was missing it before, it it didn't seem to have any negative effects.

 

[Nolan]

Alternatively, does anyone know how important this screw actually is? My carb was missing it before, it it didn't seem to have any negative effects.

I would think that with it missing, you'd be prone to an air leak, due to the fact the other two bolts would pull on the other side unequally.

 

[flx]

this ^

And the actual use of the vent on the screw is to activate the power valve when there is enough vaccum in your intake. The power valve will unlock extra fuel flow in the 1st main jet.
If you put a screw without vent, I guess you would be missing some power for the transition between the use of the 1st and secondary body. Maybe even running lean in some acceleration conditions?
If you don't put the screw you may have leak and a carb not seated perfectly correct.

 

[65swb45]

The previous replies are correct: the venting performs the essential function of providing the internal vacuum signal for the power valve.

There are four different lengths of that screw: 70-73/4, 74 Cali spec (just a hair longer than an FJ60, 75-80 &81-up. The one you need is the longest of the 4. I’ve never bothered to measure one, so I can’t offer you that info.

I suppose if I ever have need of one, I will get some shouldered 6mm bolts of appropriate length and pay to have them gun-drilled, then grind the head into a round dome and use a hacksaw to cut a slot across it. Form follows function.

 

[Golfer53]

How does air get to the power valve if the insulator blocks the hole when the base and body are assembled?

 

[mattressking]

The Power Valve does not get 'air' (you mean vacuum).

The power valve is a gatekeeper for fuel from the float bowl into the venturi fuel passage. Vacuum that travels from manifold into the vent screw retracts the power piston and mechanically compresses OR releases the power valve dependent on engine vacuum (or rpm/engine load if that helps).

Make sure the Power Piston moves smoothly in the bore and has the correct length spring. I like to polish the piston with steel wool to mirror so its smooth.

 

[Golfer53]

Thanks for the explanation. I'm still curious how vacuum (not air sorry I mis spoke earlier) gets to the power valve if the hole the screw with the hole in it is blocked. Does it feed in from somewhere else? The reason I ask is because it seems all year carbs are different and I want to be sure mine is operating.

 

[65swb45]

If you look at the base upside down, you will see a hole in the primary bore that joins with the chamber that has the screw.
This came up once before that I remember, and someone posted a cutaway drawing from a repair manual that illustrates the hole.

 

[mattressking]

This is for a an earlier 2F carb but same concept. Cutaway instead of a hole.

Later 2F/3F had a hole, circled and arrow, only picture I have on my phone I can quickly find.

 

[Golfer53]

Mine has neither. It appears I have a carb put together from more than one year. Should I cut a groove or drill a hole to give a path for vacuum? I will remove the carb today and send a pic of what I have. Thanks for all your help.

 

[Golfer53]

I'm not the best photographer but these might help.