Carburetor Conundrum (260Z)

[Mark Maras]

 Generally speaking, adding more fuel rarely increases the rpms unless the mixture is way too lean. Air is the most common cause of increased rpms.

[Captain Obvious]

9 hours ago, Patcon said:

If the front carb float bowl level was higher than the rear, wouldn't it run faster due to extra fuel?

No, it would just run richer, not faster. Unless, of course as Mark suggested, it was running really really lean and wanted the extra fuel.

I really don't think that has anything to do with what's going on here. Float levels will need to be checked before this is all said and done. but that's not what's causing the idle to be at 2500 RPM with the screws backed all the way out.

[DadAndLadZ]

Been too hot to work in the garage but we finally had a couple nice days and I think we may have solved the mystery… You’re gonna laugh. 
 

 

[Captain Obvious]

Well THERE'S your problem!! Yes, that nail was holding your butterfly open and not allowing it to close completely.

Some answers to additional questions:

That hole is intended to be a ported vacuum source to drive the distributor advance. There is supposed to be a vacuum tube nipple pressed into the carb body that allows you to connect a rubber tube between the front carb and the distributor advance diaphragm. It appears someone sheared the nipple off and decided to plug that hole with a nail instead.

And about the "insert grease" engravings... It appears that someone drilled little holes in the carb mounting face with the intent of using them as lubrication grease ports to lube the throttle shafts. Probably a waste of time, but since they are sealed off by the gasket between the intake manifolds and the carb faces, it probably doesn't cause any harm either.

So for both of those items... THAT'S something I've not seen before!! 

Bottom line... Plug that vacuum nipple with something that does not stick down into the carb throat and I bet your idle issue will be pretty much taken care of.

[Captain Obvious]

And can I just point out that when I said "I'm thinking that something within the carb itself is keeping that front throttle butterfly from closing completely"...

I nailed it!

HAHAHAHAHA!!!!!   

[DadAndLadZ]

14 hours ago, Captain Obvious said:

Bottom line... Plug that vacuum nipple with something that does not stick down into the carb throat and I bet your idle issue will be pretty much taken care of.

That’s the plan now… JFC I was losing my mind. I know I should have just taken the damn thing off earlier but it always feels like opening a can of worms to me. You would think I’d be used to it by now lol…

It does have a vacuum advance hose going to the distributor, it’s just attached a little farther back, behind the spacer on the manifold. I guess it’s a by-product of mixing and matching parts off different years of car.

Many thanks again to everyone who has chimed in on this, here’s hoping the boys and I can start working on something else soon! lol 

Edited July 31, 2022 by DadAndLadZ

[Captain Obvious]

8 hours ago, DadAndLadZ said:

It does have a vacuum advance hose going to the distributor, it’s just attached a little farther back, behind the spacer on the manifold. I guess it’s a by-product of mixing and matching parts off different years of car.

Well that vacuum connection you are using now is full manifold vacuum, but the system was designed to use a ported source that peaked advance at light cruise throttle (slightly above idle). With your direct manifold vacuum source, you will have peak advance at idle, and it will drop from there. I'm no timing expert, but I'm not sure that's what you want.

And I don't think your vacuum connection is really a remnant of mixing and matching parts from different years. I think it's a remnant of snapping off the original ported vacuum nipple and then looking for some place else to connect that vacuum line.     

Either way... So when will we get an idle report to see if taking that nail out allows the front carb to operate properly?

[DadAndLadZ]

1 minute ago, Captain Obvious said:

Either way... So when will we get an idle report to see if taking that nail out allows the front carb to operate properly?

Wow, good info, thanks! We will put it back together ASAP!

[Captain Obvious]

I have very little doubt that things will be a whole lot better now, but after four pages, I just want to hear it from you to be sure!   

[DadAndLadZ]

11 minutes ago, Captain Obvious said:

I have very little doubt that things will be a whole lot better now, but after four pages, I just want to hear it from you to be sure!   

Oh you guys will hear for sure! lol … THANKS most sincerely for your patient help!!!

[Jeff Berk]

Captain Obvious, do you think its advisable for DadAndLadZ to reestablish that return line to the fuel tank to minimize the potential for vapor lock? 

[Captain Obvious]

Yeah, I think I would recommend running a return line, but that (like a lot of other details) can wait. I just want it to idle correctly for now!  Haha!

I'm no regulator expert, but I'm not a big fan of dead-headed systems in carb applications. This non-expert would prefer a bypass regulator system with a return line.