choke part

[Iceman_70]

Already asked this in the engine and drivetrain section, but not a lot of answers there, so I'll give it a try here ...

I'm currently rebuilding my 350 engine. It has a Rochester 2GC carb (I think)... The choke was attached to a 'tube' that was sitting in the manifold.




In the other post, someone said it was a 'heat riser' ... Can someone please explain how this works ?

The tube seems loose or broken ... is it worth replacing ? or any suggestion on what to do with it ?

[1project2many]

That's actually a "choke stove." The passage it's installed in is connected to exhaust gas. When the engine is started cold, the automotic choke closes for a rich mixture. Once the engine is running, vacuum draws fresh air from outside the engine through the choke stove and into the choke housing where the choke spring sits. The exhaust gas heats the choke stove which in turn heats the fresh air going into the choke housing. The spring inside the housing is a special material known as "bimetallic" and as it warms up it unwinds, opening the choke. If the choke stove is not operating the choke will stay on longer than it should and can use extra fuel or run badly for a while after startup. If you adjust it to be open sooner the choke will not close enough when cold.

Your intake is for a 4 barrel carb which is usually a 4GC also known as a Quadrajet. That's a dual pattern intake (looks aftermarket) so it could use a Holley style carb if you wanted. I've always liked the Q-jet for economy and street driving. Those tubes are often found loose in the plate. Years of heating, cooling, and vibration can do that. If you're careful you can round off the end of a pin punch that's just a hair larger than the inside of that tube and tap it down into the tube to swell it some. You might get away with using some high temp silicone on the inside of the plate to seal it. I probably wouldn't worry if I didn't notice an exhaust leak.

"Back in the day" what we liked to do was to block the exhaust crossover ports to reduce intake heating, discard the choke stove, plug the vacuum port in the housing (tiny little hole behind the spring) and install an electric choke from an 80's model. The electric choke installation is usually easy although that older carb might require a new choke housing to accept it. Just remember to locate a wiring diagram using an oil pressure switch to turn on the choke so it doesn't start warming up until the engine is running.

HTH

PS The heat riser is the tube connected from the air cleaner to the exhaust manifold which warms up the fresh air going into the engine for better emissions.

[mknittle]

Originally Posted by 1project2many

That's actually a "choke stove." The passage it's installed in is connected to exhaust gas. When the engine is started cold, the automotic choke closes for a rich mixture. Once the engine is running, vacuum draws fresh air from outside the engine through the choke stove and into the choke housing where the choke spring sits. The exhaust gas heats the choke stove which in turn heats the fresh air going into the choke housing. The spring inside the housing is a special material known as "bimetallic" and as it warms up it unwinds, opening the choke. If the choke stove is not operating the choke will stay on longer than it should and can use extra fuel or run badly for a while after startup. If you adjust it to be open sooner the choke will not close enough when cold.

Your intake is for a 4 barrel carb which is usually a 4GC also known as a Quadrajet. That's a dual pattern intake (looks aftermarket) so it could use a Holley style carb if you wanted. I've always liked the Q-jet for economy and street driving. Those tubes are often found loose in the plate. Years of heating, cooling, and vibration can do that. If you're careful you can round off the end of a pin punch that's just a hair larger than the inside of that tube and tap it down into the tube to swell it some. You might get away with using some high temp silicone on the inside of the plate to seal it. I probably wouldn't worry if I didn't notice an exhaust leak.

"Back in the day" what we liked to do was to block the exhaust crossover ports to reduce intake heating, discard the choke stove, plug the vacuum port in the housing (tiny little hole behind the spring) and install an electric choke from an 80's model. The electric choke installation is usually easy although that older carb might require a new choke housing to accept it. Just remember to locate a wiring diagram using an oil pressure switch to turn on the choke so it doesn't start warming up until the engine is running.

HTH

PS The heat riser is the tube connected from the air cleaner to the exhaust manifold which warms up the fresh air going into the engine for better emissions.

i remember

[Iceman_70]

Many thanks for the explaination !!

Originally Posted by 1project2many

Your intake is for a 4 barrel carb which is usually a 4GC also known as a Quadrajet. That's a dual pattern intake (looks aftermarket) so it could use a Holley style carb if you wanted. I've always liked the Q-jet for economy and street driving.

Yeah the intake is an 'Edelbrock Performer' ... I think I'll keep the 2GC for now ... and maybe change it later to a Quadrajet. I didn't plan on doing a lot with the carb anyway for now ...

[mr48chev]

Well done 1p2m that's a good explanation on how it works and why it's there.

[Wrenchbender Ret]

2 Many gave a real good explanation on how it works. One correction however. A 4GC is not a Q-jet. The 4GC was a square bore carb that was fazed out in 65. The Q-jet which is a spread bore carb was used from 66 on. Yyou cant use a 2GC carb on that manifold without an adapter. That choke stove looks good. The outer tubes dont fit tight in the holes. The small amount of vacuum lost is not material & still brings up heat. I agree, the Q-jet is the best way to go & also the electric choke.

[OrrieG]

I did not think the tube types pulled any vacuum, just had air chambers on each end (manifold and carb) that passed heated air? The older engines also had the tubes coming from the exhaust manifords. Another type heats a coil instead of an air tube which pushes a rod to open the choke (see photo). That was standard on Qjets too.

[1project2many]

A 4GC is not a Q-jet. The 4GC was a square bore carb that was fazed out in 65.

I edited that post a half dozen times for typos and ended up leaving a glaring mistake right out in the open. Q jet w/ attached choke is is 4MC.

OrrieG, those "divorced choke" systems tend to have trouble as the engine ages and carbon builds up in the exhaust crossover. Carbon removal can be a task by itself and has caused more than one guy to toss the OE manifold in trade for an aftermarket part. Electric choke conversion is the cheap man's way to fix that problem, too.

[Russell Ashley]

I have a question since this topic has come up. I have a Q jet on a Summit intake that doesn't have any way to mount a choke. Is there an after market electric choke conversion that would work with this situation? I don't see any place to mount it to the carb.

[1project2many]

Here's a page that does a great job with pictures and ID info:
http://www.carbkitsource.com/carbs/t...jet-index.html

I'm not aware of any electric conversion for the divorced choke. If your carb uses the divorced choke then you're probably better off buying a newer version and installing an electric choke coil. This is one version of the divorced choke style carb:


Get the carb numbers and ID the carb from the page above and maybe it will turn out to be the correct type.

[Iceman_70]

Thanks guys for the explaination and feedback !!