Carburetor port questions Edit: Shop refused to smog

[Jforeman]

I think I have the routing mostly figured out for the EVAP systems. I unfortunately am in California.

I'm not 100% sure I have the "correct" carb on there.

For the emissions systems, it appears there are three basic systems or components (correct me if I'm wrong):
-The PCV system
-The Thermostatically Controlled Air Cleaner (or Controlled Combustion System)
-The "Others":
TVS (Christmas Tree)
Throttle Return Control

It looks like the PCV is correct.

*The Air cleaner is missing a hose that goes to port "F" on the carb. The manual also references port N, Hose to Air Cleaner. What is the functional difference between port F and N? I don't have a port N, does that matter?
* The TVS system is a mess (can absent be a mees?), but I think Jim K got me sorted out on what goes where.

* The Throttle Return system might have been deleted. I'm pretty sure I don't have the Delay Valve or Solenoid Valve, and I'm not sure about the Vacuum Control Valve. The manual shows it gets plugged into the left side; I have a thingy on the right side that's plugged into the same vacuum line as the transmission Edit: it goes directly to the carb. What is that component, and is it missing some things?

The manual refers to tube identifications for Air Horn, Float Bowl, And Throttle body. I can't find a diagram of where tubes A (vacuum hose to hose break), B (TVS to Solenoid Valve) would go. Also ports not on my carb are ports G, T, and N. FWIW.

The good news is apparently only original CA vehicles need to have all the CA equipment.
See linked album for what I have installed, and notes from manual.

GMC Emissions · Jun 1 – Jul 29, 2025 📸

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[JSanford]

Looks like you have a standard 1973-77 49-state carburetor.

Your throttle return control system is completely missing, don't worry about it you don't need it.

Looks like your vacuum break is hooked up properly.

You don't have F and N, just F which is the fresh air feed for the choke. N would be the fresh air feed for the throttle return control system (which you don't have).

Plug your thermostatic air cleaner vacuum line into the port on your intake manifold behind the carb with the blue cap on it.

Your distributor vacuum advance should be connected to carb port H which you have capped.

If your TVS is functional and you want to use it, connect the bottom port to manifold vacuum (the Tee line shared with the transmission modulator), connect the center port to the distributor, and the top port to carb port H.

 

[Jforeman]

I took it to the closest SMOG facility this afternoon, and basically they refused to test it. The reasons given were 1. it wouldn't stay running 2. They couldn't figure out how to enter it into their computer for the DMV. At least that service was free. They said I should take it to a referee. That seems a little premature to me...

Does anyone know of a shop in the North Bay that is familiar with these?
Would bumping up the idle be a good first step?

It does seem like it's idling *very* low, it smells rich, and I did get it to die on me just as I was pulling into where I live. When I first start it, it idles OK until you give it a little throttle, like enough to disable the choke maybe. Then it idles like crap hot or cold.

I haven't changed the filter in the carb, but I do have one and this will happen before the next start.

I installed an inline pump on the aux tank, and I replaced the hoses on everything from the aux tank forward. Haven't gotten to the main tank yet. Turning the pump on does not alleviate the slow idle. On the positive side, inside the aux tank looked nice and clean.

I had a shop rebuild the carb before I drove it home (about 300 miles). It ran fine on that trip, and it only died once while pulling out of a gas station to a red light. They did not exchange the carb but did the work themselves. That made it start so much better, with no issues other than the starter doesn't turn over as well when hot.

The guy I bought it from put a new starter on, and in the distance between him and the nearest RV shop, the starter went bad, presumably from a lot of previous cranking. As of now it starts well once it starts cranking, with yet another starter on it. When hot it takes an uncomfortable amount of time to start turning over, in reality it's probably just over one second.

I haven't verified the timing yet, but there is a new distributor cap so I was assuming (hoping?) it's correct.

 

[Bhart]

Call Jim Kanomata @ GMCRVPARTS.COM. He has helped several coach owners with smog issues.
(800)752-7502

 

[Jforeman]

Call Jim Kanomata @ GMCRVPARTS.COM. He has helped several coach owners with smog issues.
(800)752-7502

He's been helping me get the hose routing figured out, maybe he knows of a shop in my area. He's not exactly local but close enough I could go see him if push comes to shove. I'm looking forward to his reaction when I relay this story.

Still..."Go get an appointment with BAR" as the other shop's first and only suggestion. I think getting the government involved would be the LAST step.

 

[chris poulsen]

When I tried to get my 78 royal from Nevada smoged two years ago, I was over whelmed Trying to find parts
to match the CA smog diagram. What finally worked for me, (because I flunked the first time missing hose and hydro carbons.
Here is what I did, Rebuilt quadra jet carb, replaced all vacuum lines. plug vacuum line under hood that controls heat air distribution, Add hot air bracket and hose from header to bottom of air cleaner, Adjusted timing.
Dummied up vacuum lines that were not connected due to missing smog parts . My idle vaccum was almost 18 inches when I took it in. I went to different shop 2nd time and passed easily.
Also, make sure they know you have two gas tanks (That le.ts you bypass a check)
Most techs don't have an idea what you should have. They look for the obvious

 

[Jforeman]

If you have a ‘73 and live in CA, why are you required to smog it? My ‘74 doesn’t require smog certification and I live in San Luis Obispo County, CA.

I have a '77 with the 403.

Dummied up vacuum lines that were not connected due to missing smog parts . My idle vaccum was almost 18 inches when I took it in. I went to different shop 2nd time and passed easily.
Also, make sure they know you have two gas tanks (That le.ts you bypass a check)
Most techs don't have an idea what you should have. They look for the obvious

What is the idle vacuum supposed to be?

I think I may have to check the timing, I read on another thread that it can cause difficult starting. Though my issue seems more like a small battery or the cables have too much resistance.

The fact that the entire throttle return system is missing might have thrown them for a loop. I think they didn't want to mess with it. The fact that they couldn't find the info to match to the computer seemed odd, but I didn't want to argue.

 

[JSanford]

You need to call around and find a smog shop that is will ing to do it. Sadly most shops just don't want to bother with anything pre-OBD2. I have my shop here in Sacramento, but that wont help you.

The shop needs to know to put the vehicle in as a heavy duty Class A Motorhome Chassis, GVWR over 10,000.

The throttle return system IS NOT on the list of things they need to check. If all the components are missing, they are not going to know any better. If the throttle return system were partially present and disconnected then they might fail you.

Attached is a picture of the "Emission Control Application Guide" that the shop should have.

Here is the Smog Test manual.

https://www.bar.ca.gov/pdf/smog-check-manual.pdf

I also attached a picture of my smog check so you can see what the checked and what is not applicable.

 

[Jforeman]

You need to call around and find a smog shop that is will ing to do it. Sadly most shops just don't want to bother with anything pre-OBD2. I have my shop here in Sacramento, but that wont help you.

The shop needs to know to put the vehicle in as a heavy duty Class A Motorhome Chassis, GVWR over 10,000.

The throttle return system IS NOT on the list of things they need to check. If all the components are missing, they are not going to know any better. If the throttle return system were partially present and disconnected then they might fail you.

Attached is a picture of the "Emission Control Application Guide" that the shop should have.

Good info. BAR sent me a copy of the page in your photo and I gave it to them. I'm going to guess they were not looking under the right chassis class. I did call them and ask first so I'm not sure where the communications broke down.

Sacramento is an easier drive than San Jose, but I should be able to find something local, maybe I'll start with RV service centers & see if they can recommend someone.

 

[JSanford]

My sister got her junky old 1977 American Clipper motorhome smogged in Santa Rosa. If they could get that thing to pass then Im sure you would have good luck there!

Premium Auto Glass & Smog Check
1075 Santa Rosa Ave
Santa Rosa, CA 95404
(707) 595-1199

 

[Jforeman]

Yesterday I took it to Four Corners Smog in Sonoma, and it passed! It was just across town, which is good because I don't have the rear wheel wells in and highway speeds seem iffy in that configuration. I got new plates & tags and can now get my title. I can understand CA withholding the registration, but the title? What if I want to take it out of state?

Four Corners Smog
20500 Broadway, Sonoma, CA 95476
(707) 938-4176

 

[JSanford]

Glad you got it smogged!